GB2029016A - Vertical seismic exploration and profiling technique - Google Patents

Abstract

In a vertical seismic exploration technique, periodic signals are emitted to propagate seismic waves in subsurface earth formations penetrated by a borehole. Seismic waves in formations near the bore hole are received at determined borehole depths and their representations recorded to form a vertical seismic profile. Exploration is conducted by recording representations of seismic waves at borehole depths varying by depth intervals ( DELTA X, Fig. 1B, not shown) corresponding to a time interval DELTA t dependent upon the wave period of a given mode. Profiling is accomplished by selecting recordings from borehole depths separated by this depth interval and combining using time delays related to the wave period and the propagation direction of the mode to produce traces E preferentially suppressing seismic waves propagating in one direction (1), (2) while automatically reinforcing those propagating in an opposite direction (3), (4). A vertical seismic profile is recorded, which may also utilize this time interval between traces. Use of depth and time intervals respectively approximating one- quarter of the wave length in the formation near the borehole and one- quarter of the recorded wave period of a selected propagation mode simplifies field production of the directionally preferential suppression and reinforcement characteristics of the resulting vertical seismic profile. <IMAGE>

Description

SPECIFICATION Vertical seismic exploration and profiling technique This technique relates to method and apparatus for seismic exploration and profiling of subsurface earth formations penetrated by a borehole, and more particularly, to improved methods and apparatus for recording at different borehole depths and processing representations of seismic waves propagating in subsurface earth formations near a borehole to produce a vertical seismic profile.

Surface seismic techniques employing both seismic sources and detectors at the surface have undergone extensive development in recent years.

A recent survey of patents in this area can be found in Technology Assessment and Forecast-8th Report (December 1977), conducted and published by the U.S. Department of Commerce. More recently, borehole recording of seismic waves emitted at the surface has allowed calibration of seismic two-way travel times to borehole depths. This calibration is performed by setting a specially adapted well seismic tool at a known depth. After assuring the borehole tool is firmly clamped to the borehole, the wireline used to lower the tool is slacked off to decrease the possibility of travel paths down the wireline. A surface seismic source is then activated to emit seismic waves into the subsurface earth formation penetrated by the borehole. The surface signal and signatures of the seismic waves received at a few check-shot set points along the borehole are recorded.The travel time between emission and detection of the seismic wave is determined at known borehole depths. this time and depth information allows calibrating the time-based surface seismic profile in borehole depths corresponding to the tool setting depths.

Other information acquired at known borehole depths can now be integrated into the calibrated seismic profiles. As for example, borehole logs acquired versus depth can now be converted to the time base of the seismic profile. Formation characteristics present both on the seismic profiles and borehole logs can therefore be related. However, these relationships are often obscure, as for example, a particular velocity or density change apparent on a borehole log may not be related to a particular reflection present on the seismic profile.

It is therefore an object of the present invention to provide a seismic exploration technique which has a direct relationship between seismic profiles generated from signals acquired at the surface and signals acquired in the borehole.

One approach to relate borehole signals to seismic signals employs borehole logs, principally velocity, density, and more recently, the dipmeter logs, to generate a synthetic seismogram. This technique is described in U.S. Patent Application Serial No. 558,832, filed March 17, 1 975. Briefly, reflection coefficients are derived versus depth from the borehole velocity and/or density logs and the velocity log integrated and perhaps periodically calibrated by means of the above described check shots to provide travel times. A source signature is then selected and convolved into the reflection depths and travel time information to provide a synthetic seismic profile at the borehole. Where dip information is utilized, this profile can be extended away from the borehole in any specified direction.However, the nature of the traces depends upon the source signature assumed. This, and limitations on other information, allows realistic synthesizing of only downwardly propagating seismic waves. For example, the above borehole logs do not provide the necessary information to synthesize seismic signals reflected from subsurface formations below the bottom of the borehole.

It is therefore another object of the present invention to provide a vertical seismic exploration technique employing characteristics of actual seismic waves received in the borehole after propagating downward through subsurface earth formations penetrated by the borehole, then penetrating these formations to be subsequently reflected upward and again pass through these formations to be received at a later time at the same depth in the borehole.

One approach towards seismic exploration directly utilizing recordings of seismic waves received in the borehole at different depths is characterized as vertical seismic profiling or simply VSP. A seismic tool adapted for use in the borehole is set as described above in the technique for time-to-depth calibration or seismic reference shooting. However, many more settings are employed at depth levels systematically spaced at constant depth-intervals at perhaps 10 meters or 50 feet or more apart.At each depth, a seismic signal is emitted at the surface to propagate into the subsurface earth formation downward toward the borehole tool where it it received and recorded versus time. Now, however, in contrast to the calibration shots, the time of each recording is extended well beyond the first reception time for the downwardly propagating wave, such that waves with different modes of propagation and multiples thereof are also recorded.

In some cases, the recorded time intervals are long enough to allow recording of waves which were propagated downward past the recording depth and were subsequently reflected to propagate upward to the borehole tool. These reflected waves represent information as to the presence and position of a formation reflector far below the borehole tool depth and even below the bottom of the borehole. Consequently, the ability to detect and recognize these upwardly propagating waves is very important in VSP exploration.

The measurement of upwardly propagating waves in the borehole offers an advantage over surface seismic measurements in that the loss of signal strength due to travel path and reflection losses is minimized by avoiding part of the return trip to the surface. However, borehole measurements have the disadvantage that downwardly propagating waves, which are of much less interest, also have shorter and less lossy travel paths--only one way in the borehole measurement case-and consequently strongly interfere with the weaker signals traveling in the upward direction. In many cases, the upwardly propagating signals are apparently lost at shallower depths as they arrive later and later with time into the recordings. These weak signals are further obscured by still downwardly propagating waves, usually delayed multiples of earlier arrivals.

Further, tracking of upwardly propagating modes from trends established from deeper recordings, where identification of the upwardly propagating modes is easier, is complicated by shifts in alignment.

It is therefore a further object of the present invention to provide a vertical seismic exploration and profiling techniques which will enhance recognition of weak, upwardly propagating seismic wave modes despite the presence of stronger downwardly propagating modes.

One approach to enhance the later part of seismic recordings is to increase the gain with increasing time, compensating somewhat for the longer travel path implied by the later arrival time.

However, if relatively unattenuated downwardly propagating modes still exist at late times, as for example, still arising out of a "ringing" of the source signals between the sea surface and sea floor as in offshore exploration, these events will also be enhanced. Advantageously, the enhancement technique employed should preferentially discriminate against modes propagating in one direction while enhancing modes propagating in the opposite direction, independent of the time in which they occur.

Accordingly, it is a still further object of the present invention to provide a technique for vertical seismic exploration and profiling which preferentially suppresses seismic waves propagating in a given direction and reinforces seismic waves propagating in a direction opposite from that given direction.

It is known, as pointed out in the book, VERTICAL SEISMIC PROFILING, by E. L. Gal'perin, as translated by A. J. Hermont and edited by J. E.

White, for the Soc. of Exploration Geophysicists, on page 27, column 1 thereof, that for reliable wave correlation, the phase shift between neighboring points should not exceed above 1/3 the wave length. Since velocity varies, the intervals between points may be selected beforehand from electrical well log curves and from a general understanding of the cross-section.

While this depth interval suggestion may assure that the setting points are close enough to meet the 1/3 wave length criteria, it does not provide the substantial advantage that is found by accurateiy determining the velocity of the subsurface formation between the proposed setting points, as can only be found by direct measurements of velocity as from an acoustic log or from seismic measurements made at nearby points, and using these velocity measurements to determine a precise depth interval between the setting points which is related to a selected and consistent portion of an actual wave observed, with that portion having properties which enhance and simplify processing of the recorded seismic waves.

Accordingly, it is the principa! object of the present invention to provide a vertical seismic exploration technique which optimally utilizes wave period, wave length and velocity characteristics of the seismic waves received at one depth to control the depth interval between neighboring recording points in a manner that the recordings may be processed in the field using relatively unsophisticated electronics to produce vertical seismic profile traces which are substantially improved over the recordings themselves in that the produced traces preferentially suppress seismic waves propagating in one direction while reinforcing seismic waves propagating in the opposite direction.

These and other objects and advantages of the invention are obtained by emitting periodic seismic signals to propagate seismic waves in subsurface earth formations penetrated by a borehole. The seismic waves in formations near the borehole are received and their representations recorded at determined borehole depths to form a vertical seismic profile. The recording of representations of seismic waves is conducted at depth levels varying by determined depth intervals corresponding to a time interval dependent upon a selected portion of a seismic wave propagating in the formation and its velocity.

In one form of the invention, the above time interval is determined from velocity information derived from the same borehole. Profiling is accomplished by combining recordings from depth levels separated by this time interval using related delays and the propagation direction to produce traces preferentially suppressing seismic waves propagating in one direction while automatically reinforcing those propagating in an opposite direction.

In another form of the invention, a vertical seismic profile is recorded, also utilizing this time interval for the time scale between traces. The traces are generated by combining recordings from depth levels separated by depth intervals corresponding to the time interval of a quarter wave length of a selected propagation mode in the formation near the borehole. The combining also employs delays related to this quarter wave time to provide preferential suppression of seismic waves propagating in one direction and automatic reinforcement of waves propagating in the opposite direction.The utilization of the quarter wave length time for determining the depth interval between recording depths, the related delay in these combined recordings and the time scale between the vertical seismic profile traces substantially simplifies field processing and production of the vertical seismic profile.

These features and advantages of the invention are more fully explained in the following detailed description thereof when taken in conjunction with the accompanying drawings in which: FIG. 1 A is a schematic view of borehole seismic exploration apparatus; FIG. 1 B illustrates apparatus to determine borehole depth intervals and depth levels for recording seismic waves in a borehole in accordance with one form of the invention; FIG. 2 represents schematically possible paths of seismic waves, from surface emission and propagation through the subsurface earth formations near a borehole to two different depth levels; FIG. 3 schematically represents seismic waveforms as received and at different stages of processing and recording in accordance with one form of the invention; and FIG. 4 is a block diagram illustrating one form of an implementing embodiment for the invention.

In a particular embodiment illustrated in FIG.

1 A, the invention is applied in exploration of subsurface earth formations under the sea.

However, as will be explained hereinafter, the invention is also applicable to land based exploration, using well known sources for seismic signals suitable therefor.

The installation for marine application, as illustrated in FIG. 1 A, comprises essentially a drilling platform 11 placed over a borehole 12 penetrating subsurface earth formations 1 3. A seismic signal source 10 is immersed in the sea 1 8 suspended from a buoy 20. The source comprises, in this application, a known airgun 10 including a chamber supplied with compressed air under high pressure which is rapidly discharged by opening a release valve. The shooting of this gun can be controlled electrically from a remote location and gives rise to a high intensity seismic wave having a known waveform and frequency spectrum. The pseudo-period T of the emitted seismic wave depends upon the source used.

However, in accordance with the concepts of the invention, the pseudo-period T of subsurface seismic waves would depend upon additional factors.

As an emitted seismic wave traverses earth formations, these formations act as frequency filters absorbing to a greater or lesser extent, certain frequencies in the frequency spectra of the waves emitted by the source at the surface.

Considering the form of the waves delivered by airgun 10 and a downhole receiver 14, seismic waves reaching the level of the receiver, can in approximation, also be regarded as pseudoperiodic and having a period related to period T.

However, it is preferred that the pseudo-period T utilized in accordance with this invention be determined from seismic waves which have traveled the relatively long path between the surface source and the downhole receiver and thus experiencing the above selected frequency absorption, and further, the limitations of the particular receiver employed for downhole recording of these waves.

Pseudo-period T, as defined for use in this invention, is the time interval between two successive extreme values of the same sign or zero crossings in the same direction, for example, of a received waveform at a given borehole depth.

The pseudo-period employed in the present invention is preferably determined from the initial portions of the received waveform and therefore to the first compressional mode propagating in the downward direction. The downwardly propagating compressional mode will be used for illustrative purposes herein but it should be noted that the invention is applicable to use with a pseudoperiod T determined from other modes of propagation, such as the shear mode arriving somewhat after the first compressional mode or the tube wave mode which may arrive still later but exceed prior arriving modes in amplitude. It should also be noted that the propagation mode from which the pseudo-period T is determined need not be limited to a mode propagating in the downward direction.

The pseudo-period of a detected siesmic wave can be determined, preferably by analyzing the wave received at a given depth level; the first borehole depth, for example. It is preferred that this first depth be located within a total borehole measurement interval to be used for the VSP.

The VSP measurement interval, which includes all the successive depth levels for a complete recording of a VSP, is small (a few hundred meters) compared with the subsurface depth of receiver 14 (from 2,000 to 4,000 meters). Thus, changes in the pseudo-period over the VSP measurement interval will be small compared to the changes in pseudo-period induced by the total travel path of the seismic wave from the source to the receiver.

As an example, the recording of vertical seismic profiles may be carried out between depths of 2,850 meters and 3,250 meters, and hence over a measurement interval of over 400 meters which is short compared with the travel path from surface to subsurface depth of 2,500 meters. Generally, the pseudo-period of the detected seismic wave recorded over this short interval will not vary substantially during the recording of the vertical seismic profile. However, the pseudo-period will vary under some circumstances, as for example, when the velocity of the subsurface earth formations near the borehole are very different from each other from one level to the next, and/or when the measurement interval for the seismic profile is very long. In this case, it is preferred to know the variations in the pseudo-period of the detected waves over this interval.This can be achieved by measuring the time between two successive corresponding points on successive periods of a received seismic wave.

In the case of onshore drilling, the seismic source is placed at the surface near the borehole.

For example, the source may be placed in a shallow hole drilled into the surface, or where a suitable borehole source is available, the source may be placed in the same borehole as the receiver.

Downhole receiver 14, shown in FIG. 1A, may be a geophone or a hydrophone and contain a suitable borehole tool. Receiver 14 is raised and lowered in borehole 12 by a wireline 1 5 running over sheaves attached to drilling rig 9, allowing controlled movement of receiver 14 in borehole 12. Electrical conductors in wireline 1 5 allow transmission of seismic signals received by receiver 14 to additional equipment at the surface.

Normally, receiver 14 is designed to be clamped or mechanically coupled to the borehole wall in some manner to facilitate the detection of seismic waves which reach it. Source 10, like receiver 14, is controlled from surface equipment 1 6 via an electrical cable 8. Conventional means are also provided in surface equipment 16 to determine the depth reached in the borehole by the tool containing receiver 1 4.

A reference sensor 17, such as a hydrophone, immersed in the sea 18, is suspended from a crane 19 in marine applications, but sensor 1 7 may be suitably placed for land applications as well. Reference sensor 17 makes it possible to deliver to surface equipment 16, a reference seismic signal in the form emitted at the surface.

The reference signal is used to verify the proper operation and positioning of airgun 10. It also serves as a common starting point in time for downhole recording of the seismic waves.

However, this reference sensor and associated signal is optional, as the time reference signal so used to initiate the seismic signal may be used as a starting point; i.e., a cornmon time equals zero reference, in recording the downhole waves.

FIG 1 B illustrates in more detail method and apparatus used for vertical seismic expioration.

Surface equipment 16 contains, according to prior art techniques, a drum 1 0 for storing and reeling wireline 15 used to raise and lower the borehole seismic tool containing receiver 14. Drum 110 is driven through drive 112 buy a suitable mechanical power unit, not shown, controlled by an operator who observes the borehole depths of receiver 14 on a depth meter 114 linked via electrical or mechanical drive 1 16 to a wireline measuring unit 118 mechanically coupled to wireline 1 5. At an initial borehole depth, usually near the bottom of the borehole, as shown in FIG. 1 B at 100, the operator stops and sets borehole seismic tool via electrical signal sent down conductors in wire line 1 5.Signals from the tool are also returned via wireline 15, slip ring collector 120 and cable 1 22 to surface recorder 44.

When the recording system is readied, the operator uses shooting control 124 to activate the surface source 10 via cable 8A. As discussed in regard to FIG. 1 A, a surface reference signal received via cable 8B from geophone 17 may also be recorded by recorder 44. The recording may be in the form of an analog trace versus time showing a shot time reference SO superimposed on the downhole receiver signal. The shot reference time is normally followed by a quiet period and then the first arrival propagating directly from the source through subsurface earth formations to the downhole receiver. Since the receiver depth, as provided by depth meter 114 is known, depth can be related to the time between SO and the direct arrival.This one-way travel time is useful to time and depth calibrate both the vertical seismic profile and any surface seismic profile that may exist.

Typically, additional seismic wave modes follow the first arrival, still arriving at times up to several seconds after surface emission by source 1 0. The source of some of these waves will be described further in regard to FIG. 2.

It is difficult to identify the various propagation modes from a single trace. Consequently, additional traces at different known depths will be acquired. The surface equipment operator will unset and move the borehole tool to a new depth, perhaps some convenient depth increment up the hole, as for example, 10 meters or 50 feet above the previous depth, as shown in FIG. 1 B at 101.

The above described set, shoot and recording process will be repeated at this depth and similarly at depths 102 103, etc., separated by this same depth increment. By comparing the traces at two or more spaced depths, it becomes possible to identify different arrivals. FIGS. 2 and 3 are useful to explain the source and time for these arrivals and will be discussed now, returning thereafter to FIG. 1 B and a detailed description of the technique of the invention.

In FIG. 2, there is schematically represented the level of the sea surface 21, the level of the sea bottom 22 and a generally vertical direction 23 corresponding to the borehole which penetrates the subsurface earth formations down to the bottom 24 of the borehole. The source of the seismic waves is designated by S in the figure.

Two different borehole depth levels or elevations are designated by A and B. FIGS. 2 and 3 make it possible to illustrate certain principles utilized by this invention and references to both figures will be made in the following description.

A vertical seismic profile (VSP) is obtained by carrying out a large number of measurements for successive depth levels of the receiver. Source S may advantageously remain fixed as shown in FIG.

2 at or near the surface but is usually shifted in relation to the borehole to allow access thereto.

The deviation of the source position from the vertical projection of borehole position of the receiver is usually small compared to the great depths at which the receiver will be placed.

However, it is well known how to correct for the horizontal displacement of the source from a vertical projection of the borehole receiver.

The vertical differences in FIG. 2 are compressed to facilitate illustration. For each depth level considered, a seismic wave is emitted by actuating the source S and the received waves are detected by a receiver at a given depth level such as illustrated in FIG. 2 at A or B. The electrical signals representing the received seismic waves are transmitted to surface equipment 16, as explained above and recorded by recorder 44. The initial portion of two such waveforms received, for example at depths A and B illustrated in FIG. 2, are shown in schematic fashion in FIG. 3.

As shown in FIG. 3, variations in amplitude E as a function of increasing time represent the raw signal traces from depths A and B. The waveforms shown in FIG. 3 represent the first waves received in very simplified form, compared with those obtained in reality.

Successive depth levels A and B would be separated by some conveniently fixed depth interval in the prior art, but as will now be explained, the depth interval between successive depth levels utilized in the present technique, are not fixed but vary as a function of time rather than depth.

Recordings of waveforms taken from different depths, either at even increments of depth, as practiced in the prior art, or in the increments of time as practiced in the present technique, usually exhibit signals corresponding to seismic waves which are propagating in the downward direction and in many cases waves propagating in the upward direction. The upwardly propagating waves are usually reflections from a deeper formation.

in FIG. 2, a reflective earth formation 25 reflects upwardly seismic waves emitted bX source S after following paths approximating those of the seismic waves received directly at A on one hand and B on the other. The paths for the waves propagating downwardly from source direction are shown in FIG. 2 by the arrowed lines 1 and 2 directed at A and B, respectively.

Approximately parallel lines 1' and 2', respectively represent similar paths but for waveforms propagating past detection points at A and B to formations below these points, until reflected by a reflector formation such as iltustrated at 25. Upon reflection, the direction of propagation is reversed such that a reflection of the same seismic wave detected at A at an earlier time may be detected again at A at a later time, the earlier detected wave travels shorter path 1 and the later, a longer path 1 '+3. Thus, the first detection of a particular wave represents the wave propagating in a downward direction along path 1 while the later direction may represent a wave propagating in the opposite direction along path 3.The same is true for waves propagating downwardly along path 2 to be detected at B, but of course, since path 2 is longer than path 1 , the initial detection at B will be at a later time, relative to the shot time, than the detection at the shallower depth A. However, because a reflection path 4 is shorter for point B than reflection path 3 for point A, reflected waves will be detected earlier at B than A.

Note that downwardly propagating paths 1' and 2' for the reflected waves detected respectively at A and B are essentially the same.

Thus, the variation in path length for the direct waves is the difference between downward paths 1 and 2 represented in FIG. 2 and for the reflected waves is the difference between upward paths 3 and 4. This path difference is the same in both directions and is a function of the distance between depths B and A. Since the same formation is involved, the time difference between detections of the same modes propagating downwardly and upwardly is the same.

The waveforms of a recorded signal corresponding to the waves received at points A and B are shown in FIG. 3 using the same path and detector references. Signals (1, and (2), respectively, represent the signals received at depth levels A and B. Signals (1) and (2) are essentially identical at depth levels A and B except that signal (2), received at the deeper depth B, is delayed in time with respect to signal (1) taken at the shallower depth A. This delay is reversed for the reflected signals; i.e., signal (3) from the shallower received depth A is delayed relative to reflected signal (4) received at deeper depth B.

As is well known, and explained above, the time delay will be a function of both the depth distance between levels A and B and the velocity of the formation near the borehole between these levels.

Thus, if the levels are widely spaced, as for example, by 50 or 100 meters or several hundred feet, the delay will correspond to at least a substantial part of the period corresponding to one cycle of the wave.

For constant depth increments between these levels, the delay will vary, of course, with formation of velocity and change the alignment of both downwardly and upwardly propagating waves.

Examining the waveforms illustrated at A and B in FIG. 3, it will be noted that the particular delay equal to:t illustrated there, corresponds to a selected portion of the period of the waveform shown. This occurs when the waveforms are recorded in accordance with the invention. How this is accomplished will be explained with the aid of a given waveform G shown at the bottom of FIG. 3.

As shown at G of FIG. 3, the wave representing a seismic wave propagating in a given mode in the formation near the borehole may be regarded as pseudo-periodic. One full period of such a wave is illustrated at G and it would have a corresponding wave length in the formation. Both the period and the wave length may be divided into characteristic portions. In accordance with the invention, the selected portion corresponds to 1/4 the pseudoperiod T. Specifically, let the total time from one point on the wave to the corresponding point in the next period of that wave be regarded as T. For example, let T correspond to the time period between an initial zero crossing 0+ to the corresponding zero crossing 0+ of the following period, or similarly between corresponding negative going zero crossings 0- between successive periods or between corresponding positive peaks P+ or corresponding negative peaks B- as illustrated for the single period shown at G of FIG. 3. When successive periods or part of a given period for a selected seismic wave is distorted or obscured, the remaining portion may be employed by considering the waveform as shown at G to be divided into four approximately equal parts representing 1/4 of a wave period.

Thus, by assuming the partial periods between 0+ and P+, P+ and 0-, 0- and P-, or P- and O+ are approximately equal, one can determine the quarter wave length from any of these portions, or when several portions are available, determine the quarter wave represented by each portion and derive the average therefor to prevent distortion of a particular subperiod from introducing inaccuracies in the quarter wave determination.

Once the time period corresponding to the selected portion of a wave is determined, a corresponding depth interval may be determined if the velocity of the seismic wave in the formation near the borehole is known or can be assumed.

For example, the wave length for the seismic wave in the formation near the borehole corresponding to the pseudo-period T may be found by multiplying this time by the velocity of the seismic wave in that formation. More particularly, the length or borehole interval corresponding to a time period for a selected portion of the wave length may be determined. It is in this manner that, in accordance with this invention, a depth interval along the borehole corresponding to a time period of a selected portion of a given seismic wave is determined. This depth interval then controls the distance between successive depth levels used to acquire waveforms such that the delays shown in FIG. 3 between seismic representations propagating in a given direction are constant.In particular, the depth intervals between successive recording depths are determined using the time interval for a selected wave portion of a given seismic wave propagating at a known velocity in the earth formation near the borehole. Preferably, the selected portion corresponds to one-quarter of the wave period. The advantages of employing this particular selected portion will now be described with reference to the remaining signals shown in FIG. 3.

If the depth interval along the borehole between depth A and depth B shown in FIG. 2 correspond substantially to 1/4 the wave length of signals (I) and (2) shown in FIG. 3, the delay between signals (1) and (2) will be as shown at A and B in the upper portion of FIG. 3. As illustrated there, a given feature such as a peak, zero crossing or the like present on signal (1 ) recorded at depth A may be found on signal (2) recorded at depth B delayed by the time period At=T/4 when the distance AX between depths A and B also correspond to that same wave length portion in the formation. This relationship holds whether the selected portion is 1/4 or 1/2 or even 1 full period.

However, in accordance with the preferred implementation of the invention, and as shown in FIG. 3, the selected portion should approximate 1/4 of the wave length; i.e., the selected time and depth intervals should both correspond to 1/4 of the period and wave length, respectively. This introduces a known delay corresponding to this time period for downwardly propagating seismic modes as illustrated between signals (1) and (2) and, more particularly, assures us the same known delay will be present between upwardly propagating modes as illustrated between signals (3) and (4) later in the waveform.

Stated in another manner, the use of constant known times between depths assures that the angle sX shown in FIG. 3 will be thus known and can be determined from the velocity of that mode.

This facilitates tracking of that mode from waveform to waveform. This is particularly important for the weaker upwardly propagating modes as illustrated by signals (3) and (4).

However, it is even more important in that it simplifies and therefore advantageously facilitates processing of the seismic representations so recorded. This will be appreciated by examining waveforms C, D and E in FIG. 3.

As will be apparent from traces A and B in FIG.

3, signals (1) and (2) may be brought into alignment with each other by providing a delay for waveform B equal to At, i.e., equal to the time period for the selected wave length portion corresponding to this signal. This is illustrated by waveforms C and D. Note that this delay is introduced here in the waveform acquired at the shallower depth level, and thus aligning the downwardly propagating wave. This delay also changes the relationship between the upwardly propagating signals in that they are further out of alignment. This is indicated by the angle (L corresponding to the inclination of the time trend line for these upwardly propagating signals. It can be readily determined that these signals are now out of phase by 1/2 the pseudo-period orT/2.

Recording of the waveforms at depth intervals determined in the above described manner allows the delay At to be known in advance and introduced to one of the recorded waveforms such that the only additional processing necessary to suppress seismic waves propagating in one direction while at the same time enhancing waves propagating in the opposite direction is to subtract the two recorded representations. This results in a waveform trace as shown at E of FIG. 3. Where signals (1) and (2) previously existed, only a small artifact Ed of the downwardly propagating wave energy exists, therefore substantially suppressing that wave. Note that even though not illustrated, this suppression is effective for all waves propagating in the downward direction at the velocity employed to determine the delay At.Thus, not only the first compressional arrival from which At may be determined, but all downwardly propagating multiples or echoes thereof will be suppressed. At the same time, waves propagating in the upward direction at that velocity will be enhanced as shown for signal Eu present in the later part of Trace E of FIG. 3. the enhancement is obtained because, while in-phase components of the downwardly propagating wave subtract out, the upwardly propagating wave is in an out-ofphase relationship, and this causes the subtraction process to reinforce the out-of-phase energy and thus substantially enhance modes propagating in the upward direction.Note also, as illustrated at E of FIG. 3, the resulting wavelet corresponding to the upwardly propagating wave has a distinctive shape, further enabling its tracking through any still existing noise that may be present on the processed VSP trace.

The necessity of determining the time period At of the selected wave portion will vary but this period need not be determined between each depth level. In many cases, the time period will remain constant over the VSP measurement interval. The time period can be expected to vary if the formations in the travel path for the selected portion of the siesmic wave; i.e., in the downgoing path for the compressional mode, change in velocity or absorption characteristics. For example, if the path now excludes or includes a formation having a different selective frequency absorption characteristic, this changes the frequency of the wave received downhole.

The pseudo-period and, therefore, the time interval for the selected wave portion, can be expected to change with changes in source frequency as for example, if a change of tide changes the sea depth of airgun 10. For this reason, it is preferable that the time period be checked from time to time during the exploration.

Also, when applying the techniques of this invention to other propagating modes, such as the shear mode, the time period for this mode is likely to vary from that determined for the compressional mode, and this variation should be incorporated into the technique. Of course, the shear mode velocity will be substantially different than that for the compressional mode, and therefore, the corresponding depth interval variation differs.

In fact, it is the propagating mode velocity of the formations near the borehole that is the principal controlling factor of the variations in depth intervals between the depth levels used for recording. It should be appreciated that, while the depth intevals vary in accordance with changing velocities, the time period, however, will not necessarily vary with velocity, and thus, the processing of the recorded seismic waves remains simple and unchanged. This is a substantial advantage for field implemented processing and provides the ability to produce enhanced vertical seismic profiles at the wellsite. Preferably, the velocity information versus depth is made available prior to the survey.Then if the time period corresponding to the selected portion of the wave length for the propagation mode to be preferentially suppressed in one direction and reinforced in the opposite direction is already known, through familiarity with the source of a previous borehole recording of the seismic waveform representation made within the vertical seismic profile interval, a schedule of depth levels may be determined prior to beginning the exploration.

However, as shown in FIG. 1 B, the velocity versus depth information may be stored in the memory of a computer 1 30 and employed during the course of the exploration. Initially, a first recording is madeof a seismic wave representation as received at a first depth, as for example, depth 100 near the bottom of the borehole as shown in FIG. 1 B. From this recording or from a prior recording as discussed above, the time interval At for the selected wave portion of the seismic wave mode to be preferentially suppressed on the one hand and reinforced on the other, is determined as already discussed in regard to FIG. 3. This determination of At may also be made by computer 1 30 when properly programmed to detect the zero crossing or peaks characterizing the pseudo-period of the received wave, as discussed in regard to waveform G of FIG. 3.

Once the time interval for the selected wave portion of an initial seismic wave has been determined, a second recording depth may be determined by employing the velocity of the subsurface formation near the borehole at the first depth. This determination provides a depth interval, AX, corresponding to the determined time interval. This variable AX may then be subtracted from the first depth to determine the second recording depth.

As shown in FIG.1 B, the depth measurement system comprising measurement wheel 118 and linkage 1 16, may also be linked to computer 130 so that the computer has the necessary information to determine AX, and the second recording depth. Also, as shown in FIG. 1 B, the computer may be linked via linkage 138 to the winch drive 112 to automatically move receiver 14 by the depth interval Xa as shown in FIG. 1 B, to the second recording depth.

At this point, the computer could be employed to prepare the well tool to receive a new seismic wave and, via cable 8A, to initiate the seismic signal at the surface source. Both the reference signal received from geophone 1 7 and the seismic wave signal received from the downhole receiver 14 may be linked to a digital-to-analog converter 132 via lines 8B and 1 26 respectively. The converter is controlled by the computer via a twoway bus 1 34 to convert these signals to digital samples related to time. In this manner, the representations of the seismic waves received at both recording depths may be stored in the memory of computer 1 30. Also stored is the time period At such that the recorded representations may be provided at a subsequent time and combined employing this delay.

As previously described, the process involving the combining of waveforms recorded at depth intervals corresponding to the time interval for the selected wave portion is simplified by the novel manner by which the varying depth interval is determined. Thus, the process of combining successive waveforms recorded in the above manner merely involves delaying and subtracting one of the recorded waveforms from the other. As is now apparent from the previous descriptions, if the shallower depth recording is delayed for combining with the deeper recording, the mode propagating downward will be suppressed while the mode propagating upward at the same.veloxity will be reinforced.Alternatively, if the waveforms recorded at the deeper depth are delayed and subtracted from those recorded at the shallower depth, the preferential directivity will be reversed and waves propagating downward will be reinforced while suppressing those propagating inthe upward direction.

The preferred application of the exploration technique is for suppressing the larger and often dominant downwardly propagating compressional mode while enhancing its reflected upwardly propagating reflections. The resulting enhanced vertical seismic profile is illustrated at 140 in FIG. 1 B.

Artifacts of the suppressed downwardly propagating modes are indicated along time trend line Ed while the reinforced upwardly propagating mode is indicated along trend line Eu. By projecting these two trend lines to convergence, as indicated in FIG. 1 B, the position of the reflective formation is indicated as illustrated at line 25, which in this case, is found to be below the initial depth level 100 and in fact, below the bottom of the borehole.

In the vertical seismic profile resulting from the technique of the invention, as illustrated for example at 140 of FIG. 1 B, it should be noted that the slope of the two lines, Ed and Eu, are the same and correspond to a shift of double the time interval or 2At or the half-period, T/2 of the selected wave. This slope, as represented by the angle a in FIG. 3, is related to the angle b also indicated in FIG. 3 by the relationship tangent a=2 tangent 0.

Of course, the implementation of the method of the invention may be either by analog or digital means. An alternative embodiment to that shown in FIG. 1 B is indicated by the schematic diagram shown in FIG. 4. A control function V=f(x), representing the variations of the propagation velocity of acoustic waves as a function of depth x over the profile interval in the borehole, is stored in memory 32. This memory may be simply in the form of a tape having a previously recorded acoustic log versus depth thereon. An additional memory 31 contains the previously determined time period At approximating the selected portion of the wave which is desired to be suppressed in one direction and reinforced in another. The determination of At has been described above.A multiplier 33 carries out the operation AX=V. At after the recording of each seismic wave representation. This operation provides AX which may be automatically added to the previous depth to provide the next depth or this addition operation left to the surface equipment operator.

When successive depths determined as above are approximately complied with, the propagation time of the selected wave between two succesive depths will be equal to At since variations in velocity are considered in the determination of the depth interval AX. This considerably simplifies the processing required to perform the directionally preferential suppression and reinforcement.

The simplicity of the processing is also illustrated in FIG. 4 where it is assumed that the deeper waveform corresponds to signal B input to filter 35 and the shallower waveform recorded at a depth interval AX above is input to A to filter 38.

Both filters 35 and 38 are of the bandpass type which eliminate spurious signals. The outputs of these filters are amplified by amplifiers 36 and 39, respectively. The outputs of both amplifiers may be stored in memory. In the illustrated case, it will be recalled that depth B is deeper than depth A and that it is preferred to record from deep to shallow. For this reason, the filtered and amplified signal received at depth B will be temporarily recorded in memory 37. Subsequently, at the time the seismic waves are being received at depth A, the earlier recorded waveforms will be recalled from memory 37.Since this later production of the seismic waves received at depth B may be synchronized to the shot reference point common to each received wave, as shown by the time markers SO in the waveforms illustrated in FIG. 3, it is necessary to delay the shallower reception relative to this common time marker by an amount corresponding to locally constant time interval At.

This delay is shown at 41 of FIG. 4. The result of the synchronized and delayed waveform representations provided by memories 37 and delay 41 are illustrated respectively at D and C of FIG. 3, aligning the selected wave segments propagating in the direction from A to B, such that, as shown at differential amplifier 43 in FIG. 4, the two seismic wave representations may be simply subtracted to provide the waveform representation E preferentially suppressing the aligned portion of the waveforms such as signal (1) and (2), leaving only artifacts corresponding to the small differences in amplitude and phase between these signals, as illustrated in FIG. 3 by Ed.At the same time, there is an automatic reinforcement of waves propagating in the opposite direction at the same velocity as illustrated in FIG. 3 by signals (3) and (4), which results in a reinforced wave Eu for those portions traveiing in the upward direction. The combined signals may then be recorded by recorder 44, replacing the usual raw signals recorded in a vertical seismic profile by the enhanced signal, with the resulting profile illustrated at 140 of FIG.

1B.

The following is an illustrative example of the technique. The pseudo-period of the seismic signal provided by an airgun such as illustrated in FIG. 1A, will be about 24 milliseconds by the time it propagates from the source through the subsurface earth formations to a depth of about 3,000 meters and is there received. Therefore, the pseudo-period T=24 milliseconds and the selected portion of the wave propagating in a downward direction would correspond to approximately 6 milliseconds. Thus, the time interval employed in the technique may be taken to be this 6 milliseconds. Formation velocities may vary between 2,500 meters/second and 5,000 meters/second at these depths so that the depth interval X may be expected to vary between limits somewhat less than 1 5 meters to somewhat more than 30 meters.As a specific example, where the velocity is found to be 3,000 meters/second, the depth interval X, corresponding to the 6 millisecond time interval is 1 8 meters.

The invention should not be construed as limited to the above described embodiment. For example, it may be more practical to record at depth levels separated by a depth interval much less than 1 5 meters, the minimum depth interval normally expected, but to record at depth intervals of, for example, every 5 meters. If this should be the case, the invention may be practiced by determining the depth interval AX as described above in the usual fashion and then selecting the recordings received at depths which are the best approximation of the depth interval AX. In this manner, recordings from depth levels separated by the preferred depth interval determined as a function of the formation velocity near the borehole in the vicinity of these depths may be provided and combined as described above.

Appropriate adjustments to the delay used in this combining operation may be made for approximations in the depth intervals available between two such recordings.

In summary, and as illustrated in FIG. 1 B, seismic signals are periodically emitted to propagate in the subsurface earth formations in the proximity of the borehole. A first seismic signal is recorded at an initial depth, preferably near the bottom of the borehole. A portion of this recorded signal is then analyzed to determine a time period or a selected wave in that signal. Usually, this wave will correspond to the downwardly propagating arrival received directly from the source. The preferred time period corresponds to the quarter-wave period in the time domain and to its corresponding quarter-wave length in the spatial domain of the formations through which it is propagating. A depth interval along the borehole corresponding to this time interval or wave length is determined, utilizing the formation velocity, and the next recording depth determined therefrom.

As illustrated in FIG. 1 B, the depth interval between successive recording depths, denoted there as AX, AX2, AX3, etc. AXNT varies in accordance with the formation velocity 128 near the borehole at the same depth. This variation in depth interval as a function of the formation velocity is in distinct contrast with the constant depth interval employed in the prior art technique.

As shown in FIG. 1 B, the depth interval AX increases with increasing velocity 128. Stated another way, the depth levels are more concentrated adjacent low velocity formations than high velocity formations. This will become apparent when the resulting seismic profile traces are recorded versus depth and these recordings compared to the velocity of the formations. A vertical seismic profile recorded on a depth scale will, by the same token, show varying separations between the profile traces, when made in accordance with one feature of the invention, while those profiles made using prior art techniques will show constant separations between the traces.In the complimentary time based presentations, the traces produced using the constant depth interval technique of the prior art will show varying separations while the traces produced in accordance with the invention will show equal separations since they are, in effect, recorded at constant time intervals which, as previously pointed out, simplifies the subsequent processing; and as is now apparent, simplifies the production of the resulting vertical seismic profile on a time scale, the time interval between Profile traces corresponding to the constant At.

While the fundamental novel features of the invention have been shown and described above, it will be understood that various substitutions, changes and modifications in form and detail of the apparatus illustrated, and its manner of operation, may be made by those skilled in the art without departing from the spirit of the invention.

Thus, it is apparent that the novel recording and combining technique may be used employing different recorders and circuitry, as for example, the recordings may be made in the described manner, with the seismic waveform representations recorded on digital magnetic tape.

At the subsequent time and place, these recordings may be used to provide the representations of the waveforms recorded at the various depth levels with the depth interval between these levels being known. If the velocity of the formations near the borehole at these depth levels has not been previously determined, this may be determined by measuring the time interval between the same seismic wave at these different depth levels. In a similar manner, the time interval At corresponding to a selected portion of the recorded wave length, may be then determined and employed with the velocity determined if required from the recorded waveforms at two different depth levels, to determine the depth interval from which the recorded waveforms to be combined may be provided.With two depth levels separated by this depth interval then determined, the waveform representations recorded at depths approximating a separation by this depth interval, may be selectively retrieved and processed in accordance with the invention. This processing then would delay or offset one of these representations by the above determined At. The synchronization in time for this process can be provided by including in the recordings a time marker related to the shooting time.

After combination by delay and subtraction, the resulting signal may be recorded as a trace on a graphic recorder such as a Calcomp plotter. An additional depth interval may be determined in the above described manner, and employed to determine an additional recording depth. The seismic wave representation previously recorded at that depth may then be provided and processed in a similar manner and recorded alongside the previously processed trace. The recording may be done on a scale representing the difference in depth between the depth recording points or may be done on a time scale reflecting the equal time intervals between the selected recording depths.

In a similar fashion, analog components may be employed for specified parts of the digital functions otherwise implemented by the computer shown in FIG. 1 B. All such variations and modifications, therefore, are included with the intended scope of the invention as defined by the following claims.

Claims (35)

1. A method of vertical seismic exploration employing a borehole penetrating subsurface earth formations, comprising: emitting a periodic seismic signal to propagate as seismic waves in subsurface earth formations; recording at a first recording depth in a borehole a representation of a first seismic wave received from said seismic signal and propagating in an earth formation near said borehole; determining a depth interval along said borehole corresponding to a time interval dependent upon a selected wave portion of a seismic wave propagating in a known mode and velocity in said earth formation; and recording at a second depth in said borehole varying from said first depth by said determined depth interval a representation of a second seismic wave in a manner that said first and second recorded representations of seismic waves propagating in said earth formation near said borehole may be combined as a function of said time interval to preferentially suppress seismic waves propagating in a given direction and reinforcing said seismic waves propagating in a direction opposite from said given direction.

2. The method of Claim 1 wherein said step of recording at said depth includes providing said representations of said first and second representations of seismic waves with a delay of said time interval to provide a delayed representation of one of said seismic waves for combining with the underlaid representation of said seismic wave.

3. The method of Claim 2 wherein said delayed representation is combined with said underlaid representation of said seismic wave by subtracting to suppress seismic waves propagating in the direction from said delayed representation depth toward said underlaid representation depth while reinforcing seismic waves propagating in the direction from the underlaid depth toward the delayed depth.

4. The method of Claim 3 wherein said delayed representation depth is shallower than said underlaid representation depth and the suppressed seismic waves propagate downward and the reinforced seismic waves propagate upward in the subsurface earth formations near said borehole.

5. The method of Claim 4 wherein said selected wave portion is approximately one-fourth of the wave period of the seismic wave propagating in a known mode in the earth formation near the borehole between the first and second depths.

6. The method of Claim 4 wherein said selected wave portion is approximately one-fourth the wave period of the seismic wave propagating in a known mode in the earth formation near the borehole between the first and second depths.

7. The method of Claim 1 wherein said selected wave length portion is approximately one-fourth of the wave length of the seismic wave propagating in a known mode in the earth formation near the borehole between the first and second depths.

8. The method of Claim 7 wherein said step of recording at said second depth includes providing said representation of said first seismic wave delayed by said time interval to provide a delayed representation of said first seismic wave for combining with said representation of said second seismic wave.

9. The method of Claim 8 wherein said step of recording at said second depth includes providing said representation of said first seismic wave delayed by said time interval to provide a delayed representation for combining with said representation of said second seismic wave.

1 0. A method of Claim 9 and further including the steps of: recording said combined representations; determining additional depth intervals and recording at additional depths varying by said determined depth intervals additional seismic wave representations which are delayed and combined with a delay related to the time interval for said selected wave portion to produce additional seismic traces suppressing seismic waves propagating in said direction and recording said traces with said time interval therebetween along a vertical scale.

11. A method of producing a vertical seismic profile from borehole recordings of seismic waves propagating in earth formations, comprising: a) providing at various borehole depths representations of borehole recorded seismic waves propagating in subsurface earth formations near a borehole; b) determining a time interval corresponding to a selected portion of a wave length of a seismic wave present in one of said representations and propagating in a given mode in said formation near the borehole; c) determining a depth interval corresponding to said time interval using the propagation velocity of said mode in said formation; d) selecting from said provided representations of borehole recorded seismic wave representations varying in borehole depths approximately by said determined depth interval; and e) combining said selected representations as a function of said determined time interval to produce a vertical seismic profile trace representation preferentially suppressing seismic waves propagating in said mode at said velocities in a given direction and reinforcing said seismic waves propagating in a direction opposite from said given direction.

12. The method of Claim 11 wherein the time interval corresponding to said selected portion of a seismic wave is one-quarter of the period for said seismic wave.

13. The method of Claim 11 and further including the step of recording said produced trace.

1 4. A method of producing a vertical seismic profile from borehole recordings of seismic waves propagating in earth formations, comprising: a) providing a representation of first borehole recorded seismic waves propagating in a subsurface earth formation near a borehole at a first borehole depth; b) providing additional representations of borehole recorded seismic waves propagating in said earth formations at additional borehole depths varying in distance from said first borehole depth; c) determining a time interval corresponding to a selected portion of the period of a seismic wave propagating in said formation near the borehole at said first borehole depth; d) determining a depth interval using a propagation velocity for said formation and said determined time interval corresponding to said selected portion of said seismic wave;; e) selecting from said provided additional representations of borehole record seismic waves at additional borehole depths a representation at the borehole depth corresponding approximately to a depth varying from said first borehole depth by said determined depth interval; and f) combining as a function of said determined time interval said first and selected - representations of borehole recorded seismic waves propagating along said borehole to produce a vertical seismic profile trace representation preferentially reinforcing seismic waves propagating at said velocity in one direction and suppressing seismic waves propagating at said velocity in other directions.

1 5. The method of Claim 14 and further including the step of repeating steps (d), (e) and (f) for additional depth intervals and representations to produce additional traces.

1 6. The method of Claim 1 5 and further including the step of recording said traces in increments of said determined time interval to produce a vertical seismic profile.

17. A method of producing a vertical seismogram from borehole recordings of seismic waves propagating in earth formations, comprising: a) providing a representation of first borehole recorded seismic wave propagating in a subsurface earth formation near a borehole at a first borehole depth; b) providing additional representations of additional borehole recorded seismic waves propagating in said earth formations at additional borehole depths varying in distance from said first borehole depth; c) determining a time interval corresponding to a selected portion of the wave length of the seismic wave propagating in said formations near the borehole at said first borehole depth; d) determining a depth interval using the propagation velocity for said formation and said determined time interval corresponding to said selected portion of said wavelength;; e) selecting from said provided additional representations of borehole recorded seismic waves at additional borehole depths the representation at the borehole depth approximately corresponding to a depth varying from said first borehole depth by said determined depth interval; f) combining as a function of said determined time interval said first and selected representations of said borehole recorded seismic waves propagating along said borehole, said combining preferentially suppressing seismic waves propagating at said velocity in one direction; and g) recording said combined representations of said borehole recorded seismic waves as one trace in a vertical seismogram.

1 8. The method of Claim 1 7 wherein steps (a) through (f) are repeated using the borehole depth of the last selected representation as the first borehole depth to select, combine and record additional traces in said vertical seismogram suppressing seismic waves propagating at said velocity in said one direction.

1 9. The method of Claim 18 and including repeating step (d) to determine different depth intervals using the varying propagation velocities of said different formations to select, combine and record additional traces suppressing seismic waves propagating in said one direction at said varying velocities.

20. A method of Claim 18 and further comprising: h) determining a time interval corresponding to another selected portion of the wave length of the seismic wave propagating in another direction in said formation near the borehole at said first borehole depth; i) determining another depth interval using a propagation velocity for said formation and said time interval corresponding to said another selected portion of said wave length; j) selecting from said provided additional representations of borehole recorded seismic waves another representation at the borehole depth corresponding approximately to a depth varying from said first borehole depth by said another determined depth interval; and k) combining as a function of said determined time interval said first and another selected representations of said borehole recorded seismic waves propagating along said borehole to produce a vertical seismogram trace representation preferentially suppressing said another portion of selected seismic waves propagating at said velocity in said another direction.

21. A method of producing a vertical seismic profile from borehole recordings of seismic waves - propagating in earth formations, comprising: a) providing representations of borehole recorded seismic waves propagating in subsurface earth formations at different borehole depths; b) determining a time interval corresponding to a selected portion of the period for a seismic wave propagating in a given mode in said formation near the borehole at a given borehole depth; c) determining a depth interval using the propagating velocity of said mode in said formation and said time interval corresponding to said selected portion of said wave period; d) selecting from said provided representations of borehole recorded seismic waves at said different borehole depth representations at borehole depths corresponding approximately to depths varying by said determined depth interval;; e) delaying said one of said selected representations by said determined time interval and combining said selected representations of said borehole recorded seismic waves propagating along said borehole to produce a vertical seismic profile trace representation preferentially suppressing seismic waves propagating at said velocity in one direction.

22. The method of Claim 21 wherein the time interval corresponding to said selected portion of a seismic wave is one-quarter of the period for said seismic wave and said delay corresponds to propagation in a given direction to suppress waves propagating in said given direction.

23. The method of Claim 21 and further comprising: g) determining another time interval corresponding to a selected portion of the period for another seismic wave propagating in said formation near the borehole at said given borehole depth; h) determining another depth interval using a propagation velocity for said another seismic wave ~propagating in said formation and said another time interval corresponding to a said another wave; i) selecting from said provided representations of borehole recorded seismic waves other representations at borehole depths corresponding approximately to depths varying by said another determined depth interval; j) delaying one of said selected other representations as a function of said another determined time interval; and k) combining said other selected and delayed representations of said borehole recorded seismic waves propagating along said borehole to produce a different vertical seismogram trace representation preferentially suppressing said another seismic wave propagating in said velocity in said given direction.

24. A method of Claim 21 and further comprising: g) determining a different time interval corresponding to a said selected portion of the period of a different seismic wave propagating in said formation near the borehole at said given borehole depth; h) determining different depth intervals using a propagation velocity for said different seismic wave in said formation and said different time interval corresponding to said different wave; i) selecting from said provided representations different representations recorded at borehole depths corresponding approximately to depths varying by said determined different depth interval; j) delaying one of said selected different representations as a function said different determined time interval; and k) combining said selected different representations of said borehole seismic waves propagating along said borehole to produce a different vertical seismogram trace representation preferentially suppressing said different seismic wave propagating at the velocity of said different seismic wave in said given direction.

25. A method of vertical seismic exploration employing a borehole penetrating subsurface earth formations, comprising: repeatedly emitting periodic seismic signals to propagate as seismic waves in subsurface earth formations; recording at a first recording depth in a borehole a first seismic wave received from said seismic signal and propagating in said earth formations near said borehole; determining a time interval corresponding to a quarter of the period of a seismic wave propagating in said formation; determining a depth interval along said borehole using a known propagation velocity for said formation and corresponding to said determined time interval; recording at a second depth varying from said first depth by said determined depth interval a second seismic wave received from said seismic signal and propagating in said earth formations near said borehole; and combining as a function of said determined time interval said first and second recorded seismic waves received from said seismic signal to preferentially reinforce seismic waves propagating at said propagation velocity in said earth formation near said borehole in one direction while suppressing seismic waves propagating at said velocity in other directions.

26. A seismic exploration method for producing a vertical seismic profile of a borehole, comprising: emitting seismic waves from a source; detecting pseudo-periodic waves received by means of a tool placed in said borehole at different successive depth levels; recording signals corresponding to the detected waves so as to obtain waveforms each representing as a function of time the waves detected at said depth levels, the depth intervals between said depth levels being determined from the propagation velocity and quarter-wave period of seismic waves in subsurface formations near said borehole at said depth levels to correspond approximately to a quarter-wave length in said formation;; combining signals recorded at depth levels separated by said quarter-wave length to provide combined signals suppressing seismic waves propagating in one direction while reinforcing seismic waves propagating in an opposite direction; recording said combined signals to produce a vertical seismic profile.

27. The method of Claim 26 wherein said combining employs a quarter-wave period delay between signals.

28. The method of Claim 27 wherein the quarter-wave period delay is introduced in the signal recorded at the shallower of the two depth levels being combined to suppress signals propagating in the shallow to deep direction while reinforcing signals propagating in the deep to shallow direction.

29. The method of Claim 28 wherein said combining includes subtracting said combined signals to produce traces for said vertical seismic profile.

30. Apparatus for producing a vertical seismic profile from borehole recordings of seismic waves propagating in earth formations, comprising: a) means for providing at various borehole depths representations of borehole recorded seismic waves propagating in subsurface earth formations near a borehole; b) means for determining a time interval corresponding to selected portion of a wave length of a seismic wave present in one of said representations and propagating in a given mode in said formation near the borehole; c) means for determining a depth interval corresponding to said time interval using the propagation velocity of said mode in said formation;; d) means for moving a receiver of said borehole seismic waves to borehole levels varying in borehole depths approximately by said determined depth interval to provide recorded representations of borehole seismic waves shifted by said determined time interval; and e) means for combining said recorded representations as a function of said determined time interval to produce vertical seismic profile traces preferentially suppressing seismic waves propagating in said mode at said velocities in a given direction and reinforcing said seismic waves propagating in a direction opposite from said given direction.

31. The apparatus of Claim 30 wherein the time interval corresponding to said selected portion of a seismic wave is one-quarter of the period for said seismic wave.

32. The apparatus of Claim 31 and further including means for recording said produced trace.

33. A method of vertical seismic exploration substantially as hereinbefore described with reference to the accompanying drawings.

34. A method of producing a vertical seismic profile from borehole recordings of seismic waves propagating in earth formations, the method being substantially as herein described with reference to the accompanying drawings.

35. Apparatus for producing a vertical seismic profile from borehole recordings of seismic waves propagating in earth formations, the apparatus being substantially as herein described with reference to the accompanying drawings.