GB2133148A

GB2133148A - Vertical seismic profiling

Info

Publication number: GB2133148A
Application number: GB8312719A
Authority: GB
Inventors: Albert Alfred Fitch; Peter Bernard Dillon
Original assignee: Seismograph Service Corp
Current assignee: Seismograph Service Corp
Priority date: 1982-11-30
Filing date: 1983-05-09
Publication date: 1984-07-18
Also published as: FR2536865B1; FR2536865A1; GB8312719D0; NL8304103A

Abstract

A method of obtaining an estimate of the earliest part of a surface recorded seismogram which comprises generating a seismic shot, recording by one or more well geophones signals from said shot which have transmitted through and/or reflected from various strata of the earth, autocorrelating the z- transforms of the upgoing and downgoing waves, and determining the difference between the autocorrelated z-transforms and utilizing said difference as an estimate of the surface seismogram. A plurality of such estimates may be normalized and a central measure taken to give an enhanced estimate. The estimate obtained by the method of the invention may be deconvolved to give a band-limited estimate of the log of accoustic impedance above the top of the velocity and density logs.

Description

SPECIFICATION Vertical seismic profiling This invention relates to methods of obtaining improved seismographic data particularly under circumstances where direct readings can be obtained only with difficulty or not at all.

In obtaining a vertical seismic profile of subsurface strata, information is derived from the signals detected by geophones after the passage through and/or reflection from sub-surface strata boundaries of an acoustic shot-firing.

Conventionally, information is obtained concerning the nature of the downgoing and upgoing wave forms, the log of acoustic impedance, the reflection coefficients, wave velocities etc.

Whilst a considerable degree of precision is associated with events at some depth in the earth, it has not hitherto been possible readily to derive information relating to the vertical seismic profile at shallowest depths.

In fact, because the source may be offset a significant distance with respect to the shallow detectors, the first few members of the reflection series are obscured by refracted arrivals and, perhaps, by noise from the source. The result is that no information is obtained concerning the vertical seismic profile, the acoustic impedance log or the reflection coefficients at these shallow depths. The shallow part of the hole can be redrilled and logged but will generally produce only poor data.

According to a first aspect of the present invention there is provided a method of obtaining an estimate of the earliest part of a surface recorded seismogram which comprises generating a seismic shot, recording by one or more well geophones signals from said shot which has been transmitted through and/or reflected from various strata of the earth, autocorrelating the z-transforms of the upgoing and downgoing waves, and determining the difference between the autocorrelations and utilizing said difference as an estimate of the surface seismogram.

The method is based on a long established theoretical relationship. It is described, and its historical development given by E. A. Robinson and S. Treitel in their paper "The Spectral Function of a Layer System and the Determination of the Waveforms at Depth" published by Geophysical Prospecting, Vol. 25, p.

434, 1977.

If Dn(z), Un(z) are the z-transforms of the downgoing and upgoing waves at level n.

S(z) is the z-transform of the surface seismogram.

4 signifies the autocorrelation. Then the relations are: st Dn(z)bUn(z) is a constant waveform for all levels n scaled by the acoustic impedance at the level n.

D,(z)-4U,(z)=D,(z) where D,(z) is the ztransform of the downgoing wave in the basement below the pile of sediments.

and D(z)-U(z)=S(z) +S(1 /Z)+ 1 In other words, if one takes the difference of the autocorrelation of downgoing and upgoing waveforms, then the right-hand side of this symmetrical waveform is the surface seismogram.

In actually recording surface seismograms it is not possible to detect the earlier part of the reflection sequence because detectors must be placed away from the source, as a result of which the first reflected arrivals are obscured by refracted arrivals and perhaps by noise from the source. No such disadvantage is found with well geophone records, which present the first samples of down- and upgoing waves without any interference. Thus the application of equation (3) yields an estimate of S(z) from the first sample, the coefficient of z, onwards.

In implementing this method, the waveform ID,(Z)-4Un(Z) is determined for each level n. They are preferably normalized to remove the effects of the varying acoustic impedance at the levels n, and a central measure taken to give the best estimate of sbDfU, and hence the best estimate of S.

According to a second aspect of the present invention the estimate of the surface seismogram S obtained by the method defined hereinbefore is deconvolved, e.g. by gapped or spiking deconvolution followed by a filter to give a bandlimited estimate of the log of acoustic impedance above the top of the velocity and density logs.

The deconvolved process is applied in detail as follows: As stated previously, the difference of the autocorrelations of the upgoing and downgoing waveforms yields a symmetrical waveform=1 +S(Z)+S(1/Z) Where S(Z) is the surface seismogram which would be recorded by an ideal surface seismic survey. It can be shown that S(Z) is the upgoing waveform which exists just below the surface of the earth or the sea. It can also be shown that 1+S(Z) is the downgoing waveform which exists just below the surface. Thus a deconvolution operator designed from this downgoing waveform may be derived using methods which are well known. This operator may then be applied to the upgoing waveform for the removal of multiple energy and, after suitable filtering, the required band-limited estimate of the reflection series from the surface down revealed.

Finally, the log of acoustic impedance for the aforementioned shallowest regions can be derived by methods which will be apparent to one skilled in the art.

Claims (Filed on 30/11/83) 1. A method of obtaining an estimate of the earliest part of a surface recorded seismogram which comprises generating a seismic shot, recording by one or more well geophones, signals from said shot which have transmitted through and/or reflected from various strata of the earth,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Vertical seismic profiling This invention relates to methods of obtaining improved seismographic data particularly under circumstances where direct readings can be obtained only with difficulty or not at all. In obtaining a vertical seismic profile of subsurface strata, information is derived from the signals detected by geophones after the passage through and/or reflection from sub-surface strata boundaries of an acoustic shot-firing. Conventionally, information is obtained concerning the nature of the downgoing and upgoing wave forms, the log of acoustic impedance, the reflection coefficients, wave velocities etc. Whilst a considerable degree of precision is associated with events at some depth in the earth, it has not hitherto been possible readily to derive information relating to the vertical seismic profile at shallowest depths. In fact, because the source may be offset a significant distance with respect to the shallow detectors, the first few members of the reflection series are obscured by refracted arrivals and, perhaps, by noise from the source. The result is that no information is obtained concerning the vertical seismic profile, the acoustic impedance log or the reflection coefficients at these shallow depths. The shallow part of the hole can be redrilled and logged but will generally produce only poor data. According to a first aspect of the present invention there is provided a method of obtaining an estimate of the earliest part of a surface recorded seismogram which comprises generating a seismic shot, recording by one or more well geophones signals from said shot which has been transmitted through and/or reflected from various strata of the earth, autocorrelating the z-transforms of the upgoing and downgoing waves, and determining the difference between the autocorrelations and utilizing said difference as an estimate of the surface seismogram. The method is based on a long established theoretical relationship. It is described, and its historical development given by E. A. Robinson and S. Treitel in their paper "The Spectral Function of a Layer System and the Determination of the Waveforms at Depth" published by Geophysical Prospecting, Vol. 25, p. 434, 1977. If Dn(z), Un(z) are the z-transforms of the downgoing and upgoing waves at level n. S(z) is the z-transform of the surface seismogram. 4 signifies the autocorrelation. Then the relations are: st Dn(z)bUn(z) is a constant waveform for all levels n scaled by the acoustic impedance at the level n. D,(z)-4U,(z)=D,(z) where D,(z) is the ztransform of the downgoing wave in the basement below the pile of sediments. and D(z)-U(z)=S(z) +S(1 /Z)+ 1 In other words, if one takes the difference of the autocorrelation of downgoing and upgoing waveforms, then the right-hand side of this symmetrical waveform is the surface seismogram. In actually recording surface seismograms it is not possible to detect the earlier part of the reflection sequence because detectors must be placed away from the source, as a result of which the first reflected arrivals are obscured by refracted arrivals and perhaps by noise from the source. No such disadvantage is found with well geophone records, which present the first samples of down- and upgoing waves without any interference. Thus the application of equation (3) yields an estimate of S(z) from the first sample, the coefficient of z, onwards. In implementing this method, the waveform ID,(Z)-4Un(Z) is determined for each level n. They are preferably normalized to remove the effects of the varying acoustic impedance at the levels n, and a central measure taken to give the best estimate of sbDfU, and hence the best estimate of S. According to a second aspect of the present invention the estimate of the surface seismogram S obtained by the method defined hereinbefore is deconvolved, e.g. by gapped or spiking deconvolution followed by a filter to give a bandlimited estimate of the log of acoustic impedance above the top of the velocity and density logs. The deconvolved process is applied in detail as follows: As stated previously, the difference of the autocorrelations of the upgoing and downgoing waveforms yields a symmetrical waveform=1 +S(Z)+S(1/Z) Where S(Z) is the surface seismogram which would be recorded by an ideal surface seismic survey. It can be shown that S(Z) is the upgoing waveform which exists just below the surface of the earth or the sea. It can also be shown that 1+S(Z) is the downgoing waveform which exists just below the surface. Thus a deconvolution operator designed from this downgoing waveform may be derived using methods which are well known. This operator may then be applied to the upgoing waveform for the removal of multiple energy and, after suitable filtering, the required band-limited estimate of the reflection series from the surface down revealed. Finally, the log of acoustic impedance for the aforementioned shallowest regions can be derived by methods which will be apparent to one skilled in the art. Claims (Filed on 30/11/83)

1. A method of obtaining an estimate of the earliest part of a surface recorded seismogram which comprises generating a seismic shot, recording by one or more well geophones, signals from said shot which have transmitted through and/or reflected from various strata of the earth, autocorrelating the Z-transforms of the upgoing and downgoing waves, and determining the difference between the autocorrelation and utilizing said difference as an estimate of the surface seismogram.

2. A method according to claim 1, wherein a waveform is determined for each level under investigation in the earth, the waveforms are normalized and a central measure taken.

3. A method according to claim 1 or 2, wherein the estmiate of the surface seismogram obtained is deconvolved to give a band-limited estimate of the log of acoustic impedance above the top of the velocity and density logs.

4. A method according to claim 3, wherein gapped or spiking deconvolution is applied to the estimate, followed by application of a filter.

5. A method of obtaining an estimate of the earliest part of a surface recorded seismogram, substantially as hereinbefore described.

6. Processing apparatus adapted (a) to accept an input of signals corresponding to an upgoing and a downgoing wave obtained from a seismic shot-firing, for a particular depth in the earth; (b) to calculate the Z-transforms of the wave forms represented by said signals; (c) to autocorrelate said Z-transforms; (d) to determine the difference between the autocorrelated Z-transforms of said upgoing and downgoing waves; (e) to generate from said difference a seismograph providing information about shallowest depths in the earth.

7. Processing apparatus as claimed in claim 6, adapted to accept inputs of a plurality of signals corresponding to a plurality of upgoing and downgoing waves corresponding to a plurality of particular depths in the earth, to normalize the estimates obtained and to take a central measure to give an enhanced estimate of the surface seismogram.

8. Processing apparatus as claimed in claim 6 or 7, which is adapted additionally to deconvolve the estimate of the surface seismogram obtained, so as to give a band-limited estimate of the log of accoustic impedance above the top of the velocity and density logs.

9. Processing apparatus according to claim 8, which is adapted to apply gapped or spiking deconvoiution to the estimate, and subsequents to apply a filter to the deconvolved estimate.

1 0. Processing apparatus as claimed in any of claims 6 to 9, comprising a computer conditioned to perform the operations defined in any of claims 6 to 9.