Method for seismic exploration by vertical seismic profiling and installation for its implementation

Claim 2

A seismic exploration method for determining characteristics of a geological formation by obtaining a plurality of signals (s k ) derived from a respective plurality of different positions within a borehole that traverses said geological formation and by combining said plurality of signals, comprising: a. determining a plurality of propagation times (t k ) required for a seismic signal to propagate between a first detector position and a plurality of other detector positions; b. delaying each of the seismic signals detected at each of said other detector positions by respective propagation times; c. advancing each of the seismic signals detected at each of said other detector positions by respective propagation times; d. combining said delayed signals with the signal detected at said first detector position to produce a first combined signal (y 1 ); e. combining said advanced signals with the signal detected at said first detector position to produce a second combined signal (z 1 ); and f. generating a signal indicative of an optimum estimation of the upgoing wave component at said first detector position by combining said first and second combined signals.

Claim 3

The method as recited in claim 1 further comprising the steps of: a. repeating the steps of claim 1 for a plurality of different first detector positions to produce a plurality of optimum estimations of the upgoing wave components; and b. recording said plurality of optimum upgoing wave component estimation signals in a display which reveals the location of geological acoustic reflectors.

Claim 4

The method as recited in claim 1 wherin said signal indicative of an optimum estimation of the upgoing wave component is determined by taking the inverse Fourier transform of the expression ##EQU15## where Y 1 is the Fourier transform of y 1 which is the combination of the signal detected at the first detector position with delayed signals from the other positions; Z 1 is the Fourier transform of z 1 which is the combination of the signal detected at the first detector position with advanced signals from the other positions; n is the total number of combined signals; and α k and α k are respectively the operators exp(-it k w) and exp(it k w).

Claim 5

The method as recited in claim 1 further comprising the step of generating a signal indicative of an optimum estimation of the downgoing wave component by combining said first and second combined signals.

Claim 6

The method as recited in claim 4 wherein said optimum estimation of the downgoing wave component is determined by taking the inverse Fourier transform of the expression ##EQU16## where Y 1 is the Fourier transform of y 1 which is the combination of the signal detected at the first detector position with delayed signals from the other positions; Z 1 is the Fourier transform of z 1 which is the combination of the signal detected at the first detector position with advanced signals from the other positions; n is the total number of combined signals; and α k and α k are respectively the operators exp(-it k W) and exp(it k w).

Claim 7

The method as recited in claim 5 further comprising the steps of: a. repeating the steps of claim 1 for a plurality of different first detector positions and producing an optimum estimation of the downgoing wave components for each of said plurality of different first detector positions; and b. recording said plurality of optimum downgoing wave component estimation signals in a display which reveals the location of geological acoustic reflectors.

Claim 8

The method according to claim 1, wherein the optimum estimation of the upgoing wave component is determined from the expression: ##EQU17## where y 1 is the combination of the signal detected at the first detector position with delayed signals from the other positions; z 1 is the combination of the signal detected at the first detector position with advanced signals from the other positions; and n is the total number of combined signals.

Claim 9

The method according to claim 4, wherein the optimum estimation of the downgoing wave component is determined from the expression: ##EQU18## where y 1 is the combination of the signal detected at the first detector position with delayed signals from the other positions; z 1 is the combination of the signal detected at the first detector position with advanced signals from the other positions; and n is the total number of combined signals.

Claim 11

The method acording to claim 9, wherein seismic signals of deficient quality are eliminated.

Claim 12

A seismic exploration system for the implementation of the method acording to claim 1, comprising an acoustic wave detection system for deriving acoustic signals from a plurality of different positions in a well, an acoustic wave source, means for recording the signals produced by the detection system, said signals comprising downgoing wave components and upgoing wave components, and means for filtering these signals in order to improve the signal-to-noise ratio, wherein the filtering means comprises: a memory containing the N signals to be processed and controlled to furnish a series of n signals s 1 . . . s k . . . s n corresponding to successive levels x 1 . . . x 1 . . . s n ; a memory containing the propagation times t k of the acoustic waves from the level x 1 to each level x k ; a first shift circuit for advancing the signals s k respectively by the time t k in relation to the signal s 1 ; a first adder furnishing the sum z 1 of the signal s 1 and of the signals s k thus advanced; s second shift circuit for delaying the signals s k respectively by the time t k in relation to the signal s 1 ; a second adder furnishing the sum y 1 of the signal s 1 and of the signals s k thus delayed; computation means for determining from the signals y 1 and z 1 a signal u 1 * defined as an optimum estimation of the upgoing wave component for all the signals s 1 . . . s n ; and recording means connected to the computation means.

Claim 13

The system according to claim 11, wherein the computation means comprise devices connected to the adders and furnishing the Fourier transforms Y 1 and Z 1 of y 1 and z 1 , a computer furnishing the parameter U 1 * by the relationship ##EQU19## where α k and α k are respectively the operators exp(-it k w) and exp(it k w), and a device furnishing the inverse Fourier transform and delivering the signal u 1 * from U 1 *.

Claim 14

The system according to claim 12, wherein the computer also generates a signal D 1 * according to the relation ##EQU20## this signal being applied to the device furnishing the inverse Fourier transform which delivers a signal d 1 *, the optimum estimation of the downgoing wave component for all the plots s 1 . . . s k . . . s n .

Claim 15

The system according to claim 11, wherein the computation means comprises a shift device furnishing shifted signals z 1 (t-2t k ) from the signal z 1 (t), an adder furnishing the sum of the signals z 1 (t) and z 1 (t-2t k ), a circuit dividing this sum by n 2 , a circuit dividing the signal y 1 by n and a subcontractor related to the two dividing circuits and furnishing an approximate signal u 1 *.

Claim 16

The system according to claim 14, wherein the computation means further comprises a shift device furnishing shifted signals y 1 (t+2t k ) from the signal y 1 (t), an adder furnishing the sum of the signals y 1 (t) and t 1 (t+2t k ), a circuit dividing this sum by n 2 , a circuit dividing the signal z 1 by n and a subcontractor connected to the two dividing circuits and furnishing a signal d 1 * representing approximately an optimum estimation of the downgoing component for all the signals s 1 . . . s k . . . s n .