DE2613259A1 - METHOD AND DEVICE FOR THE EXAMINATION OF A DRILLED EARTH INFORMATION - Google Patents

Description

of the Societe de Prospection Electrique Schlumberger, 42, rue Saint-Dominique, Paris / France

concerning:

"Method and device for the investigation of a borehole penetrated earth formation"

The invention relates to a method for nuclear physics borehole investigation and to its Carrying out suitable devices, in particular on methods and devices for gaining additional and more detailed information on the location and recoverability of hydrocarbons in underground Earth formations by combined investigation of neutron characteristics data. and gamma ray spectroscopy data.

Cj C!

In general, when examining earth formations, the identification of oil or gas carrying Zones not only important to determine the presence of hydrocarbons, but also the relative Determine the recoverability (saturation) and the degradability. In this context, it is desirable that

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information is obtained regarding such formation parameters such as lithology, schist content, porosity and Salinity, as such information may be useful for making quantitative estimates regarding the hydrocarbon or water saturation level as well as the degradability of an existing one Formation.

An example of the prior art is given by U.S. Patent 3,521,064. In this known system, a detected gamma ray energy spectrum, for example the thermal neutron capture gamma ray spectrum of a Earth formation of unknown composition analyzed by adapting to a composite spectrum built up from weighted spectra of known constituent materials. By Compare the size of the detected gamma radiation energy spectrum at a large number of separate points or Energy level with the size of the composite spectrum an accurate analysis of the formation composition can be obtained to achieve the best possible fit. By Appropriate selection of the constituent spectra, one can also obtain spectroscopic output data that are representative are for interesting formation properties such as porosity, schist content, salinity, lithology and the same. Known systems for implementing this concept, however, have been spurious and statistical Subject to uncertainties which limited its usefulness in practice under certain circumstances.

Other known forms of nuclear physics borehole investigation involve determining one or more selected neutron characteristics of a formation, including for example the thermal neutron life

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duration or decay time (■), the macroscopic capture cross section ( Z ) and the neutron delay time. Systems suitable for deriving measurements of these characteristics are described in US Patents 3,566,116, 3,552,179 and in US Patent Application SN 356,151 dated May 1, 1973. A great deal of valuable information, particularly useful for distinguishing between salt water and oil, and for detecting changes in water saturation, is provided by these systems. The interpretation of the neutron characteristic logs, e.g. the ~ T ~ and 1st logs, is improved by reliable and correctable data on the formation lithology, porosity and schist content. This is especially true for low saline formations where the ~ V "and £ L logs are less reliable and where different formations give similar 7 and 1 values.

It is already generally indicated in US Pat. No. 3,413,471 that that useful correlations between the thermal neutron lifetime logs and certain gamma-ray spectroscopy logs exist, but so far there has been no integration of neutron characteristic logs and gamma radiation spectroscopic log functions done in such a way that enough information was provided in a single system under consideration of the various formation parameters of interest that a complete and accurate estimate of hydrocarbon-bearing zones would have been made possible.

The object of the invention is to provide an improved nuclear physical examination method and its Implementation to create suitable devices, «especially for use in lined boreholes,

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with additional knowledge about the location and the mining efficiency of the hydrocarbon-bearing zones should be.

The method provided according to the invention to solve this problem for examining an earth formation through which a borehole has drilled, in which a measurement of a time-dependent neutron characteristic of the formation is derived, is characterized according to the invention by the steps:

- Irradiation of the formation with a first pulse of neutrons ,

- Derivation of a representation of the energies of gamma rays generated by those with the first pulse in Related neutron interactions with nuclei of the formation,

- Analyzing at least a part of the energy spectrum of the gamma rays corresponding to those related to the neutron pulse Representations obtained during a first period of time after the formation was irradiated with the neutron pulse, and

- Control of the timing of most of the time period as a function of the derived measurement of the time-dependent neutron characteristic the formation.

In a further development of the invention is a device for examining an earth formation from a borehole is penetrated, provided with a borehole probe,

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which is movably suspended in the borehole, with a device for irradiation carried along by the probe of the earth formation with at least a first NEutron pulse and with a device for measuring a selected, time-dependent neutron characteristics of the formation, the device for performing the method according to the invention is characterized by: a detector arrangement for the detection of gamma radiation, resulting from neutron interactions with nuclei of the formation, related to the first neutron pulse and for the generation of corresponding signals representative of the energies of the detected gamma rays means for analyzing at least a part of the energy spectrum of the gamma rays, through gate means for passage to the analyzer those signals generated in response to gamma rays detected during a first period of time after the first neutron pulse have been generated, and by a control device for controlling the operation of the gate device as a function of the value of the selected neutron characteristic measured.

A further development of the invention relates to a method for maintaining accuracy within a wide temperature range, the input channel relationship of a pulse height analyzer with which the pulses received by it are sorted into a plurality of channels according to the respective pulse amplitude; the method is characterized according to the invention by the steps: generation of a first and a second test pulse of relatively large and small amplitudes, respectively, Maintaining a predetermined relationship between

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the amplitudes of the first and second test pulse within of the wide temperature range, application of the erssn and Second test pulse to the pulse height analyzer, determination the respective channels to which the first and second test pulses belong by the pulse height analyzer, formation the ratio of the respective channels and control of the pulse height analyzer depending on the channel allocation ratio in such a way that the impulses are sorted exactly into the assigned channels,

In a further development of the invention comprises a device for maintaining, «within a wide Temperature range, the accuracy of the input channel assignment of a pulse height analyzer designed for use in a system having a pulse generator, the Pulses with relative amplitudes that depend on a selected parameter are sent to the pulse height analyzer, which in turn, the pulses in a plurality of channels, sorted according to their pulse amplitudes, to carry out the The latter method has the following characteristic features: a device for the generation of a first and a second test pulse of relatively large or relatively small amplitude, a means for maintaining it a predetermined relationship between the amplitudes of the first and second test pulses within the second Tempo range, a facility for creating the and send a second test pulse to the pulse height analyzer, a device for the determination of the respective channels to which the first or second test pulses by the pulse height analyzer be assigned a device for the formation of a signal corresponding to the ratio between the respective channel assignments? and a facility for controlling the pulse height analyzer in response to the channel assignment ratio signal.

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For example, the measured neutron characteristic is a thermal neutron absorption characteristic of Formation, i.e. the thermal neutron decay time (Γ) or the macroscopic capture cross-section (/) and the analyzed gamma-ray energy spectrum is the thermal neutron absorption gamma-ray spectrum. By Selection of the fraction of the time distribution of the gamma radiation to be analyzed according to the measured value a neutron absorption characteristic, a correct

gamma
Term selection of the capture radiation energy spectrum for the particular formation under investigation. The selected spectrum is accordingly representative of the composition of the formation, regardless of the fact that the thermal neutron absorption properties of the formation, which to a large extent determine the duration and shape of the capture gamma radiation time distribution after a neutron pulse, may differ within the formations drilled by a borehole.

Further features and advantages of the subject matter of the invention will become apparent from the following description Typical embodiments of the subject matter of the invention illustrated, with reference to the accompanying drawings Is referred to.

1 is a schematic block diagram representation of an embodiment for a device according to the invention,

Figure 2 is a graph of the time distribution of trapped gamma rays, plotted as a function of ■ after irradiating a formation with a pulse of fast neutrons, with preferred time

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were the spectroscopy and decay time acquisition periods according to FIG Invention are shown,

Fig. 3 shows an embodiment for a basic operating cycle for combined decay time spectroscopic functions according to the invention, and

Fig. 4 illustrates a typical scheme for the transmission of calibration pulses the spectroscopy circuitry.

As mentioned above, the present invention in its broadest sense is intended to provide expanded or improved information in connection with the location and assessment of oil-bearing and gas-bearing zones in earth formations. In particular, the invention has application in cased wellbores, that is, production wells for identifying undeployed oil zones and the like, but it can also find application in open wellbores. An apparatus for carrying out the method according to the invention, which gives particularly advantageous results, is shown schematically in FIG. It includes apparatus for simultaneously deriving readings relating to | ~~ and 1 of the information of interest, as generally described above in the aforementioned prior art patents, and for analyzing selected portions of the trapped gamma ray spectrum of the formations, particularly in accordance with the curve fitting technique as described in the aforementioned U.S. Patent 3,521,064. It is pointed out, however, that the device to be described can only be taken as an example that it is used

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can find for the measurement of other neutron characteristics and that gamma radiation energy spectra deviating or in addition can be analyzed for the capture gamma radiation spectrum.

According to Fig. 1, the device comprises a fluid tight pressure- and temperature-resistant probe 1o, which is attached an armored cable 14 is suspended in a borehole 12 in order to examine underground earth formations 16. The borehole 12 is filled with borehole fluid 18 and with a steel lining 2o and a cementation 22 provided. While no tubing string is shown in the borehole, the probe can also be used for can be dimensioned for use with a drill string.

The probe 1o contains a pulsed neutron source 24 and a radiation detector 26, which in the distance with respect to the source 24 is arranged. A neutron shield 28 of conventional composition is preferably between the source 24 and the detector 26 to reduce the direct neutron irradiation of the detector. The neutron source 24 is designed to generate discrete pulses of fast neutrons, e.g., 14 Mev, and suitable ones Types of such neutron sources are described in U.S. Patents 2,991,364 and 3,546,512. The detector 26 may be of any construction suitable for the detection of gamma rays and for the Generating a pulse signal in response to each detected gamma ray having an amplitude representative of is for the energy of the gamma ray in question. Typically the detector comprises a scintillation crystal 3o, which is optically coupled to a photomultiplier tube 32 in a conventional manner. The crystal 3o is preferred of the thallium activated sodium iodide type, though any

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suitable crystal could be used, for example Thallium or sodium activated cesium iodide. Alternatively, you can also use a solid-state detector, for example with a germanium (lithium) crystal. The operating power for the probe 1o becomes is supplied via cable 14 from a surface source, not shown, and it understands ensure that appropriate power supply devices (not shown) are provided in the probe 1o to the neutron source 24, the detector 26 and other probe equipment to operate.

A sleeve 34 impregnated with boron carbide encloses the probe 10 in the region of the source 24 and the detector The sleeve is slotted lengthways to allow the passage of borehole fluid along the probe, and preferably the sleeve has a diameter which allows the free movement of the probe within the lining 2ο just allowed. The sleeve 34 reduces the number of undesirable gamma rays that the detector 26 at various Ways to reach; it displaces the borehole fluid 18 from the vicinity of the probe, whereby Gamma rays resulting from neutron interactions with the borehole fluid are minimized, and after all, it serves as a derivation for neutrons in the immediate area of the detector, so as to produce gamma rays to reduce that result from neutron interactions with the liner or pipe string or from activation of iron or other elements in the probe itself.

Pulses from the photomultiplier 32 are amplified in a preamplifier 36 and then on the one hand via a

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Line 38 is applied to the input of an amplifier 4o to derive 'f and on the other hand via a conductor 42 to the input of an amplifier 44 for the spectrum analysis. If desired, separate detectors can be used used for the "\ ~ and spectroscopy functions will. In this case, the detector intended for spectroscopy should be sensitive to gamma radiation Type as described, but this need not apply to the detector intended for the \ measurement; example the detector used for the I measurement could be sensitive to thermal neutrons, include a proportional counter filled with helium-3. In either case, an extremely strong one becomes extremely strong during and immediately after a pulse of neutron generated by source 24 Incidence of gamma rays at detector 26 (or the detectors) are present, with a correspondingly high rate of pulses in the photomultiplier 32 would be generated. If not exceptionally gamma radiation during this time period is to be measured, the photomultiplier is preferably short during the neutron pulse and during one The following period is blocked to increase the number of photos 32, amplifiers 40 and 44, and other signal processing holding circuits protect against adverse effects resulting from unusually high count rates. As later to explain in detail, the operation of the source is advantageously controlled by signals from a Programming device 46 are generated and transmitted to it via a conductor 48. These signals can also, as indicated by the conductor 5o, to switch off the photo multiplier for the desired period of time to be used.

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Pulses from amplifier 4o are applied to a discriminator 52, which only allows those pulses to pass are above a predetermined amplitude level, and the incoming pulses to standardized output pulses brings. These pulses are then transmitted through a conductor 54 to the ·. -Computing circuits 56 supplied, whose more precisely Structure can be found in US Pat. No. 3,662,179.

As described in that document, the i calculator circuit 56 comprises gates, counters, comparators and Oscillator circuitry responsive to the discriminator 52 during a variable time period of detection Occurrence of passed impulses in the following selected Neutron pulses for the purpose of solving the equation:

N = 1/2 (N ₁ + N ₃ ) - N ₂ (1)

N _{1 is} the number of counts during a first acquisition interval or gate (I) of 1 ι duration, which begins 2 ι after the end of the preceding neutron pulse,

N "is the number of counts during one second acquisition interval or gate (II), which begins immediately after the first interval and has a duration of 2 ι owns,

N _{3 is} the number of counts during a third detection interval or gate (III), which is triggered 6 ι after the end of the previous neutron pulse and has a duration of 3 ι.

N ₃ is intended to correct the counts N and N of the main intervals I and II with regard to the

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basic gamma radiation and can therefore be omitted if the detector designated for '[does not respond to gamma radiation.

If the solution of equation (1) reveals that N = O, the apparent value of the decay time can be derived by circuits 56, may be considered to be the actual decay time of the formation under investigation. If N deviates from 0, an error signal is made variable within the circuits 56 by regulating a variable Frequency oscillator generated with the tendency to return the system to the state N = O. As in the one mentioned As explained in U.S. Patent No. 3,662,179, the variable frequency oscillator may be a digital oscillator with a 100 KHz clock signal circuit and a desired number of oscillator flip-flop circuits. The flip-flop circuits change the switching state in accordance with the 100 KHz clock signal, and the holding states of the flip-flops are continuously compared with a corresponding number of flip-flops in the detector signal counting circuits (the i -computing circuits 56) by means of a corresponding number of operators. When the counter and oscillator flip-flop circuits are in the same switching state, a pulse is generated which resets all oscillator flipfbps. This technique actually converts a count rate signal into a time signal as the oscillator flip-flop circuit controls the switching state change individually until they have adapted to the switching states of the counter flip-flop circuits, which the represent observed radiation registered by the meter. Accordingly, the output signals represent generated from the oscillator, the time it takes for the oscillator to reach a given level of radioactivity to be counted and are therefore representative of the thermal

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mix formation neutron decay time. The oscillator output signals are sent via a line 58 as an error or control signal for controlling the operation of the Programming device 46 coupled.

A greater degree of reliability in the derived value for ^r "can be achieved by increasing the frequency of the clock or by adding more stages of the clocked network of flip-flop circuits. For example, a frequency of 300 KHz can be used. The clock-driven network of flip-flops be dimensioned so that a high initial counting rate results, which decreases exponentially with time after a neutron pulse. This would result in a greater stability of the ι "" calculation circuits 56, especially at low T values, since the changes "\ ~~ , caused by the generated error signal, would always be relatively small relative to the absolute value of '\ " . This would also make it possible to use a counter of smaller capacity.

The programmer 46 may include a counter with suitable diode matrix logic circuitry. Programmers of this type are described in U.S. Patents 3,566,116, 3,6o9,366 and 3,662,179. The programming facility can use the usual logic and signal combination circuits , which are described in U.S. Patent 3,662, which are required to make dependent of the output signal sequence from the oscillator of the Γ computer circuits 56 gate unlock signals for the 7 ~ acquisition intervals I, II and III to generate. These signals are transmitted to the T-computer circuits 56 via a Line 6o where they operate the gate circuits

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control and the pulse counter circuitry described in the above Way. The programming device 46 also includes circuitry for generating control signals, which regulate the duration and repetition frequency of the neutron impulses emitted by the source 24. as the time of occurrence (beginning and ending) of the ι -Erfassungsintervalle I, II and III in relation stand with the occurrence time of the assigned neutron pulses, the i-gate unlock pulses are on line 6o also synchronously related to the control signals transmitted via line 48 to the neutron source 24 are transmitted. A preferred pulse sequence for the source 24 and gate sequence for the ι computer circuits 56 is described below in connection with FIGS. 2 and 3.

The programmer 46 can also be configured to generate a signal representative of 7 (or Ϊ) for transmission to the earth's surface, and this can also be done as described in US Pat. No. 3,662,179. Alternatively, the number of times the neutron source 2 4 is pulsed, and preferably just those times after which i detection periods are sequenced, can be counted for a given period of time. Since the repetition frequency of the neutron pulses as a function of

i "is controlled, the number of pulses generated during the given time period is proportional to < and accordingly to ι. The counting time period should of course have a sufficiently long duration for statistically reliable measurements and should be repeated at a frequency corresponding to the desired vertical resolution in the ^ * - \ logs. Corresponding circuits for this

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Purpose could include an AND gate 62 according to FIG. 1, which is responsive to an unlock signal on line 63 from programmer 46 for the passage of pulses on line 65 generated by the programmer 46, in synchronism with the control signals for the respective neutron pulses to be counted. The gated pulses are is applied to a binary counter 69 via a line 67. After the end of the counting period, a deceleration monovibrator instructs 71, for example by the falling edge of the gate enable signal on line 63, energizes the counter 69 to output binary-coded parasitic signals, which are indicative of the registered pulse count above a trunk group 73 to the signal processing and cable driver circuits 64 and thus reset to the zero position to become.

Consider now the spectroscopic portion of the device. Output pulses from amplifier 44 are over a line 66 is led to a pulse height analyzer 68. The pulse height analyzer 6 8 can be of conventional design be, such as the Wilkinson type with a single ramp, and the pulse height analyzer 6 8 is used to pulse accordingly their amplitude and preferably in binary-coded parallel form to corresponding channels to be applied in an output line bundle 7o for delivery to the signal processing and cable driver circuitry 64. It will be understood that the usual low level and high level discriminators will provide for the selection of the energy range to be analyzed, and linear gate circuits for controlling the time portion of the pulses generated by the detector to be analyzed. For this purpose, appropriate signals are generated by the Programming device 46 and given on lines 72, 74 and 76, respectively, to adjust the discriminators and the

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to unlock linear gate switching circuits. During the Proportion of the time distribution of the signals after each neutron pulse, selected for the analysis, each desired proportion may correspond to the time distribution of gamma rays resulting from the neutron pulse, they will preferably correspond to the gamma rays generated by the thermal neutron capture interactions with nuclei of the formation. The programming device 46 is therefore preferably constructed in such a way that it activates the unlock signals on line 76 in such a way generates the linear gate circuits to open for a period of time following each neutron pulse when the formation trapping gamma rays are predominant. In accordance with a feature to be described according to the invention the time of occurrence of these gates or detection periods is controlled in relation to the previous one Neutron momentum measured as a function of 'Γ through the circuits 56. For this purpose, the Programmer 46 suitable logic and signal combination circuitry for generating the desired Gate unlock signals synchronous with the occurrence of the associated neutron pulses.

The proportion of the entire energy spectrum of the gamma rays corresponding to the gated detector signals, which is to be analyzed can also be selected as desired and can extend from, for example 1.5 MeV to 7.5 MeV. The number of channels used over the energy range of interest depends Of course, this depends on the desired analysis accuracy and the quality of resolution of the scintillation crystal used. With a thallium-activated sodium iodide crystal it has an order of magnitude of 200 channels over

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the energy range from 1.5 MeV to 7.5 MeV has been shown to be suitable for providing a satisfactory spectral analysis. However, fewer channels, e.g., about 50, can be used if desired. In general, the number of the channels, the channel width, the total energy range and other characteristic values of the analyzer 6 8 are determined in accordance with the teaching of US-PS 3,521 o64.

The processing and driver circuitry 64 can be conventionally constructed for encoding, time division multiplexing or otherwise processing the data representative signals applied to them in any desired way Manner for transmission on cable 14, and the specific forms of these circuits which are employed, are not decisive for the invention. Preferred embodiments of the circuits 6 4 are shown and described in US patent application SN 563 5o7 of March 31, 1975.

On the surface of the earth the ^ -related signals from binary counter 69 and the count-per-channel signals from Pulse height analyzer 68 amplifies, decodes and otherwise processes, as required, by means of circuitry 78 for coupling to a computer 82 via a conductor bundle 80. The computer develops values for

Ϊ and / or \ and the desired spectroscopic output values, such as values that are typical for water saturation, shale content, lithology, porosity, water salinity, etc. Preferred embodiments of the spectroscopic output values are to be described later. Digital displays of these values are provided on lines 84A-84H to a tape recorder 86 and a digital-to-analog converter 88 which generates analog signals proportional to the respective inputs for application to a visual recorder 9o. Above-

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monitoring data (not shown), such as the mean count rate during the first '■ acquisition interval (I) or, for example, the head tension, can also be recorded will. The magnetic tape recorder and the vision recorder 9o are conventional and suitable for generating the usual logs as a function of the probe depth. The usual with the cable 14 mechanically coupled linkage is shown diagrammatically at 9 2 in Fig. 1 and serves this purpose.

The calculator 82 may be of any configuration for performing the "L. and T" calculations above and the spectrum adjustment, the component proportion determination and ratio formation as described in the aforementioned US Pat. No. 3,521,063. For example it may comprise a commercially available general purpose digital computer manufactured by the Digital Equipment Corporation, Maynard, Massachusetts, with the type designation PDP-11. The calculator can connect to the drilling location are, as indicated in Fig. 1, or it can be set up remotely and deceived with recorded Data relating to the counts per channel, the and £ data, and the record would for example comprise the decoded signals from processing circuitry 78 on magnetic tape.

It is a feature of the present invention that the perturbation effects for the analysis of the formation trapping gamma ray spectra of gamma rays resulting from neutron interactions with materials in the immediate vicinity Neighborhood of neutron source 2 4 and detector 26 (probe housing, borehole fluid 18, lining 2o, cement annulus 22 etc.), the so-called "borehole effects",

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be reduced by controlling the time of occurrence of the spectroscopic detection period relative to the time of occurrence of the associated neutron pulse in accordance with a measured value of '· each of the analyzed formation. This is illustrated in Figure 2, which shows schematically how the capture gamma ray count rate may typically change with time, expressed in terms of the decay time after a burst of 9 8 fast neutrons. On the extreme left side of the distribution curve 100 there is a region of rapid decay as a result of early borehole effects, ie as a result of high absorption rates of thermal neutrons in the borehole media which directly surround the source and detector. This is followed by a practically straight-line region which, in accordance with Nelligan's investigations, is the useful part for determining the thermal neutron decay time and, on a semi-logarithmic plot, corresponds to the exponential decay of the neutron density in the formation. Finally, the curve 100 is flattened to the right, the counting rates in this area corresponding to the background radioactivity in the formation and in the borehole.

For the calculation of ι, Nelligan uses the above-mentioned U.S. patent has been determined to be a 2 \ "delay after the neutron pulse, most of the undesirable borehole effects are eliminated before the start of the acquisition period and the beginning part of the first detection interval (I) places the exponential decay region of curve 1oo; and accordingly the gate circuits are also the "T" computing circuitry 56 is controlled. The "T gate sequence described above (two contiguous main intervals I and II of 1 T ~ or. 2 V "and a background interval III from 3 "duration by a duration of 1 \ from

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Interval II separated for a total acquisition period of 7 'is shown in FIG. 2, but, as the cited document describes, other total acquisition periods and other acquisition intervals can be used within the total period. In either case, the timing of the V "acquisition period is automatically controlled in accordance with the ' \ of the particular formation being examined.

For the purpose of capture gamma spectroscopy, it has been found that a spectroscopic detection period, which is shown in Fig. 2 with 1o2, and a Has a duration of 2 Ί, starting after a delay of 1> after termination of the preceding neutron pulse 9 8, early borehole effects eliminated in a satisfactory manner and at the same time delivers high count rates for improved statistics and better vertical resolution. Since accordingly the Time slot of the spectroscopic recording period also with a measured neutron characteristic of the formation is linked, the spectroscopic period is automatically properly controlled from formation to formation. Other time slots of the acquisition period 1o2 can of course can also be used. In general, the timing of the period should be chosen so that formation capture gamma count rates relative to the background count rate can be maximized in a manner that also includes the suppression of Borehole effects is taken into account.

In order to further improve the effectiveness of the examination system and to further increase the counting rates, the duration and the repetition rate of the neutron pulses are also _{controlled in accordance with (} . According to the invention

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For example, the individual neutron pulses are preferably of 1 "" duration. In addition, a predetermined number of spaced neutron pulses are preferably generated cyclically during each of a series of time intervals or cycles. A preferred basic cycle of 31 "" duration is shown schematically in FIG It includes a 20 spectroscopy sub-cycle, a 10 T decay time sub-cycle, and a 1 " pause period. The spectroscopic sub-cycle consists of a total of 5 intervals, labeled A, B, C, D, D and E, each of which has a duration of 4 Γ 'and each has a neutron pulse 1o4A-1o4E of 1 \ duration and one allocated spectroscopic acquisition period I06A-I06E of 2 V "duration. Each acquisition period of I06A-I06E is fixed in time with respect to the associated neutron pulse 1o4A-104E in the manner described above in connection with the spectroscopic gate 1o2 according to FIG Neutron pulse 1o4A-104E generated jointly with the termination of the previous spectroscopic acquisition period I06A-I06E, so that during the spectroscopic sub-cycle the neutron pulses occur at intervals of 4 '.

The decay time sub-cycle begins with the sixth pulse of neutrons 104F in the 31T overall cycle. This pulse preferably occurs immediately after the previous spectroscopic acquisition period I06E. Although the main function of the decay sub-cycle is to enable the determination of, a sixth spectroscopic acquisition period I06F is also built into this sub-cycle. The decay time sub-cycle naturally also includes two main detection periods for T (I and II) and, if the detector intended for \ ~

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is a gamma ray detector, a background detection interval (III), which is placed as described in FIG. The 1 * 'interval at the end is in the 31 j cycle built in to reset the programmer 46 and to stabilize the spectroscopy, as explained below.

The 31 total cycle described above is repeated successively as the probe moves through the Borehole, the decay time is determined by the repeated

Computer circuits 56, and under the control of the programming device 46, the times of occurrence of the neutron pulses 1o4A-1o4F, the associated spectroscopic acquisition periods I06A-I06F and the acquisition intervals (I, II and III) are continuously readjusted in accordance with the measured value of. The spectroscopic acquisition periods I06A-I06F and the acquisition period are accordingly automatically placed in the correct area of the gamma ray time distribution curve that follows each neutron pulse for optimal measurement of gamma ray activity for the respective spectroscopic and determination functions. In addition, the neutron source pulse control and the detector gate sequences, which form the 31 'overall cycle, provide both a high spectroscopic duty cycle (12 \ of 31 each) with correspondingly high counting rates for the spectral analysis and a continuous "scanning" (once every 31) for sfanelles response for changes in formation conditions.

The resolution quality of the gamma radiation energy spectra, delivered by the probe 1o, is of course ab-

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dependent on the stability of the energy dependency of the detector analyzer system. In addition, when using the spectrum matching technique discussed in the above-mentioned publication, it is important for accurate results that the energy dependence characteristic of the system be substantially the same when making calibration spectra of known composition and when determining a formation spectrum. It is therefore desirable to check the response characteristics repeatedly during use and to immediately compensate for any instabilities determined in the process. Such instabilities usually result, for example, from the sensitivity of the detector crystal, the photomultiplier and the electronics located in the borehole with regard to temperature changes in the borehole as well as due to other operating parameters, and very often they lead to changes in the pulse gain of the detector analyzer circuit or in the pulse height-channel relationship the pulse sorting circuitry of the pulse height analyzer. Detection and compensation of such errors can be effected in a suitable manner at any point in time. However, it is convenient to perform this during the period from 24 · "to 31 t (shown as the stabilization period in FIG. 3) within 31 [ total cycle and in accordance with one of the procedures described below.

A natural gamma ray emitter, preferably with an energy peak below the gamma energy range, which is to be analyzed, e.g. 1.5 to 7.5 MeV, becomes close to the Detector 26 is placed / and a certain channel or channel portion is the peak energy within the pulse height analyzer s 68 assigned. For example, a Zinc source with a 1.11 MeV gamma radiation for this

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Purpose to be used. The energy sensitivity of the detector analyzer system in operation can then be related to The accuracy can be checked by determining whether the specified energy / channel relationship is applicable to the zinc Peak was intended to be maintained. This can be done by counting the number of zinc pulses that into a certain number of channels (energy bands) on both sides of the energy level assigned to the 1.11 MeV Canal fall. Compare the respective counts with each other and generate an error signal if one Total count is greater than the other, so so does the sensitivity to readjust the detector analyzer system, e.g. by readjusting the voltage of the power supply for the photomultiplier 32, with which the 1.11 MeV energy level is restored for the intended channel localization will. This procedure corrects for gain changes occurring anywhere in the detector analyzer system.

Circuitry for performing this gain control function in a borehole probe are described in US Pat. No. 2,956,165. Alternatively, the counting and Vagleichs steps are easily carried out by the computer 82, which then sends an error signal accordingly Height and polarity for controlling the photomultiplier power supply would generate. In another embodiment, the computer could be designed so that he calculates the centroid of the zinc tip in terms of its channel location from counts, the one represent small range of gamma-ray energies covering the tip, either with or without Subtract the background, after which the desired error signal is generated, by the prescribed 1.11 MeV channel relationship restore.

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In order not to "clog" the pulse height analyzer 68 during the spectroscopic acquisition periods I06A-I06F due to the contribution of the zinc source, its lower discriminator level is preferably set so that only pulses above 1.11 MeV are allowed through. The lower level can be set close to 1.5 MeV, for example, and would normally be held at this level for every 31 * cycle with the exception of the stabilization period from 24 1 to 31 ■. At 24 * in each gain control cycle, the lower discriminator level is shifted downward to pass pulses corresponding to the zinc gamma rays, and it is held at the lower level until 31ν, after which it is shifted wMer to the high level. Control signals for this purpose are generated by programmer 46 which includes logic and signal combining circuitry designed for this purpose and coupled to pulse height analyzer 68 via line 72 (FIG. 1). Similarly, gate Rent inhibiting signals are generated from the programming device 46 and the pulse height analyzer 68 (not shown) via line 76 to de-linear gate circuits coupled to the passage of zinc 5-i _m p _U i _{se to} cU_ _e to unlock pulse sorting circuits during Stabilisierperiode.

In addition to the gain control described above, it is also desirable to have drift or operating point shifts to compensate in the pulse height / channel relationship of the pulse height analyzer. An embodiment of a device that is particularly suitable for Such an operating point shift control is explained in US patent application SN 563 51o of March 31, 1975.

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According to this older proposal, a pulse generator circuit 77 is coupled to the pulse height analyzer 6 8 via an amplifier and, upon receipt of a command signal from the programming device via line 79, alternately transmits a low-amplitude pulse and a high-amplitude pulse, the ratio of which is essentially constant over a wide temperature range is. The transmission of the pulse to the analyzer can be phased with respect to the transmission of the zinc pulses in any convenient manner. For example, the analyzer's discriminator can be adjusted so that the zinc pulses are input during the stabilization periods of every other 31 'cycle, as shown in the first and third 31' cycles of the sequence of FIG. 4, and the pulses are lower and higher Amplitude are transmitted during the stabilization periods of each or every other one of the remaining 31 \ cycles. 4 illustrates the transmission of high amplitude pulses during the second 31 Γ cycle and the transmission of low amplitude pulses during the fourth 31 1 cycle. The channel assignment to the corresponding low and high amplitude pulses by the analyzer 6 8 is determined, for example by the same techniques that are used for the assignment of the zinc tip, and the ratio of the channel assignments is established. Since the pulse sorting circuits of the analyzer 68 receive the pulses after they have been influenced by any operating point shifts in the pulse height / channel relationship, the ratio of the channel locations to which they are assigned is likewise affected by such operating point shifts. By comparing this ratio with the known ratio of the pulsing circuit outputs, a

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809851/0683

Error signals are generated if any difference between the two ratios is detected, such as that the corresponding operating point shift is eliminated and the correct pulse height / channel relationship is restored will. This can be done conveniently by adjusting the zero discriminator level accordingly of the pulse height analyzer.

As an alternative procedure, a second typical gamma ray peak can be used in conjunction with the zinc tip or any other power tip in place of the pulsing circuitry as described above. The second peak need only differ from the first in terms of energy and could be below, within, or beyond the energy range to be analyzed. The o.51 MeV peak of zinc, for example, or sodium 24 with 1.38 MeV and 2.76 MeV gamma peaks can be used can be used instead of zinc. You can also use the capture gamma tip for oxygen at 6.1 MeV along with a natural low energy emitter. The channel assignment of the second peak would be on determined the same way as the zinc 1.11 MeV peak, and the relationship between the two channel locations is established. This ratio is then compared to that calibrated channel ratio for the two relevant energy peaks, and an error signal for the purpose of correction any disparity between the detected relationship and the correct relationship will be developed. As in the pulsation system could send the error signal to the zero discriminator of the pulse height analyzer 6 8 are applied to the operating point shift causing the error or to eliminate drift.

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609851/0663

If one or both of the oak energy peaks that have been selected fall within the energy range to be analyzed drop, as for example for the 6.1 MeV oxygen scavenger gamma peak in the case of the 1.5 to 7.5 MeV range mentioned above, it would naturally be one of the detected ones Gamma rays contribute to the spectroscopic acquisition period I06A-I06F. You would therefore have to take into account if the analysis is done using the spectrum matching technique of US Pat. No. 3,521,064. This could be done by inserting the capture gamma radiation spectra of that emitter into the constituent spectra used are used in building the composite spectrum with which the acquired spectrum is compared.

Let us now turn to the analysis of the captured gamma radiation spectra and the spectroscopic ones Output signals developed by computer 82 are received. The calculator does essentially the same thing Functions such as the component share calculator 39 according to US Pat. No. 3,521,064. That is, he determines the composition an unknown formation by comparing the heights of the detected capture gamma ray spectrum of the formation at a large number of energy points or levels with a composite spectrum formed from weighted Proportions of individual spectra from a number of material components or specific combinations £> materials that can be expected to be formed from them, so as to make the best possible Finding adaptation, preferably by least squares and derivative of indications with regard to the approximate proportions of the material components for which the adjustment applies. The factors to consider are when selecting the respective material components for the composite spectrum, the Art

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B 0 9 8 5 1/0 6 δ 3

and way of measuring the individual spectra of the constituents, building the composite spectrum, the Application of the least squares criterion to determine a best fit between the detected Spectrum and the composite spectrum, and other teachings of U.S. Patent 3,521,064 for deriving the desired Information regarding the unknown proportions W. of the postulated material components in the formation also apply here and therefore do not need to be repeated in detail.

However, it should be briefly stated here that it is then assumed that the formation is predominantly from not consists of more than a number of η postulated constituents or elements, e.g. hydrogen, chlorine, silicon, Calcium, iron and oxygen, and the composite spectrum to which the detected formation spectrum is compared accordingly consists of the individual capture gamma ray spectra for these elements (preferably including the secondary activation radiation spectra for oxygen). Through combination functions G, the gamma radiation count amplitudes at a selected number of energy levels (not less than n) in the detected one Gamma radiation spectrum with amplitude coefficients

· - <. ., which are determined beforehand from the individual beik

known spectra and entered into the computer 82, as disclosed by the above-referenced US Pat Set of linear equations in the form of equations 3a-3n solved by the computer:

X = I 112 122 133 Inn

2n ^G n ^(3b)

^w i = 3 = '% 1 ^G 1 ⁺ % 2 ^G 2 ⁺ ^ 33 ^G 3 ⁺ ^ 3n ^G n ^(3c)

609851/0683

x = n n1 1 n2 2 n3 3 nn η (3η)

Since the number of equations 3a-3n is the same number η of educational materials from which it is postulated, that they are sowing present in the formation, the computer 82 automatically develops all unknown w.-weight

or proportional component factors. Since these w. Factors are representative of the proportions of the respective constituents Is materials on the composite spectrum, they are equally representative of the relative Proportions of the individual constituent materials in the formation. Information on these factors for each of the n-constituents are fed to tape recorder 86 via line 84C and recorded in use the probe depth.

As further stated in US Pat. No. 3,521,064, the calculator 82 can be constructed to have ratios selected constituent proportion factors w. determined to derive information about certain properties of the formations. For example, spectroscopic Output signals from the computer 82 are developed and the converter circuit 88 and the visual recorder 9o and the magnetic tape recorder 86 supplied via line 84D-84H for such formation properties, such as salinity, porosity, lithology, slate content and water saturation. Advertisements regarding other properties can also be developed if desired.

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6 0 9 8 5 1/0683

_{The ratio w r} , / w _R , ie the weighting factor w for chlorine divided by the weighting factor w for hydrogen, can be cited as an example of the derivation of an indication of the salt content. The numerical value of this ratio can therefore be recorded as a salinity indicator. This indicator can be quantified by correlating it with calibration curves developed in formations of known salinity. If desired, the correlation can be carried out by the computer 82 or also in the recorder 9o by applying an appropriate conversion factor, and recordings can be made directly of the quantitative values thus obtained either in addition to or in place of the ratio indicator.

A suitable ratio for specifying the porosity can, for example, have the form aw / (bw _q . + Cw _c )

with w and w ^ as calculated proportional factors for silicon Sx approx

or calcium. The coefficients a, b and c (and further below d) etc. stand for different gamma radiation emission intensities of the respective elements for the same neutron flux (due to different microscopic Capture cross-sections and different gamma radiation / neutron capture interaction values) and are selected so that the individual terms in which they appear, e.g. (bWg. + cw_) are constant, independent of the specific Sets of elements of the formation. Generally, the porosity ratio should be such that it corresponds to the amount of Represents fluid in the formation relative to the amount of matrix material, and any other form of ratio, which is suitable for this case can be used. For example, a ratio in the

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809851/068 3

Form of aCbw ^, + cw «-.) / (Dw _o . + Ew _p ) are used anil V- ^ JL oil L» a

place of the aforementioned relationship. The latter relationship is more precise than the former because it has both Hydrogen as well as chlorine are taken into account, which have predominant proportions in the formation fluid. The former relationship however, allows a good approximation because the ratio of hydrogen in water does not change rapidly changes with a change in the saline content (chlorine). As With the aforementioned salinity data, quantified values of the porosity can also be obtained by using calibration curves can be derived, and this could easily be done in the computer 82 or in the recording device 9o will.

Either or both of two ratios can be used to determine the lithology of a formation use to indicate whether the formation is limestone or sandstone. A suitable relationship for viewing limestone can have the form

w _c ./w, while an approximate sandstone indicator ox approx

can be obtained by using w _a . / aw _o . + bw_,)

bi bi oa

An alternative form of a ratio that can be used as a lithology _{indicator is w G} ./(aw + bw "

al Dl approx

+ cw), where w for one or more other components, such as oxygen, iron, etc., which should be taken into account. Indicators of sandstone or limestone different litohelogies can of course also be provided will.

Experimental results have shown that the ratios w / w., Where W is the proportion factor for iron, and w _p / w_, are both higher in shale formations than in non-shale formations

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609851/068 3

Accordingly, one can derive a slate content indicator by directly examining one of these ratios. Alternatively, the ratio w / (aw _c . + Bw_) could be formed. This ratio has the advantage over both ratios W "/ w". or w / w _r that it can be taken into account that a slate matcLx can contain either silicon or calcium.

Generally speaking, the evaluations of <ί - - logs are improved both quantitatively and qualitatively by the availability of precise data on the properties of the formation, such as litology, slate content, porosity and salinity. Combined examination of the foregoing characteristics and spectroscopic data in accordance with the present invention accordingly improves the knowledge that can be gleaned from decay time logs regarding the formation content of hydrocarbons and their recoverability. In addition, one gains specific advantages under certain conditions that previously led to difficulties. For example, certain formations have distinctly different properties, such as shale and high-salinity sands, similar Γ and <i values and therefore cannot be easily distinguished by "%" logs alone Another area of improvement arises in connection with formations of low salinity (e.g. on the order of 20,000 ppm) where the thermal neutron decay time log is known to be of little reliability , eg the ^w ci ^ ^w H ~ ratio, more precise salinity measurements

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609851/0683

can then be used instead of the salinity values, derived from the decay time log, for water saturation values to win in a known manner. The calculations of the water saturation values are performed using the spectroscopic salinity measurements in the computer 82, and the Water saturation values obtained thereby are recorded by means of the recorders 86 and 9o, the are shown in FIG.

The invention was described above in principle in terms of the analysis of thermal neutron capture gamma radiation energy spectra described, but it is understood that gamma spectra resulting from other neutron interactions, can be analyzed just as well. Such other spectra can be, for example, those include those generated by non-elastic fast neutron scattering, and those representative of activation gamma rays. In the case of non-elastic scattering gamma spectra, the electroscopic Gate circuits are unlocked by signals from programmer 46 for an appropriate period of time during and or or immediately following the occurrence of a neutron pulse, as generally described in U.S. Patent 2,991,364. If desired, the entire 31f sequence could be 3 can be varied to include the 2o] capture gamma spectroscopy sub-cycle omitting and keeping only the 1o '"f thermal decay sub-cycle, This results in an additional time gain for the decay of trapping gamma rays between neutron pulses thus reducing the level of residual trapped gamma rays during the subsequent inelastic scattered gamma ray detection period.

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609851/0683

Baseline spectroscopic data developed from the analysis of inelastic scattered gamma radiation spectra would preferably include baseline data for carbon and oxygen. Such output data can be obtained, as in the above-mentioned US Pat. No. 2,991,364, by entering the carbon and oxygen peaks of inelastic gamma radiation, ie 4.4 MeV for carbon and 6.9 and 7 _f 1 MeV for oxygen. Preferably, however, the analysis of the inelastic scattering gamma spectra would be performed using the spectrum fitting methods described above. If so, separate constituent spectra would be selected for the non-elastic scattered gamma analysis. These could include, for example, spectra for carbon, oxygen, silicon, calcium, and hydrogen. It may also be desirable to consider constituent spectra for residual capture gamma rays or other background gamma rays. In either case, one would follow the spectrum fitting procedures noted above to derive the desired output spectroscopic data which would include, for example, proportional factors such as for calcium, carbon, oxygen, silicon and hydrogen and appropriate ratios thereof. If both thermal neutron gamma capture spectroscopic signals and non-elastic scattered gamma spectroscopy signals are desired, the different types of spectra can be acquired and analyzed simultaneously, that is, during the same pass of the probe, or each analysis can be performed separately, during multiple, e.g., alternating, passes.

Simultaneous acquisition and analysis could be performed using additional linear gate circuits (not shown) between the discriminators and the pulse sorting circuits of the pulse height analyzer

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609851/0683

68, which would be unlocked in a corresponding manner by Signals from programmer 46 to that part the time distribution of the detector signals emanating from each neutron pulse or from selected ones Neutron pulses corresponding to the inelastic scattered gamma rays. The capture gamma radiation detection periods could be determined in their time slot as described above. In this case, the computer 82 would be designed in this way that it uses the correct set of constituent spectrum coefficients ν., respectively detected assigns inelastic scattering or capture gamma spectra.

The separate probe pass method could easily be performed by using of switches that can be operated from the surface of the earth to select the appropriate detector gate sequences and to send commands to computer 82 to input the correct constituent spectrum coefficients are used for the purpose of analyzing the respective recorded gamma radiation energy spectra.

If desired, the number of individual spectra can be provided in the composite spectrum must be reduced by subtracting certain individual spectra from the detected spectrum the matching of the detected and the composite spectrum. For example, the gamma ray spectrum can be for oxygen or iodine or both of that The capture gamma ray spectrum generated during the spectroscopic periods can be subtracted. Again, the contributions to the detected gamma radiation spectrum from relatively long-lived emitters (such as oxygen) and from other background sources (such as the calibration source or several calibration sources), natural gamma

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609851/0683

radiation, radioactive salts in the borehole, etc.) can be taken into account by detecting such contributions during a time period following the spectroscopic period and subtracting the background spectrum thus obtained from the detected spectroscopic spectra. In the capture gamma radiation measurement cycle according to FIG. 3, for example, the background spectrum can expediently be measured during the 24 ~ ~ - to I 31 stabilization period, this spectrum then being subtracted on a proportion basis from the capture gamma spectrum which was developed during the previous spectroscopy periods I06A-I06E is. This can easily be done in the computer 82. The resulting background corrected spectrum would then be used to develop the desired output spectroscopic data.

Numerous modifications and improvements to the exemplary embodiment described can, without from the The principles of the invention may differ from those of ordinary skill in the art. For example, whichever Neutron source have a fixed pulse duration and a fixed pulse repetition frequency, and the time of occurrence of the

Γ detection period relative to the neutron pulses could also be fixed or controlled in some way other than automatic in accordance with a measured value of a neutron characteristic of the formation.

(Patent claims)

609851/0883

39 -

Claims (38)

Claims 1.) Procedure for examining a borehole : euften earth formation in which a measurement of a time-dependent The neutron characteristic of the formation is derived, characterized by the steps: - irradiation of the formation with a first neutron pulse, - Derivation of a representation of the energies of gamma rays generated by those of the first pulse resulting neutron interactions with nuclei of the formation, - Analysis of at least part of the energy spectrum of the gamma rays corresponding to those of the Representations derived from neutron momentum interactions, - Obtained during a first period of time after irradiating the formation with the neutron pulse, and - Control of the timing of the first time period as a function of the derived measurement of the time dependent Neutron characteristics of the formation. 2. The method according to claim 1, characterized in that for the derivation of a measurement of the time-dependent Neutron characteristics of the formation are as follows - 4o - 609 851/0683 Steps to be carried out: - Deriving representation of neutron interactions with nuclei of the formation after irradiation the formation with a second neutron pulse, and - Deriving the mentioned measurement of a neutron characteristic of the formation from the second neutron pulse associated representations. 3. The method according to claim 1 or 2, characterized in that that the step of controlling the timing of the first time period comprises: Generating a control signal in accordance with the derived measurement of the neutron characteristic, and - Determination of the time of occurrence of the first time period as a function of the control signal. 4. The method according to claim 3, characterized in that the control signal is based on the time of occurrence of the first Neutron pulse is obtained in accordance with a function of the derived neutron characteristic measurement. 5. The method according to claim 3 or 4, wherein the measured neutron characteristic is the thermal neutron decay time of the formation, characterized in that the control signal is generated so that the first time period is initiated at a first selected point in time after the termination of the second neutron pulse. 6. The method according to claim 5, characterized in that the first time period is selected to a second 609851/0683 - 41 - Time after it has started. 7. The method according to any one of claims 1 to 6, characterized in that the step of spectrum analysis includes: - Comparison of the analyzed portion of the formation gamma energy radiation spectrum with a composite Eneigies spectrum, built up from weighted spectra of Components which are postulated to make up the irradiated formation for the purpose of determining the Proportions of the postulated components that produce a composite spectrum that most closely matches with the analyzed portion of the formation gamma radiation energy spectrum. 8. The method according to claim 7, characterized in that representations of the measured neutron characteristic and the constituents identified are recorded as a function of depth in the formation. 9. The method according to any one of claims 1 to 4, characterized characterized in that the earth formation with at least said first and second pulses during each one Sequence of time intervals is irradiated - the comprising the step of deriving a measurement of a neutron characteristic during the second period of time, and that the time of occurrence of each first time period from a second following with respect to a first time interval Interval is controlled from a sequence of intervals as a function of the neutron characteristic xk measurement, the is derived during the second time period of the first time interval. - 42 - 609851/0683 10. The method according to claim 8, characterized in that that the second time period follows the first time period in each time interval. 11. The method according to claim 9 or 1 o, characterized in that that the occurrence time of each first time period of the second time interval is controlled in dependence on the time of occurrence of the associated neutron pulse as a function of that measured in the first interval Neutron characteristics. 12. The method according to claims 9, 1o or 11, characterized characterized in that the time of occurrence of each neutron pulse of the second time interval as a function the first inte is controlled. the neutron characteristic measured in the first interval 13. The method according to any one of claims 9 to 12, characterized through the steps: - Control of the occurrence time of a second time period, which is a neutron pulse of the second time interval follows, as a function of the neutron characteristic measured in the first interval, and - Derivation of a further measurement of the decay time as a function of signals generated during the second time period of the second time interval. 14. The method according to any one of the preceding claims, in which the representations of the gamma-ray energies are derived from thermal neutron capture interactions with nuclei of the irradiated formation and others 43 - 609851/0683 Borehole medium, characterized in that the control of the first period time slot is used for the selection of those gamma-ray energy presentations that predominantly result from properties of the irradiated formation. 15. The method according to any one of claims 1 to 8, characterized by controlling the time of occurrence of the first neutron pulse as a function of the measured neutron characteristic . 16. The method according to claim 9, characterized in that: the measured neutron characteristic the thermal Neutron decay time is that neutron pulse control signals are generated because the aforementioned first and second Neutron pulses each approximately have a measured decay time, the first neutron pulse Shifted by intervals of about four measured decay times and every other neutron pulse of that occurs next neutron pulse by an interval of at least ten decay times away that approximately two measured decay times are provided for the mentioned eBth time period that is triggered about one measured decay time after termination of the previous neutron pulse, and that about seven measured decay times are provided for the second time period, which is triggered about two measured Decay time periods after the termination of the previous selected neutron pulse. 17. The method according to claim 6, characterized in that that the first and second selected times approximate a measured decay time and two measured decay times, respectively be. - 44 609851/0683 18. Device for performing the method according to one of claims 1 to 17 with a movable suspended probe in the borehole, with housed in the probe Facilities for irradiating the earth formation with at least one first neutron pulse and with devices for measuring a selected, time-dependent one Neutron characteristics of the formation, characterized by a detector arrangement (2 6) for the detection of Neutron interactions with nuclei originating from the formation Gamma radiation, related to the first neutron pulse, and for the generation of the corresponding, for the energies the detected gamma radiation representative signals, by an analyzer (6 8) for the analysis of at least a part the spectrum of the gamma radiation, through gate circuits (44) for transmission to the analyzer of those signals, the response to sensed gamma radiation during a first period of time following the first neutron pulse have been generated, and by a controller (46) for controlling the operation of the gate circuits in Function of a value of the selected, measured neutron characteristic . 19. The device according to claim 17, characterized in that the measuring device is designed to be responsive Representations of neutron interactions with nuclei of the formation after irradiating the formation with a second neutron pulse. 20. Apparatus according to claim 19, characterized in that that the neutron characteristic measuring device is designed to be responsive to representations of thermal neutron capture interactions with nuclei of formation, - 45 - 609851/0683 and that the measured neutron characteristic is a thermal neutron capture characteristic of the formation. 21. Device according to one of claims 18 to 2o, characterized in that the gate circuits a Each variable gate includes, in response to a control signal, depending on the time of occurrence of the first Neutron pulse for controlling the time of occurrence of the first time period, and that the control device Circuitry includes the selected one for generating the first gate control signal as a function of the measured value NEutron characteristics. 22. The device according to claim 21, characterized in that the selected neutron characteristic is the thermal Is neutron decay time of the formation, and that the first gate control signal is generated to be the first Time period initiates, with at least one measured value of the decay time after termination of the first neutron pulse and the first time period ends at at least two measured values of the decay time after it has started. 23. Device according to one of claims 18 to 22, characterized in that the control device has switching circuits contains for the generation of a control input for controlling the time of occurrence of the first neutron pulse as a function of the measured value of the neutron characteristic, 24. Device according to one of the preceding claims, characterized in that the neutron characteristic measuring device is designed to be responsive to signals generated by the detector arrangement during a second Time period that one neutron pulse after the second - 46 - 609851/0683 ITeutron pulse follows “and second variable gate circuits comprises, in response to a control signal in dependence on the time of occurrence of the subsequent control pulse for controlling the occurrence time of the second time period, and in that the control means further includes circuitry for generating the second gate control signal as a function the measured value of the selected neutron characteristic. 25. Device according to one of the preceding claims, characterized by means (86,9o) for recording representations of the measured value of the selected neutron characteristic and the analyzed part of the detected gamma radiation energy spectrum as Function of the formation depth below the earth's surface. 26. Device according to one of the preceding claims, ■ characterized by means for comparing the analyzed portion of the detected gamma radiation energy spectrum with a composite energy spectrum, built up from weighted spectra of components that are primary are postulated in the irradiated formation for the purpose of determining the proportions of the postulated constituents below Generate a composite spectrum that best matches the analyzed portion of the detected gamma-ray energy spectrum . 27. Device according to one of the preceding claims, characterized in that a plurality of the neutron pulses is generated by the irradiation device (2 4) during a sequence of time intervals, and that devices are provided for the generation of calibration signals and for the transmission these calibration signals to the analyzer during a predetermined period of time within each time interval. - 47 609851/0683 28. Device according to claim 27, characterized in that that the equipment includes: - a source of gamma rays of known energy, a device for the transmission of signals generated by the detector arrangement in response on the calibration gamma radiation to the analyzer during the predetermined time periods within alternating Time intervals, - Means for generating a first series of calibration signals of low amplitude and a second series of calibration signals of high amplitude with essentially constant amplitude ratio, and for alternately transmitting the first series of signals and the second series of signals to the analyzer during the predetermined time periods of the remaining time interval in the sequence of intervals, - Means for the formation of the ratio of the channel positions of the first and second series of Calibration signals, - Means for generating a control signal depending on the difference between the Channel position ratio and the amplitude ratio, and - Means designed responsive to this relationship to correct any error in the analyzer. 29. The method, in particular according to one of the claims 1 to 17, in which the accuracy within a wide temperature range of the input channel relationship of a pump height analyzer which sorts the pulses received from it onto a plurality of channels - 48 - 609851/0683 according to the respective pulse amplitudes, characterized by the following steps: a) Generation of a first and a second test pulse of relatively large and small amplitudes, respectively, b) Maintaining a predetermined ratio between the amplitudes of the asten and the second test pulse within the wide temperature range, c) Applying the first and second test pulse to the pulse height analyzer, d) Determination of the respective channels to which the first and the second test pulse from the pulse height analyzer be assigned, e) Formation of the ratio of the respective channels, and f) Controlling the pulse height analyzer in response to the channel position ratio so as to make the pulses accurate to sort into the assigned channels. 30. The method according to claim 29, characterized in that that the first and the second test pulse are alternately applied to the pulse height analyzer. 31. The method according to claim 29 or 3o, characterized in that a constant ratio between the test pulse amplitudes is maintained. 32. The method according to claim 29, characterized in that the channel position ratio for the analyzer control 609851/0683 - 49 - is compared with a signal that is within the second Temperature range is kept constant to derive a control signal that is representative of the difference between the channel position relationship and that signal, and that the pulse height analyzer is controlled in dependence on the control signal such that the channel position ratio and the aforementioned signal are substantially be made consistent. 33. The method according to claims 31 and 32, characterized in that the constant signal is the test pulse amplitude ratio is. 34. Device for performing the method according to one of claims 29 to 33, in which over a wide Temperature range to maintain the accuracy of the input channel ratio of a pulse height analyzer which is adapted for use in a system having a pulse generator which provides pulses with corresponding amplitudes, depending on a selected parameter for the pulse height analyzer, which in turn sorts these pulses onto a plurality of channels in accordance with the respective pulse amplitudes, marked by: - A device for generating first and second test pulses relatively large or even smaller Amplitude, a device for maintaining a predetermined ratio between the amplitudes of the first and second test pulses within the wide temperature range, - a device for the creation of the first resp. second test pulse to the pulse height analyzer, 609851/0683 - A device for the determination of the respective channels, which the first and second test pulses from be assigned to the pulse height analyzer, - a device for the formation of a signal corresponding to the relationship between the respective channel positions, and means for controlling the pulse height analyzer in response to the channel position ratio signal. 35. Apparatus according to claim 34, characterized in that the device for applying the test pulses to the pulse height analyzer includes means for alternately applying the first and second Test pulses to the pulse height analyzer. 36. Apparatus according to claim 34 or 35, characterized in that that the predetermined ratio between the test pulse amplitudes is a constant ratio. 37. Apparatus according to claim 34, characterized in that the devices for controlling the Pulse height analyzer circuitry includes, for comparing the channel position relationship with a signal, which is held constant within the temperature range for the purpose of generating a control signal which is representative for the correspondence between the channel positional relationship and that signal, and that circuitry is provided are for controlling the pulse height analyzer in such a way that the channel position ratio and that signal are essentially be brought into agreement. 38. Device according to claims 36 and 37, characterized in that that that signal is the pulse amplitude ratio. 609851/0683