SU1539700A1 - Method of bibroseismic prospecting - Google Patents

Abstract

The invention relates to a vibroseis scan over a multiple overlap system and can be used to search for oil and gas fields. The purpose of the invention is to improve the adaptability of field work and reduce the cost of vibroseismic exploration. For this, at each point of excitation only one of the linear-frequency-modulated signals is excited, and the parameters of the signals are chosen in accordance with the condition that their total spectrum represents a function inverse to the spectrum of the target waves during wideband excitation.

Description

The invention relates to vibroseismic exploration through a multiple overlap system and can be used to search for oil and gas fields.

The purpose of the invention is to improve manufacturability and reduce the cost of work.

The method of vibroseismic intelligence is implemented in the following sequence of operations.

A wideband linear-frequency-modulated (chirp) signal is generated and recorded in the studied area, whose maximum frequency of sweeps is at least 80-100 Hz. By spectral analysis of correlograms obtained using wideband signals, the spectrum of the target waves H (f) is determined. A combination of vibroseismic sweeps is selected, providing the best compensation for the frequency distortion of the signal in the vibrator-soil system, recording equipment and in the geological environment. This condition is fulfilled if the amplitude spectrum of the sum of the autocorrelation functions of the vibration seismic signals used to excite oscillations included in the sum of the records summed over the common depth point (GBS) coincides with a sufficient degree of accuracy with the function 1 / H (f ). In the method, the autocorrelation function of the probing signal of the probing signal is synthesized by summing the autocorrelation functions of the p vibroseis signals, and

WITH

with

each point of the excitation, i.e. is defined by the expression

H- |, mu5

where R () is the main maximum of the ACF i-ro control signal.

The amplitude spectrum of the sum function R is computed as a module of the forward JQ Fourier transform, whose analytical expression can be written as follows

I I1 (tR. (C e- ЈCdt | --1 / H (f),, 5

 L i

where O. and U are the time shift values of 2, which limit the region of the main maxima of the autocorrelation functions and are approximately equal, respectively, to +100 ms.

When using scans of equal duration, the amplitude spectra of the control vibroseis signals are characterized by the same area. A simple algorithm for determining the parameters of vibration seismic signals, at which this requirement is fulfilled, is as follows.

The area nQ under the 1 / H (f) curve is determined in the frequency range fn - fk, where f, - fK is the frequency range of the widest sweep itself (f, -fk - is considered as a priori information and is selected according to specific seismological conditions Solvable geological problems, the final frequencies fk for all the sweeps are assumed to be the same).

The height h 1 P / (lower rectangle approximating the amplitude spectrum of the FAA of the widest-band signal is calculated.

By enumerating the values of height Qb / (fk - f7) of the next box from the conditions | Q 1 - Qz | -, min, the initial boundary frequency of the second sweep fg is determined, where Q 1 and Q i are the areas of curvilinear triangles.

By enumerating le Q / (f, - Ј3) from the condition | Q3 - the left cutoff frequency of the next sweep f3, etc. is determined.

Thus, the considered algorithm provides a search for the initial cut-off frequency of the sweeps, at which the spectrum of the sum of the FAA of the used signals approximates the function 1 / H (f) by the minimum of the algebraic sum of the approximation error. The selected sweeps are alternately processed as the vibrators move from one drive point to another.

Claims (1)

Invention Formula A method of vibroseis reconnaissance, which includes excitation at n excitation points with different frequency ranges of sweeps, linear-frequency-modulated signals, recording over the system of multiple overlaps and correlating them with control vibroseis signals and then summing manufacturability and reducing the cost of work, each of the excitation points excites one of the linear-frequency-modulated signals, and the parameters of the signals are chosen in accordance with the condition 2 w I (Ј Rj ()) e irJfCdcUl / H (f),  p i-1 M where R, (Ј) are the main maxima of the autocorrelation functions of the linear-frequency-modulated signals, H (f) is the spectrum of the target waves, determined with the aid of a highband signal, f is the oscillation frequency, Both and Dj are the time shift values C, limiting the region of the main maxima of the autocorrelation functions.