SU1679429A1 - Method of vibration seismic prospecting - Google Patents

Abstract

The invention relates to geophysical prospecting and can be used for prospecting and exploration of mineral deposits by a vibroseismic method, as well as for vibrating the screening of the Earth. The purpose of the invention is to increase labor productivity and increase the resolution of vibroseismic prospecting. The method is based on the excitation of one or several non-linear frequency-modulated or several linear-frequency-modulated vibro-seismic signals with specially defined parameters for recording and correlation processing of vibrograms. The frequency sweep functions of one or more combinable signals are determined on the basis of the pre-sending of a given frequency-modulated signal, the registration of the vibrograms, and calculations based on the proposed dependency. This allows, with vibration effects with the frequency sweep function obtained in this way, to obtain a seismic recording of the optimal resolution and to eliminate the search for parameters of the vibration signals during experimental work. 2 hp f-ly. J

Description

The invention relates to geophysical prospecting and can be used for prospecting and exploration of oil, gas, and ore deposits by vibroseismic methods, as well as for vibrating the transmission of the Earth.

The purpose of the invention is to increase productivity and increase resolution,

The method is carried out as follows.

A priori frequency sweep function and amplitude modulation function are specified. The frequency sweep functions at a constant amplitude correspond to the spectrum of the correlation pulse I N (co) I or I Ni (f) I2. The amplitude modulation is defined by the function A (o) or Ai (t).

A vibrator with a mode of operation specified in this way excites oscillations that are registered, subjected to correlation processing, as a result of which

Os

4 o

YU

Yu

receive a correlation pulse on the correlum frame K (t),

Using the two selected correlograms, the autocorrelation function (FAA) and cross correlation (EF K) are calculated, and then the signal-to-noise ratio is determined and its value is used to select the sweep signal parameters.

The FAK spectrum of a linear frequency-modulated (LFM) signal approximately has the shape of a rectangle whose height is equal to the square of the amplitude of the sweep signal divided by the frequency sweep rate f (t) df (t) / dt, where f (t) is a function ( law) frequency sweep.

The spectrum of the registered correlating frame due to the different attenuation of the different-frequency components of the vibroseis signal is different from the rectangle.

The maximum resolution of the reflection coefficients on the correlogram is when the spectrum of the correlation pulse, which is determined by the ACF of the correlogram with a sufficient degree of accuracy, is rectangular, i.e. the widest in the given frequency band and aligned, provided that the signal exceeds the interference by a specified number of times. The correlation pulse spectrum can be aligned (in the original frequency band) using inverse filtering, and ideally a rectangular spectrum should be obtained. But the rectangular spectrum is also in the original correlation pulse (swept signal ACF), so this reverse filter is both a compression filter, whitening the spectrum in a given frequency band, and a shaping filter.

The compression filter - Hc (ftJ) (in the frequency domain) compensates for the change in the shape of the spectrum of the correlation pulse, which has the original rectangular spectrum, and brings it almost to its original form. This alignment, carried out during processing, can be done in another way, sending such a sweep signal to the medium, which, as a result of propagation in the medium, registration, etc. gives as a result a correlation pulse with a rectangular spectrum. Since Hc (G)) aligns the spectrum of the correlation pulse, the vibration signal sent to the medium must have a spectrum equal to

VlHc (o) t. Obviously

Hc (") V | OAKK (t) | / (FAK K (t) + R2), (1)

where FAK K (t) is the spectrum of FAK of the registered correlogram K (t);

R is the regulating term determined by a known method taking into account the signal-to-noise ratio, estimated from the cross-correlation function of two paths.

Thus, the task of obtaining a correlogram with the highest resolution in a given frequency band is reduced to the formation of a sweep signal with a frequency sweep function such that the sweep signal has a spectrum equal to

15

At | NCI.

The solution of problems of determining the mode of operation of a vibrator from a given spectrum is known, and

IS (bo)

2 years A (o)) 2 STiT

(2)

where iS (ftji) I is the correlation pulse spectrum;

A (SU) is the amplitude modulation of the signal; ft (t) is the frequency sweep rate of the signal whose spectrum is S (tw);

w (t) 2 l f (t), and for the chirp signal f (t) const (f maxImin) / T and A (w) const C.

In this case, the desired, specified spectrum should be the spectrum of the inverse compression filter Hc (w).

Therefore, using the correlogram, the inverse filter Hc (o) (or Hc (m)) is calculated using formula (1).

Substituting Hc (co) to the left side of formula (2) instead of IS (o) I2, we find the frequency sweep function f (t) (U (t) / 2 l of the desired vibration signal. To do this, we set A priori considerations A (co) ( for example, A (w) C) and integrate (2), meaning that for a monotone function f (t) there exists an inverse of it t (f), so f (tH / tr (f):

(lHic (f) ldt. O)

where fMHH is the initial sweep frequency;

Hic (f) is a function of Hc (w) at the frequency scale f.

The frequency sweep f (t) is reversed by the function t (f), varying from 0 to T as f changes in the limits

fmihrfMaKC.

A vibration signal with a frequency sweep f (t) and amplitude C excited by a vibrator, etc., results in a correlation pulse on the correlogram with a spectrum aligned. With him and held

The second production series of vibration effects.

If a nonlinear frequency-modulated signal is excited in the first series, then the He (m) filter, calculated according to (1) from the results of sending such an LFM signal, will simultaneously compensate for the distorting effect of the medium and the unevenness of the spectrum of the original vibration signal. The latter should not be done from considerations of expediency if further we calculate f (t) by (3). When used in the first series of actions, the LFM signal according to (1) calculates Hc (a)). If lN (ft) | 2 is the spectrum of the correlation pulse of this LFM signalses, then we get the NA (PG) S) l I M (s) G (4)

This equality follows from the fact that in the spectral region the inverse filter is the inverse of the spectrum:

Hc (w) is the reciprocal of the frequency response of the medium;

Hc (co) is the inverse of the product of the frequency response of the medium and the spectrum of the correlation pulse, which in this case is equal to the autocorrelation function of the cybroseismic LFM signal | M (o) | 2.

The signal amplitude is constant, i.e. A (w) c. Obviously, asking for some a priori considerations A (o)) in the first series and saving A (w) in the second series of actions, as well as substituting (4) in (3), receive

t (f) H4. {five)

4 fMMHA2, (f)

u

where Ai (f) is an analogue of А (ш) on the scale of frequencies f; Ni (f) - corresponds to N (w), Hic (f) -Hc ((O).

Frequency sweep and amplitude modulation control is possible on vibrators equipped with special control units.

For vibrators that work out only chirp signals, the reverse filter is approximated by a step broken line, which corresponds to several (by the number of steps) combined sweep-sig nals.

The effectiveness of the proposed method is associated with an increase in resolution due to the expansion of the spectrum of excited vibrational vibrations.

Claims (3)

1. A vibration seismic survey method based on the excitation of a series of vibration effects with a priori given law of frequency and amplitude change, recording, accumulation and correlation processing of seismic vibrations, characterized in that subsequent vibration effects use the law of frequency sweep, which is inversely related to the expression t ((f) fdt, V GninA2, (f) where Ni (f) is the spectrum of a priori given effect; Ai (f) is a function of amplitude modulation of a priori given effect; H1C (f) reverse compression filter spectrum for the resulting correlogram; Cmin - the minimum value of the frequency f of the frequency range of the excited oscillations. 2. A method according to claim 1, characterized in that the excitation is performed by linear frequency modulated signals with matching frequency bands, the integrand function is approximated by a step broken line, each step of which corresponds to a single signal. 3. The method according to claim 1, characterized in that the excitation is produced by non-linear-frequency-modulated signals with connecting bands, and the integrand is approximated by connecting pieces of a curve.