FR2536865A1 - METHOD AND APPARATUS FOR OBTAINING A VERTICAL SEISMIC PROFILE - Google Patents

Abstract

PROCESS FOR OBTAINING AN EVALUATION OF THE FIRST PART OF A RECORDED SURFACE SISMOGRAM, WHICH CONSISTS OF GENERATING A SEISMIC SHOT, RECORDING BY ONE OR MORE WELL GEOPHONES THE SIGNALS FROM THIS SHOT WHICH WERE TRANSMITTED THROUGH THE VARIOUS EARTH LAYERS ARE REFLECTED BY THEM, PERFORM THE AUTOCORRELATION OF THE Z-TRANSFORMATIONS OF THE UPWARD AND DOWNWARD WAVES, AND DETERMINE THE DIFFERENCE BETWEEN THE AUTOCORRELATION AND USE THIS DIFFERENCE AS AN EVALUATION OF SURFACE SISMACEOGRAM. A PLURALITY OF THESE EVALUATIONS MAY BE HOMOGENIZED AND A CENTRAL MEASUREMENT CARRIED OUT TO PROVIDE AN IMPROVED EVALUATION. THE RATING OBTAINED BY THE PROCESS OF THE INVENTION MAY DECONVOLUTION TO GIVE A LIMITED-BAND RATING OF THE REGISTERED ACOUSTIC IMPEDANCE SIGNAL ABOVE THE TOP OF THE REGISTERED SPEED AND DENSITY SIGNALS.

Description

The present invention relates to methods of

  dice for obtaining better seismographic data, in particular in conditions where direct readings can be obtained only with difficulty or not at all In obtaining a vertical seismic profile of layers of the subsoil, the information comes from signals detected by geophones after passing through the limits of the layers of the subsoil, and or the reflection by

  these, from an acoustic shot Usually an informa- :.

  tion is obtained regarding the nature of the waveforms -

  top and bottom, the impedance measurement pushes

  tick, reflection coefficients, wave velocities, etc. Although a high degree of precision is associated with phenomena at a certain depth in the ground, it has so far been impossible to obtain information relating to the vertical seismic profile at depths

weaker dors.

  In fact, since the source can be offset by a significant distance compared to the shallow detectors, the first few elements of the Qn-reflexion series are obscured by refracted arrivals and, perhaps, by

  source noise The result is that no information

  tion is not obtained concerning the seismic profile, the acoustic impedance curve or the reflection coefficients; at these shallow depths The shallow part of the well

  can be reformed and controlled, but it generally provides-

  only poor data.

  According to a first aspect of the present invention, this provides a method for obtaining an evaluation of the first part of a recorded surface seismogram, which comprises the emission of a seismic shot, the recording by a or more signal well geophones of this

  shot that were passed through the various layers of -

  the earth and / or reflected by them, the self-

  lation of the z-transforms of the rising waves and

  and the determination of the difference between the autocorrelations and the use of this difference as an evaluation of the surface seismogram. The process is based on a theoretical relationship

  long established It is described and its development

  development is given by E A Robinson and S Treitel in

  their document "The spectral function of a coupling system

  ches and determining deep waveforms "

  published by Geophysical Prospecting, Vol 25, p 434.1977.

  If Dn (z), Un (z) are the z-transforms of the descending and ascending waves at level-n, S (z) is the z-transform of the surface seismogram, O means autocorrelation, then the relations are 0 Dn (z) Un (z) is a constant waveform for all levels N graduated by the acoustic impedance at level n,

  0 Dn (Z) -Un (z) = O D ((z), o Da (z) is the transform-

  mé in z of the downward wave, in the base under the pile of sediments,

and O Dn (Z) -O Un (z) = S (z) + S (1 / Z) + 1.

  In other words, if we take the difference from the other

  tocorrelation of falling and rising waveforms

  tes, then the right side of this symmetrical waveform

is the surface seismogram.

  In the actual recording of surface seismograms, it is not possible to detect the first

  part of the reflection sequence because the detectors must

  can be placed far from the source, so that

  thoughtful first arrivals are obscured by ar-

  refracted rivets and perhaps by noise from the source.

  No similar disadvantage is observed with the recordings.

  well geophone records, which present the pre-

  better samples of the falling and rising waves without any interference Thus, the application of equation (3)

  provides an evaluation of S (z) from the first sample

  tillon, coefficient'de z, and the following -

  In the implementation of this method, the waveform O Dn (z) -0 Un (z) is determined for each level nn n They are preferably homogenized to eliminate the effects of the variable acoustic impedance at the levels n , and a central measurement is made to give the

  best evaluation of OD-0 U, and therefore the best evaluation

tion of S

  According to a second aspect of the present invention

  tion, the evaluation of the surface seismogram S obtained by the process defined above undergoes a deconvolution, for example by 'deconvolution at intervals or peaks, followed by a filter, to give a limited band evaluation of the recorded signal of impedance acoustics above

  peak of recorded speed and density signals.

  The deconvolution process is applied in

tail as follows:

  As already indicated, the difference in self-

  rising and falling waveform relationships provides a symmetrical waveform: 1 + S (Z) + S (1 / Z), where S (Z) is the surface seismogram that would be recorded by a surface seismic measurement ideal We can show that S (Z) is the rising waveform that exists just below the surface of the earth or the sea We can also show that 1 + S (Z) is

  the falling waveform that exists just below the surface

  this So a deconvolution operator designed from

  this falling waveform can be obtained by pro-

  which are well known This operator can then be applied to the rising waveform for the elimination of

  multiple energy and, after appropriate filtering, assess it

  limited band tion sought reflection series at

from the surface is obtained -

  Finally, the acoustic impedance curve for the shallow regions mentioned above can be deduced, by methods which will appear to those skilled in the art.

* 4 2536865

  The invention includes a processing apparatus,

  characterized in that it is intended to: (a) accept an input of signals corresponding to an ascending and descending wave obtained by means of a seismic blast, for a particular depth in the earth;

  (b) calculate the Z transforms of the waveforms represented

  sensed by said signals; (c) autocorrelate said transforms into Z; (d) determine the difference between the autocorrelated Z transforms of the rising and falling waves; (e) generate from this difference a seismogram providing information on the shallow depths in

  earth This device can accept inputs from a variety of

  lity of signals corresponding to a plurality of ascending waves

  dantes and descendants corresponding to a plurality of pro-

  special founders in the earth; it can standardize the evaluations obtained and perform a central measurement,

  to give an improved assessment of the surface seismogram

this.

  The device can also carry out the deconvolu-

  tion of the evaluation of the surface seismogram obtained, of

  so as to give a limited band evaluation of the signal received

  acoustic impedance above the top of the signals

  recorded speed and density It can also apply

  quer a deconvolution at intervals or peaks at the evaluation

  tion, and then apply a filter to the assessment that has undergone

deconvolution.

  Preferably, the following treatment apparatus 1 '

  invention includes a computer designed to 'perform the o-

operations defined above.

2536865

Claims (7)

Claims   1 Method for obtaining an evaluation of the first part of a recorded surface seismogram, which comprises the emission of a seismic shot - the recording by one or more well geophones of the signals of this shot which have been transmitted through the various layers of the earth and / or reflected by them, the autocorrelation of the transforms   rising and falling waves in Z, and determining it   tion of the difference between autocorrelations and utility   reading of this difference as an assessment-of the seismo- gram of surface.   2 Method according to claim 1, characterized in that a waveform is determined for each level under examination in the earth, the waveforms are homogenized   and a central measurement is performed.   3 Method according to claim 1 or 2, character-   se in that the evaluation of the surface seismogram obtained undergoes deconvolution to give a band evaluation   limited of recorded acoustic impedance signal above   above the top of the recorded speed and power signals site.   4 Method according to claim 3, characterized in   what an interval or spike deconvolution is   applied to the evaluation, followed by the application of a thread-   be. 5 Processing apparatus, characterized in that it is designed to (a) accept an input of signals corresponding to an ascending and descending wave obtained by means of a seismic shot, for a particular depth in the earth;   (b) calculate the Z transforms of the waveforms   presented by said signals; (c) autocorrelate said transforms into Z; (d) determine the difference between the autocorrelated Z transforms of the rising and falling waves;   (e) generate from this difference a seismogram-   providing me with information on shallow depths in the earth, 6 A processing apparatus according to claim 5, characterized in that it can accept inputs of a plurality of signals corresponding to a plurality of ascending and descending waves corresponding to a plurality of particular depths in the earth, homogenize the assessments obtained and carry out a central measurement,   to give an improved assessment of the sur- seismogram face.   7 treatment apparatus according to claim 5 or 6, characterized in that it can further perform the   deconvolution of the evaluation of the surface seismogram ob-   held, so as to give a limited band assessment of the   recorded signal of acoustic impedance above the   puts recorded speed and density signals.   8 Treatment apparatus according to claim 7, characterized in that it can apply a deconvolution to   intervals or peaks at assessment, and apply in-   following an evaluation filter that has undergone deconvolution.   9 Processing apparatus according to any one of   Claims 5 to 8, characterized in that it comprises a   calculator designed to perform the defined operations   in any one of claims 5 to 8.