Seismic Exploration Using Non-Impulsive Vibratory Sources Activated by Stationary, Gaussian Codes, and Processing the Results in Distortion-Free Final Records Particularly Useful in Urban Areas

This application relates to U.S. Ser. No. 316,006, filed Oct. 28, 1981, for "Seismic Exploration Using Non-Impulsive Vibratory Sources Activated By Stationary Sign-Bit Recording Gaussian Codes and Processing That Results in Distortion-Free Final Records Particularly Useful in Stratigraphic Trap Determination", assigned to the assignee of the present application now U.S. Pat. No. 4,486,866; and also relates to the following U.S. applications filed simultaneously herewith, viz., Ser. No. 525,587 for "Seismic Exploration using Non-impulsive Vibratory Sources Activated by Stationary, Gaussian Codes, Detecting Vibrations via Receivers Within a Wellbore and Processing the Results Into Distortion-free Final Records", and Ser. No. 525,761 for "Seismic Exploration Using Non-impulsive Vibratory Sources Activated By Stationary, Gaussian Codes, To Simulate an Impulsive, Causal Generating, Recording and Pre-processing System and Processing the Results Into Distortion-free Final Records".

This invention relates to seismic exploration in which seismic vibrations, generated by one or more non-impulsive vibratory sources, are injected into the earth, and subsequent vibrations are recorded by one or more detectors at a location displaced from the source. More particularly, the invention relates to the enhanced recovery of all desired seismic waveform information even though either representations of the waves injected into the earth, or representations of the subsequent vibrations of the earth, are subject to non-linear distortion. In a further aspect, the present invention has the surprising capability of providing identification of trends within the resulting records so as to indicate accumulations of oil and gas, even though such accumulations underlay urbanized areas.

The terms "urban urea" or "urbanized area" are used herein in their broadest sense: areas at the earth's surface where human activities of a non-rural nature predominate. Further characteristics of urban areas are: complexed transporation, residential and business systems that include standing streets, sidewalks, homes and business buildings whose construction is subject to governmental control through operations of codes and statutes; and business activities within such areas which are also subject to similar laws promulgated by governmental agencies. With respect to seismic prospecting within such areas, typically the statutes require that the operator obey laws and/or ordinances dealing with noise limits to avoid inflicting undue stress upon the populace and avoid activities that produce too much seismic energy output that could damage structures in the exploration areas. A particular agency limits exploration activities through issuance (or denial) of exploration permits within a given area under that agency's jurisdiction.

Since oil and gas accumulations within such areas are often difficult to discover, mapping of such structures by conventional non-impulsive sources where output and/or signal duration have been governmentally limited, is extremely difficult to achieve.

That is to say, governmental agencies not only can limit the maximum signal output (usually in decibels) but the characteristics of the signals themselves may also be limited to given frequencies which do not damage man-made structures. Likewise, activities within the urban area can also effect the nature of the collected signals in an adverse way; for example, an auto can pass close by the detector array during collection and cause a cultural noise burst which will adversely affect final results (even though redundancy techniques are used).

The term "distortion-free, after processing" to describe the final records relates to the fact that there should be no undesired change in waveform either of the recorded field signals or of the final data after processing has been completed.

Prior to my invention, such distortion could be brought about (i) by the use of generating recording techniques in the field as described hereinafter wherein harmonics of the fundamental dissipate its energy, or (ii) accidentally, where, for example, components of the vibrator pad and the adjacent surface of the earth do not linearly interface, or where the dynamic limits of one or the other are exceeded so that signal clipping invariably results.

Doty et al., U.S. Pat. No. 2,688,124, issued Aug. 31, 1954, for "Method and Apparatus for Determining Travel Time of Signals" describes the well-known VIBROSEIS.RTM. system of Continental Oil Company. In such a system, seismic waves are generated by mechanical vibrators on the earth's surface. Each of the vibrators is firmly anchored to the earth by the combined weight of the source. Peak forces in the neighborhood of 10 to 20 tons (and up to 36 tons) can be developed by the rapid, non-explosive interaction of the base-plate and piston system of each vibrator. Consequently, the weight of each vibrator is proportionally large to maintain the desired, continuous vibrator-earth contact during operations. The waves sent into the earth consist of long, sinusoidal wave trains of predetermined frequency and time duration characteristics rather than the much sharper wave impulses sent into the earth by the explosive sources used prior to the VIBROSEIS.RTM. system, or by "weight drop" methods including those provided by various impulse-coded systems, e.g., the so-called "pulse-coded" techniques.

There may be some confusion as to the differences of the signals produced by the VIBROSEIS.RTM. system and those produced by impulsive sources such as provided by exploding dynamite, exploding mixtures of propane and air, or by "weight drop" methods, including pulse-coded techniques.

It is well known that the capacity of any signal (including seismic signals) to carry information can be measured in a manner analogous to determining the volume of a container. Since volume is the product of height times width times length; similarly, information capacity of a signal is related to a product of amplitude, frequency bandwidth and the length of the signal.

Dynamite as a seismic energy source produces an input signal having considerable amplitude (height) and bandwidth, but has very short length. On the other hand, "non-impulsive" vibrations generated in the manner of a VIBROSEIS.RTM. system have limited amplitude, but such is compensated for by the long length of the input signal and a faithful, continuous reproduction of the control signal over the frequency spectrum of interest. That is to say, in the VIBROSEIS.RTM. system, the amplitude and phase spectra are carefully and continuously controlled so that the resulting energy spectra changes smoothly as a function of time. Thus, a smoothly varying output of desired frequency and duration characteristics is provided in contra-distinction to the binary-coded (ON-OFF) square wave output generated by pulse-coded methods in which the energy-per-blow is substantially constant and cannot be so controlled.

A further essential part of the VIBROSEIS.RTM. system lies in the processing of the received data to produce records that tend to show short pulses representing reflections from subsurface interfaces. Such responses are provided by cross-correlating the recorded representation of the vibratory waves sent into the ground with the recorded representation of the waves received subsequently.

The use of cross-correlations, as taught by Doty et al. and many others since, has now become so well known in vibratory seismology that it will be presumed to be well known in the following parts of the present specification; and the description will concern itself only with differences from the prior art.

Erich, U.S. Pat. No. 4,234,053, for "Seismic Exploration Method Using a Rotating Eccentric Weight Seismic Source", describes an exploration method in which a rotating eccentric weight source is used (as a power impactor) to transmit a coded, non-Gaussian impulse input signal into the earth on a substantially constant energy-per-blow basis. An improved representation of the pulsed input signal is correlated with the raw seismic data to provide the field record of interest. But since the impulsive source is also only discontinuously coupled to the earth (i) the interaction of the mass of the eccentric weight source with the spring constant of the earth produces an output dominated by low frequency components and (ii) the pulse shape of the output can vary non-linearly with time. Hence, such a system is limited to those uses where a conventional VIBROSEIS.RTM. system cannot be employed.

Multi-array use of such sources is likewise limited.

Another relevant patent is that of Crook et al., U.S. Pat. No. 3,264,606, issued Aug. 2, 1966, for "Method and Apparatus for Continuous Wave Seismic Prospecting", which teaches driving of vibratory sources (in conjunction with conventional full-wave recording equipment) with pseudo-random codes which, although differing in detail from the preferred codes prescribed here, does share the desirable generic property of "a code sequence which may be represented as a reference time series having a unique auto-correlation function comprising a single major lobe having no side lobes of greater amplitude than the side lobes of the auto-correlation function of statistically unrelated noise components of the composite signal detected at said detecting location" (column 13, lines 32-44).

In my U.S. Pat. No. 4,346,461 for "Seismic Exploration Using Vibratory Sources, Sign-Bit Recording, and Processing That Maximizes the Obtained Subsurface Information", issued Aug. 24, 1982 and assigned to the assignee of the present application, I describe a non-impulsive vibratory system that uses a class of vibrator signals best characterized as Gaussian, zero-mean, and stationary, in conjunction with sign recording of both the injected and received vibrations at the sources and receivers. The stated advantages relate to the channel-capacity economy of sign-bit recording (at both the sources and receivers), and to the distortion-free quality of the final processed records.

I have now discovered that use of the above class of vibrator signals not only does not sacrifice information in the final processed records even though the data is collected by sign-bit recording methods, but such type of vibrator signals also favorably impacts seismic recording and processing operations within urban areas where full-wave recording is contemplated.

Aside from the above, a paper of A. B. Cunningham, Geophysics, December 1979, Vol. 44, No. 12, pages 1901 et seq., for "Some Alternate Vibrator Signals", works out in mathematical detail expected types of cross-correlation functions from various types of vibrator sweeps, including certain types of pseudo-random sweeps, but not in the context used herein.

The present invention uses a class of vibrator signals best characterized as Gaussian, zero-mean, and stationary, in conjunction with conventional recording techniques in an urban setting. The surprising result: because the power and character of the generated wave, as measured at any point away from the sourcepoint of the vibratory source, is constant in time; hence, the drive level of the source can be held at an unusually high level throughout the activation cycle and still be consistent with and not exceed the drive level requirements as proscribed by statute. Note further that the frequencies injected into the earth can also be used in their full ensemble; that is, they need not be limited by possibilities of damage to man-made structures. This is because, inter alia, the source of the present invention spreads the energy of each component of frequency evenly over the full length, in time, of the source signal. Hence, man-made structures are less likely to vibrate at dangerously high levels than when the latter are subject to conventional "chirp" signals, that is, where the energy of particular frequencies is concentrated and localized in time. Subsequent cross-correlation of the injected vibrations with the received vibrations provides cross-correlation records that are actually, provably superior in expectation to cross-correlation records that use conventional seismic sources, due, inter alia, to the fact that use of Gaussian, zero-mean, and stationary codes for driving the vibrator, completely invert usual seismic signal processing and recording requirements and prevent unknown, frequency-dependent loss of energy from the fundamental to harmonics. Result: The final records are distortion-free irrespective of non-linear operations occurring in either the collection or processing mode of such records.

In this aspect, the distortion-free nature of the final records results from the injection of the vibrations associated with the above-identified Gaussian codes (rather than a conventional "chirp" signal of predetermined frequency and time duration characteristics) and recording of a full-wave representation of the injected signal. Reflected signals are next recorded at the array of receivers using full-wave preservation methods, and thereafter processed using conventional correlation techniques. Thus, absolute scale of each received trace is maintained.

Since the final correlation records are surprisingly distortion-free, they have a superior capability of, say, indicating oil and gas accumulations associated with the surveyed earth formation within the urban setting.

These and other features and advantages of the present invention will become evident to exploration seismologists in light of the following specification, including the appended drawings briefly described here.

FIG. 1 is a side elevation of an earth formation in an urban area, and shows major pieces of apparatus used in a survey system for carrying out the present invention, the survey system including a vibratory source adapted to be driven by a Gaussian, zero-mean, stationary code, a series of detectors arranged to receive the propagated portions of the injected vibrations, and processing and recording means for the generated and recorded signals.

FIG. 2 illustrates records associated with a conventional "chirp" signal conforming with existing urban noise laws, in an urban area found in the Western portion of the United States, and a Gaussian, zero-mean, stationary input.

FIG. 3 shows a sequence of auto-correlations, first that of one random sweep, then a sum of the auto-correlations of two different random sweeps, then a sum of the auto-correlations of 4, then 8, then 16 different random sweeps.

FIG. 4 is a schematic diagram in block form of the system of FIG. 1 showing paths along which signal information flows, in accordance with the present invention.

Refer now to FIG. 1, 10 illustrates the survey system of the present invention in an urban area. System 10 includes vibratory source 11 positioned on a street 12 at a location distant from a series of detectors 13. In operation, vibratory source 11 injects seismic vibrations in a continuous and non-impulsive manner into the earth formation 14 in a manner that does not inflict damage upon humans or to adjacent buildings 16. Amplitude-versus-time characteristics of the vibrations of the source 11 vary over a predetermined time duration and frequency spectrum, under control of a control signal generated by sweep generator 15. The sweep control signal fed to vibratory source 11 is also fed through a separation information channel to processing and recording unit 17.

In FIG. 1 the vibratory waves going into earth formation 14 are represented by ray paths 20, the waves traveling obliquely downward toward a pair of stratal interfaces 21, 22 where they undergo partial reflection and travel back upwards along paths 23, again obliquely, toward the series of detectors 13 at the earth's surface. While only three symbolic detectors are shown in FIG. 1, in actual current field practice, hundreds of detectors may be employed. From detectors 13, the final operations represented in FIG. 1 are transmissions of full-waveform data to the processing and recording circuit 17. In the unit 17, cross-correlations are made of the full-wave received data and the injected vibratory sweeps.

Reference is now made to FIG. 2, which illustrates recordings 30, 31 of full-wave signals generated by a conventional "chirp" signal and signals obtained via a Gaussian, zero-mean input code in accordance with the invention, respectively. Both operate at the same drive level.

When waveforms are cross-correlated and stacked with a high multiplicity, the stacked cross-correlations of recording 31 of vibrations associated with the Gaussian code of the present invention are superior to those of recording 30 associated with the chirp signal. But the mere appearance of superiority could not convince seismologists in general that the desirable information is due to the vibratory input of the present invention.

But it is now possible to state that for currently practiced seismic exploration in urban areas, if the specified types of driving functions are used as taught in the present invention, the resulting seismic information will be surprisingly distortion-free.

The physical steps of the present invention, when properly combined, make possible the utilization of the theoretically provable, maximum amount of information that can be recovered from correlated waveforms.

That it should be possible to better recover distortion-free information from correlations of a stationary, zero-mean, Gaussian function was obliquely implied in theoretical results obtained by Bussgang. These results are concerned with noise for the purpose of data communication and, to my knowledge, have remained relatively unknown among exploration geophysicists. Their geophysical implications require an unobvious twist of thought in that it is necessary to think of Bussgang's "noise" as a possible seismic "signal", just the opposite from that would then be called the "noise" in the seismic context. [Chapters 5 and 6, e.g., of An Introduction to Statistical Communication Theory, J. B. Thomas, John Wiley & Sons (1969), are seen to express "noise" and "signal" in such conventional terms.]

The results of Bussgang that bear upon the present invention are equations given here in notation different from that originally used notation, but more like that used in current theoretical seismology.

Bussgang's result can be stated as follows:

Let

W, Y be stationary, jointly Gaussian, zero-mean, random processes (continuous in nature or sampled representations of such continuous processes);

X(.,.) be the cross-correlation operator; and

k(.) be a characteristic of any non-linear function (i.e., k is a fixed mapping function or table that converts input values to output values but not necessarily one-to-one).

Then

where a and b are constants depending only on k and the sequence affected by k.

In words, equation (2) states that given two input signals (W and Y) with certain properties, their cross-correlation X(W,Y) is unaffected, except to a scale factor, by any distorting mechanics that either one might suffer.

In such case, X(W,Y) represents the correlated output of a true amplitude recording system (i.e., without non-linear distortion); a X[W,k(Y)] represents the correlated output of a system in which the data undergoes non-linear distortion as it comes through the geophone, and then is correlated with the sweep signal; and b X[k(W,Y] represents the correlated output of a system in which the sweep undergoes non-linear distortion, and then is correlated with the unclipped output of the geophone.

Interpreted in this fashion, we can distinguish the two cases where:

(i) Y, the recorded signal, is subjected to the non-linearity, k, before correlation; and

(ii) W, the source signal, is subjected to k before correlation.

These results apply directly to seismic prospecting where W is the motion of a vibratory source and Y is the consequent motion of the earth picked up by a geophone of a series of geophones.

In this case it is well known that W and Y are related through the usual convolutional equation:

where E is the impulse response of the earth and N represents any uncorrelated noise present at the geophone.

Interpreted in this fashion, the extension of Bussgang's result to seismic prospecting implies that if the sweep and recorded data have the right properties, non-linear operations produce no distortion; but in case (i) above there is loss of absolute trace amplitude information. Absolute amplitude data is lost because Bussgang's extension applies to normalized cross-correlations only. The loss is the same as that which occurs when data are trace equalized using a single gate containing the whole trace. It is not equivalent to an AGC operation. In case (ii), the correlated output is undistorted and, since the scaling constant is only a function of the particular non-linearity and the particular sweep statistics, if these are held constant from profile-to-profile, then true scaling both for the traces within a profile and for the traces from different profiles, is assured.

With the above as a guide, briefly, the operational steps of method of the present invention may now be specified using vibratory sweeps and conventional full-wave recordings, so as to produce final cross-correlation traces that are surprisingly distortion-free and so are superior to conventional cross-correlation traces. In accordance with method aspects, a preferred embodiment of the present invention includes the steps of:

A. Injecting seismic vibrations into the earth in an urban area by driving at least one vibratory source using a control code of preselected Gaussian, zero-mean, stationary characteristics;

B. Recording the full-wave representations of the injected seismic vibrations and received full-wave vibrations that have propagated through the earth from the vibratory source(s) to the receiver(s); and

C. Cross-correlating the results of Step B, i.e., cross-correlating the full-wave injected vibrations with the received signals.

The result of such cross-correlation, without more, is the seismic record of interest that is surprisingly useful in oil and gas exploration in urban areas.

The above three steps, performed in combination, give the result never achieved before: the final cross-correlation traces that are superior to cross-correlation traces that could have been made heretofore using an impulsive or a chirp source signal.

Briefly, superiority of the method of the present invention results from the fact that the use of vibrations, generated by a Gaussian, zero-mean, stationary drive code, invert the signal collection and processing requirements for producing distortion-free final correlated records. Thus, where the prior art performs any form of non-linear operation on the source or receiver signals, distortion would occur. But note that in my method, all such non-linear operations are performed in an inverse domain (the "noise" domain) and hence do not distort the essential nature of the final seismic records. Hence, the present invention can operate with superiority in urban areas.

Within the above-identified area of usage, the step of the present invention related to the injection of vibrations into the earth by driving the vibrator with a Gaussian, zero-mean, stationary code, plays a very important role.

Hence, it is appropriate therefore to go into some detail concerning the nature of such vibrations and especially the control codes used in conjunction with sweep generator 15 to drive the vibratory source 11 of the system shown in FIG. 1.

In the art of driving vibratory sources, extensive use has already been made of activating codes embodied on magnetic tape, so it is unnecessary to discuss in detail how the activating codes are used to drive the vibratory source 11. It should be commented, however, that sufficiently detailed equipment is now presently available for this purpose. For example, in Payton, Waters and Goupilland's U.S. Pat. No. 4,168,485 for "Simultaneous Use of Pseudo-random Control Signals in Vibrational Exploration Methods", a digital sweep generator is described that uses a digital control code to produce the desired control signal. Essentially, the digital control signal is placed in the memory of a digital computer as digital words which can be shifted out, in seriation, to a digital-to-analog converter. The converter, in turn, generates an alternating signal to drive the vibratory source, such a vibratory source 11. The output signal from the D/A converter is initially in the form of a series of stepped electrical functions in which amplitude changes, in steps, as a function on incremental time change. But after smoothing via a filter, the stepped signal becomes a more conventionally looking alternating control signal those amplitude varies smoothly with time, to provide the desired output signal.

It should also be noted that the art has further progressed where it is no longer necessary to use main-frame type computers on-board the field equipment truck. Advances in both areas of software and hardware technology now allow the explorationist a choice of numerous mini- and micro-computers for this purpose. Additionally, he is no longer required to generate the activating codes at a remote computer center and transport them on tape to the field location. Codes can now be generated in real time at the field site using the above-identified equipment and it seems possible that in the future this will become the preferred way to generate them.

The discussion here will concern itself with some of the possible options in preparing a code particularly suited for the practice of this invention; a code that results in the injection of an associated signal into the earth. Its duration can be longer than that of conventional "chirp" signals. For discussion purposes, 32 seconds is a possible time interval. The magnetic tape devices used in geophysical prospecting in urban areas have various sampling intervals. Two milliseconds is a possible choice. Thirty-two seconds of code at a two-millisecond sampling rate require 16000 random numbers. These may be generated in several ways. One way is to use, in succession, two of the well-known subroutines in the IBM Scientific Subroutine Package: RANDU, to generate a set of uniformly-random numbers, then GAUSS, to convert those numbers into a set with Gaussian distribution. Other ways of generating suitable sets of numbers have been discussed by Lewis in the IBM Systems Journal No. 2 (1969) and by Knuth in his book "Seminumerical Algorithms" (Vol. 2 of the "Art of Computer Programming", Addison Wesley Pub. Co.). However it is done, the desired objective for purposes of this invention is a set of random numbers with a zero-mean, Gaussian, stationary vibratory signal.

Another way is to employ a controlled code of preselected characteristics, but which is modified in a fast, feedback mode as a function of the output vibrational signals. Result: irrespective of the characteristics of the control code, the total vibratory system interacts so as to provide the desired output signal.

A set of random numbers, 500 per second, may contain, in frequency terms, components up to 250 Hertz, and such frequencies are higher than those usually considered useful in current field vibrator work. So a possible next step is to put the set of random numbers through a digital band-pass filter to restrict the frequency content to a band, say, between 10 and 80 Hz. Many phase characteristics are possible. Minimum phase is preferred, but zero phase may be satisfactory.

The last condition that the code should satisfy in order that the entire method of the present invention will correspond as nearly as practicable to the ideal conditions indicated by Bussgang's results, is the condition of "stationarity". Statistical stationarity, by definition, is the maintenance of the same statistical characteristics throughout the time interval of interest. A signal generated as described above has inherent stationarity, e.g. as long as program control parameters are not varied during the time it is activating the vibrator(s). Stationarity of both the source and receiver data is approached as closely as practicable by (1) beginning the actuation of the vibrator(s) long enough before the starting of the recording for transients associated with the impulse response of the earth to die down substantially (e.g., 6 seconds), and (2) continuing the non-impulsive actuation of the vibrators, such as 11, throughout the long-time interval conventional for the vibrations to propagate through the earth to the receivers, such as 13, in usual VIBROSEIS.RTM. operations.

A field test is illustrative of the method of the present invention. In the test, the receiver group nearest the shot is offset from the source. Spacing between groups is 30 feet. Each group consisted of 6 receivers spaced 5 feet apart along the direction of survey.

Structure undergoing survey in the present example is conventional structural situation prescribed in an urban setting.

In one instance, comparative data is generated using a conventional "chirp" signal of about 16 seconds duration of the form: ##EQU1## where the signal consists of a sinusoidal function of gradually and uniformly increasing frequency, starting at w.sub.o =2.pi. a (f.sub.o =a) and ending at w.sub.f =2.pi. (a+bT). Thus, a sweep going from 10 to 100 Hz in 16 seconds has values of a=10 and b=5.6. The purpose of the function A(t) is to have a multiplying factor in the sinusoidal function so that the sweep does not start or stop abruptly in amplitude.

In an urban area, the permitting process would limit the drive level to a maximum value F.sub.m (t) with, in addition, certain frequency ranges being omitted because of possible resonance effects.

The spectrum of the system input is assumed to be completely white so the shape of the amplitude spectrum of the received signal is the shape of the earth signal.

In the other instance, the driving code of the vibrator is Gaussian, zero-mean, and stationary. Duration: 16 seconds long. Sample rate: 2 milliseconds. The sweep is generated by passing a random number sequence generated with RANDU and GAUSS through a zero-phase filter. The amplitude response of the filter increases linearly from 0 Hz to 125 Hz at a rate of 12 db/octave. Above 125 Hz, the filter amplitude response decreases linearly at a rate of 72 db/octave.

Both sets of data are correlatable. I.e., to say a full amplitude representation of the conventional "chirp" signal is cross-correlatable with the full-wave signal received at each receiver group. Similarly, the full-wave representation of the Gaussian, zero-mean, stationary sweep signal is cross-correlatable with the full-wave receiver signal of each receiver group.

Comparing the sets of data, it is evident that the data developed by the method of the present invention is far superior to that provided by the more conventional "chirp" method. Better resolution of adjacent waveform changes (in the traces provided by the method of the present invention) is the main advantage. Hence there is strong evidence that the present method provides final records that are superior to final processed records obtained using a conventional "chirp" signal, especially for indicating the presence of structure within an urban setting. In this regard, note that because the power and character of the generated wave, as measured at any point away from the sourcepoint of the vibratory source, is constant in time; hence, the drive level of the source can be held at an unusually high level throughout the activation cycle and still be consistent with and not exceed the drive level requirements as proscribed by statute. Note further that the frequencies injected into the earth can also be used in their full ensemble; that is, they need not be limited by possibilities of damage to man-made structures. This is because, inter alia, the source of the present invention spreads the energy of each component of frequency evenly over the full length, in time, of the source signal. Hence, man-made structures are less likely to vibrate at dangerously high levels than when the latter are subject to conventional "chirp" signals, that is, where the energy of particular frequencies is concentrated and localized in time, at similar levels and of like characteristics.

From this point on, the discussion will be concerned with refinements and variations not necessary to make the basic method operable, but may be desirable for optimum performance.

Sometimes in vibrator seismology, in order to increase the effective signal-to-noise ratio, it is desirable to make repetitive sweeps and add the records from the successive repetitions expecting that desired signals will be reinforced and the noise will tend to cancel itself out. When Gaussina, zero-mean, stationary sweeps are being used, and such repetitions are made, it is desirable to use a different time sequence of vibrations for each repetition. FIG. 3 illustrates this, using auto-correlated random sweeps recorded as traces 36a, 36b, 36c, 36d, and 36e. Trace 36a is the auto-correlation from a single sweep. The other traces 36b, 36c . . . 36e are the sums of the auto-correlations of successively 2, 4, 8, and 16 different sweeps. (The vertical lines are 100-millisecond timing lines.) The events seen in addition to the main peak are due to correlation noise. Since the correlation noise of any one sweep is different from that of any other sweep, the level of this noise decreases as more sweeps are summed.

Some of the variations and alternatives in the method of the present invention are illustrated in FIG. 4. Alternate data paths are shown along which signal information may flow during practice of the method. The original Gaussian code from the sweep generator 15 may flow through filter 37 on its way to the vibrator 11, or it may not, depending on the state of switch 38a. The same code may or may not flow through filter 39, depending on switch 38b. Subsequently, the signal passes to the correlator and recorder unit 18. Representations of the full-wave received signals are seen to pass into the recorder unit 18. Within unit 18, the multi-bit correlation functions go directly to form the final record.

The invention is not limited to the specific embodiments described above, because variations will be readily apparent to those skilled in the art. Thus, the invention is to be given the broadest possible interpretation within the terms of the following claims.