Abstract: The invention relates to a vibroseis analysis method in which frequency-sweep signals (10) are emitted into a subsurface, the signals reflected on the substrata of such a subsurface (10, 20) are logged and the logged signals are processed, a method in which the harmonics (20) of the fundamental signal (10) initially emitted are eliminated from the logged signals, by applying the steps consisting in: a) providing a time/frequency plot, showing the respective contributions of the fundamental (10) and of the harmonics (20) in the logged signal, b) providing a time/frequency plot also showing these contributions of the fundamental (10) and of the harmonics (20) in the logged signal, this plot having been stretched in the direction of the frequency axis such that the fundamental (10) of this plot is over the location of a harmonic (20) chosen from the plot; c) adapting the power amplitude of this stretched plot to make this amplitude correspond to that of the said chosen harmonic (20) of the plot; d) subtracti

Abstract: A method and system for transferring synchronization data between a central control and seismic recording station and dependent stations in all the cases where the time of transmission through the connection channels between the elements may be liable to too great fluctuations is disclosed. The invention provides a central station (CCU) and intermediate local control and concentration stations (RRS), each connected to local acquisition units (RTU), with a common time reference or clock (H) (that can for example be provided by a satellite positioning network of a well-known type such as the GPS system, or based on the carrier frequency of a radio transmitter) which is picked up by specialized receivers associated with the elements of the seismic device, and in using this common reference to calculate the real transmission time and to local readjust seismic acquisition units (RTU).

Abstract: An apparatus (10) may implement a method (160) for creating (162) a data-domain sampled network (206). In certain embodiments, a method in accordance with the invention may define (166) a data domain, provide (172) points in a data-domain space or sub-space, followed by analyzing (174) data in the data domain (100). Analysis (174) may involve selecting (175) the dimensions or variables of interest, evaluating (176) or determining (176) the cycles per dynamic range, selecting (177) an interpolation method, and selecting (178) a number of sample points (223) in each respective dimension (227, 228). Providing (180) a data-domain network (206) typically includes applying (184) sampling theory to a native data domain (100). Calculating (188) waits for this by an interpolation module (212) specifies the data-domain network in its native domain (100).

Abstract: The invention relates to a two-phase method of transmitting, to a central station (CS), seismic signals received by seismic receivers (R) and picked up by collecting units (A) disposed in the field. In the first phase, a compression is applied to each seismic trace, the ratio of which is adapted to the duration of a transmission window placed in the time intervals between successive seismic transmission-reception cycles, as well as to the rate of the transmission route used. The compressed trace as a whole is thus transmitted, allowing an operator at central station (CS) to check qualitatively that recording took place correctly in each collecting unit (A) even though he does not have the exact trace available.

Abstract: Monofrequency continuous wave signals of long duration are used as inputs form sources of known characteristics and location. These signals are reflected off subterranean features and serve to paint the features for display and evaluation. A primary feature is the use of continuous wave, long duration signals as opposed to the standard short pulse technique.

Abstract: A plurality of seismic traces are arranged in order of increasing of feet and features therein are analyzed so as to identify corresponding features in each of the curves. The travel time to each of the features is derived in order to extract an actual travel time curve as a function of offset.

Abstract: A method for seismic data interpretation for the purposes of identification of oil, gas and water accumulation locations and delineation of their contours uses a model of discrete media with inhomogeneous stresses. For a selected reflection horizon, the relative value of overall pressure gradient is calculated on the basis of instantaneous amplitudes and frequencies of seismic signals using a derived relationship between the reflection coefficients of elastic waves and the stress condition parameter. The calculation technique uses estimates of instantaneous amplitudes and frequencies smoothed over the entire seismic horizon or its fragment. A generalized model of normal (lithostatic) pressure is excluded, and the final interpreted parameters have the sense of relative estimates of anomalous pressure caused by basin dynamics.

Abstract: A conventional CMP gather is Radon transformed from t-x space to the .tau.-p domain. The Radon-transformed data are windowed or zoned into a plurality of data sets, each having an common independent range of selected characteristics. All except one data set are muted. The zero ordinate of the p-axis is shifted to become centered through the contents of the retained data set which is then inverse-transformed back to t-x space. The process is repeated for each of the remaining windowed data sets. The processed windowed data sets are summed.

Abstract: An apparatus and method are provided for ensuring good coupling and vertically planting of seismic receivers while substantially reducing the need for interconnecting wires and costly surveys. The apparatus includes an acquisition unit (AU) 20 for measuring seismic energy and a planting member 12 for engaging and planting the AU 20. An electronic surveying instrument 14 is coupled to the planting member 12 for determining the location of the AU 20. An installer 16 plants the AU 20 in the terrain 22 in near vertical alignment using the planting member 12. Once the AU 20 is planted in near vertical alignment, coordinates are downloaded from the electronic survey unit to locate the planted AU 20.

Abstract: A method for identifying the boundaries between sequences of rocks laid down in different depositional regimes. The approach used is to search, in well logs or seismic data, for a statistic characteristic of such regimes, and for breaks in that statistic. The preferred statistic used in the method of the present invention is the first moment of the power spectrum of the wavenumber of a suitable physical property as a function of depth. This statistic is calculated rapidly and with excellent depth resolution from the complex-valued autocorrelation function at its zero and unit lag values, respectively, and displayed as a single variable in black-and-white or color. The method of the present invention further includes manipulations that achieve the approximate conversion of the scale (and corresponding display) from its initial dependence on depth to one that is displayed as a function of geologic time.

Abstract: Marine seismic data sets are generally under-sampled spatially because of the relatively long listening times required in deep water. It is customary to use very long spreads in the field thereby enhancing aliasing and interference from coherent noise. A seismic-signal data processing method is proposed that applies a combination of a forward parabolic Radon transform and a linear Radon transform to the data, followed by a further transform to a three-dimensional frequency domain. In this domain, a deterministic operator is applied to the data to sharpen the Radon-domain response thereof. The data are then scavenged of noise in the Radon domain and inversely transformed back into the time-space domain.

Abstract: A reverse VSP system is provided and uses a frequency-controllable modulator of an MWD tool as a downhole seismic source. The modulator has a rotor and stator arranged to provide substantially sinusoidal outputs, and is controlled to provide a "Vibroseis"-type frequency sweep (preferably from 1 Hz to 50 Hz). With the bit of the MWD tool in contact with the bottom of the borehole, the force generated by the frequency-controllable modulator will generate measurable axial compressional (P-) waves. These waves which move through the formation are detected by an array of geophones on the formation surface, and a reference signal is obtained by a kelly-mounted accelerometer or strain gauge. The reference signal and signals obtained by the geophones are processed using known signal processing techniques.

Abstract: Long-term changes in the distribution of a mineral in a subsurface deposit, due to commercial production of the mineral, are monitored by suitable instrumentation. The monitoring instrumentation is installed in combination with commercial mineral production equipment but the monitoring instrumentation is transparent to production. The monitoring instrumentation measures long-term changes in selected petrophysical attributes that are due to re-distribution of the mineral within the deposit in response to production.

Abstract: A method for attenuating source-generated surface wavetrains in a set of seismic data traces. According to the method, the seismic data traces are Fourier transformed with respect to time to determine the frequency components of the surface wavetrain. A phase matching operation is then performed using estimates of the wavenumbers for each of the frequency components to approximately align the surface wavetrain in the seismic data traces. Next, a spatial low-pass filter is used to remove the approximately aligned surface wavetrain. Finally, the phase matching is removed and an inverse Fourier transformation is performed to return the seismic data traces to the time domain.

Abstract: The present invention relates to a method of extrapolating a set of traces recorded by means of an array of sensors, in the context of a geophysical prospecting process, the method comprising the steps consisting in: i) approximating a minus Laplacian L.sub.0 (k.sub.x,k.sub.y)=k.sub.x.sup.2 +k.sub.y.sup.2 by the sum of two one-dimensional filters; and ii) approximating an extrapolation operator F.sub.0 by a polynomial of the minus Laplacian L.sub.0. The invention also provides a system for geophysical prospecting.

Abstract: The amplitude spectrum is generated within a selected time window of a seismic time scale recording. The rate of change, Q, of the natural logarithm of the signal amplitude is measured with respect to frequency. A Q-derived static time shift is computed and applied to the selected seismic time scale trace to correct the raw seismic times for dispersion-induced time distortion. Amplitude-correction and phase-correction filters are applied to the static-corrected trace to provide a new time scale trace corrected for velocity dispersion which is processed to provide a physical model of the subsurface of the earth.

Abstract: The present invention relates to a method and to a system for analyzing drilling conditions and/or the behaviour of a drill string element. It comprises the stages as follows:acquisition of a signal,calculation means are operated, which determine the wavelet coefficients of segments of the signal, the segments are merged to form a homogeneous segment,a magnitude associated with each homogeneous segment is determined.The invention further relates to a system for implementing the method.

Abstract: A method for determining a unique signal representing the earth's response between a transmitter and receivers when conducting a seismic survey involves the steps of receiving a seismic signal set that comprises a plurality of different output frequency signals having known amplitude and phase characteristics, selectively combining the different output frequency signals to form the unique signal, and recording values of amplitude, phase, and frequency of the unigue signal. A method of conducting a modulated seismic survey involves the steps of providing seismic sensors capable of receiving a modulated seismic signal comprised of different frequency signals, transmitting modulated seismic energy information into the earth, and recording indications of reflected and refracted seismic waves sensed by the seismic sensors in response to transmission of the modulated seismic energy information into the earth.

Abstract: Apparatus and methods for determining an indication of a nonlinear property of a formation traversed by a borehole are provided. An acoustic tool is used in a borehole to generate first and second signals of different frequencies. As a result of nonlinearities in the formation, the mixing of the first and second signals results in a third signal having a frequency equal to the difference of the frequencies of the first and second signals. The amplitude of the third signal is measured by a detector, and the indication of nonlinearity of the formation is determined according to a relationship which relates the measured amplitude at the measured frequency to the amplitudes of the first and second signals, the frequency of measurement, the velocity of the measured wave, the distance of the receiver from the source, and a function of the nonlinear and linear parameters of the formation.

Abstract: Excellent likeness of impulse response function r.sub.o (t) of a given linear space is obtained by exciting the space through a transmitting transducer with a compressible probing signal x(t) which is a string of lowpass narrow p(t)-wavelets polarity-controlled by a single cycle of an m-sequence and processing received raw-data by cross-correlating it with a special reference s(t) which is cyclical repetition of the same m-sequence. Cross-correlating x(t) directly with s(t) yields compressed version of x(t) (denoted e(t)) which is p(t) multiplied by length of the m-sequence. e(t) equivalently excites the space to give processed output which is the likeness of r.sub.o (t). e(t) is free from side-lobes and readily made arbitrarily slim by choosing p(t) slim; keeping duration and energy of x(t) fixed, e(t) characterizes an area invariant of the slimness of e(t) or p(t).

Abstract: The invention is a method and apparatus for conducting seismic analysis of underground formations that offers improved accuracy in locating strata that contain natural gas in paying quantities. The method analyzes the resonant response generated when a seismic wave passes through a given stratum. Seismic data are collected using conventional field acquisition methods. Following conventional preprocessing, the seismic response data are mapped onto the quefrency domain in order to separate the resonant and non-resonant components of the reflected energy. Strata that consist of predominantly elastic materials (solids and liquids) resonate in discrete frequency bands, which are represented by a quefrency spectrum with a large amplitude at high-quefrency values. Gas-bearing strata have more plastic properties, and emit a more uniform response that lacks distinct resonant peaks. The quefrency spectrum of gas-bearing strata contains a relatively higher amplitude at low-quefrency values.

Abstract: A seismic sensor platform comprises an open frame and a plurality of seismic motion sensors vertically positioned in a common horizontal plane in spaced relationship around the periphery of the open frame. A spike associated with each of the sensors extends beneath the open frame for anchoring each of the sensors in the earth. The frame includes a shield for protecting the sensors from physical damage and for shielding them from seismic noise. Second and third pluralities of horizontally positioned sensors may be provided on the frame, orthogonally positioned with respect to each other to sense orthogonal components of reflected seismic waves. The sensors may each be of a different natural frequency to optimize the multi frequency output of the sensor array.

Abstract: Seismic impulses irregularly distributed in time (according to a random or a pseudorandom code for example) are transmitted into the soil, and the direct waves and those reflected by the subsoil discontinuities are picked-up by a reception system (G). The frequency spectrum of the emitted and the received signals is divided into several bands, and the components of the signals in each one of the bands are reduced to elementary signals indicating the change of sign thereof (sign bit) and are recorded. Cross-correlation of the elementary signals respectively associated with the emitted and the received signals and stacking of the cross-correlation products associated with each sensor are carried out thereafter for each sensor of the reception system, and seismic sections very comparable to what would be obtained by digitizing the signals received with full precision, as in conventional methods, are achieved with the obtained stacks.

Abstract: Method for removing coherent noise from a series of seismic traces. Each seismic trace included in the series is Fast Fourier Transformed into the frequency domain. For each frequency of the resultant f-x representation, a series of N complex numbers, one corresponding to each trace, is selected. The selected series of complex numbers corresponding to each frequency are processed to produce autoregressive model coefficients from which a polynomial rooting provides complex roots related to wavenumbers and decay constants. Amplitudes and phases for complex non-stationary spatial sinusoids associated with the estimated wavenumbers are then determined. For all frequencies f, the collection of determined wavenumbers, decay constants, amplitudes and phases constitute an f-k representation of the seismic traces.

Abstract: A method for simultaneous collection of seismic data of shallow and deep targets, in which two seismic streamers are towed at different water depths (1 and 2 in FIG. 2), and only one acoustic source of seismic signals is used. The seismic data recorded by the different streamers are mixed in a subsequent processing step, seismic data being weighted before being mixed by a weight which is made dependent on the recorded two-way travel time for the respective streamers, so that data from one or several streamers located at the least water depth receive a weight which decreases with travel time, whereas data from a corresponding number of streamers of greater depths receive a weight which increases with travel time.

Abstract: Continuity of subsurface formations or beds in the earth between existing wells is investigated. Seismic energy is sent from a seismic source in one of the wells at various selected fixed depths. A number of sensing geophones deployed at selected fixed depths in one or more adjacent wells detect the seismic energy as data. A frequency domain decomposition process is performed on the data in order to determine if any of the formations located between the wells function as waveguides for seismic energy within the frequencies of interest. Those formations exhibiting waveguide properties are indicated as continuous between the wells.

Abstract: A method of geophysical exploration is provided for identifying and evaluating stratigraphic prospects associated with step-like changes in elastic impedance from gradational changes in elastic impedance. In particular, the seismic data are bandpass filtered with a plurality of filters each having a different pass band. Measures of the amplitude are obtained for selected reflection events in the seismic data as a function of their associated pass band frequency content. The changes in amplitude as a function of changes in frequency content of the reflection event can thus be employed to identify and evaluate subtle stratigraphic prospects in the seismic data.

Abstract: A method for high-resolution seismic recording with increased bandwidth using detectors planted at shallow depths below the earth's surface. In one embodiment, an optimum depth where the amplitudes of high frequency signals increase relative to that of the noise during high-resolution recording is determined, geophones are then planted at that optimum depth and signals are recorded at that depth. To find this optimum depth, detectors are planted at various depths below the earth's surface in order to record the signals at those various depths. The frequency spectra for each detected signal is generated and a velocity profile of the depths covered by the detectors is also generated from the signals. The bandwidth of the frequency spectra and the velocity profile are used to determine a depth for planting the detectors that is closer to the earth's surface than the depths previously considered.

Abstract: In seismic exploration, seismic reflections from subsurface formations are recorded in the form of a time-distance array of seismic traces. This array is transformed into a frequency-distance domain. Frequencies at which coherent noise resides within the frequency-distance domain are determined. Wavenumbers corresponding to such frequenices are removed by filtering. The wavenumber-filtered, frequency-distance domain of seismic traces are then transformed back into a time-distance array of seismic traces.

Abstract: In seismic exploration, seismic reflection from subsurface formations are recorded in the form of a time-distance of seismic traces. This array is transformed into a time-wavenumber domain. Wavenumbers at which coherent noise resides within the time-wavenumber domain are determined. Frequencies corresponding to such wavenumbers are removed by filtering. The frequency-filtered, time-wavenumber domain of seismic traces are then transformed back into a time-distance array of seismic traces.

Abstract: The invention is a method for improving quality of seismic data generatd with a source positioned substantially at the lower end of a drill string within a wellbore. A pilot signal is detected at the upper end of the drill string for cross-correlation with signals detected by field sensors positioned at the earth's surface displaced from the wellbore. Noise is reduced in the pilot signal by detecting the noise wih a sensor positioned to be sensitive to the noise but substantially insensitive to the acoustic signal transmitted up the drill string from the source. The frequency band within which there is high coherence between the energy in the noise signal and the pilot signal is determined. The noise signal is then amplified by a factor equal to the average ratio of the energy amplitude of the pilot signal to the noise signal within this frequency band, and this weighted noise signal is substracted from the pilot signal to reduce the noise in the pilot signal.

Abstract: A blast recorder and method for monitoring and processing vibrations from blasts, and for displaying the results in a nearly real time basis and in a manner which is easily interpreted by a relatively unskilled field worker and corresponds to a form which closely correspond to the real damage causing aspect of the blast than heretofore.

Abstract: In a method of acquiring seismic data in a prospecting area, an array of excitation vibration receivers is placed on or over the terrestrial surface. A plurality of excitation vibration sources is disposed in the vicinity of the array, each source emitting excitation vibrations with a frequency spectrum in a specific frequency band. These bands are adjacent and together cover the frequency band of usable seismic data signals. When the excitation vibrations are emitted by the plurality of sources, the excitation vibration echoes reflected from strata beneath the terrestrial surface are registered by the receivers. The above steps are repeated for a plurality of successive trace increment displacements to produce an emission figure and to cover the prospecting area. The method and the device implementing it can be used for acquisition of terrestrial or marine seismic data.

Abstract: A transmission from a hydroacoustic source, especially for underwater geophysical exploration, is generated in accordance with two binary codes (provided by two pulse trains or sequences having two levels and which provide a spectrum of harmonically related components). One of these codes has one more or one less harmonic component than the other. The transmission is provided with a frequency spectrum which extends between an upper and lower frequency limit by sweeping the pulse trains over an octave (2:1) frequency range, far shorter than the range of the spectrum, and utilizing one of the codes and then the other of the codes during the sweep. The spectrum of the transmission extends between two arbitrary frequencies which need not be harmonically related, notwithstanding that the sweep is limited to an octave in frequency range.

Abstract: A method for predicting the ground vibration and airborne noise at any position resulting from a blasting operation using the data obtained from one or more single-charge low level blasts. By using convolution in the frequency domain, large charge weight nonlinear vibrations are considered, such that the optimum excitation frequency (detonation delays) is induced for minimizing objectionable ground and air vibrations and noise. The method also allows optimization of the number, position and loading of the blast holes, and the number and position of charges detonated per delay period, to produce maximum removal of material without damage to surrounding structures.

Abstract: A method of seismic prospecting in which there is employed conventional upsweep and downsweep and opposed polarity of signals that are being sent into the earth as seismic signals and in which there is introduced a taper at the common frequency at the common time on the respective upsweep and downsweep signals to reduce the most significant part of the correlation noise, yet provide the advantages of each of the methods of using opposite phase polarity, to cancel and using opposite upsweep and downsweep to achieve reduction in the noise experienced, as well as the reduction of the most significant part of the correlation noise that distorts the desired zero-phase Klauder wavelet.

Abstract: A method for preparing a display of seismic data obtained from multiple-point coverage of a subterranean earth formation using an array of seismic sources and acoustic receivers. A set of seismic traces is obtained in such a manner that each trace is associated with a source-receiver pair separated by a particular offset distance, and all the traces are associated with a common subsurface reflection point. Each of the traces is filtered through a bank of band-pass filters to produce a set of filtered traces, each having a different frequency content, for each offset value. The filtered traces associated with a particular offset value are displayed in order of progressively changing frequency content, and the filtered traces associated with a particular frequency band are displayed in order of progressively changing offset value.

Abstract: In portable seismic data acquisition units, an adaptive seismic signal processor and method for resampling and bandpassing selected frequencies of an input digital signal. The adaptive seismic signal processor and method provide an output digital signal having a frequency domain and resampling rate which can be selected from one of a plurality of predetermined frequency domains and sampling rates during the course of acquisition of the input digital signal without substantial loss or deterioration thereof.

Abstract: A method and apparatus for generating a nonlinear sweep signal to be supplied to a seismic vibrator. A seismic sweep time is determined and is divided into a plurality of preselected time intervals. The rate of change of the frequency is determined for each of these time intervals to generate an output signal of a desired shape. The rate of change of the frequency for each time interval is calculated in response to the starting frequency of the sweep, the range of the sweep, the sweep time, and the desired shape of the output signal. The rate of change is supplied to a frequency generator which generates a signal to be supplied to the vibrator.

Abstract: A time variant filter system is employed for removing spurious noise content from a received seismic data signal. The filter passband tracks delayed versions of the pilot signal of a vibrator seismic source. The resulting recorded portion of the received seismic signal is thus restricted to a desired time-frequency band (which is expected to contain the significant mapping information) prior to being recorded. The band is made adjustable in order to allow for optimization with respect to the particular geologic region being prospected.

Abstract: Acoustic energy is propagated through earth material between an electro-acoustic generator and a receiver which converts the received acoustic energy into electrical signals. A closed loop is formed by a variable gain amplifier system connected between the receiver and the generator. The gain of the amplifier system is increased until sustained oscillations are produced in the closed loop. The frequency of the oscillations is measured as an indication of the acoustic propagation velocity through the earth material. The amplifier gain is measured as an indication of the acoustic attenuation through the earth materials. The method is also applicable to the non-destructive testing of structural materials, such as steel, aluminum and concrete.

Abstract: A novel method for electronically determining the gross characteristics of he top layer of the earth's crust at a given location by using a passive seismic transducer as a sensing device for sensing background seismic noise in conjunction with as assembly of generally conventional miniature low power electronic components arranged in a system containing analog and logic circuits with a read-out meter calibrated to represent the detected background noise as seismic resonant frequency. The electronic determination of the crust character is in lieu of various conventional soil refractometry tests which are lengthy time-consuming procedures. The seismic resonant frequency has been found to be representative of the thickness and of the property or character of the crust at that particular geographical location.

Abstract: A seismic signal is created, processed and recorded and then analyzed to determine the nature of subterranean formations.

Abstract: A method and apparatus for use in marine seismic data gathering comprises a vessel (1) is equipped for deploying and retrieving trailer cables and tows a plurality of cables (3) which are provided with at least one array of transmitters (5), which array constitutes a point source for a transmitter, and an acoustic cable, over the region to be investigated. The transmitter cables (3) are caused to become distributed laterally in relation to the tow direction, the distance between them, the firing power and firing timing being controlled from the vessel in accordance with data collected previously regarding the nature of the sea bed. The point source-formed transmitter arrays can be distributed both laterally and longitudinally in relation to the tow direction. The transmitter cables (3) are distributed laterally with the aid of otterboards (8).

Abstract: A receiver array for detecting seismic reflections is composed of geophones of several different natural frequencies divided into subarrays having lengths compatible with the natural frequencies of their geophones and the seismic wavelengths at the recording site. This system provides both frequency and spatial filtering at the sensors so as to enhance the signal-to-noise ratio of broad-band seismic signals.

Abstract: A method for migration of velocity spectra is disclosed wherein migration of common depth point gathers is accomplished without prior determination of the velocity/depth field.

Abstract: In seismic exploration a seismic source transmits seismic energy into the earth and seismic detectors record the seismic waves returning to the earth's surface from reflecting subsurface interfaces. The downgoing transmitted seismic energy and the upcoming reflected seismic waves are transformed into the frequency domain, downward continued, and summed to produce a migrated seismic depth section.

Abstract: A method of seismic prospecting is disclosed in which the seismic source is excited in such a manner as to maximize the use of the energy generated by the seismic source. In certain cases it may be desirable to convert the received seismic signals to their frequency domain counterparts before performing subsequent processing. Such conversion may be performed using the discrete Fourier transform with the result that transformed values are obtained only at certain discrete frequencies. It may further be desirable that processing be performed only at subsets of the total set of discrete frequencies with the values at the remaining frequencies being discarded. In the practice of the present invention, source energy is generated only at those discrete frequencies at which subsequent processing is to be performed. As a result there is substantially no source energy in the transform values at the frequencies which are discarded.

Abstract: A sonic transmitter and receiver are employed to locate and evaluate zones of significant porosity in the proximity of a borehole. Continuous wave and pulse techniques are used to locate the zones and to determine the void dimensions by means of frequency domain signatures. Both through-transmission and reflection methods are employed. The technique is considered especially useful for locating fluid-filled vugular zones.

Abstract: In magnetotelluric exploration, channels of measurements of the earth's electric and magnetic field are digitized. Blocks of digital samples are transformed into frequency domain representations of the power spectra, including amplitude and phase, at selected harmonics. The coherence between the power spectra from different channels is determined and compared to a threshold. The power spectra which have a coherence exceeding the threshold are stacked and recorded on magnetic tape.