US2265768A - Method of logging bore holes - Google Patents

Description

Dec. 9, 1941.

L. F. ATHY ETAL METHOD 0F LOGGING BORE HOLES AFiled July 2o, 1940 6 Sheets-Sheet 1 i A- M/ 3 MNM fm www lNVENToRs aw/'ence E #dro/d E presea Dec. 9, 1941. L, F. ATHY ETAL. 2,265,768

METHOD 0F LOGGING BORE HOLES Filed July 20, 1940 6 Sheets-Sheet 3 ZOO ZIO

l Dec. 9, 1941. 1 F. ATHY Erm. 2,265,768

METHOD 0F LOGGING BORE HOLES Filed July 20, 1940 6 Sheets-Sheet 4 BY u? f/aes ATTORN f I Dec. 9, 1941.

L. F. ATHY ETAL METHOD OF LOGGING BORE HOLES Filed July 20, 1940 6 Sheets-Sheet 5 F//u//es ATTR Y Dec. 9, 1941. l.. F. ATHY ETAL 2,265,768

METHOD OF LOGGING BORE HOLES TTRm Y of said buried formations.

Patented Dec. 9, 194.1

METHOD F LOGGING BOBE HOLES Lawrence F. Athy, Harold lt. Prescott, and Roland F. Hughes, Ponca City, Okla., assignors to Continental Oil Company, Ponca. City, Okla., a corporation o! Delaware Application July 20, 1940, Serial No. 346,608

19 Glaims. (Cl. 181-05) Our invention relates to a bore holes.

This application is a continuation in part of our co-pending applications, Serial Nos. 202,483, led April 16, 1933, Patent No. 2,207,281, July 9, 1940, and 202,741 led April 18, 1938.

More or less parallel beds of rock materials are pierced by bore holes drilled for exploratory purposes in seeking oil or gas. It is essential to prospectors to be able to recognize certain identifying characteristics of the beds penetrated in order that the geological structure of the buried formations may be accurately determined. This geological structure is a guide in seeking accumulations Vof oil, gas, or other valuable deposits. By recognizing and logging a sequence of identifying characteristics with respect to depth or sea level elevation in individual bore holes, it is possible to determine the relative structural attitude and position of the various rock formations contributing to these recognizable characteristics.

In geological explorations, core drilling has been resorted to in order to determine the characteristics of the beds penetrated. A record .or log is kept showing the diieient formations traversed. This log is obtained by sampling the drill cuttings to determine their variation in mineral content or rock type. It is a common practice to,sample the drill cuttings taken from bore holes and from a careful study of these cuttings to determine their variation of mineral content or rock type with depth or elevation and thereby provide a subsurface map of the structure CommonLv, holes are drilled to relatively shallow depths solely for purposes of determining the structure of the subsurface. In many areas it is impossible or diicult to correlate cuttings from one well with those of another, thereby rendering structural determination by this method ineiective. Sometimes actual cores or chunk samples of the various formations penetrated by the drill are taken in order that the beds may be recognized and correlated. This procedure is slow and expensive, and frequently necessitates deep drilling in order to penetrate recognizable marker beds which may be correlated from hole to hole.

One object of our invention is to provide a novel method and apparatus for logging bore holes, usingstanding Waves.

Another object of our invention is to provide a method of logging bore holes by resonance methods in which the velocity of motion causing resonance may be logged, independent of phase velocity.

method of logging fio Another object of our invention is to provide nomena in various strata. in the bore hole.

Another object of our invention is to provide a method which may be used simultaneously with the impedance method disclosed in our co-pending application, Serial No. 202,741.

Another object of our invention is to provide a method of logging resonance phenomena simultaneously with the phenomena disclosed in our co-pending application, Serial No. 202,741, and with the phase velocity method disclosed in our co-pending application, Serial No. 202,483.

Another object of our invention is to provide a. method of logging bore holes utilizing transverse waves in addition to compressional or longitudinal waves.

Other and further objects of our invention will appear from the following description.

In the accompanying drawings which form part of the instant specification and are to be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various views;

Figure 1 is a diagrammatic view used in connection with the explanation of the theory of standing waves.

Figure 2 is a schematic view representing the instantaneous picture of the direction and magnitude of compressional or longitudinal Waves in one position and a position one half wave length later in time. y

Figure 3 is a schematic view of a log strip of amplitude of longitudinal waves placed opposite the position from which it was obtained.

Figure 4 is a, diagrammatic view of a log strip of amplitude of transverse waves placed opposite the position from which it was obtained.

Figure 5 is a diagrammatic sectional view of a bore hole with an emitter and two receptors.

Figure 6 is a diagrammatic view of a log strip made by the upper receptor, the log strip being placed opposite the position from which it was obtained.

Figure 7 is a schematic view of a log strip of amplitudes received at the lower receptor, the log strip being placed opposite the position from which it was obtained.

Figure 8 is a cross sectional view of a bore hole taken between the depths of feet and 260 feet.

Figure 9 is a drawing of envelopes of energy or amplitude logs made with the emitter at various positions in the bore hole.

Figure 10 is a cross sectional view of a bore hole between the depths of 160 feet and 240 feet.

Figure 11 shows a plurality of superimposed reproductions of envelopes of energy or amplitude received in the bore hole shown in Figure under varying conditions.

Figure 12 is a schematic cross sectional view of one form of emitter and receptor capable of use in carrying out our invention.

Figure 13 is a sectional view taken on the line I3-I3 of Figure 12.

Figure 14 is a schematic view showing an ampliiier capable of use in carrying out the method of our invention.

Figure 15 is a schematic view of a bor'e hole iitted with apparatus capable of carrying 'out the method of our invention.

Referring now to Figure 1, a slab I represents isotropic material having good elasticity with a. lower interface 2 bonded to a rm foundation 3 such that it is greatly restricted and. having a free interface 4 at the top. It will be clear that,

since the bottom is restricted, the motion at thev restricted interface will be at a minimum and that, since the top interface 4 is free, that this surface will have maximum motion.

If a motion of proper frequency is impressed upon the slab. I it will resonate. The waves in slab I will be of complex nature, consisting of a longitudinal or compressional component, and of a transverse component-also possibly of other more complex components. The arrows 5 represent an instantaneous picture 'of a direction and magnitude of the compressional or longitudinal waves. This picture may be represented by the sine curve 6. The sine curve 'I represents the instantaneous picture of longitudinal waves one half wave length later in time from the situation shown by curve 6. The wave length of motion of the longitudinal waves is shown in Figure 2 by a symbol 7u.. g

If a system responsive to longitudinal waves is lowered through the slab I, the log strip shown in Figure 3 wilbbe obtained in which the distance di. between nodes or loops is one half the wave length, that is in a. direction at right angles to tlie'arrows 5 represent the instantaneous motion of these transverse waves. The wave length n of the .transverse wave motion is less than the wave length n. of the longitudinal wave motion. If a. system responsive to transverse Waves is lowered along the lateral interface, the log strip shown in Figure 4 willA be obtained in which the distance dr between nodes or between loops is one half the wave length,

with the bore hole uid instead of the material of slab I. Because it has no shear rigidity, the

bore hole fluid itself cannot sustain the trans- Regardless of the position at which excitation energy of a constant frequency is delivered to the slab I or the position at which the energy is received, the position of the nodes and loops will remain xed. The position of the nodes and loops should not be confused with their magnitude. While the position of the nodes and loops will remain xed, the magnitude of the nodes and loops will depend upon the position at which excitation takes place. If the slab is excited at a node, the magnitude will be small. If the slab is excited at a loop, the magnitude will be large.

The velocity of the longitudinal waves, VL may be represented by the expression:

The velocity, VL will be referred to as the resonance longitudinal velocity to distinguish it from the phase" velocity described in our copending application Serial No. 202,483.

The transverse component of the wave in slab I travels in a direction at right angles to the direction of motion of the individual particles of slab I as indicated by the arrows 5 in Figure 1. The velocity of the transverse wave is lower than that of the longitudinal wave. This fact is well known. The arrows 8 drawn on Figure 1 verse wave. However, the energy of the transverse wave, or more complex wave, which exists in the slab near the walls of the bore hole can be transferred to the responsive means through the short fluid path between the walls of the hole and the responsive means. In this short fluid path the energy undoubtedly is transferred as a compression wave. Hereafter we shall present experimental proof that the energy of such a transverse wave, or more complex wave, actually is transferred in this manner and that the characteristics observed are different from the characteristics 'of longitudinal waves. Hence we say that the case discussed for transverse waves approaches the case wherein the material is pierced by a bore hole, as must be actually done.

In an actual case, where a bore hole pierces various geological strata of non-isotropic media, of various thicknesses, various hardnesses, and various velocities, the theoretical case shown in Figure l does not strictly apply for the reason that there are no completely free interfaces and no completely restricted interfaces. However, it is to be observed that the walls of the bore hole, although in contact with the bore hole fluid, give a fairly good case approaching the free side interface. A strict and comprehensive theoretical treatment of the various phenomena involving compressional waves, transverse waves, renections from interfaces, reflections from walls of the bore hole, and involving multiple reflections including various paths involved, is very diillcult and quite beyond the scope of this speciilcation.

In spite of the fact that the theoretical case of Figure 1 does not strictly apply, the fundamental ideas there represented are useful and we have found that a close approximation to the theoretical phenomena. is observed under the complicated conditions in a bore hole.

Referring now to Figure 8, we have shown a section of an actual bore hole between 10U-feet and 260 feet. The layer I0 comprises chiey dark maroon calcareous clay shale containing some sharp limestone fragments. .Ihe layer II was very fine grained sandstone with paper-thin laminations of maroon clay shale. The layer I2 comprised ilne to medium grained sandstone with paper-thin laminations of gray shale. The sandstone was somewhat calcereous. The layer I 3 was of dark gray to gray shale, some of which was slightly calcareous. The layer I4 was dark gray and gray to greenish gray shale. The layer I comprised greenish gray calcareous shale, grading to mottled brown and greenish gray calcareous shale. The layer I6 is composed of a light gray to white, nely crystalline limestone. The layer Il comprised mottled brown and ochre to greenish gray shale. 'I'he layer I8 was composed of gray, slightly calcareous osslliferous shale, grading to mottled gray and greenish grayl calcareous shale. The stratum I9 was composed of finely laminated, light green calcareous shale and crystalline limestone, grading to dark gray. calcareous shale having thin streaks of sandstone. The layer 2d is finely laminated, dark gray shale and buff to white nely crystalline shaley limestone grading to gray, greenish, calcareous shale. The stratum 2l is maroon, slightly calcareous shale with occasional limestone concretions.

The layer 22 is composed of grayish green calcareous shale, grading to light gray, iinely crystalline marl, which was very fossiliferous grading to nely, then coarsely crystalline limestone containing some shale fragments. The layer 23 was lavender, `maroon and ochre, varicolored nely crystalline shaley limestone. The layer 24 was composed of maroon shale. The stratum 25 was varicolored shale interbedded with thin layers of fine grained sandstone grading to fine grained calcareous sandstone interbedded with varicolored shale grading to white, line to medium grained, slightly calcareous sandstone. The stratum .26 was composed of greenish gray fossiliferous shale interbedded with thin layers of line grained sandstone. The layer 2'! was'composed of gray to greenish shale with occasional interbedded thin layers of sandstone. The layer 28 was gray shale, very fossiliferous in its nature. The layer 29 was composed ofl greenish gray shale, somewhat carbonaceous and containing streaks of sandstone. The layer 30 was composed of gray calcareous fossiliferous shale. The layer 3l was composed of gray, medium crystalline fossiliferous limestone, shaley in the middle part. 'I'he layer 32 was gray to greenish gray shale with occasional thin layers of sandstone. The layer 33 comprised gray limestone grading to gray calcareous, fossili/ferous shale. The layer .34 was` gray shale with occasional thin sandstone streaks. The layer 35 comprised gray medium crystalline shaley fossiliferous limestone, grading to shale. The layer 36 was gray fossiliferous shale, grading to gray medium crystalline shaley limestone. The stratum 3l Was interbedded gray shale and limestone, grading to resinous gray, medium crystalline fossilif'erous limestone. The layer 38 was fine to medium grained angular sandstone, very porous in its nature, grading to White ne to medium grained, slightly calcareous sandstone. The layer 39 was finel grained, calcareous sandstone, slightly carbonaceous with shale grading to dark gray shale interbedded with thin sandstone streaks. The layer 40 was composed of gray medium crystalline limestone, somewhat shaley and grading to same with streaks of dark gray shale. The layer 4| was gray, nely crys talline, fossiliferous limestone, some shaley, fusulinidae abundant. The layer 42 was gray shale interbedded with hard gray, ne grained calcareous limestone, grading to gray shale with occasional limestone streaks. The layer i3 was gray, nely crystalline, fossiliferous limestone, interbedded with gray shale. The layer 44 was gray, carbonaceous shale. The layer 45 was composed of maroon shale mottled to light gray.

To determine the various phenomena for an actual case, test logs were obtained in the bore hole shown in Figure 8.

, Figure 9 shows a superimposed reproduction of the envclcpes of the amplitude received in the bore hole 8. A frequency of 100 cycles was used for this series of tests.

First an emitter of continuous 100 cycle energy was lowered to a depth of 254 feet and held constant for the test. A receptor responsive to 100 cycle energy was raised from the starting point 245 feet and a continuous log strip of amplitude of motion obtained as it traversed the bore hole. A reproduction of the envelope is represented by the curve 46 in Figure 9. Y

Next, the transmitter was raised to position #2, indicated by the reference numeral 41 in Figure 9 and held xed for the second test. It is to be Observed that, at 237 feet, a loop occurred as shown by envelope 43.

For the second test, the receptor was started at 225 feet and a continuous log of the amplitude of motion was obtained as it traversed the b'ore i hole. A reproduction of this envelope is shown in Figure 9 by the curve 48.

Next the transmitter was raised to 225 feet at position #3 and held fixed for the third test. It is to be observed that, at 225 feet a node occurred as shown by both envelopes 46 and 48. For this third test, the receptor was started at 220 feet and a continuous log strip of the amplitude of motion was obtained as it traversed the bore hole. A reproduction of this envelope is shown by the curve'49. It will be observed that the level of the amplitude received was greatly lessened when the transmitter was placed at the node. v

In the fourth test, the transmitter was moved to position #4 in Figure 9, where a loop occurred. For the fourth test, the receptor started at 205 feet and a continuous log strip of the amplitude of motion was obtained as the receptor traversed the bore hole section. A'reproductlon of the envelope is shown in Figure 9 as envelope 50.

It will be observed that, while the transmitter A was held at the loop at 217 feet, the amplitude received as indicated by envelope 50 was large overall. 'I'he four envelopes thus obtained are superimposed in Figure 9 and the following facts will be appreciated:

l. Regardless of the position of the transmitter or of the receptor, the position of the nodes and loops remain substantially iixed in the bore holeso long as the frequency is held constant.

2. The overall amplitude of motion in the bore hole will be less when the transmitter is placed at a node and greater When it is placed at a loop.

3. The distance between nodes or the distance between loops in slow velocity material is less than in high velocity material. This is apparent by comparing Figures 8 and 9, Figure 9 being placed opposite the layers in Figure 8, which produce the nodes and loops.

4. The ratio of amplitude between loops and nodes is greater in lime sections than in shale sections; that is, there is less damping in lime sections than there is in shale sections.

5. Nodes and loops of standing waves are clearly set up throughout the varous strata tested at the one frequency of cycles. This is true, even though theoretical reasons indicate that a given strata with its own elasticity, density, and dimensions, should respond more prominently at a given frequency only.

To determine for an actual case in a bore hole what effect a change in frequency has, a second series of tests were conducted in the same bore hole, a section of which between 160 feet and 240 feet is reproduced on a somewhat larger scale in Figure 10. 'Ihe reference numerals indicate the same layers described in connection with Figure 8.

Figure 11 is a superimposed reproduction of envelopes of the individual test records made in a similar fashion to that described for Figure 9, employing a frequency of 480 cycles per second. Envelope 5I is a reproduction of the amplitude received at receptor with the transmitter moving and the receptor held xed at 260 feet. Envelope 52 is a reproduction of the amplitude received at receptor with transmitter held fixed at 230 feet. Envelope 53 is a reproduction of amplitude received at receptor with transmitter or emitter held fixed at 190 feet with the receptor moving.

In all curves, the depth scale refers to position of the removing element.

It will be apparent from the curves that the facts observed from the curves shown in Figure 9 still hold true and that a further fact is apparent namely that the distance between nodes or the distance between loops lessens as the frequency increases.

It will be clear to those skilled in the art that the theoretical considerations discussed with respect to Figures 1 to 4 in conjunction with the actual phenomena observed, as shown in Figures 8, 9, 10 and ll, conclusively demonstrate that the resonance effect or standing waves, with their characteristics, may be used to indicate the physical properties of strata pierced by bore holes, thus enabling us to correlate bore holes in a simple, convenient, accurate and expeditious manner.

When carrying out our invention in practice, by means of our resonance method, it is desirable to have several nodes and loops in the thinnest strata which it is planned to log and identify. Sometimes this involves the use of fairly high frequencies in the order of 1,000 to 2,000 cycles per second, if the waves utilized have the velocity of compressional or longitudinal waves.

Since transverse waves have less velocity than longitudinal waves, a transmitter and receptor system responsive to and arranged to utilize the transverse waves, may employ lower frequencies and still have as many nodes and loops as desired in the thinnest strata it is planned to log.

From actual observations in the bore hole shown in Figures 8 and 10, we have found in the shale sections of the bore hole that the trans.

verse waves have a velocity of about one-half that of the longitudinal waves. Accordingly, apparatus may be employed having a range of 500 to 1,000 cycles per second, thus simplifying the problem over the use of apparatus in a 1,000 to 2,000 cycle range.

There is another important advantage gained from the employment of transverse waves. With compressional waves, the bore hole fluid conducts the waves regularly in parallel with the conduction of those waves through the strata themselves. Y Although it is true that the cross section of the hole is small compared with the cross sections of the various strata which conduct the waves, still it is highly desirable to minimize this conductivity, if possible, if proper relationship is to appear on the log strip between strata traversed.

It is well known that transverse waves cannot, theoretically, travel in fluid because of the lack of rigidity of uids to shear forces. 'Ihis means that, if transverse waves are utilized, they will have passed through the strata surrounding the borehole rather than through the borehole fluid. The use of transverse waves is, in the practical case, an appreciable help in this respect, though not a complete cure, for the reason that complex conditions exist in a bore hole, with the interface at the walls and various interfaces between strata. all ,of which tends to translate transverse wave motion into components, some of which can travel in the uid.

In our preferredapparatus for carrying out our invention, the transmitter delivers harmonic energy to the stratum opposite the transmitter through the medium of the bore hole fluid. The construction of an emitter discourages harmonic vertical compressional waves in the fluid in favor of lateral harmonic waves in the strata. Transyerse waves having nodes and loops are set up near the walls of the bore hole and extending back into the virgin strata. The transverse waves, after traveling through the strata intervening between the stratum opposite the emitter and the stratum opposite the receptor, exhibit characteristics opposite the receptor which are an index of the strata through which they traveled. The transverse wave motion at the bore hole interface sets up wavemotion in the fluid. The receptor will respond to the fluid motion set up from the stratum opposite the receptor. In this manner, a large portion of the energy which arrives at the receptor to which it is responsive has traveled through geological strata and not the bore hole fluid.

This will be more clearly understood by reference to Figures 5, 6, and 7. In Figure 5, the bore hole 54 passes through a limestone stratum 55, a shale stratum 56 and a sandstone stratum 51, and contains a bore hole fluid 58. An emitter 59 is supplied a harmonic potential at a fixed frequency through a cable 60 so that the emitter 59 will emit harmonic vibrations. The construction of the emitter is such that the vibrations travel principally in a lateral direction as indicated by the arrow 6|. At suitable distances below the emitter 59 are positioned receptors 62 and 63. These receptors are responsive to vibrations which come to them in a lateral direction from the stratum opposite them through the bore hole fluid. The receptors are such that vibratory energy is converted into electrical energy, in sympathy therewith, which energy is conducted respectively by cables 6l and 65 to suitable amplifying, filtering and recording apparatus, which will be hereinafter more fully described.

An instantaneous conception of the compressional wave which the transmitter delivers to the fluid is indicated by the arrow 6|. When this compressional wave strikes the bore hole interface, a small part of the energy will be reflected laterally but a large part will enter the limestone stratum 55 as indicated by the arrow 66. Later in time the compressional wave has traveled laterally in the limestone stratum as indicated by the arrow 61.

Referring again to the lateral compressional wave 6|, although it does transmit compressional movement as indicated by arrows 66 and 61, a particle of Water at position indicated by the reference numeral 68 cannot obtain any lateral motion from the wave 6I for the reason that there is no rigidity in the fluid to this shear However, afterfa compressional wave force, as indicated by the arrow 6I, has been established in the limestone stratum 55, which does have a rigidity to shear forces, a different situation exists. When a compressional wave exists in the stratum as indicated by the arrow 66, the rigidity to shear causes a component indicated by the arrow 69 to move'in sympathy with the compressional wave indicated by the arrow 6B.

This sets up a wave motion which travels outwardly at right angles to the compressional wave at 66 and 61. This right angle wave is called the transverse wave and travels atless velocity than the compressional wave. These transverse waves exist back within the virgin part of the stratum as a result of the compressional wave at 66 and 61 passing out laterally. One place at which the transverse waves express themselves is as shown by the curve 'l0 which is an envelope of the instantaneous picture of the motion at and near the bore wall interface.

We have already shown that the i'luid itself will not carry transverse motion to the receptor from the lateral compressional wave 6I. Now if the transmitter is designed such that very little compressional waves are set up or if the receptor is made such that it is substantially nonresponsive to vertical compressional waves which do travel in the fluid, and if the supporting cables 60, 64 and 65 are designed to carry very little energy from the transmitter to the receptor, then the only Waves to which the receptor can respond are those in the stratum directly opposite the receptor.

Referring again to Figure 5, the transverse wave motion in the shale stratum 56 is represented by an arrow 1I. 'I'his motion causes a compressional wave in the fluid represented by the arrow 'l2 which carries to the receptor 62. The receptor is designed to be primarily responsive to the movement represented by the arrow '12.,

From the foregoing, it will be clear that the characteristics of the wave motion arriving at the receptor 62 have been primarily influenced by the section represented by the reference numeral 13 in Figure 5, for the reason that the distance between the transmitter 59 and the receptor 62 is large compared to the uld path from the wall of the bore hole to the transmitter or receptor.

As the assembly comprising the transmitter and receptors is lifted or lowered, the receptors 62 and 63 will receive the nodes and loops existing opposite respective receptors. Figure 6 shows a log strip of the amplitude arriving at receptor 62 and Figure 7 is a log strip of the amplitude of energy arriving at receptor 63.

In general, it will be necessary to use an amplier having a somewhat greater gain for amplifying the energy received by receptor 63 than that for amplifying the energy received by receptor 62 in order to obtain sufficient amplitude dimensions for proper recording.

We find that, for the frequency range between 100 and 1,000 cycles per second, in which we prefer to operate, that limestone carries the Wave energy at these frequencies quite well. Sandstone is not quite so good a conductor of energy of this range, while shale is a poor ccnductor of energy lying within this range.

Referring to Figure 6, we flnd that the amplitude a is the largest, the amplitude e somewhat less, and the amplitude c appreciably smaller. Likewise, referring to the log strip shown in Figure 7, we find that the amplitude a' is the largest, the amplitude e' somewhat less, and the amplitude c' appreciably smaller.

As indicated above, the resonance velocity of the transverse waves can be computed from a knowledge of the distance between nodes or between loops; that is y1 being the greatest has the greatest resonance velocity; ya is less than y1 and has an intermediate resonance velocity, While ya indicates the smallest resonance" velocity.

It will be observed that limestone, sandstone and shale are respectively materials of decreasing hardness and would, therefore, be expected to have decreasing resonance velocity, the harder material having the greater resonance velocity.

'Ihe log strips, Figure 6 and Figure 7 yield other useful and pertinent informtion regarding the strata traversed. The ratio of a to b, that is, the ratio between the amplitude of the loop and the amplitude of a node for limestone is greatest, showing the least damping of the resonance phenomena. The ratio of the amplitude of the loop e tothe node f is intermediate for sandstone, showing intermediate damping. The ratio of the loop c to the node d for shale is still less, indicating maximum damping.

Since th'e receptor 63 is a greater distance from the transmitter 59 than the receptor 62, data becomes available to determine the rate of amplitude decay, that is attenuation of the wave motion with distance. In other words, the ratio of a to c from the log strip in Figure 6, may be compared with the ratio of a' to c from the log strip shown in Figure 7 to obtain an index of attenuation of amplitude with distance.

As has been pointed out above, it is desirable to use a frequency suiciently high' to obtain enough nodes and loops in the thinnest strata to be located and identified to avoid confusing a node between two large loops as a thin bed of shale having poor conductivity to the transverse waves.

To prove experimentally the feasibility of employing transverse waves in bore holes, a series of tests were carried out.

We first determined the velocity of the longitudinal or compressional waves by the use of explosives. Explosives were detonated at various places in the bore hole and the first arrival impact of the longitudinal wave was recorded at receptors variously spaced in th'e bore hole. In this manner, we found that the velocity of compressional waves in the shale section of the bore hole was about 6,700 feet per second, while the velocity of the compressional waves in the limestone sections was higher. i

Referring now t0 Figure 9, where a frequency of cycles was used, we found a distance of about 15 feet between nodes (or loops) in the shale section lying between feet and 140 feet. Using the formula outlined above; we find that, Vr=2:c distance between nodesX=2X15X100= 3,000 feet per second. This velocity is definitely not that of the compressional waves but is reasonable for transverse waves. In a log obtained by using a, frequency of 480 cycles in the shale section lying between 110 and 140 feet we found that the distance between loops was about 3 feet. Applying the formula. above, we obtain, V: 2 3 480=2,880 feet per second.

The above substantiates the fact that the waves producing the energy were not longitudinal,

Further work was done in the same shale section with a frequency of 750 cycles per second. The spacing of the Vloops was found to be in the order of 2 feet. Applying th'e formula, we obtain, V=2X 2 X750=3,000 feet per second.

Here we obtain another conilrmation oi the fact that transverse waves are utilized.

In another test, one receptor was held iixed in the bore hole at 150 feet which was the bottom of the shale section. A transmitter and a receptor with a spacing of 111/2 feet between them were moved as a unit up the bore hole as the record was taken. Connected with' each receptor was an ampliner system, such as shown in our appllcation Serial No. 202,483, capable of holding the output amplitude substantially constant regardless of variation of amplitude received kat the receptors. The output of the two amplifiers was connected to a single oscillograph element in such manner that the amplitude of the resulting trace indicated phase changes. Thatis, the maximum required points occurwhen the motions at the receptors are in phase, and minimum when the motions of the receptors are out of phase. This is fully described in our co-pending application, Serial No. 202,483.

It will be clear that the distance between two maximum or between two minimum points is one wave length of the motion involved. We found this Wave length to be 6.1 feet, which corresponds to a velocity of, VT=6.1 480=2,928 feet per second.

The above test gives additional conrmation th'at the waves recorded were transverse waves.

Since thewave length in the shale section at o 480 cycles is 6 feet, the distance between loops should be 3 feet at this frequency. This we found to be the case.

Further actual log strips were taken in adjacent bore holes in which the geological logs were well identied.k A frequency of about 750 cycles was employed. An emitter and two receptors such as o shown inl'igure 15 were used in eacli' bore hole.

Referring now to Figure 15, an alternating current generator 15 supplies electric current at a constant frequency. This frequency is impressed by conductors 16 and 11 upon the emitter 18 through a cable 19 and suitable brushes. The cable 19 is normally wound about a reel .80 mounted on a shaft 8| and rotated by suitable means. A self-synchronous motor 82 is driven by the shaft 8| and is interconnected with a second self-synchronous motor 83 which drives a photosenstive strip 84 as a function of the movement of the emitter 18. VA receptor 85 is supported by a cable 8B wound about a reel 81. The reel 81 is secured to the shaft 8| so that it turns with the reel 80 in such fashion that a predetermined spacing is maintained between 18 and 85. Similarly, the receptor 88 is supported by a cable 89 wound about a reel 90, mounted on the shaft 8| for movement along with reels 80 and 81. The receptors 85 and 88 will be more fully described hereinafter and are adapted to convert wave motion into electrical energy which is conducted through the cables. Through suitable brushes the output of the receptor 85 is impressed through conductors Bl and 82 upon the amplification and lter channel indicated diagrammatically by the reference numeral 93. In similar manner, the output of the receptor 88 is impressed by conductors 84 and 95 upon the amplification and lter channel 96. The instantaneous voltage of the alternating current generator 15 is impressed across the galvanometer loop 91 by conductors 98 and 99 through a resistance |00. A mirror |0| carried by the loop g1 is adapted to reiiect a light beam from the incandescent lamp |02 along the path |03 to form the trace I 04 which is a trace oi' the voltage produced by the generator 15. A resistance |05 is placed in one oi the leads going to the emitter. Conductors |08 and |01 across this resistance are connected to a galvanometer loop |08. The voltage drop across the resistance ,|05 will be a function of the currentfso that the current passing through the galvanometer loop |08 will be a i'unction of the current owlng to the emitter. Galvanometer loop |08 carries a mirror |08 adapted to reiiect light from incandescent lamp |0 along the path to form the trace 2 which is a trace of the current delivered to the emitter 18. 'I'he motion received by receptor 88 is converted by the receptor into electrical impulses in sympathy with the motion received, Vandthese electrical impulses are transmitted through cable 89 which is wound on drum 80, through suitable brusheathrough conductors 84 and 95, through the amplifier and filter assembly 95, through conductors ||3 and ||4 to the galvanometer wire |15. The galvanometer wire carries a mirror I8, upon which is focused a light beam from incandescent light ||1. This light beam is reflected by the mirror I6 along the path |8 upon photosensitive strip 84, and produces trace I9. Similarly, the output of the amplifier 93, which amphi-les electrical impulses generated by receptor 85, is impressedupon galvanometer loop |20, positioned within the .field of permanent magnet |2| through conductors |22 and |23; 'I'he galvanometer element 28 carries a mirror |24 upon which is focused light from incandescent lamp |25 for reflection along the path |28 upon the photosensitive strip 84 to produce trace |21. The trace |21 is produced by the wave motion at receptor 85. The trace ||9 is produced by the wave motion at receptor 88.

In order to provide means for indicating the relative phase changes of the harmonic motion received at receptors 85 and 88, we provide an arrangement in which two additionel ampliers 20| and 202 have their output terminals connected through conductors |3|, |32 and 203 in series with a single oscillograph element |28 carrying mirror |28 and positioned in the iield of magnet |30. Amplifier 20| is connected in parallel with amplifier 83 by conductors 203 and 205 which are joined to conductors 9| and 92 respectively so that it receives the same electrical impulses from receptor 85 as it received by amplier 93. However, amplifier 20| has incorporated in it an arrangement to provide automatic volume control so that the amplitude of output from arnpliiier 20| will be maintained substantially constant although the amplitude of electrical impulses from receptor 85 may vary within Wide limits. Likewise ampliiier 202 is connected in parallel with amplier 96 by means of conductors 206 and 201 so that it receives the same electrical impulses from receptor 88 as are received by amplifier 9B. Also, ampliiier 202 provides automatic volume control in the same manner as described for amplier 20|. The output of ampliiier 20| is adjusted to be equal in amplitude to the output of amplier 202; however the phase diierence between the output of amplifiers 20| receptors 08 and 68 are out of phase with each other the output from amplifiers 20| and 202 will cancel, resulting in a minimum width of oscillograph trace |33. Thus trace |33 affords a measure of the phase relationship between the two wave motions recorded individually as traces I9 and |21. This method is covered in copending application 202,483. It is not necessarily limited to recording the phase relationship between the electrical impulses from4 the two receptors, but may also be used to record the phase relationship between the electrical impulses and a constant frequency such as that of generator 15.

In our recording system, we therefore obtain the following traces:

Trace |04 which is the impressed harmonic voltage.

Trace ||2 which is the impressed harmonic current.

Trace I9 which is the wave motion received at one receptor.

Trace |23 which is the wave motion received at another receptor.

Trace |33 which is the phase relationship of the wave motions of traces ||9 and |21,

From what has been said hereinabove, it will be clear to those skilled in the art that the following results are obtained:

a. From the general appearance of traces |21 and l i9, the logs taken in the various bore holes can be readily correlated.

b. The nodes and loops may be readily seen throughout the length of the log in all of the strata.

c. The level of amplitude of the wave motions is lowest in shale and most in limestone. (This can be checked quite readily by observing the impedance changes as portrayed by traces |04 and H2. The energy required to vibrate the emitter in the region of hard material will be greater than that required to vibrate the emitter in the region of softer material. This gives an additional check.)

d. The distance between loops in the shale sections may be as small as two feet when a frequency of '750 cycles is used, while the distance between loops (or nodes) in the limetone sections may be three feet or more, indicating the higher velocity of the limey portions.

e. The ratio of amplitude of loops and amplitude of nodes (damping) is greatest in shale and least in lime.

f. In addition to the specific characteristics that can be readily described and identified, the overall characteristics of amplitude, standing Waves and ratio of amplitudes gives a general character diiilcult to describe but which carries over lfrom one bore hole to the next, enabling ready correlation of bore holes. y

g. The lters permittingv passage of wave motion' of predetermined frequency only, remove nearly all visible extraneous disturbances caused same and damping might be the same, though other factors may be diiferent. The simultaneous recording of resonance velocity, phase velocity, energy conductivity and damping, as is shown in Figure 15, gives a very complete story of the various characteristics of the layers enabling bore hole logging to be practiced with a degree of certaintyheretofore unknown.

In our method, we may log the same hole several times using different frequencies. Though we generally prefer a frequency range between 500 and 1,000 cycles per second for transverse waves, our method is not limited to these frequencies.

Any suitable emitter or receptor may be employed. A preferred type is shown in Figure 12 and Figure 13, in which the housingl |36 is a cylindrical tube in which a permanent magnet |31 is held in neutral position by flexible diaphragms |38 and |39. The diaphragms are seated on peripheral shoulders |40 by non-magnetic pressure disks |4|. 'I'he pressure disks in turn are held in position by core pieces |42 which are, in

turn. held in position by upper threaded closure member 43 and lower threaded closure member |44, respectively. The conductor cable |45 is arranged to completely close and seal the passage in the upper closure member |43 and carries conductors |46 and |41. The conductors are connected to an upper coil |48 and a lower coil |49.

When harmonic vibrations are received by the case or housing |36, the air gaps between the permanent magnet |31 and the magnetic cores |42 are varied in sympathy with the harmonic vibrations received. The variation of the air gaps induces electrical potentials in the coils |48 and |49.

Since we, in the instant case, are interested in lateral motions, the case |31 is sufficiently ilexible to permit the diaphragms |36 and |39 to deform the case harmonically in a Alateral direction as the magnet vibrate when the arrangement shown in Figure l2 is used as an emitter. It is understood, of course, that, if harmonic potential is furnished to the conductors |46 and |41, the

permanent magnet |31 will vibrate in a vertical direction, deforming the iiexible vdiaphragms |38 and |39, thus setting up the ripples in the case |36. These ripples will have a large lateral component. It is also desirable to increase the mass of the case system such that the ratio of its mass to the magnet is large. It will be noted that the closures |43 and |44 are quite heavy. The provision of a large mass of the case system to the magnet mass discourages vertical movement of the case, thus tending to suppress vertical vibrations in favor of lateral ones. This principle is useful for both transmitter and receptor.

When the arrangement is used as a receptor, the impression of lateral energy on the case will cause ripples in the case which, in turn, produces a harmonic vibration of the permanent magnet supported by the flexible diaphragms 38 and |39. These harmonic motions set up induced currents in sympathy with the harmonic motions received which are conducted to the amplifier and filter and thence tothe recorder.

We have found that a transmitter and receptor such as shown in Figure 12 is satisfactory for a frequency range from 50 cycles to a thousand cycles for both transmitter'and receptor.

We prefer, when logging at a fixed frequency, to tune the transmitter to this frequency by adjustment of material and dimensions of the iiexible diaphragms |38 and |39 and to operate without any damping fluid or damping medium inside of the case |36. This permits the delivery of maximum motion output with a minimum of electrical power input when using the device as an emitter.

In the case ofv receptors, however, we prefer to use a damping iiuid to obtain the order of 25 per cent to 100 per cent critically damped. .This prevents a small change in frequency, near that of the resonance frequency used, from producing a large change of phase of the electrical output of the receptor.

Referring now to Figure 14, the electrical energy generated by the receptors is conducted by conductors such as 9| and 92 from a receptor directly across the primary winding |50 of the input transformer The winding has a low impedance to match the impedance of the receptor and to minimize induced voltage in the conductors from the source system conductors. The secondary winding |52 of the input transformer, one end of which is connected by conductor |53 to the grid |54 of the thermionic tube |55, provides a step up for the voltage received by the receptor. The cathode |56 of the tube |55 is provided with a filament heater |51, which is' furnished energy by an A battery |58 through leads |59 and |60. The cathode |56 is biased by a C battery |6|. The plate |62 of the thermionic tube |55 is connected by conductor |63 to impress the/output of tube |55 through a high pass filter |64 and a low pass filter |65, the filtered energy passing through conductor |66 to the grid |61 of thermionic tube |68. The condenser |69 of the high pass filter |64 is adjusted to reject frequencies below those of the predetermined frequency generated by the generator (Figure 15). The condenser |10 of the low pass filter is adjusted to reject frequencies higher than those which it is desired to receive. The volume control comprising the resistance |1| and the variable arm |66 adjusts the overall gain. The grid |61 of the thermionic tube |68 is biased by a C battery |13. The resistances |14 and |15 are connected across reactances |16 and |11 ofI the high pass filter while the reactances |18 and |19 of the low pass iilter are shunted by resistances |80 and |8|. These resistances suitably damp the electrical network in order to prevent self-oscillation. This damping is desirable when transient impulses are received in order that the impulses will be amplified with reasonable faithfulness. The cathode |82 of the thermionic tube 68 is provided with a lament heater |83 which is supplied with energy from the A battery |58. Plate Voltage is supplied from the B battery |84, the positive terminal thereof being connected to the plate |62 of the thermionic tube |55 by conductor |85, reactance |11 and conductor |63. The positive terminal of the B battery |84 is connected to the plate |86 of the thermionic tube |68 through conductor |81, the primary winding |88 of the output transformer and conductor |89. The secondary winding |90 of the output transformer is connected by leads |22 and |23 to the oscillograph element as shown in Figure 15. The amplifier and filter assembly 96 in Figure 15 is similar to that just described` The ampliiier and filter assemblies 20| and 202 in Figure 15 are also similar to those just described, and in addition have incorporated in them an arrangement for providing automatic volume control. Such an arrangement is shown in copending application 202,483, Figure 4.

While we have described one form of amplifier and filter, it is to be understood that any suitable form of amplifier may be employed and, if desired, an amplitude limiter or volume control arrangement may be used to give a predetermined amplitude of the standing waves on the record. A suitable arrangement is shown in our co-pending application, Serial No. 202,483, in which a portion of the output energy is rectified and used to charge a condenser which is shunted by a variable high resistance. The charge in the condenser is led to the grid of the input tube so that the amplification factor'of the channel will vary as a function of the voltage of the output, the variable resistance across the condenser giving the time factor during which biasing of the input tube is effective.

It will be seen that We have accomplished the objects of our invention. We have provided a method for logging bore holes using resonance methods or standing waves. We have provided a method of logging bore holes wherein the velocity of motion causing resonance may be logged independent of phase velocity. Our method provides a means of logging the damping of resonance phenomena in various strata traversed. Our method is such that we utilize transverse waves which may be used simultaneously with compressional or longitudinal Waves to facilitate logging.

We have provided a method in which various spacings may be used between emitter and receptors to bring out more clearly the characteristics desired.

It will be understood that certain features and subcombinations are of utility and may be employed Without reference to other features and sub-combinations. This is contemplated by and is within the scope of our claims. It is further obvious that various changes may be made in details within the scope of our claims without departing from the spirit of our invention. It is, therefore, to be understood that our invention is not to be limited to the specific details shown and described.

Having thus described our invention, we claim:

1. A method of logging bore holes including the steps of generating a harmonic potential, impressing said potential upon a vibratory means adapted to respond thereto to create standing waves in the strata traversed, lowering said vibratory means into a bore hole the log of which is being made, measuring changes of impedance in said vibratory means caused by the counter effects of motions created as said vibratory means is lowered into the bore hole, and plotting impedance changes to determine the character of the standing waves created.

2. A method of logging bore holes including the steps of generating harmonic potential, impressing said harmonic potential upon a vibratory means adapted to respond thereto and to create standing Waves of transverse motion in the strata traversed, lowering said vibratory means into the bore hole the log of which is being made, measuring changes of impedance in said vibratory means caused by the counter eiects of motions created as said vibratory means is lowered into the bore hole, and plotting impedance changes on the bore hole log to determine the character of the standing waves created.

3. A method of logging bore holes including the steps of generating a harmonic potential of predetermined frequency, impressing said potential upon a vibratory means adapted to respond thereto and to create longitudinal waves, lowering said vibratory means into said bore hole the log of which is to be made and permitting said longitudinal waves to pass through the adjacent stratum whereby to create standing waves in the strata traversed, measuring changes of impedance in said vibratory means as it is lowered into the bore hole, plotting impedance changes to obtain a desired bore hole log showing the character of the standing waves created, changing the frequency of the harmonic potential impressed upon said vibratory means and repeating the above steps to obtain a plurality of logs showing the characteristics of standing waves created at a plurality of frequencies.

4. A method of logging bore holes including the steps of generating a harmonic potential of predetermined frequency, impressing said potential upon a vibratory means adapted to respond thereto and to create standing waves in the strata traversed, lowering said vibratory means into the bore hole in predetermined fixed relation to means responsive to transverse vibratory energy, recording the motion received by said responsive means and plotting said motion against depth to obtain a bore hole log of the standing waves created.

5. A method as in claim 4 in which the steps are repeated with a diiferent predetermined frequency to obtain a plurality of logs at different frequencies.

6. A method of logging bore holes including the steps of generating a harmonic potential of predetermined frequency, impressing said harmonic potential upon a vibratory means adapted to respond thereto, lowering said vibratory means into a bore hole to create standing waves within the strata pierced by the bore hole, simultaneously lowering energy responsive means into the bore hole a predetermined distance from said vibratory means, said energy responsive means being adapted to receive transverse motion, converting transverse motion received by said energy responsive means into electrical potentials in sympathy therewith and recording said potentials against depth to obtain a log of resonance velocity characteristics of the standing `waves plotted against depth.

7. A method of logging bore holes including the steps of generating a harmonic potential of predetermined frequency, impressing said harmonic potential upon vibratory means, lowering said vibratory means into a bore hole to create standing waves in the strata, simultaneously lowering ya plurality of energy responsive means into said bore hole at different predetermined distances from said vibratory means, said energy responsive means being adapted to receive transverse motions in the strata traversed, converting the respective transverse motions into electrical potentials in sympathy therewith, recording said potentials as a function of depth to obtain a bore hole 10g.

8. A method of logging bore holes including the steps of creating a harmonic potential of predetermined frequency, impressing said harmonic potential upon a vibratory means, lowering said vibratory means into a bore hole to create standing waves of motion in the strata adjacent said vibratory means, receiving transverse motions thus generated at a plurality of points within the bore hole at diierent predetermined distances from said vibratory means, converting said transverse motions into respective electrical potentials in sympathy therewith, recording said potentials whereby the attenuation between wave motion traveling from said vibratory means to the remote energy responsive means and that traveling to the proximate energy responsive means may be determined.

9. A method of logging bore holes including the steps of creating a harmonic potential of predetermined frequency, impressing said harmonic potential upon vibratory means responsive thereto, lowering said vibratory means into the bore hole to create standing waves in the strata adjacent said vibratory means. said frequency being suchv that standing waves will be created in the thinnest stratum to be identified, receiving vibratory motions within the bore hole at a plurality of points therewithin, converting said motions into harmonic potentials in sympathy therewith and recording said potentials as a function of depth to determine the 'characteristics of the standing waves created within the bore hole.

10. A method of logging ,bore holes including the steps of generating a harmonic potential of predetermined frequency, impressing said harmonic potential upon a vibratory means responsive thereto, lowering said vibratory means into the bore hole to create standing waves within the bore hole, simultaneously lowering means responsive to transverse energy into said bore hole a predetermined distance from said vibratory means, receiving transverse motion at said responsive means, converting said transverse motion into electrical potential in sympathy therewith, rejecting frequencies higher and lower than the predetermined frequency, amplifying and recording the resultant electrical potential as a function of depth to obtain a log of the standing waves created.

11. A method of logging bore holes including the steps of creating standing waves in the strata in which the bore hole is formed, receiving standing waves within the bore hole, and recording the standing wave motion' along the bore hole.

12. A method of logging bore holes including the steps of creating standing waves of transverse motion in the material. of the strata in which the bore hole is formed, receiving said standing wave transverse motion along the bore hole, and recording said motion.

13. `A method of logging bore holes including the steps of creating a harmonic potential of predetermined frequency, impressing said harmonic potential upon a vibratory means, lowering said vibratory means into a bore hole to create standing waves of motion in the strata adjacent said vibratory means, receiving transverse motions` thus generated at a plurality of points within the bore hole at different predetermined distances from said vibratory means, converting said transverse motions into respective electrical potentials in sympathy therewith, recording said potentials whereby the damping between wave motion traveling from said vibratory means to the remote energy responsive means and that traveling to the proximate energy responsive means may be determined.

14. A method of logging bore holes including the steps of creating a harmonic potential of predetermined frequency, impressing said harmonic potential upon a vibratory means, lowering said vibratory means into a bore hole to create standing waves of motion in the strata adjacent said vibratory means, receiving transverse motions thus generated at a plurality of points within the bore hole at diierent predeter-

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