US3060371A - Geological prospecting process and apparatus - Google Patents

Description

Oct. 23, 1962 3,060,371

J. TOWNSEND ETI'AL GEOLOGICAL PROSPECTING PROCESS AND APPARATUS Filed July 20, 1955 Ti-EH.

4 Sheets-Sheet 1 an Ed 9 f i gm I 2.80 I 3f 59 I :2 2E .smeusm 0F H new 5% 0.1 I E- b'm I smsuam 0F H new gE o I D t rv=28o Ho Ill @5 FREQUENCY OF n. 0 l H FIELD 2f 51' i at; 0" I ll] I h? :2!- 3." I

FREQUENCY OF H, FIELD SIGNAL AMPLITUDE STRENGTH OF Hp FIELD NV E N TO R Juazikam Tow/ism Egg Commonefi TIME ATTORNEY Oct. 23, 1962 J. TOWNSEND ETAL 3,060,371

GEOLOGICAL PROSPECTING PROCESS AND APPARATUS 4 Sheets-Sheet 2 Filed July 20, 1955 MQDFIQE 44206 Ho CURRENT ATTORNEY Oct. 23, 1962 J. TOWNSEND ,ETAL 3,060,371

GEOLOGICAL PROSPECTING. PROCESS AND APPARATUS Filed July 20, 1955 4 Sheets-Sheet 3 RECTIFIER A 29 .31 .55 RECTIIFIER I INPUT \n r b'g' AMPUFIER l FREQUENCIES :5 Fan d T FREQUENCY T P SIGNAL fifi of m a DEMODUIATOR LOW-PASS RECTiFlER FILTER 3 o 3.

. MIXER-P m qrs- FROM 'sou'Rc-E-W IN ENTORS .fo a/ms'end 23 Com/zen Oct. 23, 1962 GEOLOGICAL PROSPECTING PROCESS AND APPARATUS Filed July 20, 1955 4 Sheets-Sheet 4 l CARBONACEOUS DEPOSIT CD (I II I I I INVENTORS fanaikam 76101250120 I 01719 Commoner REFLECTING Y INTERFACE I m ATTORNEY J. TOWNSEND ETAL 7 3,060,371

United atent 3,060,371 GEOLOGICAL PROSPECTING PROCESS AND APPARATUS Jonathan Townsend, 5942 Horton Place, St. Louis 12,

Mo., and Barry Commoner, 50 Arundel Place, Clayton, Mo.

Filed July 20, 1955, Ser. No. 523,213 26 Claims. (Cl. 324.5)

This invention relates to the detection of carbonaceous materials and materials geologically associated therewith. More particularly it relates to the location of subterranean or inaccessible geological deposits of carbonaceous materials and materials often associated therewith, such as uranium. By carbonaceous materials as used herein is meant petroleum, coal, natural bitumens including tars and asphalts, partially carbonized animal and vegetable matter and carbonaceous geological deposits and formations including oil-bearing shales.

As is well known, valuable deposits of carbonaceous materials may occur in subterranean locations which become accessible only through drilling or mining operations. Uranium ores also may occur in subterranean deposits. Such uranium deposits are frequently found in association with carbonaceous materials including partial ly carbonized animal and vegetable matter, bituminous materials, such as pitchblende and gilsonite, and carbonaceous shales. Typical of this relationship is the presence of uranium salts among fossilized plant materials or other organic geological debris collected in depressed areas, such as in ancient stream beds, swamps or shoreline areas. For example, this type of geological situation is characteristic of the uranium-bearing Morrison, Entrada and Shinarump formations of Utah and Colorado.

Prior to this invention there has been no way of directly and positively detecting from a distance the presence of subterranean deposits of carbonaceous materials, such as petroleum, coal, and the other carbonaceous materials hereinabove identified. In the case of petroleum, for example, apart from the expensive method of boring or digging into the earths crust, most, if not all, other methods used for locating deposits of petroleum indicate only the presence of geological structures capable of trapping petroleum, but leave open the question whether the traps actually contain the material. Such other methods include surface observations of geological structures and electrical, seismic, gravity, magnetic, and radioactive methods. The electrical methods usually depend upon differences of electrical conductivity or dielectric properties between the deposit sought and the surrounding earth, use electrical signals of one or more frequencies, and depend for detection upon these same frequencies. The magnetic methods usually depend upon a distortion of the earths magnetic field by iron-containing compounds in parts of the geological structures.

Prior to this invention, the art of detecting useful underground deposits of uranium has also been based chiefly on indirect geophysical methods which describe the distribution of geological formations believed to contain uranium. The direct method of detecting uranium ores by means of the gamma rays which they emit is effective only in connection with uranium ore deposits located at or very near the surface of the earth. How ever, most of the useful ore deposits and particularly those in the United States of America are located in scattered deposits at depths of fifty to two hundred feet more or less. Gamma radiation will not penetrate through such a thickness of overlying soil and therefore cannot be used for direct localization of these subterranean uranium deposits. Gamma radiation detectors serve only to locate outcroppings of uranium ore which may bear some stratigraphic relationship to other deposits lying at a depth. Thus, even where a valuable outcropping has been located by gamma radiation detection, the problem still remains to find other nearby deposits which lie underground. Prior to this invention this has been accomplished by drilling numerous and expensive bore holes in the neighborhood of the detected outcropping in an endeavor to find useful ores at a depth. Since even near an ore outcropping the underground deposits may be scattered at random, the present methods require a great deal of drilling, much of which is fruitless.

It is among the objects of this invention to provide process and apparatus for detecting directly the presence of inaccessible geological deposits of carbonaceous materials and/ or of uranium found in association therewith.

A further object is to provide such process and apparatus for detecting directly the presence of carbonaceous materials and/ or uranium found in association therewith in a mixture of such materials with other materials and where the presence of the carbonaceous materials is not readily apparent.

Still a further object is to provide such process and apparatus for detecting, locating and determining the size of subterranean deposits of carbonaceous materials to the end that geographical distribution of the deposits may be mapped and drilling operations guided by the information thus obtained.

Still a further object is to provide process and apparatus for locating scattered underground uranium deposits found in association with carbonaceous materials, eliminating the drilling of unnecessary or profitless bore holes in the location of such uranium deposits.

Still another object of the invention is to provide electrical geophysical exploration method and apparatus for locating deposits of carbonaceous materials and/or of uranium associated therewith which method and apparatus is more certain in its performance than prior known methods and apparatus.

Other objects and advantages of this invention will be apparent from the following detailed description thereof.

In accordance with this invention, inaccessible deposits of carbonaceous materials and/or of uranium associated therewith are located by utilizing or creating, in the subterranean zone or region being explored a magnetic field, desirably a unidirectionally pulsating magnetic field (such field being herein for convenience referred to as the H field), creating an alternating or rotating magnetic field in such subterranean zone or region (such field being herein for convenience referred to as the H field), which field is transverse and usually at right angles to the H field, with a frequency (1) related to the strength of the H field in accordance with the formula ;f=KH where K is about 2.8 megacycles per second per gauss, or in accordance with the formula In the above formula and throughout the specification the frequencies referred to are in megacycles per second; the strengths of the magnetic fields are given in gauss. The invention also involves noting, detecting, or measuring at a remote point magnetic changes resulting from the presence of carbonaceous material in the subterranean zone or region being explored, if such material is present, as by utilizing a detector actuated by energy from the H field to give a signal or otherwise indicate the presence of such carbonaceous material. The above Formula 1 represents the limits of the broad range of frequencies of the H field and strengths of the H field; the limits of the preferred range are indicated by the formula The H, field may be the earths magnetic field, a component thereof, or a specially created magnetic induction field or magnetic component of a radiation field as hereinafter more fully described. Induction fields do not permanently leave the region surrounding the apparatus as do those fields classed as radiation fields. The latter become detached from the apparatus and propagate themselves away indefinitely until reflected, refracted, or absorbed by obstacles. Radiation fields consist of an inseparable mixture of electric and magnetic fields, both of which are usually perpendicular to each other and to the direction of propagation. A beam of electromagnetic radiation in which the electric fields are oriented in one direction is called a polarized beam, and the direction of the electric fields is called the direction of polarization.

The H field may also be of the induction or radiation type and should have a frequency of from 0.1 to 10,000, preferably from 0.25 to 250 megacycles per second, the latter range of frequencies being particularly useful when employing this invention for geological prospecting.

An alternating magnetic field of the induction type may be produced by a coil supplied with an alternating current. A rotating magnetic field may be produced by superimposing two alternating magnetic fields, mutually perpendicular, and having a constant phase difierence of 90 or one-fourth of a cycle.

In accordance with a preferred embodiment of this invention either the strength of the H field or the frequency of the H field is modulated in the region of resonance to improve the clarity of the signal or other sign of detection given by the detector. Best results are obtained by modulating the strength of the H field in the region of resonance. By the region of resonance is meant the region of the above mentioned formulas represented by the range of plus or minus 10, preferably plus or minus 2.5 gauss. Stated otherwise, the region of resonance, in the case of modulation of H field strength, is the range of values of H field strength in which the ordinates of the curves of FIGURES 1 and 2, hereinafter more fully described, are appreciably different from zero. In the case of modulation of the H field frequency the region of resonance is the range of values of the H field frequency of the curves of FIGURES 3 and 4, hereinafter more fully described, in which the ordinates of these curves are appreciably different from zero.

With the thought that it would be helpful to a better understanding of the invention, the following explanation and description of certain scientific principles used in the invention are believed to be explanatory of its unexpected results, and are given before proceeding to a detailed description of the drawings and of exemplary embodiments of the invention. It will be understood, however, that this explanation and discussion, as noted, is advanced in the interests of facilitating a better understanding of the invention and the invention is not to be limited by this explanation.

The discussion which follows will use the terms of classical physics rather than those of quantum physics because it is recognized that in the subjects to be discussed, both lead to the same result and classical physics is more familiar to the general reader.

In the course of research conducted by us it was discovered that carbonaceous materials hereinabove enumerated are exceptional as compared with other materials in that they have a molecular structure comprising a small fraction of unpaired electrons. Concentrations of unpaired electrons in coal and petroleum samples tested by us ranged up to 0.001 mol per liter.

As is well known, electrons are magnetic. If an isolated electron be placed in a magnetic field, it will experience a torque tending to align its magnetic axis parallel to the direction of the magnetic field, in the manner of a compass needle in a magnetic field.

In addition, each electron behaves as if it were spinning about its magnetic axis. A spinning object, whose axis is acted on by a torque, undergoes precession, the familiar motion of a spinning top or gyroscope. Hence, an electron placed in a magnetic field does not immediately align its axis parallel to the field "but undergoes a precession in which its axis maintains a nearly constant angle between itself and the direction of the magnetic field. The frequency of this precession depends upon the magnetic moment, the angular momentum of the electron and the strength of the magnetic field. Since all electrons are known to have identical properties, all electrons precess at the same frequency in the same magnetic field. This frequency, often called the Larmor frequency for electrons, is given by the equation where is in megacycles per second, and H is the strength of the magnetic field in gauss. Stated otherwise, the rate, or frequency, of precessionis proportional to the strength of the magnetic field, being about 2.80 megacycles per second per gauss of field strength.

Most matter is so constituted that the electrons occur in pairs in such a way that the magnetic properties of each electron are cancelled by those of its partner. The electrons of most substances therefore do not partake of the action described above. As above noted, the carbonaceous materials, hereinabove set forth, are unusual or exceptional in that they have a small fraction of their electrons not paired.

When a carbonaceous material is placed in a magnetic field its unpaired electrons are acted on by the field, as described above, and also are acted on by dissipative forces, which transfer the energy of precession into heat energy. As this happens, the axes of the electrons become more nearly parallel to the direction of the magnetic field, until finally complete alignment occurs and precession ceases to exist. It is possible, however, to supply, from an outside source, energy to start or steadily maintain the precession despite the effects of dissipative mechanisms. This is done by the second magnetic field, H oriented perpendicularly to the first, and either rotating or oscillating at a frequency f, equal to or near the Larmor frequency f The first magnetic field, whose direction is fixed, is the H field, above mentioned, and the second rotating or oscillating field is the H field, above mentioned. The agency supplying the H field is required to furnish energy to the processing electrons, which, in turn, pass the energy, through the dissipative mechanisms mentioned, into heat energy. This phenomenon is termed resonance absorption. If the agency producing the H field is a current-carrying coil, then the phenomenon of resonance absorption increases the loss factor of the coil.

In the phenomenon of resonance, the unpaired electrons are precessing predominantly in unison, that is, their axes predominantly point in the same direction at any given instant. Their individual magnetic moments then add to produce an overall magnetic moment of the substance. This overall magnetic quantity precesses about the direction of the H field, and does so at the same frequency as the applied H field. The absorption of energy from the H field may be viewed as being a result of the fact that the precessing magnetic moment vector adjusts itself so that it has a component opposite in direction to the vector representing the time rate of change of the H field.

By measuring the power absorbed from the H field, while holding its amplitude and frequency f constant but varying the Larmor frequency f by varying the H field strength, in accordance with Equation 3, the resonance curve of FIGURE 1 of the later described drawings is obtained when a carbonaceous substance is in the fields.

The value of H at the center of the curve is that given by Equation 3 together with the resonance requirement f =f. The width, AH of the resonance curve,

depends upon the strength of the dissipative mechanisms mentioned above. We have found that for representative samples of coal and crude oil the value of AH, is is approximately gauss.

The precessing magnetic moment vector also has, 1n general, a component, either positive or negative, in the direction of the H field vector. The effect of this is to change the magnetic susceptibility of the sample with respect to the H field. The behavior of this susceptibility as a function of the strength of the H field is depicted in FIGURE 2. This change of susceptibility will hereinafter be called resonance dispersion. If the agency producing the H field is a current-carrying coil, then the phenomenon of resonance dispersion effects the inductance of the coil.

Still another phenomenon may be observed by creating an oscillating H field in a direction perpendicular to the H field and establishing a detector which will detect a component of the magnetic moment oscillating in a direction perpendicular to both the H and H fields. A plot of the amplitude of oscillation of this component of the magnetic moment as a function of the strength of the H field will closely resemble the curves of FIG- URES l and 3. This phenomenon will be hereinafter called resonance induction.

A beam of radiation, having a frequency f, travelling through a carbonaceous material can be affected by resonance in the material if a steady field H the strength of which lies in the region of resonance, exists. For example, if the H field is in the same direction as the direction of propagation of the radiation, a progressive rotation, known as Faraday rotation, of the direction of polarization occurs. If the H field is in the direction of the electric field of radiation, then a change of wave length of the radiation occurs if the H field passes through the resonance range. In both of these cases there is also a progressive attenuation of the beam due t resonance absorption. If the H field is in the direction of the magnetic field of radiation, then no interaction occurs.

One embodiment of our invention for the detection of an inaccessible deposit of carbonaceous material involves: (1) a source of alternating electrical power communicating with a system of coils or antennas powered by the source which establishes an alternating (or rotating) magnetic field at a frequency f, the H field, at right angles to the H field which may be the earth's magnetic field or a magnetic field specially created, both fields being in the locality of exploration; and (2) a suitable detector actuated by energy from the H field when a carbonaceous substance is present in both fields.

In a typical case, a plot of the amplitude of the signal given to the above mentioned detector vs. the H field strength may appear as curve 10 in FIGURE 5.

Such a curve may be plotted by taking successive readings of the detector at progressively higher values of H However, the sensitivity of this method is limited by disturbances and fluctuations of the apparatus which occur between the times at which the readings are taken.

An enhancement of detection sensitivity may be obtained by superimposing upon the constant or slowly varying H field strength a sinusoidal variation with time, as shown in curve 12, FIGURE 5, or by modulating the frequency of the H field in the region of resonance. Such a variation, or modulation, of the H field strength or of the H field frequency will produce an amplitude modulation of the signal to the detector, as shown in curve 13, FIGURE 5. The modulation envelope, represented by curve 13, may consist predominantly of a frequency component of the same frequency as the modulation of H as shown, or, if the modulation of H, has a greater amplitude, causing H to oscillate over most of the resonance region, higher harmonics may predominate in the modulation envelope.

It is a fact well known in the art of electrical communication that a signal of frequency f which has undergone amplitude modulation no longer consists of a single frequency, but has other frequency components, or sidebands, two for each frequency component in the modulating signal. The sidebands of a component of the modulating signal of frequency 111 have frequencies f-l-m and m, respectively.

When the H field (but not the frequency of the H field) is modulated at a frequency In, the detector receives a modulated wave having, in general, frequencies f, f+m, f-m, f+2m, f2m, etc., in the presence of resonance and only the frequency f in its absence. A suitable detector could then have two possible forms, with respect to frequency considerations: (1) a demodulator acting upon the total input signal, producing a signal at the modulation frequency plus its harmonics, followed by a filter selecting one of these frequencies to provide indication; and (2) a filter selecting one of the sideband frequencies to produce indication directly. In either case, the presence of sideband frequencies is necessary for the detector to indicate resonance. These sideband frequencies are produced not by the apparatus but by the carbonaceous material itself.

In the accompanying drawings, forming a part of this specification and showing, for purposes of exemplification, preferred forms of this invention Without limiting the claimed invention to such illustrative instances:

FIGURE 1, as noted, is a typical resonance curve in which the power absorbed from the H field is plotted against the strength of the H feld;

FIGURE 2 is a curve showing the change in the magnetic susceptibility to the H field as the strength of the H field is varied;

FIGURE 3 is a typical resonance curve in which the power absorbed from the H field is plotted against the frequency of the H field;

FIGURE 4 is a curve showing the change in the magnetic susceptibility to the H field as the frequency of the H field is varied;

FIGURE 5 shows plots of amplitude of the signals with variation in the strength of the H field;

FIGURE 6 illustrates apparatus embodying this invention, for prospecting, by means of resonance absorption or dispersion, using electromagnetic induction fields;

FIGURE 7 is a block diagram showing the relationship between the well known parts of one form of detector;

FIGURE 8 illustrates a modified form of apparatus embodying this invention, for prospecting by means of resonance induction, using electromagnetic induction fields;

FIGURE 9 illustrates still another modified form of apparatus for prospecting by resonance absorption or dispersion, using an electromagnetic radiation field for H and an electromagnetic induction field for H FIGURE 10 illustrates still another modified form of apparatus for prospecting, in accordance with this invention, and involves the use of Faraday rotation, an electromagnetic radiation field for H and an electromagnetic induction field for H FIGURE 11 illustrates still another modified form of apparatus for prospecting, in accordance with this invention, by means of resonance induction using electromagnetic radiation fields for both H and H FIGURE 12 illustrates an embodiment of this invention for logging a bore hole by means of resonance induction, using electromagnetic induction fields; and

FIGURE 13 shows plots of amplitude of the signals with variations in the value of current which flows in the coil which generates the H field.

In the several figures of the drawings, like parts are indicated by the same reference characters. Referring to FIGURE 6, a coil 15 is placed at the surface of the ground with axis vertical and is energized by a source of direct current 16 and a source of alternating current 17. This coil produces a magnetic field, the H field, beneath the surface, represented by arrow H on FIGURE 6 in the vertical direction. The current supplied by source 16 is varied until the magnetic field at a given selected depth of exploration is brought into the region of resonance of petroleum or other carbonaceous substance at the selected frequency f, of operation (given by Equation 3 above). The alternating current source 17 is adjusted to provide a suitable modulation of H as explained above in connection with FIGURE 5. Let the modulation frequency be called m.

The two coils 19, 20, which are symmetrically located with respect to the center of coil and connected in such a way as to produce magnetic fields at their centers having opposite directions, are employed to produce the H magnetic field below the surface 21 in a horizontal direction. The directions of the H and H fields are indicated by the arrows on FIGURE 6.

Coils 19, are energized, at the frequency f, by source 22 through bridge- arm impedances 23, 24 and 25. The condenser 26, together with coils 19, 20, form a circuit resonant at the frequency 1, but the resonance is broad enough to transmit the sidebands at frequencies fm and f-l-m. Impedances 23, 24 and 25 may be adjusted, by well known methods, so that the signal fed to detector 27 is determined by (a) changes of the loss factor of coils 19, 20 or (b) changes of the inductance of coils 19, 20. As previously explained, (a) then permits detection of resonance absorption, and (b) permits detection of resonance dispersion. The detection sensitivity achievable will not be appreciably different for the two cases.

In FIGURE 7, is shown one form of detector designed to detect sidebands in the signal having frequencies f-m and f-l-m. As shown in FIGURE 7, a preferred arrangement for this detector consists of an amplifier 28 to amplify the frequency f and the sidebands, a demodulator 29 to produce a signal of frequency m from the sidebands, a further amplifier 31 to amplify the frequency m, a suitable mixer 32 to mix this signal together with one derived directly from source 17 circuit to produce a zero-frequency signal, and a low-pass-filter 33 to remove as much noise as possible by limiting the effective noise band-width of the system to a suitable low value. The resulting output is a D.C. signal the value of which is indicative of the presence of resonance, e.g., resulting from a deposit of carbonaceous material at the selected depth. As these parts are well known in the electronic art, further description thereof is believed unnecessary. This arrangement of the detector, and also the arrangement of the bridge circuit consisting of impedances 23,

24 and 25 are illustrative, and other well known means of accomplishing these functions could be employed. For convenience the reference numeral 27 is used to indicate a detector in the several views. It Will be un derstood, however, that any suitable form of detector may be used and different forms of detectors may be used in the different modifications.

A variation of the arrangement of FIGURE 6 may be had by omitting direct-current source 16 and depending upon the earths field for the constant part of H The frequency i must then be chosen accordingly. This variation results in a saving of the power otherwise needed for source 16. A further saving of power may be made by replacing the sinusoidal source 17 by 'a pulse generator furnishing short pulses of large current. In this case the frequencies supplied by source 17 consist of the pulse repetition frequency m, plus many harmonics or multiples of m. Each harmonic, including the frequency In as well as its multiples, produces a pair of sidebands in the received signal. The detector may be made to re spond to one or more of these sidebands.

The roles of the coil 15 and the pair of coils 19, 20 can be interchanged, thereby producing a horizontal H field and a vertical H field without substantially altering the operation or result.

Alternatively, source 17 may be omitted, thus employing a: constant H field produced by direct-current source 16, and the frequency of the H field modulated through the region of resonance by proper variation of the frequency of the current produced by source 22. A current of varying frequency in the ranges given by the above Formulas l or 2, i.e., in the region of resonance, may be produced by any means well known in the electrical field and accordingly a description of such means would serve no useful purpose.

Referring now to FIGURE 8, which is a diagram of apparatus for exploration by resonance induction, coil 15 and power sources 16 and 17 perform the same functions as in FIGURE 6, namely, the production of a modulated H field. The coils 19, 20 and source of alternating current 22 also serve, as in FIGURE 6, to produce an H field, indicated by the arrow on FIGURE 8, oscillating at the desired operating frequency, f. If there is present a carbonaceous substance, a precessing magnetic moment is produced, which will have an oscillating component perpendicular to H and H and indicated by the arrow 35 on FIGURE 8. This component will induce a voltage in coils 36, 37, which are similar to coils 19, 20 except that they are oriented in such manner that a line joining their respective centers is at right angles to a line joining the respective centers of coils 19, 20, in order to minimize the direct induction of a Voltage in coils 36 and '37, due to the current in coils 19, 20. Detector 27 is connected to coils 36, 37; the description of this detector is given above in connection with the description of FIGURES 6 and 7.

The functions of coils 15, 19, 20, 36 and 37 could be permuted in any manner with substantially equal results. As in the case of FIGURE 6 hereinabove described, source 17 may be omitted and the H field frequency modulated (instead of the strength of the H field effected by source 17) by proper variation of the current generated by source 22.

In FIGURE 9 is shown apparatus employing an induction field for H and a radiation field for H Coil l5, D.C. source 16 and A.C. source 17 produce a vertical modulated H field, as in FIGURE 6. The H field is produced by a directional antenna 49, which, for convenience of illustration, is shown as a dipole antenna 41 with a parabolic reflector 42, although any of the types of directional antennas known to the art of radio communication might be used. The antenna directs a beam of radiation into the earth at the center of coil '15.

Antenna 40 is energized by A.C. source 22 through a bridge circuit consisting of impedances 23, 24 and 25. Detector 27, of the type discussed in connection with FIGURE 7, is connected to the bridge as shown.

The beam of radiation penetrates the earth and is partially reflected at interfaces between layers of earth having different electrical properties. If it passes through a carbonaceous material of the type hereinabove identified, it will suffer an absorption, the value of which will depend upon the value of the H field, and the radiation reflected from lower interfaces will suffer further absorption on its way back to the antenna. This radiation, now modulated by resonance absorption, enters antenna 40 and actuates detector 27. The bridge circuit is balanced so that the power entering the detector directly from source 22 is minimized.

FIGURE 10 shows an arrangement for making use of the Faraday rotation of the plane of polarization of a beam of radiation by carbonaceous materials in accordance with our invention. As before, coil 15 and sources 16 and 17 create a modulated H tfield in the vertical direction. Source 22 energizes a dipole antenna 46, which, together with parabolic reflector 44, emits a beam of radiation downward into the earth. In the presence of a carbonaceous material the direction of polarization is rotated, the direction of rotation depending upon the direction of the H field. A part is reflected upward from some lowerlying interface, again passes through the material, and is rotated further in the same direction. The rotated beam enters the antenna and induces a signal into the dipole antenna 45, which is perpendicular to antenna 46, and therefore has little direct coupling to it. The signal induced in antenna 45, which is modulated at the frequency of source 17, is detected by detector 27, which is similar to that of FIGURE 7. Instead of the set of dipole antennas with a parabolic reflector, other well known directional antennas may be used.

FIGURE 11 shows an arrangement for prospecting by means of radiation fields for both H and H The H field consists of a steady component, contributed by the earths magnetic field, plus an alternating component, furnished as a radiation field by directional antenna 50 and generator 51. Preferably, antenna Stl is so oriented that the magnetic components of its radiation field are substantially parallel to the earths field.

The H field is the magnetic component of the radiation field created by directional antenna 52, which is powered by generator 53. Antenna 52 should be oriented so that the field is substantially perpendicular to the H field.

The beam of radiation from antenna 52. is reflected to the surface by one or more interfaces between subterranean layers of different electrical properties. A part of it is intercepted by antenna 54 and the signal is fed to detector 27, which is similar to the detector shown in FIGURE 7. Antenna 54 and detector 27 are designed to detect radiation with frequency lying in one of the sidebands, preferably at f+m or f-m, where f is the frequency of source 53, and m is the frequency of source 51. These sidebands are the result of amplitude modulation produced by a deposit of a carbonaceous material in the earth. The amplitude modulation may arise in one or both of two ways, depending upon the geometrical relationships.

One way is due to absorption of the electromagnetic wave as it passes through the deposit. The amount of this absorption is varied by the modulation of the H field. The second Way is due to a variation in the refractive index of the deposit of carbonaceous material, also produced by the modulation of the H field. The beam of radiation will be deflected through certain angles upon centering and leaving the deposit, and these angles will vary with H Hence, the position and possibly the direction of travel of the emergent beam will vary with H If antenna 54 is placed near the edge of the reflected beam, any small variation of the position of the beam will produce an amplitude modulation of the signal to detector 27.

FIGURE 12 illustrates a device to be lowered into a bore hole for locating deposits of carbonaceous materials, particularly petroleum, near the hole at various depths. The principle of operation of this device is identical to that of the device of FIGURE 8, and corresponding parts bear the same numbers. The coils are smaller, and are arranged, with axes mutually perpendicular, upon a cylindrical support 60 of suitable size to be lowered into a hole. The connections are made through cables 61.

The device of FIGURE 12 is particularly useful in locating deposits of petroleum and of other carbonaceous materials laterally near the bore hole but through which the bore hole passes. It is not uncommon in the digging of wells in an endeavor to locate underground deposits of petroleum, for the bore hole to pass through a deposit of petroleum located laterally adjacent the bore hole but separated from the bore hole by rock or other earth formations which prevent the petroleum from entering the bore hole. The device of FIGURE 12 gives a direct positive indication of the presence of such deposits within an appreciable distance laterally of the bore hole. Also the device may be lowered to the bore of a dry well to give an indication of the presence of carbonaceous material including petroleum below the base of the well and thus indicate whether or not it is desirable to deepen the bore hole.

In determining the east-west and north-south location of a deposit of carbonaceous material, such as a deposit of petroleum, the set of coils or coil and antenna of FIG- URES [6, 8, 9 and 10 are moved over the surface, and the successive indications obtained at different positions are noted. This will give for all practical purposes the important confines or border areas of the deposit. By varying the current supplied to the coil i15, one can explore various depths of the earths surface. The regions of the earth being explored are those in which coil 15 sets up an H field in the region of resonance as previously defined; this depth will vary as the current supplied to the coil 15 is varied in a well known way. The arrangement of FIG- URE 11 is particularly suited for exploring different levels of the earths surface. This arrangement, as above noted, uses the earths field as the H field, thereby saving much power and also using a lower value of 1, which penetrates the earth more readily. The antennas 50 and 52 can be made large enough to send into the earth narrow beams of radiation 'whose axes are shown by the dotted lines on FIGURE 11. The region of the earth being explored lies at the intersection of these beams and its location can easily be found from the locations and orientations of antennas 50 and 52. By changing the positions of the antennas 50 and 52 to cause the beams of radiation emanating therefrom to intersect at different points, the region of exploration can readily be changed.

In the foregoing discussion, it has been assumed that the H field is substantially uniform over the entire spatial extent of the deposit of carbonaceous material to be detected. By this it is meant that the maximum variation of H over the extent of the deposit is small compared to AHO- This is true, according to electromagnetic theory, in cases in which the greatest linear dimension of the deposit is small compared with the smallest distance between a point in the deposit and a point on the coil (or any of a set of coils) producing the H field.

Thus, the above discussion applies directly to the detection of relatively small deposits of carbonaceous material at relatively large distances from the apparatus. For deposits in which this is not the case and hence the H field is not substantially uniform over the deposit, one of two alternative procedures may be employed, namely, (a) restrict the extent of the H field to a limited portion of the entire deposit, such that the H field is substantially uniform in the limited portion, or (b) not so restrict the H field but induce electron procession over a range of Larmor frequencies corresponding to the range of H in the deposit. These alternatives are discussed below in turn.

Alternative (a) can be practiced with the apparatus of FIGURE 6 or FIGURE 8 by making coil 15 much larger than coils r19, 20, 36 and 37. Thus the H field, which will be restricted to a region near coils 19 and 20, will lie entirely within a region near the center of coil 15, in which the H field produced by coil 15 will be substantially uniform.

Alternative (a) can be practiced with the use of the equipment shown in FIGURE 9 or FIGURE 10 in the relatively common case in which the deposits of carbonaceous material lie in thin horizontal veins. Here the antenna (40 in FIGURE 9, 44 in FIGURE 10) is designed to direct a narrow beam of radiation downward. The intersection of this narrow beam of radiation with the thin vein of carbonaceous material will outline a sufficiently small region such that the H field can be kept uniform within its boundaries.

In FIGURE 11, the H field is the earths field which is sufficiently uniform, with a superimposed modulation consisting of the magnetic component of the radiation field produced by antenna 50 and source 51. When practicing alternative (a) with the apparatus of FIGURE 11, the beam from antenna 52 should be narrower than that from antenna 50, and the thickness of the carbonaceous 1 l deposit should be less .than about one-fourth of a wavelength of the radiation from antenna 50.

Alternative (b) may be practiced using any of the apparatus of FIGURES 6 to 12, inclusive; the bore hole instrument of FIGURE 10 will usually be operated to practice alternative (b).

The following explanation should aid in understanding alternative (b).

Assume the carbonaceous deposit to be detected is divided by imaginary surfaces into a large number of small parts, called deposit elements, such that the H field, at any instant, is substantially uniform in the sense previously described, throughout each such deposit element. (For example, these surfaces could be three orthogonal sets of planes which divide the deposit into small cubes.) The H, field at any given instant may, of course, have appreciably different values at two widely separated deposit elements.

To a very good approximation, each deposit element makes a contribution to the detected signal, called a signal element, which is independent of the presence of the other deposit elements. In other words, the entire signal, due to the entire deposit, is a sum of the signal elements, each due to a deposit element.

Referring now to FIGURE 13, in this plot the values of current which flows in the coil which generates the H field are plotted as the abscissae (this current is called herein the H current, and it flows in coil of FIGURES 6, 8, 9, l0 and 12). In this FIGURE 13, the horizontal curve a is simply a baseline, as in the case of the horizontal curve in FIGURE 5. A signal element due to a particular deposit element is represented by dip b. A second signal element, due to a second deposit element farther away from the coil which generates the H field is shown as clip 0. That this dip lies to the right of the first is, of course, a consequence of the fact that a larger H current is required to bring the H field into the region of resonance at the greater distance of the second deposit element.

The fact that dip b is larger than dip c is meant to illustrate that the first deposit element will usually be closer to the coils which produce the H field and which detect the magnetic moment of the deposit elements, and will therefore usually produce a greater signal element.

If all of the signal elements are added together on this graph, the result is a curve such as d, representing the signal due to the entire deposit. The exact shape of curve d depends upon a large number of geometrical relationships between the deposit of carbonaceous material and the various coils. The particular curve of FIGURE 13 was drawn With the maximum depression nearer the lower values of the H current to illustrate the usual situation, wherein the nearer deposit elements contribute a stronger signal than those farther awa due to above mentioned proximity effects, despite the fact that usually there are fewer deposit elements nearer the coils than farther away. In general, however, curve d may be expected to have portions in which its slope (rate of change of signal amplitude with respect to a change of H current) is not zero.

Such a portion having non-zero slope permits the use of the detection methodp reviously discussed, namely, a method in which the H current is cyclically varied and the corresponding cyclical variations in signal strength are detected, as shown in FIGURE 13.

It should be clear that the invention is applicable to the direct detection of subterranean masses of carbonaceous material, whether the purpose of the exploration is the location of such masses per se or of uranium frequently found associated therewith. As noted, underground or otherwise inaccessible deposits of asphaltic materials like gilsonite, carbonized fossil wood, organic debris, pitchblende, and carbonized shales, with which uranium is frequently associated, may be located by the present invention and hence this invention can be used to detect directly such uranium deposits.

Moreover, while the invention has been described hereinabove and has been exemplified in the drawings in connection with the location of inaccessible deposits of carbonaceous materials and/ or of uranium found associated therewith, it will be understood it is not limited thereto. In the location of inaccessible deposits of carbonaceous material, the invention involves the principle of actuating a detector employing a property of the carbonaceous material being sought which property is peculiar to the material and is not possessed by other materials in the earths crust or located in the neighborhood of the carbonaceous materials. Hence this invention can be employed to detect the presence of carbonaceous materials including petroleum present in a mixture with other materials, such as sand, mud, etc., where the presence of the carbonaceous material, either because of the small amount present or for other reasons is not readily evident to the observer.

Since many apparent widely ditferent embodiments of this invention could be made without departing from its scope, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A process of geophysical exploration for subterranean deposits of carbonaceous materials, which process includes varying the relation between frequency and field strength of two magnetic fields intersecting in a subterranean zone of exploration, said fields being respectively unidirectional and alternating magnetic fields, said variation being through a relationship where f is the frequency of the alternating field in megacycles per second, H is the strength of the unidirectional field in gauss, and K is approximately 2.8 megacycles per second per gauss and detecting at a position remote from said subterranean zone magnetic fluctuations resulting from the presence of such carbonaceous material in said subterranean zone of exploration.

2. A process as defined in claim 1 including the step of varying the field strength of said unidirectional magnetic field through said relationship and measuring changes in energy produced by such change in field strength as an indication of the presence of such carbonaceous material in said zone.

3. A process as defined in claim 1 including the step of modulating the amplitude of said unidirectional field at a frequency m, any substantial amount of said carbonaceous material in said subterranean zone resulting in a signal with sideband frequencies f+m and fm, and detecting at a remote point the presence of said sidebands in said signal.

4. A process of geophysical exploration for subterranean deposits of carbonaceous materials including petroleum, coal, natural bitumens including tars and asphalts, partially carbonized animal and vegetable matter, and oil-bearing shales, which process comprises establishing a magnetic field of known frequency in a subterranean zone while there is present therein a unidirectional magnetic field, said fields crossing each other in said subterranean zone, varying the relation between frequency and strength of said fields through a range including the region of resonance of electrons present in the carbonaceous material being sought, to produce a signal which develops a resonance peak if carbonaceous material is present in said zone and as a result of such presence of carbonaceous material, and detecting the presence of said peak in said signal as indicative of such presence of carbonaceous material in said zone.

5. A process of geophysical exploration for subterranean deposits of carbonaceous materials, which process comprises establishing a second electromagentic field at right angles to the direction of a first magnetic field in the subterranean region of exploration, the said second field having a frequency related to the strength of the said first field indicated by the formula in which formula, 1 is the frequency of the said second field in megacycles per second and H is the strength of the said first field in gauss, and detecting magnetic fluctuations due to the presence of said carbonaceous material in the locality being explored.

6. A process of geophysical exploration for inaccessible subterranean deposits of carbonaceous materials, which process comprises creating in the subterranean region of exploration an electromagnetic field having a frequency of from 0.1 to 10,000 megacycles per second at right angles to another magnetic field having a strength indicated by the formula f=2.80 [H t in which formula 1 is the frequency of the first-mentioned field in megacycles per second and H is the strength of the second-mentioned field in gauss, and detecting a change in the energy content of the first-mentioned field caused by the presence of a carbonaceous material in said magnetic fields.

7. A process of locating an inaccessible subterranean deposit of petroleum, which process comprises creating in the subterranean region of exploration an electromag netic field having a frequency of from 0.25 to 250 megacycles per second at right angles to another magnetic field, the strength of which is related to the first-mentioned field in accordance with the formula f=2.80[H :2.5] in which formula 1 is the frequency of the first-mentioned field in megacycles per second and H is the strength of the second-mentioned field in gauss, and detecting magnetic fluctuations due to presence of said petroleum in said magnetic fields.

8. A process of locating an inaccessible subterranean deposit of carbonaceous material, which process comprises creating in the subterranean region of exploration an electromagnetic field having a frequency of from 0.25 to 250 megacycles per second at right angles to a second magnetic field, the strength of which is related to the firstmentioned field in accordance with the formula f=2.80[H L2.5] in which formula 1 is the frequency of the first-mentioned field in megacycles per second and H is the strength of the second-mentioned field in gauss, and detecting magnetic fluctuations resulting from the presence of such carbonaceous material in said region of exploration.

9. A process of geophysical exploration for inaccessible subterranean deposits of a material from the group consisting of petroleum, coal, natural bitumen, partially carbonized animal and vegetable matter and carbonaceous geological deposits, which process comprises establishing a first magnetic field in a subterranean locality to be explored, varying the strength of said magnetic field, establishing a second magnetic field at right angles to the direction of the first magnetic field, both fields being located in the same locality of exploration, and the second magnetic field having a frequency related to the strength of the first-mentioned magnetic field in accordance with the formula f=2.80[H :10] in which formula is the frequency of the first-mentioned field in megacycles per second and H is the strength of the second-mentioned field in gauss, and detecting magnetic fluctuations due to the presence of said material in said magnetic fields.

10. A process as defined in claim 9, in which the second-mentioned field has a frequency of from 0.1 to 10,000 megacycles per second.

11. A process as defined in claim 9, in which the second-mentioned field has a frequency of from 0.25 to 250 megacycles per second and the frequency-strength relationships of the two fields is in accordance with the formula 12. A process of geophysical exploration for inaccessible subterranean deposits of a material from the group consisting of petroleum, coal, natural bitumen, partially carbonized animal and vegetable matter and carbonaceous geological deposits, which process comprises estab lishing a first magnetic field in a subterranean locality to be explored, varying the strength of said magnetic field, establishing a second magnetic field at right angles to the direction of the first magnetic field, both fields being located in the same subterranean locality of exploration, and the second magnetic field having a frequency related to the strength of the first-mentioned magnetic field in accordance with the formula f=2.80[H :10] in which formula 1 is the frequency of the first-mentioned field in megacycles per second and H is the strength of the second-mentioned field in gauss, and detecting the presence of a third electromagnetic field produced by the presence of said material in the locality being explored.

13. A process of geophysical exploration for inaccessible deposits of a carbonaceous material to detect said carbonaceous material, which process comprises establishing a first subterranean magnetic field, varying the strength of said magnetic field until it is brought into the region of resonance of electrons present in said carbonaceous material at a selected frequency of operation, establishing a second magnetic field at right angles to and in the same subterranean locality as the said first magnetic field and having a frequency the same as said selected frequency, and inducing in a circuit above the surface of the earth a current due to resonance existing in a carbonaceous material in said subterranean locality and arising when the strength of said second magnetic field is altered as a result of such resonance, said current being indicative of the presence of said carbonaceous material at said subterranean locality.

14. A process of geophysical exploration for inaccessible deposits of a carbonaceous material, which process comprises establishing a first beam of electromagnetic radiation in the region to be explored, having the strength of its magnetic component within the region of resonance of said carbonaceous material at a selected frequency of operation, establishing a second beam of electromagnetic radiation intersecting the first-mentioned beam at a selected depth of the region to be explored and having a frequency within the region of resonance of the carbonaceous material acted on by the eanths magnetic field, a portion of said second beam being reflected from at least one subterranean position and detecting variations in the reflected portion of said second beam due to resonance caused by the presence of said carbonaceous material in the locality of said radiation fields.

15. A process of geophysical exploration for inaccessi ble deposits of a carbonaceous material, which process comprises the steps of establishing in the location to be explored a first magnetic field varying at a first selected frequency, establishing in said location a second magnetic field substantially perpendicular to that of the firstmentioned magnetic field and varying at a second selected frequency, and detecting the presence of another magnetic field produced by said carbonaceous material due to the presence of said carbonaceous material in the locality where the said second-mentioned magnetic field is substantially perpendicular to the first-mentioned mag netic field, said other magnetic field varying at a frequency chosen from the class consisting of the first frequency plus an integral multiple of the second frequency and the first frequency minus an integral multiple of the second frequency.

16. Geophysical exploration apparatus for detecting inaccessible subterranean deposits of a carbonaceous material including: a first field-producing means for producing a magnetic field directed into the earth in a first direction; a second field-producing means for producing a magnetic field in the earth in a second direction substantially at right angles to said first direction, both of said fields being present in a subterranean location suspected of containing a deposit of said carbonaceous material, and said second field-producing means including a directional antenna for producing a radiation field with a magnetic component in said second direction; means for varying the strength-frequency relationship of said fields through a zone including the region of electron resonance of such carbonaceous material at such location; and detecting means electrically connected to said antenna for detecting such resonance.

17. Apparatus as defined in claim 16, in which said second field-producing means includes a first directional antenna for producing a radiation field directed into the earth with a magnetic component in said second di rection to produce a polarized beam subject to Faraday rotation, and including a second directional antenna responsive to the rotated direction of polarization.

18. Geophysical exploration apparatus including: a first directional antenna and means for supplying thereto a high frequency energizing potential, said first antenna being beamed into the earth along a first axis to create a magnetic field consisting of a steady component contributed by the earths magnetic field and an alternating component contributed by said beam from said first antenna; a second directional antenna and means for supplying thereto a high frequency energizing potential, said second antenna being beamed into the earth along a second axis in such direction that said beams intersecting in a subterranean location under test and the magnetic field created thereby is substantially perpendicular to the first-mentioned magnetic field, a portion of the beam from said second antenna being reflected from subterranean structures; and a detector responsive to such reflected portion.

19. A process of detecting the presence of petroleum in a petroleum-containing mud, which process comprises subjecting said mud to two magnetic fields at right angles to each other, one of said magnetic fields having a frequency of from 0.1 to 10,000 megacycles per second and the other of said magnetic fields having a strength related to the frequency of the first-mentioned magnetic field in accordance with the formula in which formula f is the frequnecy of the first-mentioned field in megacycles per second and H is the strength of the second-mentioned field in gauss, and detecting magnetic fluctuations due to the presence of petroleum in said mud within said magnetic fields.

20. A process of detecting the presence of petroleum in a petroleum-containing mud, which process comprises subjecting said mud to two magnetic fields at right angles to each other, both fields being located in the same cality as the petroleum mud, one of said fields being a unidirectionally pulsating field and the other of said fields being an alternating field, varying the strength of said unidirectionally pulsating magnetic field, the strength frequency relationship of said fields being in accordance with the formula f=2.80[H :10] in which formula H is the strength of the said unidirec tionally pulsating field in gauss and f is the frequency of the alternating field in megacycles per second, and de- 16 tecting magnetic fluctuations due to the presence of petroleum in said mud within said magnetic fields.

21. A process of geophysical exploration for inaccessible subterranean deposits of petroleum, which process comprises establishing a second electromagnetic field at right angles to the direction of a first magnetic field in an inaccessible subterranean region of exploration, the second field having a frequency related to the strength of the said first field indicated by the formula f:2.80[H i10] in which formula, 1 is the frequency of the said second field in megacycles per second and H is the strength of the said first field in gauss, and detecting a change in the energy content of said second electromagnetic field due to the presence of petroleum in said fields.

22. In the art of exploring the earths crust for detection of carbonaceous materials, a process using two magnetic fields crossing each other at a subterranean position, said process being characterized by the steps of varying the strength frequency relationship of the two fields in accordance with the formula:

in which formula is the frequency of one of said two fields in megacycles per second and H is the strength of the other of said fields in gauss, to produce magnetic fluctuations as a result of the presence of said carbonaceous material in the subterranean zone of exploration and detecting said fluctuations as an indication of the presence of said carbonaceous material in said zone.

23. The process as defined in claim 22, in which one of said magnetic fields is the earths field.

24. A process as defined in claim 22, in which one of said magnetic fields is an induction field and the other of said magnetic fields is a radiation field.

25. A process as defined in claim 22, in which both of said fields are induction fields.

26. A process as defined in claim 22, in which both of said fields are radiation fields.

References Cited in the file of this patent UNITED STATES PATENTS 2,018,080 Martienssen Oct. 22, 1935 2,139,460 Potapenko Dec. 6, 1938 2,455,941 Muskat et a1. Dec. 14, 1948 2,561,489 Bloch et al. July 24, 1951 2,657,380 Donaldson Oct. 27, 1953 2,660,703 Herbold Nov. 24, 1953 2,661,466 Barret Dec. 1, 1953 2,720,625 Leete Oct. 11, 1955 OTHER REFERENCES Gabillard: Academic des Sciences, Comptes Rendus, vol. 237, No. 14, pp. 705 to 708, October 1955.

Beringer et al.: Physical Review, vol. 78, No. 5, June I, 1950. Pp- 581 m 586.

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Smaller et al.: The Review of Scientific Instruments, vol. 24, No. 10, October 1953, pp. 991 and 992.

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Bloembergen et a1. (II), Physical Review, vol. 93, No. 1, January 1954, pp. 72-83.

Hirschon et al.: The Review of Scientific Instruments, vol. 26, No. 1, pages 344l, January 1955.

Darrow: Electrical Engineering, vol. 70, No. 5, pp. 401404, May 1951.

Relaxation Effects in Nuclear Magnetic Resonance Absorption," by Bloembergen et 211., Physical Review, vol. 73, No. 7 (1948), PP- 679-712.

Paramagnetic Resonance Absorption in Organic Free Radicals, by Holden, Bell Laboratories Record, vol. 31, No.4 (1953), pp. 121-126.

:UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,060,371 October 23, 1962 Jonathan Townsend. et. a1,

Column 5, line 3, strike out "15" column 9, line 43, for "centering" read entering column 10, line 48, for "procession" read precession column 11, line 62, for "methodp reviously" read methgd previously Signed and sealed this 26th day of March 1963.,

(SEAL) Attest:

ESTON G. JOHNSON DAVID L. LADD Attesting Officer Commissioner of Patents

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