WO2000019240A2 - Amplitude variation as a function of offset attribute and rock property contrast analysis for seismic survey data - Google Patents

Abstract

AVO attributes are determined from seismic survey wave data and a relationship derived between the AVO attributes and specific rock property contrast including inter-face (cap/reservoir), inter-location (A/B), or inter-time (T1/T2). At least two amplitude variation as a function of offset ('AVO') attributes are determined for at least one set of seismic traces. Linearized equations providing curve fit parameters and basic functions are disclosed for determining the AVO attributes. Incident compression wave to reflected compression wave (P-P), incident compression wave to reflected shear wave (P-S), isotropic horizontal reflected wave (S-SH), isotropic vertical reflected wave (S-SV) and joint P-P and P-S AVO attributes are determined and a relationship derived to directly relate the AVO attributes to specific rock property contrasts. The obtained rock property contrasts are used to detect hydrocarbons in the geologic formation at the location of interest.

Description

AMPLITUDE VARIATION AS A FUNCTION OF OFFSET ATTRIBUTE AND ROCK PROPERTY CONTRAST ANALYSIS FOR SEISMIC SURVEY DATA

BACKGROUND OF THE INVENTION Field of the Invention

The present invention is directed, in general, to the use of amplitude variation as a function of offset (AVO) attribute analysis of common mid point (CMP) seismic reflection data, and in particular to the relation of AVO data to rock properties.

Description of the Related Art

In conventional seismic exploration of the earth's geologic structures, acoustic energy is applied to the earth's surface. As the energy travels downward, it is reflected from subsurface interfaces back to the earth's surface. The amplitude of the reflected energy is normally recorded in the form of a series of time samples. By plotting these amplitudes versus a time scale, a representation of the locations and shapes of the subsurface interfaces is generated. The depths of the various interfaces correspond generally to the time of arrival of the various signals.

Techniques such as common depth point (CDP) or common midpoint (CMP) processing improve signal to noise ratio and therefore have improved seismic exploration techniques. In such techniques, a source transmits a seismic wave into the earth's subsurface, and the seismic waves reflected from the underlying rock layers are recorded on receivers at the earth's surface. The recorded seismic signals are organized into gathers of traces, each corresponding to a CDP or a CMP. All of the seismic traces in a gather result from source receiver pairs equally spaced about a CDP or a CMP along a path. The CMP technique assumes that all the source-receiver pairs with a common mid-point on the earth's surface reflect seismic waves from a common depth point (CDP) in the earth's subsurface as illustrated in Figure 1 (labeled prior art).

It is generally known that the amplitude of reflected waves in a CMP gather will vary with increasing distance or offset from its common mid point as illustrated in Figure 2 (labeled prior art). This variation is due to the rock properties at the common depth point in the earth's surface that is being imaged. However, the variation of the amplitude with offset is not directly related to any one rock property. It is, in fact, affected by many rock properties. Current AVO attribute analysis uses two characteristics: (1) the zero offset reflectivity ("A") and (2) the AVO gradient or slope ("B"). The A characteristic represents the strength of the reflected signal where the location of the receiver matches that of the source or the amplitude of the normal incidence wave. The B characteristic represents the rate at which the reflected signal amplitudes vary relative to the squared sine of the incident angle or the rate of change of the amplitude with the offset. While the A and B characteristics resulting from traditional amplitude variation as a function of offset analysis are dependent upon subsurface structure (that is, the interfaces between rock layers of differing types), they do not indicate whether any of the layers contain hydrocarbons, nor can they be interpreted to determine specific rock properties. Therefore, there is a need for a method and system for detern ining rock property information from AVO data.

A number of methods have attempted to analyze the AVO attribute response of CMP seismic gathers to predict the presence of hydrocarbons in the earth's subsurface. Previous techniques have sought to analyze the AVO attribute response in terms of the compression waves (P -waves) that propagate through the earth's subsurface and are reflected from the subsurface layers, using a line to represent the attributes of the AVO data. In such a method, the amplitude variation with offset of pressure wave (a.k.a. "P-wave") data is plotted, approximated as a line. The intersection of the line and the slope are used to interpret likelihood of hydrocarbon structure. However, the intersection attribute of AVO data and the slope attribute cannot be related to rock properties, such as density, change in shear-wave (a.k.a. "S- wave") velocity (ΔVs/Ns) or change in P-wave velocity (ΔVp/Vp). Further, a portion of the propagating P-waves are converted into reflected shear waves (S-waves) at each subsurface layer and, at the offsets used in seismic exploration, the P-S converted waves can be significant reflections. See Castagna, J.P., "AVO Analysis-Tutorial and Review", pp 3-9 of Offset Depth Reflectivity Theory and Practice of AVO Analysis edited by Castagna and Backus; 1993, Society of Exploration Geophysicists, incorporated herein by reference. There is significant information in the S-waves which is not used in AVO analysis, due to a lack of ability to relate the attributes of S- wave AVO to rock properties.

Accordingly, it would be an advancement in the art to provide an AVO attribute analysis method using shear wave and/or compression wave reflections in a manner to determine rock properties from the data and for identifying the location of hydrocarbons within the earth's geological structures.

Further, it would be an advancement to allow for simultaneous display of seismic data and rock properties associated with a given horizon of interest.

SUMMARY OF THE INVENTION

The methods and systems of the present invention include determining AVO attributes from seismic survey wave data and deriving a relationship between the AVO attributes and specific rock property contrast such as compression velocity (Vp), shear velocity (Vs), and density (p), whether inter- face (cap/reservoir), inter-location (A/B), or inter-time (T1/T2), and the AVO attribute values and their crossplot positions. The rock property contrast is used for detection of hydrocarbons in a geologic formation of interest.

The methods and systems of the present invention further include techniques for determining and analyzing the connection between rock property contrasts such as compression velocity (Vp), shear velocity (Vs), and density (p), whether inter-face (cap/reservoir), inter-location (A/B), or inter-time (T1/T2), and the attribute values and their crossplot positions. Linearized equations providing curve fit parameters and basic functions are disclosed for deriving incident compression wave to reflected compression wave (P-P), incident compression wave to reflected shear wave (P-S), isotropic horizontal reflected wave (S-S_H ), isotropic vertical reflected wave (S-S_v) and joint P-P and P-S AVO attributes. According to the invention, various attributes are directly related to specific rock properties. Crossplots and other displays are also generated using the relationships of the attributes to the rock property. Joint AVO attributes, crossplots, and other displays, are interpreted using a combination of relationships between P-P and P-S attributes. The relative displacement of two points in attribute space can be related uniquely to the contrast of the rock properties Vp, Vs, and density, between the two points in real space.

According to one aspect of the present invention, at least two amplitude variation as a function of offset ("AVO") attributes are determined for at least one set of seismic traces. The seismic traces include, for example, compression wave traces, shear wave traces, or joint compression wave and shear wave traces, wherein the seismic traces are of a first type and a second type, and at least two AVO attributes are determined for each of the first and second types of seismic traces. The at least two AVO attributes are determined for seismic traces corresponding to a location of interest, and the presence of hydrocarbon properties of the geologic formation at the location of interest are detected using (1) the derived relationship to obtain a rock property contrast associated with the seismic traces corresponding to the location of interest and (2) the obtained rock property contrast to detect hydrocarbons in the geologic formation at the location of interest.

According to another aspect of the invention, a method is provided for determining a rock property from seismic data comprising: (a) assigning an AVO attribute to the rock property based on a combination of coefficients of a reflectivity equation approximating amplitude variation as a function of offset at a first data point; and

(b) comparing the AVO attribute at a first data point to the AVO attribute at a second data point. According to one example embodiment of this aspect, the coefficients consist essentially of coefficients based on p-p wave data. In a more specific example, the reflectivity equation consists essentially of:

Ampp.p = R(θ) = B0 + B 1 *Tan(θ)² + B2*Tan(θ)² * Sin(θ)

In another specific example, the attribute consists essentially of B2.

Alternatively, the attribute comprises the sum of Bl and B2, twice the sum of Bl and

B2, and/or B0-B1-B2. In still a further alternative, the attribute consists essentially of

2*(B0-B1-B2), and, in an even further alternative embodiment, the attribute consists essentially of (B1+B0) + B2*(l+((l/4)*(Vp/Ns)²).

In another embodiment of the current aspect, the coefficients consist essentially of coefficients based on p-s wave data. In one example, the reflectivity equation consists essentially of:

Amp_P._s=R(θ)=C0*(Sin(θ)*Cos(θ)-Sin(θ)³/(g² -Sin(θ)²)^1/2) +C1 *Sin(θ)/(g² Sin(θ)² )^1/2).

In a more specific example embodiment, the attribute consists essentially of the negative of Cl. In other alternative embodiments, the attribute consists essentially of one-half the difference between Cl and CO, Bl -BO-CO, and/or 2*B0+C1. In even further alternative embodiments, the attribute consists essentially of twice the difference between Bl and CO, C1+B0-B1 and/or - 2*(Vs Vp)*Cl. In even further alternative embodiments, the attribute consists essentially of, in the alternative: 2*(B0 - Bl) + (4*(Ns/Np)* CO,

2*B0 + (2*(Vs Vp)*Cl), 2*(Bl - (2*(Vs/Vp)*C0),

( )*(Np/Ns)² *(B0 - Bl) + ((Vs Vp) + ('/4)*(VptVs))*Cl, (Vs/Np)*Cl - (l/2)*(Vp/Ns)*C0, and/or 2*B0 + (2*(Vs Vp)*Cl).

In a further example, the coefficients consist essentially of coefficients based on s-s wave data, and the reflectivity equation consists essentially of:

Amp_s._SH= R(θ) = DO + Dl*Tan(θ) 2

According to various alternative embodiments, the attribute consists essentially of:

- 2*(D0 + D1), and/or

- 2*D1. In still another alternative, the reflectivity equation consists essentially of:

Amp_s.sv = R(θ) = E0 + El *Tan(θ)² + E2*Tan(θ)² * Sin(θ)²

In alternative embodiments, the attribute consists essentially of: (3/5)*EO + (1/5)*E1, and/or - (4/5)*E0 + (2/5)*El.

In still a further example embodiment of the present aspect, the comparing comprises subfracting the value of the attribute at the first data point from a value of the attribute at the second data point. In yet another example embodiment of the present aspect the comparing comprises plotting the value of the attribute at a first data point and at a second data point. In still another example, the first data point and the second data points are on opposite sides of a reflection interface; while, in yet a further example, the first data point and the second data point are on the same side of a reflection interface. Alternatively, the first data point and the second data point are both in the same reservoir.

According to an even further example of the present aspect, further steps are provided comprising determining a further rock property from the seismic data by a method comprising: assigning a further ANO attribute to the further rock property based on a further combination of coefficients of a reflectivity equation approximating amplitude variation as a function of offset; and comparing the further AVO attribute at the first data point to the further AVO attribute at the second data point.

According to various embodiments, the further coefficients consist essentially of coefficients based on p-p wave data, p-s wave data, and/or s-s wave data. In some embodiments, the comparing steps comprise cross-plotting the attributes, wherein the cross-plot is oriented such that displacement along an axis of the cross-plot is directly related to a change in a single rock property. In certain situations displacement along a first axis of the cross-plot is directly related to a change in a first single rock property and displacement along a second axis of the cross-plot is directly related to a change in a second single rock property.

In still a further embodiment of the invention, the first data point comprises a first spatial data point and the second data point comprises a second spatial data point.

Alternatively, the first data point comprises a first spatial data point at a first time and the second data point comprises the same spatial data point at a different time from the time from the first data point.

According to another aspect of the invention, a method is provided for determining invalid seismic data acquisition assumptions, comprising: determining AVO attributes from seismic data, wherein the attributes are dependant upon multiple modes of wave propagation; comparing attributes which are dependant upon similar physical properties at similar locations, wherein a comparing result is generated; and assigning an error to a comparing result that is outside a predetermined acceptance range.

In one example embodiment of this method, the comparing comprises cross- plotting the attributes; while, in another example, the comparing comprises subtracting a first of the attributes from another of the attributes. In still another example embodiment, the comparing comprises a statistical comparison.

In other example embodiments, the determining comprises determining an AVO attribute from a p-p wave mode and determining an AVO attribute from a p-s wave mode, determining an AVO attribute from a p-s wave mode and determining an AVO attribute from a s-s wave mode, or determining an AVO attribute from a s-s_v wave mode and determining an AVO attribute from a s-s_H wave mode, wherein both attributes are dependent upon a similar physical property. In one such embodiment, the similar physical property comprises a rock property (e.g. density, ΔVp/Vp, and/or ΔVs/Ns).

According to still another aspect of the present invention, a system is provided for determining a rock property from seismic data comprising: an assignor of an AVO attribute to the rock property based on a combination of coefficients of a reflectivity equation approximating amplitude variation as a function of offset at a first data point; and a comparator of the AVO attribute at a first data point to the AVO attribute at a second data point.

Coefficients described above are acceptable according to various embodiments. In an further embodiment of this aspect of the invention, the comparator comprises a subtractor of the value of the attribute at the first data point from a value of the attribute at the second data point. Alternatively, the comparator comprises a plotter of the value of the attribute at a first data point and at a second data point.

As before, in alternative embodiments, the first data point and the second data points are on opposite sides of a reflection interface, the reflection interface comprises a cap/reservoir interface, the first data point and the second data point are on the same side of a reflection interface, or the first data point and the second data point are both in the same reservoir.

In a further embodiment of the present aspect of the invention, there is provided a determiner of a further rock property from the seismic data using: an assignor of a further AVO attribute to the further rock property based on a further combination of coefficients of a reflectivity equation approximating amplitude variation as a function of offset; and a comparator of the further AVO attribute at the first data point to the further AVO attribute at the second data point.

In one such example, the assignor of a further AVO attribute and the assignor of an AVO attribute comprise the same assignor. In another embodiment, the comparator of the further AVO attribute and the comparator of an AVO attribute comprise the same comparator.

According to still a further aspect of the invention, a system is provided for determining invalid seismic data acquisition assumptions, comprising: an AVO attribute determiner, dependent upon seismic data, wherein the attributes are dependant upon multiple modes of wave propagation; a comparator of attributes which are dependant upon similar physical properties at similar locations, the comparator generating a comparator result; and an assignor of an error to a comparator result that is outside a predetermined acceptance range.

In one example embodiment of the present aspect, the comparator comprises a cross-plotter of the attributes. In another example, the comparator comprises a subtractor of a first of the attributes from another of the attributes. In still another example, the comparator comprises a statistical comparator. In a further example embodiment, the AVO attribute determiner is dependent upon an AVO attribute from a p-p wave mode and an AVO attribute from a p-s wave mode, wherein both attributes are dependent upon a similar physical property. In one example, the similar physical property comprises a rock property (e.g. density, ΔVp/Np, and ΔVs/Ns). According to still a further aspect of the present invention, a method is provided for determining rock properties from multiple recordings of seismic data, wherein the multiple recordings are of differing types, the method comprising: determining an amplitude relationship from each of the recordings, and determining a set of rock property relationships from the amplitude relationships.

In one example, the types comprise P-P and P-S. In another example, the types comprise S-S_v and S-S_P In a further example, the determining and amplitude relationship comprises interpolation of the recordings (e.g. cubic spline) and amplitude extraction from the interpolation. In a still further example, application is made of a least square fit to the extraction whereby an amplitude equation for the data results.

In one embodiment, the extraction comprises "isotime" extraction (a term known to those of skill in the art). In an alternative embodiment, the extraction comprises application of a window around a selected time, and the peak within the window is used an extracted amplitude pick, wherein the window comprises a width of about 10 ms, plus or minus the selected time. In still another example, the extraction comprises: selection of a zero crossing closest a selected time and determining a peak closest the zero crossing.

According to still a further aspect of the invention, a system is provided for determining rock properties from multiple recordings of seismic data, wherein the multiple recordings are of differing types, the system comprising: means for determining an amplitude relationship from each of the recordings, and means for determining a set of rock property relationships from the amplitude relationships.

In one example of this aspect, the types comprise P-P and P-S. In another example, the types comprise S-S_v and S-S_P In still a further example, the means for determining and amplitude relationship comprises means for interpolation of the recordings and means for amplitude extraction from the interpolation. In another example, the means for interpolation comprises a means for application of a cubic spline process. In still a further example, the means for amplitude extraction comprises means for application of isotime extraction. Alternatively, the means for amplitude extraction comprises a means for application of a window around a selected time and a means for assigning the peak within the window as an extracted amplitude pick. In yet a further alternative, the means for extraction comprises: means for selection of a zero crossing closest a selected time and means for determining a peak closest the zero crossing. In any such examples, there is further provided, as an option, a means for application of a least squares fit to the extraction whereby an amplitude equation for the data results. According to yet another aspect of the present invention, a system is provided for display of rock properties comprising a first display area showing a seismic data set of a first type a second display area showing a seismic data set of a second type, the second display area being viewable concurrently with the first display area. According to one example of such an aspect, the first display is of a P-P synthetic data set and a second display area is of a P-S synthetic data set. There is further provided a means for selecting a horizon of interest from at least one of two data sets of different types, wherein a data set of a first type is displayed in the first display and a data set of a second type is displayed in the second display. In a further embodiment, the means for selecting comprises a mouse.

In still a further embodiment, rock properties of the horizon are displayed concurrently with the horizon upon selection of the horizon., wherein P-P type data is displayed in one of the first and the second display areas and P-S type data is displayed in the other of the first and the second display areas. According to yet another embodiment, there is further provided a means for displaying rock properties determined from data represented in both the first and second display areas concurrently with the first and the second display areas, wherein the means for displaying rock properties determined from data represented in both the first and second display areas concurrently with the first and the second display areas comprises means for displaying rock properties determined jointly from data represented in the first and the second data sets.

According to an even further aspect of the present invention, there is provided a method for display of rock properties comprising: displaying a first display area showing a seismic data set of a first type, and displaying a second display area showing a seismic data set of a second type, the second display area being viewable concurrently with the first display area.

According to one example of the present aspect the first display is of a P-P synthetic data set, and the a second display area is of a P-S synthetic data set. According to another example, there is further provided the step of selecting a horizon of interest from at least one of two data sets of different types, wherein a data set of a first type is displayed in the first display and a data set of a second type is displayed in the second display; and, in one specific embodiment, the selecting comprises indication of the horizon of interest with a mouse. In a further embodiment, rock properties of the horizon are displayed concurrently with the horizon upon selection of the horizon, wherein P-P type data is displayed in one of the first and the second display areas and P-S type data is displayed in the other of the first and the second display areas. In still a further embodiment, there is provided the step of displaying rock properties determined from data represented in both the first and second display areas concurrently with the first and the second display areas, wherein the displaying rock properties determined from data represented in both the first and second display areas concurrently with the first and the second display areas comprises displaying rock properties determined jointly from data represented in the first and the second data sets.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. Figure 1 (labeled prior art) illustrates the CMP and CDP technique;

Figure 2 (labeled prior art) illustrates the amplitude of reflected waves in a CMP gather will vary with increasing distance or offset from its common mid point;

Figure 3 illustrates the curve shape interpretation of an incident P-wave to a reflected P-wave (P-P) ANO attributes; Figure 4 illustrates the curve shape interpretation of an incident P-wave to a reflected S-wave (P-S) ANO attributes;

Figures 5 and 6 are crossplots illustrating the ANO attribute combinations of P-P wave and P-S ANO attributes;

Figures 7 through 11 illustrate the crossplotting techniques according to the present invention;

Figure 12 illustrates the relationship between and definitions of the interface, spatial and temporal contrasts;

Figure 13 illustrates the relationship between interface and spatial contrast;

Figure 14 illustrates a geologic reservoir having a gas, oil and brine leg using time lapse ANO attributes;

Figure 15 is a crossplot of two temporal contrast attributes;

Figure 16 illustrates a geologic reservoir having a gas, oil and brine leg in a reservoir having water saturation;

Figure 17 illustrates the values of Np and Vs for a porous Gulf of Mexico ("GOM") sand with pay saturation values of 5%, 50%, and 100%;

Figures 18-23 illustrate optimum crossplots according to the present invention and corresponding to the reservoir of Figure 16; and

Figure 24 is a flowchart illustrating the system of the present invention wherein AVO attributes and rock property contrasts are determined from data for seismic traces including shear wave traces.

Figure 25 A and 25B illustrate further embodiments of the invention.

Figure 26 and 27 illustrate even further embodiments of the invention. Figure 28 is a block diagram of an example embodiment of the invention.

Figure 29 is a block diagram of an example embodiment of the invention.

Figures 30A - 30C are block diagrams of example embodiments of the invention. Figure 31 illustrates a example embodiment of the invention.

The use of the same reference symbols in different drawings indicates similar or identical items.

DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

Linearized P-P AVO Attributes

The linearized equations related to the incident P-wave to the reflected P-wave (P-P) AVO attributes, from which the P-P attributes are derived, consider only linear contributions from the exact equations due to rock property contrasts. The equations do not explicitly assume angle limitations, although deviations generally become most extreme at large angles of incidence. These deviations occur when large rock property contrasts are involved or critical angle effects are present, because both require nonlinear contributions to describe their behavior. Two example forms of the linear equations useful according to example embodiments of the invention are as follows:

Amp_P.p= R(θ) = BO + Bl*Tan(θ)² + B2*Tan(θ)² * Sin(θ)²

wherein R(θ) is the theoretical reflectivity amplitude. The first two terms of such an equation are commonly known in the art as the Bortfeld equation. The three-term equation above (BO, Bl and B2) shall be referred to herein as a three-term P-P equation method of the present invention is also practiced according to an alternative embodiment:

Amp_P.p - R(θ) = A + B* Sin(θ)² + C*( Tan(θ)² - Sin(θ)² ),

the first two terms of which are commonly known as Shuey's approximation. Here, the three-term equation comprises another three-term P-P equation. Any other algebraically equivalent reflectivity equation is also useful according to the P-P embodiment of the present invention.

In the above equations θ is the P-wave angle of incidence and the coefficients B0, Bl, B2, and A, B, and C are the P-P AVO attributes. The method of the present invention discusses particularly the use of the Bortfelt-type equation. However, the equations are related wherein: B0=A,

B1=B, B2=B-C, and

C=B1+B2.

The P-P attributes BO, Bl, and B2 have a simple geometric or curve shape interpretation, as illustrated in Figure 3.

According to one aspect of the present invention, the relationship of the attributes to the rock property contrasts across an interface can be determined. The novel relationships are given by linearized equations as follows:

BO = ¹/2*ΔNp/Np + V₂ *Δp/p;

Bl = '/2*ΔVp/Np - 2*(Vs/Vp)² * (2*ΔVs/Vs + Δp/p);

B2 = 2*(Ns/Np)² * (2*ΔVs/Vs + Δp/p); and

wherein Vp is compressional velocity, Vs is shear velocity, p is bulk density, Δ represents contrast across an interface and the Vs/Vp ratio is the average across the interface. These relationships are used, according to an example embodiment of the invention, to relate rock properties to the P-P AVO attributes, as follows:

ΔVp/Np is associated with 2*(B1+B2);

Δp/p is associated with 2*(B0-B1-B2); and

ΔNs/Ns is associated with (Bl-B0)+B2*(l+((l/4)*(Np/Ns)²).

According to still a further embodiment, the rock properties are cross plotted, wherein a change in the rock property will be associated with hydrocarbon presence, as discussed more fully below. Linearized P-S AVO Attributes

According to an alternative embodiment of the present invention, a linearized equation related to the incident P-wave to the reflected S-wave (P-S) AVO attributes is provided, wherein the converted wave reflections (S-waves) are a function of the P- wave angle of incidence (θ) as follows:

Amp_P._s =R(θ)=C0*(Sin(θ)*Cos(θ)-Sin(θ)³/(g² -Sin(θ)² )^1/2)

+Cl *Sin(θ)/(g² Sin(θ)²)^1/2);

wherein g is the average Vp/Vs ratio and wherein the P-S attributes, CO and Cl, are related to the rock property contrast by;

CO = -(Vs/Np)*(2*ΔVs/Vs + Δp/p); and

Cl = -(l/2)*(Vp/Vs)*(Δp/p).

Therefore, according to this embodiment of the invention, the following rock properties are directly related to CO and Cl according to the following relationships:

Δ p/p = -Cl; and

Δ Vs/Vs = (l/2) * (Cl - C0)

Note that the above equations provide curve fit parameters for cases in which the rock property contrasts are not large. The linearized equation is parameterized using the P-wave incident angle only. The P-S wave equivalent curve shape interpretations are illustrated in Figure 4. The individual curves are plotted separately since this shows their geometric nature more clearly. The horizontal axis as illustrated in Figure 4 is a function of θ rather than θ² as in the P-P wave case. The Cl term contributes a linear slope out to roughly forty degrees while the CO term contributes a non-linear curvature term. Also, the horizontal axis is a function of θ rather than θ² as in the P-P wave case, and there is not a conesponding intercept term. Joint P-P and P-S AVO Attributes

According to still a further example embodiment of the present invention, P-P and P-S AVO attributes are combined jointly, and relationships are developed between the rock property contrasts and the joint attributes. According to one specific embodiment of such a joint combination rock property attributes are related to the P-P and P-S AVO attributes as follows:

Δp/p = Bl - B0 - C0;

ΔVp/Vp = 2*B0 +C1;

ΔVp/Np = 2*(Bl - C0);

ΔVs/Ns = C1+ BO - Bl; and

wherein Vp/Ns is approximately two (2).

The relationship wherein Vp/Vs is approximately two (2) is appropriate for various types of media (e.g., any depositional system whether clastic or carbinate where depth of burial is greater than 2 or 3 thousand feet; older depositional basins satisfy this criteria at sallower depths than younger basins as Gulf of Mexico) and also allows for faster processing of data. However, in an alternative specific example of the invention, the relationship between the rock property contrasts and the combined P-P and P-S AVO attributes comprise:

Δp/ρ = - 2*(Vs/Vp)*Cl;

Δp/p = 2*(B0 - Bl) + (4*(Vs/Vp)* CO;

ΔVp/Vp = 2*B0 + (2*(Vs/Vp)*Cl);

ΔVp/Vp = 2*(B1 - (2*(VsNp)*C0)); ΔVs/Vs = (¹/₄)*(Vp/Ns)² *(B0 - Bl) + ((Vs/Vp) + (¹/₄)*(Vp/Vs))*Cl; and

ΔVs/Vs = (Vs/Np)*Cl - (l/2)*(VpNs)*C0;

wherein Vp/Ns is not limited, and the relationships are exact to linear order changes of rock properties.

As before, the above equations are used, according to one aspect of the invention, to directly relate the rock properties to the AVO attributes. Further, according to yet another embodiment of the invention, more fully discussed later, crossplots are formed involving these joint attributes that a displacement of a point along any axis can be directly related to the contrast of a single rock property; and, because there is a redundancy in the calculation of any rock property contrast, crossplots are made of any two redundant combinations, in yet another embodiment, producing a collection of points falling along a line at 45 degrees (as illustrated in

Figures 5 and 6 for a ΔVp/Np). Figure 6 illustrates an actual response for a water saturation case showing the points falling along the expected 45-degree line. Any deviation from the 45-degree line indicates either processing enors or the presence of anisofropy. The joint AVO attribute combinations provide a clear separation between desirable and undesirable geological structures and a displacement along either axis is associated with the contrast of a single rock property.

Linearized S-S„ AVO Attributes According to still another embodiment of the present invention, a method and system are provided in which linearized equations related to the isotropic horizontal reflected waves (S-S_H ) are used. According to this embodiment, isotropic horizontal wave reflections (S_H -waves) are described as a function of the P-wave angle of incidence (θ) as follows:

Amp_s._SH = R(θ) = DO + D 1 *Tan(θ)² ; wherein the S- S_H attributes DO and Dl are given by;

DO = - ('A)* (ΔVsNs + Δp/p); and

Dl = (¹/₂)*(ΔVsNs).

Further, the rock property contrasts in terms of the S-S_H attributes according to the present invention are as follows;

Δp/p = - 2*(D0 + Dl); and

ΔVs/Vs = 2*D1.

As before, a further embodiment of the invention comprises the cross plotting of the attributes, as described more fully below.

Linearized S-Sy AVO Attributes

According to an even further embodiment of the invention, a method and system are provided that comprise linearized equations related to the isotropic vertical reflected waves (S-S_v) wherein the isotropic vertical wave reflections (S_v -waves) as a function of the P-wave angle of incidence (θ) as follows:

Amp_s.sv = R(θ) = E0 + El *Tan(θ)² + E2*Tan(θ)² * Sin(θ)² ;

wherein the S- S_v attributes E0, El and E2 are given by;

E0 = - (H)* (ΔVs/Vs + Δp/p);

El = (7/2)*(ΔVs/Ns) + 2*(Δp/p); and

E2 = -2*(2*(ΔVs/Vs) + Δp/p). Further, the rock property contrasts in terms of the S-S_v attributes according to the present invention are as follows;

Δp/p = (3/5)*EO + (1/5)*E1; and

ΔVs/Ns = - (4/5)*E0 + (2/5)*El.

Yet again, another embodiment comprises the crossplotting of the attributes.

Joint AVO Inversion Analysis

According to an even further embodiment of the invention, the amplitude equations described above are used to determine "joint attributes" and to determine the rock properties based on multiple types (e.g. P-P, P-S, S-S_v, and S-S_H) of waves. Such analysis is possible due to the fact that the amplitude equations are expressed in term of the rock properties: ΔVp/Vp, ΔVs/Vs, and Δp/p. Therefore, a simultaneous solution of various combinations of the Amp_x._x equations will provide, directly, a value for the rock properties. In embodiments in which the various attributes (e.g. BO, Bl, B2, CO, Cl, DO, Dl, E0, El, E2) are also desired, they are then calculated according to the above equations relating the attributes to the rock properties. Such a simultaneous solution of amplitude equations from multiple recordings has the advantage that the result is not dependent upon just one recording or on one type of data, but on multiple recordings and/or multiple types of data. It has been found that if the rock property is determined from a process dependent upon only one recording (e.g. only P-P data), the accuracy of the determination is less than if it is determined from a simultaneous solution from multiple types of recordings. In a preferred embodiment, the Amp_P._P and Amp_P._s equations are used to determine the rock properties.

According to one example embodiment of joint P-P, P-S AVO inversion, the following steps are performed in a computer system: 1) Parameters for a program are selected, including: a) P-P and P-S angular apertures b) Noise reduction methods (e.g. Robust Fit percentage, Use of super gathers, both of which are understood by those of skill in the art, and other methods which will be known to those of skill in the art.) c) Amplitudes are interpolated d) Amplitudes are extracted e) A Vp/Vs value is selected 2) Inversion (i.e. simultaneous solution of the Amp_P.P and Amp_P.s equations) is performed, resulting in rock property values. It should be noted that no single combination of the above steps is necessary according to the invention, and some steps are eliminated entirely in some embodiments of the invention. For example, no noise reduction might be applied when the signal to noise ratio is substantial.

Referring now to step lc, even with a small sample size in a digital recording, the peak amplitude may not be present. Accordingly, in one embodiment of the invention (although not all, necessarily), the amplitudes are interpolated. In one such embodiment, a "cubic spline" process (a process known to those of skill in the art) is used. According to other embodiments, other interpolation processes are used, which will also occur to those of ordinary skill.

Referring now to step Id, the amplitudes for a given horizon of interest are extracted. In one example embodiment, the so-called "isotime" method, known to those of skill in the art, is used. According to this process, a time in the record is selected and the value across the gather or bin is picked. According to a further example, a window around the selected time (e.g. 10 ms, plus or minus the selected time) is used, and the peak within that window is used as the pick. According to still a further embodiment, the closest zero-crossing near the selected time is found, and the peak from that zero-crossing is used as the selected peak. It will be noted that such a peak may or may not occur within a specific window. Once the amplitudes are interpolated, a least squares fit is applied to determine the amplitude equation for the data. Referring now to step le, normally the VpNs ratio is presumed to be 2.

However, in certain situations, such a presumption does not hold. Therefore, according to some embodiments of the invention, low frequency estimates of the velocity functions are used to determine the Vp/Vs ratio to be used in the inversion. The methods for such low frequency estimates are known to those of skill in the art.

The process is repeated in further embodiments to refine parameter selection for a particular set of data. Further, cross-plotting, as described below, and other comparison operations are performed, according to still further embodiments, to test data quality. For example, certain combinations of attributes yield the same rock property. A cross plot of such combinations results in a substantially 45 degree angle for good data. If it is not, there is an indication of a problem with the data.

Further, it should be noted that so-called "true amplitude" data (i.e. data in which the relative strength of reflection events from one reflector to another has not been equalized or otherwise distorted) is preferred.

Referring now to Fig. 28, a further aspect of the invention is disclosed in which a system for determining rock properties from multiple recordings of seismic data is provided, wherein the multiple recordings are of differing types, the system comprising: means 2901 for determining an amplitude relationship from each of the recordings, and means 2902 for determining a set of rock property relationships from the amplitude relationships. Various embodiments, both hardware and software, of means for determining amplitude relationships will occur to those of skill in the art without departing from the invention. An example of one specific embodiment useful according to the invention is seen in Figure 29. Here, the means 2901 for determining an amplitude relationship comprises: means 2903 for interpolation of the recordings means 2904 for amplitude extraction from the interpolation. Again, many such means are known to those of skill in the art. In a more specific embodiment, the means 2903 for interpolation comprises a means 2905 for application of a cubic spline process. Also seen in Fig. 29, the system further comprises a means 2906 for application of a least squares fit to the extraction whereby an amplitude equation 2907 for the data results. Referring now to Fig. 30A, an example means 2904 for amplitude extraction is seen. In this embodiment of the invention, the amplitude extractor comprises means for application of isotime extraction. As before, the extraction is followed by application of a least squares fit to the extraction whereby an amplitude equation for 5 the data results. 159. Alternatively, as seen in Fig. 30B, the means 2904 for amplitude extraction comprises a means 2910 for application of a window around a selected time and a means 2911 for assigning the peak within the window as an extracted 10 amplitude pick. 160.

Again, such means, both in hardware and software, are known to those of skill in the art. In some examples, the window comprises a width of about 10 ms plus or minus the selected time

As before, a means 2912 is provided for application of a least squares fit to the 15 extraction whereby an amplitude equation for the data results.

Referring now to Fig. 30C, a further alternative is seen in which the means for extraction comprises: means 2920 for selection of a zero crossing closest a selected time, and means 2921 for determining a peak closest the zero crossing. 20 Again, such means are known; and, again, a means 2922 is provided for application of a least squares fit to the extraction, whereby an amplitude equation for the data results.

Crossplotting Techniques

25 The displacement in any crossplot space associated with the contrast of any single rock property contrast is determined using the relationships according to the present invention for the joint P-P and P-S AVO attributes. The crossplots are generated using software crossplotting and processing programs, such as, for example, the NUCLEUS RESPACK™ by PGS Seres, Inc. Figures 7 through 11 illustrate the

30 displacements for the standard P-P and optimum P-S and joint AVO attributes. Figure 7 illustrates P-P AVO attributes B0 vs. Bl and Figure 8 illustrates P-P AVO attributes B0 - Bl vs. B0 + Bl wherein the displacement associated with each rock property contrast is shown. For both Figures 7 and 8, a displacement in any given direction cannot be uniquely tied to a single rock property contrast. In Figure 8, however, showing an example embodiment of the invention, either of a pair of rock properties to the displacement. Figure 9 illustrates a further embodiment, in which P- S AVO attributes Cl vs. (1/2)*C1 + CO are crossplotted, and the displacement associated with each rock property contrast is shown. Figures 10 and 11 illustrate still a further embodiment in which joint P-P and P-S AVO attributes Cl vs. 2*(B1-C0) and Cl vs. (2*B0 - Cl), are crossplotted respectively, wherein the displacement associated with each rock property contrast is shown. Figures 9, 10 and 11 illustrate optimal crossplots having joint attributes wherein a displacement along any axis is directly related to the contrast of a single rock property.

The crossplot displacements illustrated in Figures 7 through 11 and the concepts related to the illustrated crossplots apply to interface contrast (cap/reservoir) using absolute crossplot location. The concepts apply to location contrasts (A/B) using a Vp/Ns substantially the same at A and B. The concepts apply to temporal contrasts (T1/T2) using an absolute crossplot location and crossplotting Attributes as described below.

Interface. Spatial and Temporal Contrasts

It will be noted that the description of the above embodiments is focused on determination of rock property differences between locations on differing sides of an interface (for example, between a cap and a reservoir). For purposes of determining whether a particular reservoir contains hydrocarbons, such information is quite interesting. More interesting, however, in cases in which a producing reservoir is known, is the difference between rock properties within the reservoir. For example, in a given reservoir, the density may vary from one horizontal location to another. Knowledge of the variations in rock properties and/or AVO attributes would aid an interpreter in determining the "sweet spot" in the reservoir, greatly increasing the chances of drilling a commercially viable well. Further, knowledge of variations in the rock properties and/or AVO attributes with time for the same spacial location would aid interpreters in reservoir monitoring and development

(so-called "4D" or "time lapse" processing and analysis). From such information, hydrocarbon and other fluid flow within the reservoir is obtained. Accordingly, there is a need for determining differences in rock properties and AVO attributes spacially and temporally.

Therefore, according to a further aspect of the invention, a general expression of an AVO attribute at a location A has been discovered which can be compared to the same attribute at another location (across an interface or within the same zone) or at the same location, but at a different time. For example, referring to Figure 12, presume that points A and B are located at different spacial locations along a cap/reservoir interface. The interface contrast at A is (ΔVp/V, ΔVs/s, and Δp/p)_A The interface contrast at B is (ΔVp/N, ΔNs/s, and Δp/p)_B Therefore, the spatial reservoir or cap contrast is:

δVp/Np = (Vp_B - Vp_A)Np

δVp/Np = (Vp_B - Vp_A)/Np

δp/p = (p_B - p_A)/p

Further, any of the attributes B0-B2, C0-C1, D0-D1, E0-E2, and others that may occur to those of skill in the art), is given by:

Attrib_A = (α,*ΔVp/Np + α₂*ΔVs/Vs + α₃ *Δp/p) A

For example, for the Bl attribute at location A, α, = ^lΛ, ₂ = 4, and α₃ = 2. At another location or time B, the same attribute is given by:

Attrib_B = (β,*ΔNp/Np + β₂*ΔVs/Vs + β₃ *Δp/p) B '

This equation presumes that α = β, which are both a function of Vp/Vs and has been found to be substantially stable across time and the spacial boundries used in typical surveys. Accordingly, for the case where points A and B are at the interface, then the difference between the attributes is:

Attrib_A - Attrib_B = (α,*δVp/Vp + α_:*δVs/Ns + α₃ *δp/p)_{AB(RESERV0IR)} - (α,*δVp/Vp + α₂*δNs/Vs + α₃ *δp/p)_AB(CAP).

Note that the δVp/Np term for the attribute at A is different from the δVp/Vp term at B.

Referring now to Figure 13, as a further example, a cross-plot of a first attribute (ATTRTBl) and a second attribute (ATTRIB2) is seen for two points in the reservoir (point A and point B). The change in the position of the cross-plot of the two attribute shows that there is a rock property difference in the reservoir and is given by:

Attrib_B - Attrib_A = (α,*δVp/Np + α₂*δVs/Ns + α₃ *δρ/p)_{BA(RESERVOIR)} As an example of temporal processing, if joint AVO attributes are calculated from seismic traces acquired at times Tl (Attl_T1 and Att2_T1) and T2 (Attl_T2 and Att2_T2) then differences between the above described optimum joint attributes are as follows:

Temporal Contrast Attribute = dAttl - Attl_T2 - Attl_T1 ; and

Temporal Contrast Attribute = dAtt2 = Att2_T2 - Att2_T1 . In some embodiments, the temporal contrasts are used for crossplotting in a manner similar to the other attributes. An advantage of these temporal attributes is that they are related directly to the change in rock properties of the reservoir due to fluid movement caused by the reservoir being produced. Figure 14 illustrates a geologic reservoir with a gas, oil and brine leg. The positions of the legs are indicated for times Tl and T2. It is expected that there will be a zone in which the fluid has changed due to production, while the properties of the reservoir frame and the cap remain constant. Figure 15 illustrates two temporal contrast attributes crossplotted wherein the preprocessing was performed at true amplitude. The displacement direction and distance directly indicate the change in rock properties in that zone, which is directly related to the degree to which the reservoir is swept and whether there are bypass zones or areas of inefficient sweep.

As a more specific example, the relationship between the time lapse attribute dAtt calculated from combinations of the B0, Bl, B2, CO and Cl values for temporal contrasts according to another embodiment of the present invention is as follows: BO = '/2*dVp/Np + ^lΛ*dplp;

Bl = = ¹/₂*dNp/Np - 2*(Vs/Np)² (2 dVs/Vs + dp/p);

B2 = 2*(Vs/Np)² *(2 dVsNs + dp/p);

CO = - (Ns/Np)*(2*dVs/Ns + dp/p); and

Cl = - (¹/₂)*(Vp/Vs)*(dp/p).

Quality Control

Commonly, high quality true amplitude processing is desired to obtain accurate AVO attributes. Therefore, it is advantageous to use crossplots as a method of quality control processing and calibration of seismic traces. A further embodiment method of the present invention provides the quality control processing and calibration using the rock property contrasts calculated from various combinations of attributes. According to a specific example of such an embodiment, the attribute combinations as discussed for joint P-P and P-S AVO attributes, which coπespond with a single rock property contrast, are crossplotted. Here, the points fall on a 45- degree line as shown in Figures 5 and 6. The points deviate from the line if there are processing problems or if the zones are anisotropic. According to still a further embodiment, processing is also checked by producing three crossplots that form each combination of joint attributes indicated above as follows:

- Cl versus Bl - B0 - CO;

2*B0 + Cl versus 2*(B1 - CO); and

Cl + B0 - Bl versus = (¹/₂)* C1 - CO. The ability to robustly determine the percentage of pay vs. brine in any part of a reservoir has been difficult using typical AVO techniques. The method of the present invention provides for robustly detecting hydrocarbons, even in geologic formation having water saturation problems. The discrimination of high from low pay saturation is possible using the method and system of the present invention for any type of reservoir in which the in-situ rock properties of a high pay saturation version of the reservoir are noticeably different than the low pay saturation version, such as in GOM pay sands and North Sea reservoirs. Further, the present invention is not adversely affected whether the reservoir is a low impedance type III GOM pay sand or a high impedance type II carbonate as long as the degree of pay noticeably affects the reservoir's rock properties.

To further illustrate the application of an embodiment of the present invention, a high porosity GOM pay sand is considered in Figures 16 - 23. Figure 16 illustrates this reservoir with a brine, oil, and gas leg. In the case in which the oil leg is missing, the brine sand goes to a variable water saturation gas sand. Figure 17 shows the variation of the values of Vp and Vs for a porous GOM sand having pay saturation values of 5%, 50%, and 100%. The density values vary from 2.35 to 2.10 g/cc linearly with water saturation. The coπesponding optimum crossplots are shown in Figures 18 - 23 wherein a small circle indicates brine and a small "x" indicates pay. Figure 18 shows the B0 vs. Bl crossplot for the case where density variations only are considered for the various pay saturations. Figure 19 shows the B0 vs. Bl crossplot for the case where Vs and density variations are considered for the various pay saturations. Figure 20 shows the B0 vs. Bl crossplot for the case where Vp, Vs and density variations are considered for the various pay saturations. The crossplots of Figures 18, 19, and 20 correspond to the expected displacements shown in Figure 7.

Figure 21 shows the Cl vs. (1/2)*C1 - CO crossplot for the case where Vp, Vs and density variations are considered for the various pay saturations. The crossplot of Figures 21 coπesponds to the expected displacements shown in Figure 9. Figure 22 shows the Cl vs. 2*(B1 - CO) crossplot for the case where Vp, Vs and density variations are considered for the various pay saturations. The crossplot of Figures 22 corresponds to the expected displacements shown in Figure 10. Figure 23 shows the Cl vs. 2*B0 + Cl crossplot for the case where Vp, Vs and density variations are considered for the various pay saturations. The crossplot of Figures 23 corresponds to the expected displacements shown in Figure 11.

According to still a further example embodiment of the present invention, a system is provided which includes a plurality of source-receiver pairs for transmitting seismic signals and receiving seismic signals from a plurality of common depth points in the geologic formation, and, for example, a processor and a memory coupled to the processor for processing the seismic signals. The system includes a recording module, an attribute module, a relationship module, and a hydrocarbon detection module wherein hydrocarbons are detected in a geologic formation using AVO attributes and rock property contrasts. The modules are, for example, either hardware based, software based or a combination of hardware and software.

Figure 24 is a flow chart illustrating the operation of the recording module 100, the attribute module 105, the relationship module 110, and the hydrocarbon detection module 115. Process starts at block 120 wherein the plurality of source- receiver pairs transmits and receives the seismic signals from a plurality of common depth points in the geologic formation of interest. As per block 125, the recording module 100 records the seismic signals and organizes the seismic signals, per block 130, into gathers of seismic traces wherein each seismic trace corresponds to one of the common depth points. The seismic traces include, for example, shear wave traces only or, for example, include a first type and a second type of seismic traces such as joint compression wave and shear wave related traces.

As per block 135, the attribute module 105 determines, for at least one set of seismic traces, at least two amplitude variation as a function of offset ("AVO") attributes using the appropriate linearizing reflectivity equations as previously described, per block 140. The linearizing reflectivity equations define a set of basic functions and curve- fit parameters. As per block 150, the relationship module 110 derives a relationship between the at least two AVO attributes and a specific rock property contrast. As per block 155, the hydrocarbon detection module 115 determines the at least two AVO attributes for seismic traces coπesponding to a location of interest. Using the derived relationship to obtain a rock property contrast associated with the seismic traces corresponding to the location of interest per block 160, the hydrocarbon detection module 115 detects the presence of hydrocarbons in the geologic formation at the location of interest using the obtained rock property contrast.

In the present invention, the P-P, P-S, and joint P-P and P-S AVO attribute crossplots provide unique advantages in extracting from seismic interface, spatial and temporal contrast values for each of the rock properties such as V, Vs, and density (p). The method and system of the present invention provides advantages in determination of water saturation, reservoir quality, fluid type discrimination and time lapse reservoir monitoring.

Referring now to Fig. 26, according to still a further aspect of the invention, a system 105 is provided for determining a rock property from seismic data held in memory 100. According to this aspect of the invention, an example embodiment comprises: an assignor 101 of an AVO attribute to the rock property based on a combination of coefficients of a reflectivity equation approximating amplitude variation as a function of offset at a first data point; and a comparator 103 of the AVO attribute at a first data point to the AVO attribute at a second data point.

According to one embodiment, the coefficients consist essentially of coefficients based on p-p wave data. According to an alternative embodiment, the coefficients consist essentially of coefficients based on p-s wave data; while, in even further alternative embodiments, the coefficients are based on s-s wave data. The coefficients are taken from reflectivity equations such as, for example, the following:

Amp_P.p- R(θ) = B0 + Bl*Tan(θ)² + B2*Tan(θ)² * Sin(θ)²

wherein the attributes consists essentially of: B2, the sum of Bl and B2, twice the sum of Bl and B2, B0-B1-B2 (or some multiple, thereof, for example: 2*(B0-B1-B2), and/or (B1+B0) + B2*(l+((l/4)*(Vp/Ns)²).

Amp_P.s=R(θ)=C0*(Sin(θ)*Cos(θ)-Sin(θ)³/(g² -Sin(θ)²)^1/2) +C1 *Sin(θ)/(g² Sin(θ)²)¹'²); wherein the attributes consist essentially of: the negative of Cl, one-half the difference between Cl and CO, B1-B0-C0, 2*B0+C1, twice the difference between Bl and CO, and/or

C1+B0-B1, for Vp/Ns = 2. In other embodiments, however, the assumption of NpNs = 2 is not necessarily accurate. In such embodiments, the attributes consists essentially of: 2*(Ns/Np)*Cl, 2*(B0 - Bl) + (4*(Vs/Vp)* CO,

2*B0 + (2*(Vs/Vp)*Cl),

2*(Bl - (2*(Vs/Vp)*C0)),

(! )*(Vp/Ns)² *(B0 - Bl) + ((Vs/Vp) + (^>/₄)*(Vp/Ns))*Cl,

(Vs/Np)*Cl - (l/2)*(Vp/Vs)*C0, and/or 2*B0 + (2*(Ns/Np)*Cl).

Further example reflectivity equations from which the coefficients are taken in even further embodiments include: Amp_s._SH= R(θ) = DO + Dl*Tan(θ)², wherein the attributes consists essentially of: 2*(D0 + D1), and/or 2*Dl; and

Amp_s._sv = R(θ) = E0 + El*Tan(θ)²+ E2*Tan(θ)² * Sin(θ)², wherein the attributes consists essentially of: (3/5)*E0 + (1/5)*E1, and/or - (4/5)*E0 + (2/5)*El.

Referring now to Fig. 25A, a more specific example embodiment is seen in which the comparator 103 comprises a subtractor of the value of the attribute at the first data point from a value of the attribute at the second data point. Referring to Fig.

25B, an alternative embodiment is seen in which the comparator comprises a plotter 104 of the value of the attribute at a first data point and at a second data point.

It should be noted that in some embodiments, the first data point and second data points are on opposite sides of a reflection interface, for example, a cap/reservoir interface. While in alternative embodiments, the first data point and the second data point are on the same side of a reflection interface, for example, in the same reservoir.

According to still a further embodiment, referring again to Fig. 26, a determiner 105' of a further rock property from the seismic data using: an assignor 107 of a further AVO attribute to the further rock property based on a further combination of coefficients of a reflectivity equation approximating amplitude variation as a function of offset; and a comparator 109 of the further AVO attribute at the first data point to the further AVO attribute at the second data point. As above, in more specific embodiments, the further coefficients consist essentially of coefficients based on p-p wave data, p-s wave data, and s-s wave data, with equations as described above.

Referring now to Fig. 27, an alternative to the embodiment of Fig. 26 is seen, in which only one assignor 110 and one comparator 112 are used for each of the first AVO attribute and the further attribute. The modutes 110 and 112 alternate which data is input in such an embodiment. In some embodiments, the further coefficients comprise coefficients from the same reflectively equation as in the assigning of the first attribute while in alternative embodiments, the further coefficients comprise coefficients from a reflectively equation different from the reflectively equation used in the assigning of the first attribute. Thus, the system allows for attributes to depend on, for example, both p-p coefficients and p-s coefficients.

In example the embodiments in which a cross-plotter 104 is used, the cross- plot is oriented wherein displacement along an axis of the cross-plot is directly related to a change in a single rock property. Such orientation allows for direct detection of rock property change, as opposed to an orientation in which the single rock property is not axis-aligned. In still further embodiments, displacement along a first axis of the cross-plot is directly related to a change in a first single rock property and displacement along a second axis of the cross-plot is directly related to a change in a second single rock property. It should also be noted that the term "plot" as used herein includes both physical plotting (e.g. on paper or another display medium (e.g. CRT)), and the term also includes mapping within a computer memory, wherein the "plot" of the data within the machine is available for use by further automated processing. It should also be noted that the term "data point" refers to both spatial and temporal data points. The general method of the payment invention works on

"location" of AVO analysis which are either spatially different (i.e. on different sides of a reflection interface), or temporally different (some spatial location, but different times).

Referring now to Fig. 27, according to an even further aspect of the invention, a system is provided for determining invalid seismic data acquisition assumptions (e.g. acquisition errors, anisofropy, etc.), comprising: an AVO attribute determiner 120, dependent upon seismic data 122, wherein the attributes are dependant upon multiple modes of wave propagation (e.g. p- p, p-s; and or s-s propagation).

In such an embodiment, a comparator 124 of attributes which are dependant upon similar physical properties at similar locations, generates a comparator result 126, and an assignor 128 of an enor to any comparator result that is outside a predetermined acceptance range is provided.

In one embodiment, the comparator comprises a cross-plotter of the attributes; while, in another embodiment, the comparator comprises a subtractor of a first of the attributes from another of the attributes. In an even further embodiment, the comparator comprises a statistical comparator. Specific example embodiments of an eπor assignor 128 will be known to those of skill in the art.

While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. For example, while the prefeπed embodiment is set forth as software, it is anticipated that the invention could be implemented in hardware such as an application specific integrated circuit. Accordingly it is to be understood that the present invention has been described by way of illustrations and not limitations.

Further Analysis and Display

According to an event further embodiment of the invention, it has been determined that a particular method of display is particularly beneficial. Referring now to Figure 31, an embodiment of the display is seen in which a first display area 2801 shows a P-P synthetic data set with noise on the left and a second display area 2804 shows a P-S synthetic data set with noise. A mouse or other indicator is used to select the horizon of interest, shown by anows 2806 and 2808. On the right, in display area 2810, three different sets of analysis are seen in display areas 2810a, 2810b and 2810c. In area 2810a, the rock properties are determined from the P-P amplitudes only. In area 2810b, the rock properties are determined from the P-S amplitudes only. In area 2810c, the rock properties are determined from the P-P and P-S amplitudes, jointly. In each of the display areas 2810a and 2810b, there is a graph of the amplitude of the P-P data or the P-S data, respectively, as a function of offset. Also, there is a bar chart of the rock properties from the P-P and the P-S data, respectively. In area 2810c, by contrast, there is only a bar chart, showing the rock property determination based on the joint analysis of the P-P and P-S data. The above embodiments of the various aspects of the invention are given by way of example only. Other acts, structure, hardware, and/or software, will occur to those of skill in the art which do not depart from the spirit of the invention as defined by the claims below.

Claims

WHAT IS CLAIMED IS:

1. A method of determining a rock property from seismic data comprising: (a) assigning an AVO attribute to the rock property based on a combination of coefficients of a reflectivity equation approximating amplitude variation as a function of offset at a first data point; and (b) comparing the AVO attribute at a first data point to the AVO attribute at a second data point.

2. A method as in claim 1 wherein the coefficients consist essentially of coefficients based on p-p wave data.

3. A method as in claim 2 wherein the reflectivity equation consists essentially of: Ampp.p = R(θ) = BO + B 1 *Tan(θ)² + B2*Tan(θ)² * Sin(θ)².

4. A method as in claim 3 wherein the attribute consists essentially of B2.

5. A method as in claim 3 wherein the attribute comprises the sum of B 1 and B2.

6. A method as in claim 3 wherein the attribute consist essentially of twice the sum ofBl and B2.

7. A method as in claim 3 wherein the attribute consist essentially of B0-B1-B2.

8. A method as in claim 3 wherein the attribute consists essentially of 2*(B0-B1- B2).

9. A method as in claim 3 wherein the attribute consists essentially of (B1+B0) + B2*(l+((l/4)*(Vp/Vs)²).

10. A method as in claim 1 wherein the coefficients consist essentially of coefficients based on p-s wave data.

11. A method as in claim 10 wherein the reflectivity equation consists essentially of: Amp_P._s=R(θ)=C0*(Sin(θ)*Cos(θ)-Sin(θ)³/(g² -Sin(θ)²)^,/2) +C1 *Sin(θ)/(g² Sin(θ)² )^{1 2}).

12. A method as in claim 11 wherein the attribute consists essentially of the negative of Cl.

13. A method as in claim 11 wherein the attribute consists essentially of one-half the difference between Cl and CO.

14. A method as in claim 11 wherein the attribute consists essentially of B1-B0- CO.

15. A method as in claim 11 wherein the attribute consists essentially of 2*B0+C1.

16. A method as in claim 11 wherein the attribute consists essentially of twice the difference between B 1 and CO.

17. A method as in claim 11 wherein at least one of the coefficients consists essentially of C 1 +B0-B 1.

18. A method as in claim 11 wherein at least one of the coefficients consists essentially of - 2*(Vs/Vp)*Cl.

19. A method as in claim 11 wherein the attribute consists essentially of 2*(B0 - Bl) + (4*(Vs/Vp)* C0.

20. A method as in claim 11 wherein the attribute consists essentially of 2*B0 + (2*(Vs/Np)*Cl).

21. A method as in claim 11 wherein the attribute consists essentially of 2*(B1 - (2*(Ns/Np)*C0)).

22. A method as in claim 11 wherein the attribute consists essentially of (¹/₄)*(Vp/Vs)² *(B0 - Bl) + ((Ns/Np) + (¹/₄)*(Vp/Vs))*Cl.

23. A method as in claim 11 wherein the attribute consists essentially of (Vs/Vp)*Cl - (l/2)*(Vp/Vs)*C0.

24. A method as in claim 11 wherein the attribute consists essentially of 2*B0 + (2*(Vs/Np)*Cl).

25. A method as in claim 1 wherein the coefficients consist essentially of coefficients based on s-s wave data.

26. A method as in claim 25 wherein the reflectivity equation consists essentially of: Amp_s._SH = R(θ) - DO + Dl *Tan(θ)².

27. A method as in claim 26 wherein the attribute consists essentially of - 2*(D0 + D1).

28. A method as in claim 26 wherein the attribute consists essentially of 2*D1.

29. A method as in claim 25 wherein the reflectivity equation consists essentially of: Amp_s._sv = R(θ) = E0 + El *Tan(θ)² + E2*Tan(θ)² * Sin(θ)².

30. A method as in claim 29 wherein the attribute consists essentially of (3/5)*E0 + (1/5)*E1.

31. A method as in claim 29 wherein the attribute consists essentially of - (4/5)*E0 + (2/5)*El.

32. A method as in claim 1 wherein the comparing comprises subtracting the value of the attribute at the first data point from a value of the attribute at the second data point.

33. A method as in claim 1 wherein the comparing comprises plotting the value of the attribute at a first data point and at a second data point.

34. A method as in claim 1 wherein the first data point and the second data points are on opposite sides of a reflection interface.

35. A method as in claim 34 wherein the reflection interface comprises a cap/reservoir interface.

36. A method as in claim 1 wherein the first data point and the second data point are on the same side of a reflection interface.

37. A method as in claim 36 wherein the first data point and the second data point are both in the same reservoir.

38. A method as in claim 1 further comprising determining a further rock property from the seismic data by a method comprising: (c) assigning a further AVO attribute to the further rock property based on a further combination of coefficients of a reflectivity equation approximating amplitude variation as a function of offset; and (d) comparing the further AVO attribute at the first data point to the further AVO attribute at the second data point.

39. A method as in claim 38 wherein the further coefficients consist essentially of coefficients based on p-p wave data.

40. A method as in claim 38 wherein the further coefficients consist essentially of coefficients based on p-s wave data.

41. A method as in claim 38 wherein the further coefficients consist essentially of coefficients based on s-s wave data.

42. A method as in claim 38 wherein the further coefficients consist essentially of coefficients based on p-p wave data.

43. A method as in claim 38 wherein the further coefficients comprise coefficients from the same reflectivity equation as step (a).

44. A method as in claim 38 wherein the further coefficients comprise coefficients from a reflectivity equation different from the reflectivity equation of step (a).

45. A method as in claim 38 wherein the comparing step (b) and the comparing step (d) comprise cross-plotting the attributes.

46. A method as in claim 45 wherein the cross-plot is oriented wherein displacement along an axis of the cross-plot is directly related to a change in a single rock property.

47. A method as in claim 45 wherein the cross-plot is oriented wherein displacement along a first axis of the cross-plot is directly related to a change in a first single rock property and displacement along a second axis of the cross-plot is directly related to a change in a second single rock property.

48. A method as in claim 1 wherein the first data point comprises a first spatial data point and the second data point comprises a second spatial data point.

49. A method as in claim 1 wherein the first data point comprises a first spatial data point at a first time and the second data point comprises the same spatial data point at a different time from the time from the first data point.

50. A method for determining invalid seismic data acquisition assumptions, comprising: determining AVO attributes from seismic data, wherein the attributes are dependant upon multiple modes of wave propagation; comparing attributes which are dependant upon similar physical properties at similar locations, wherein a comparing result is generated; assigning an error to a comparing result that is outside a predetermined acceptance range.

51. A method ad in claim 50 wherein the comparing comprises cross-plotting the attributes.

52. A method as in claim 50 wherein the comparing comprises subtracting a first of the attributes from another of the attributes.

53. A method as in claim 50 wherein the comparing comprises a statistical comparison.

54. A method as in claim 50 wherein the determining comprises determining an AVO attribute from a p-p wave mode and determining an AVO attribute from a p-s wave mode, wherein both attributes are dependent upon a similar physical property.

55. A method as in claim 54 wherein the similar physical property comprises a rock property.

56. A method as in claim 55 wherein the rock property comprises density.

57. A method as in claim 55 wherein the rock property ΔVp/Vp.

58. A method as in claim 55 wherein the rock property ΔVsNs.

59. A method as in claim 50 wherein the determining comprises determining an AVO attribute from a p-s wave mode and determining an AVO attribute from a s-s wave mode, wherein both attributes are dependent upon a similar physical property.

60. A method as in claim 59 wherein the similar physical property comprises a rock property.

61. A method as in claim 60 wherein the rock property comprises density.

62. A method as in claim 60 wherein the rock property ΔVp/Vp.

63. A method as in claim 60 wherein the rock property ΔVsNs.

64. A method as in claim 50 wherein the determining comprises determining an AVO attribute from a s-s_v wave mode and determining an AVO attribute from a s-s_H wave mode, wherein both attributes are dependent upon a similar physical property.

65. A method as in claim 64 wherein the similar physical property comprises a rock property.

66. A method as in claim 65 wherein the rock property comprises density.

67. A method as in claim 65 wherein the rock property ΔVp/Vp.

68. A method as in claim 65 wherein the rock property ΔVs/Vs.

69. A system for determining a rock property from seismic data comprising: an assignor of an AVO attribute to the rock property based on a combination of coefficients of a reflectivity equation approximating amplitude variation as a function of offset at a first data point; and a comparator of the AVO attribute at a first data point to the AVO attribute at a second data point.

70. A system as in claim 69 wherein the coefficients consist essentially of coefficients based on p-p wave data.

71. A system as in claim 70 wherein the reflectivity equation consists essentially of: Ampp._P = R(θ) = B0 + B 1 *Tan(θ)² + B2*Tan(θ)² * Sin(θ)².

72. A system as in claim 71 wherein the attribute consists essentially of B2.

73. A system as in claim 71 wherein the attribute comprises the sum of Bl and B2.

74. A system as in claim 71 wherein the attribute consists essentially of twice the sum ofBl and B2.

75. A system as in claim 71 wherein the attribute consists essentially of B0-B1- B2.

76. A system as in claim 71 wherein the attribute consists essentially of 2*(B0- B1-B2).

77. A system as in claim 71 wherein the attribute consists essentially of (B1+B0) + B2*(l+((l/4)*(Np/Vs)²).

78. A system as in claim 69 wherein the coefficients consist essentially of coefficients based on p-s wave data.

79. A system as in claim 78 wherein the reflectivity equation consists essentially of: Amp_P._s=R(θ)=C0*(Sin(θ)*Cos(θ)-Sin(θ)³/(g² -Sin(θ)² )^1/2) +C1 *Sin(θ)/(g² Sin(θ)² )^m).

80. A system as in claim 79 wherein the attribute consists essentially of the negative of Cl.

81. A system as in claim 79 wherein the attribute consists essentially of one-half the difference between Cl and CO.

82. A system as in claim 79 wherein the attribute consists essentially of B1-B0- CO.

83. A system as in claim 79 wherein the attribute consists essentially of 2*B0+C1.

84. A system as in claim 79 wherein the attribute consists essentially of twice the difference between B 1 and CO.

85. A system as in claim 79 wherein at least one of the coefficients consists essentially of C 1 +B0-B 1.

86. A system as in claim 79 wherein at least one of the coefficients consists essentially of - 2*(Ns/Np)*Cl.

87. A system as in claim 79 wherein the attribute consists essentially of 2*(B0 - Bl) + (4*(Vs/Np)* C0.

88. A system as in claim 79 wherein the attribute consists essentially of 2*B0 + (2*(Vs/Vp)*Cl).

89. A system as in claim 79 wherein the attribute consists essentially of 2*(B1 - (2*(Vs/Vp)*C0)).

90. A system as in claim 79 wherein the attribute consists essentially of (¹/₄)*(Vp/Vs)² *(B0 - Bl) + ((Vs/Np) + (¹/₄)*(Vp/Vs))*Cl .

91. A system as in claim 79 wherein the attribute consists essentially of (Ns/Vp)*Cl - (l/2)*(Np/Ns)*C0.

92. A system as in claim 79 wherein the attribute consists essentially of 2*B0 + (2*(Vs Vp)*Cl).

93. A system as in claim 69 wherein the coefficients consist essentially of coefficients based on s-s wave data.

94. A system as in claim 93 wherein the reflectivity equation consists essentially of: Amp_s._SH = R(θ) = DO + Dl *Tan(θ)².

95. A system as in claim 94 wherein the attribute consists essentially of - 2*(D0 + D1).

96. A system as in claim 94 wherein the attribute consists essentially of 2*D1.

97. A system as in claim 93 wherein the reflectivity equation consists essentially of: Amp_s._sv - R(θ) - E0 + El *Tan(θ)² + E2*Tan(θ)² * Sin(θ)².

98. A system as in claim 97 wherein the attribute consists essentially of (3/5)*E0 + (1/5)*E1.

99. A system as in claim 97 wherein the attribute consists essentially of - (4/5)*E0 + (2/5)*El.

100. A system as in claim 69 wherein the comparator comprises a subtractor of the value of the attribute at the first data point from a value of the attribute at the second data point.

101. A system as in claim 69 wherein the comparator comprises a plotter of the value of the attribute at a first data point and at a second data point.

102. A system as in claim 69 wherein the first data point and the second data points are on opposite sides of a reflection interface.

103. A system as in claim 102 wherein the reflection interface comprises a cap/reservoir interface.

104. A system as in claim 69 wherein the first data point and the second data point are on the same side of a reflection interface.

105. A system as in claim 104 wherein the first data point and the second data point are both in the same reservoir.

106. A system as in claim 69 further comprising a determiner of a further rock property from the seismic data using: an assignor of a further AVO attribute to the further rock property based on a further combination of coefficients of a reflectivity equation approximating amplitude variation as a function of offset; and a comparator of the further AVO attribute at the first data point to the further AVO attribute at the second data point.

107. A system as in claim 106 wherein the an assignor of a further AVO attribute and the an assignor of an AVO attribute comprise the same assignor.

108. A system as in claim 106 wherein the comparator of the further AVO attribute and the comparator of an AVO attribute comprise the same comparator.

109. A system as in claim 106 wherein the further coefficients consist essentially of coefficients based on p-s wave data.

110. A system as in claim 106 wherein the further coefficients consist essentially of coefficients based on p-p wave data.

111. A system as in claim 106 wherein the further coefficients consist essentially of coefficients based on p-s wave data.

112. A system as in claim 106 wherein the further coefficients consist essentially of coefficients based on s-s wave data.

113. A system as in claim 106 wherein the further coefficients consist essentially of coefficients based on p-p wave data.

114. A system as in claim 106 wherein the further coefficients comprise coefficients from the same reflectivity equation as step (a).

115. A system as in claim 106 wherein the further coefficients comprise coefficients from a reflectivity equation different from the reflectivity equation of step (a).

116. A system as in claim 106 wherein the comparator comprises a cross-plotter of the attributes.

117. A system as in claim 116 wherein the cross-plot is oriented wherein displacement along an axis of the cross-plot is directly related to a change in a single rock property.

118. A system as in claim 116 wherein the cross-plot is oriented wherein displacement along a first axis of the cross-plot is directly related to a change in a first single rock property and displacement along a second axis of the cross-plot is directly related to a change in a second single rock property.

119. A system as in claim 69 wherein the first data point comprises a first spatial data point and the second data point comprises a second spatial data point.

120. A system as in claim 69 wherein the first data point comprises a first spatial data point at a first time and the second data point comprises the same spatial data point at a different time from the time from the first data point.

121. A system for determining invalid seismic data acquisition assumptions, comprising: an AVO attribute determiner, dependent upon seismic data, wherein the attributes are dependant upon multiple modes of wave propagation; a comparator of attributes which are dependant upon similar physical properties at similar locations, the comparator generating a comparator result; an assignor of an enor to a comparator result that is outside a predetermined acceptance range.

122. A system as in claim 121 wherein the comparator comprises a cross-plotter of the attributes.

123. A system as in claim 121 wherein the comparator comprises a subtractor of a first of the attributes from another of the attributes.

124. A system as in claim 121 wherein the comparator comprises a statistical comparator.

125. A system as in claim 121 wherein AVO attribute determiner is dependent upon an AVO attribute from a p-p wave mode and an AVO attribute from a p-s wave mode, wherein both attributes are dependent upon a similar physical property.

126. A system as in claim 125 wherein the similar physical property comprises a rock property.

127. A system as in claim 126 wherein the rock property comprises density.

128. A system as in claim 126 wherein the rock property ΔVpNp.

129. A system as in claim 126 wherein the rock property ΔVs/Ns.

130. A system as in claim 121 wherein the AVO attribute determiner is dependent upon an AVO attribute from a p-s wave mode and an AVO attribute from a s-s wave mode, wherein both attributes are dependent upon a similar physical property.

131. A system as in claim 130 wherein the similar physical property comprises a rock property.

132. A system as in claim 131 wherein the rock property comprises density.

133. A system as in claim 131 wherein the rock property ΔVp/Np.

134. A system as in claim 131 wherein the rock property ΔVs/Vs.

135. A system as in claim 121 wherein the AVO attribute determiner is dependent upon an AVO attribute from a s-s_v wave mode and an AVO attribute from a s-s_H wave mode, wherein both attributes are dependent upon a similar physical property.

136. A system as in claim 135 wherein the similar physical property comprises a rock property.

137. A system as in claim 136 wherein the rock property comprises density.

138. A system as in claim 136 wherein the rock property ΔVp/Vp.

139. A system as in claim 136 wherein the rock property ΔVsNs.

140. A method for determining rock properties from multiple recordings of seismic data wherein the multiple recordings are of differing types, the method comprising: determining an amplitude relationship from each of the recordings, and determining a set of rock property relationships from the amplitude relationships.

141. A method as in claim 140 wherein the types comprise P-P and P-S.

142. A method as in claim 140 wherein the types comprise S-S_v and S-S_P

143. A method as in claim 140 wherein the determining and amplitude relationship comprises interpolation of the recordings amplitude extraction from the interpolation.

144. A method as in claim 143 wherein the interpolation comprises a cubic spline process.

145. A method as in claim 143 further comprising application of a least squares fit to the extraction whereby an amplitude equation for the data results.

146. A method as in claim 143, wherein the amplitude extraction comprises isotime extraction.

147. A method as in claim 146 further comprising application of a least squares fit to the extraction whereby an amplitude equation for the data results.

148. A method as in claim 143 wherein a window around a selected time is determined and the peak within the window is used an extracted amplitude pick.

149. A method as in claim 148 wherein the window comprises a width of about. 10 s plus or minus the selected time

150. A method as in claim 149 further comprising application of a least squares fit to the extraction whereby an amplitude equation for the data results.

151. A method as in claim 143 wherein the extraction comprises: selection of a zero crossing closest a selected time, determining a peak closest the zero crossing.

152. A method as in claim 151 further comprising application of a least squares fit to the extraction whereby an amplitude equation for the data results.

153. A system for determining rock properties from multiple recordings of seismic data wherein the multiple recordings are of differing types, the system comprising: means for determining an amplitude relationship from each of the recordings, and means for determining a set of rock property relationships from the amplitude relationships.

154. A system as in claim 153 wherein the types comprise P-P and P-S.

155. A system as in claim 153 wherein the types comprise S-S_v and S-S_P

156. A system as in claim 153 wherein the means for determining and amplitude relationship comprises : means for interpolation of the recordings means for amplitude extraction from the interpolation.

157. A system as in claim 154 wherein the means for interpolation comprises a means for application of a cubic spline process.

158. A system as in claim 155 further comprising a means for application of a least squares fit to the extraction whereby an amplitude equation for the data results.

159. A system as in claim 154, wherein the means for amplitude extraction comprises means for application of isotime extraction.

160. A system as in claim 157 further comprising application of a least squares fit to the extraction whereby an amplitude equation for the data results.

161. A system as in claim 154 wherein the means for amplitude extraction comprises a means for application of a window around a selected time and a means for assigning the peak within the window as an extracted amplitude pick.

162. A system as in claim 159 wherein the window comprises a width of about. 10 ms plus or minus the selected time

163. A system as in claim 160 further comprising means for application of a least squares fit to the extraction whereby an amplitude equation for the data results.

164. A system as in claim 143 wherein the means for extraction comprises: means for selection of a zero crossing closest a selected time, means for determining a peak closest the zero crossing.

165. A system as in claim 162 further comprising a means for application of a least squares fit to the extraction whereby an amplitude equation for the data results.

166. A system for display of rock properties comprising a first display area showing a seismic data set of a first type a second display area showing a seismic data set of a second type the second display area being viewable concurrently with the first display area.

167. A system as in claim 164, wherein the first display is of a P-P synthetic data set.

168. A system as in claim 164 wherein the a second display area is of a P-S synthetic data set.

169. A system as in claim 164 further comprising a means for selecting a horizon of interest from at least one of two data sets of different types, wherein a data set of a first type is displayed in the first display and a data set of a second type is displayed in the second display.

170. A system as in claim 167 wherein the means for selecting comprises a mouse.

171. A system as in claim 167 wherein rock properties of the horizon are displayed concurrently with the horizon upon selection of the horizon.

172. A system as in claim 169 wherein P-P type data is displayed in one of the first and the second display areas and P-S type data is displayed in the other of the first and the second display areas.

173. A system as in claim 169 further comprising means for displaying rock properties determined from data represented in both the first and second display areas concurrently with the first and the second display areas.

174. A system as in claim 171 wherein the means for displaying rock properties determined from data represented in both the first and second display areas concurrently with the first and the second display areas comprises means for displaying rock properties determined jointly from data represented in the first and the second data sets.

175. A method for display of rock properties comprising displaying a first display area showing a seismic data set of a first type displaying a second display area showing a seismic data set of a second type the second display area being viewable concurrently with the first display area.

176. A method as in claim 173, wherein the first display is of a P-P synthetic data set.

177. A method as in claim 174 wherein the a second display area is of a P-S synthetic data set.

178. A method as in claim 175 further comprising selecting a horizon of interest from at least one of two data sets of different types, wherein a data set of a first type is displayed in the first display and a data set of a second type is displayed in the second display.

179. A method as in claim 176 wherein the selecting comprises indication of the horizon of interest with a mouse.

180. A method as in claim 176 wherein rock properties of the horizon are displayed concurrently with the horizon upon selection of the horizon.

181. A method as in claim 178 wherein P-P type data is displayed in one of the first and the second display areas and P-S type data is displayed in the other of the first and the second display areas.

182. A method as in claim 178 further comprising displaying rock properties determined from data represented in both the first and second display areas concurrently with the first and the second display areas.

183. A method as in claim 180 wherein the comprising displaying rock properties determined from data represented in both the first and second display areas concurrently with the first and the second display areas comprises displaying rock properties determined jointly from data represented in the first and the second data sets.