RU2169382C1 - Method of multilevel vibration-seismic prospecting - Google Patents

Abstract

FIELD: seismology, prospecting for mineral resources. SUBSTANCE: seismic vibrations are excited by vibration source and are recorded. Levels of depth and time windows corresponding to them are brought out on vibration records. Values of characteristics of frequency-dependent damping and dispersion of velocities are selected per each level. Characteristics of filters used to filter emitted signal are computed by them. Test correlograms are computed and parameters of optimum filters per each level are determined. Vibration record with reference signal is correlated finally after its preliminary processing by filters optimum for each level of depth. EFFECT: increased quality and informativity of seismic material. 5 dwg

Description

 The invention relates to vibroseismic exploration and is intended to improve the quality and information content of seismic material.

 There is a known method of seismic exploration in accordance with which the vibrator function is passed through a filter that is continuously tuned to the instantaneous frequency of the vibrator after it is multiplied with registered vibrations and filtered on a band-pass filter, the frequency response of which is moved along the frequency axis by an amount proportional to the deviation of the frequency of the vibrator from the vibrator its average value [1].

 The disadvantage of this method is that it takes into account only distortion of vibration signals that occur in the vibrator-soil system, as well as the distorting effect of spurious harmonics. Distortions of the vibrational signal in the medium associated with frequency-dependent attenuation and velocity dispersion are not taken into account. This reduces the effectiveness of the method. In addition, it requires the manufacture of sophisticated equipment based on a two-three-track tape recorder of a special type, multiplying devices and delay lines.

 A known method of generating seismic data using a seismic vibrator and its variants. According to this method, the first cascaded scan sequence is formed containing n identical segments, and the second same sequence with an additional segment that is located and phased so as to suppress harmonic interference during correlation [2].

 The disadvantage of this method is that, while suppressing correlogram distortions associated with harmonic noise in the calculation of the correlation integral, it does not take into account the negative effect on the correlation results of sweep distortions in the medium. This reduces the efficiency of the allocation of pulsed signals against interference. In addition, the method is characterized by complex technology and high complexity, requiring the use of multiple scans when working out one physical point.

 Closest to the invention in technical essence is a method of vibroseismic exploration and a device for its implementation. In accordance with this method, the reference signal used in correlation is further processed by forming the first, second, etc., based on it. harmonics that sum with the original reference signal and the sum of these signals correlate with vibrograms. In this case, the information of harmonics is determined by their weight coefficients, as a result of which enumeration information on the studied geological environment is selected [3].

 The disadvantage of this method is that it does not take into account the manifestation of the effects of frequency-dependent attenuation and the dependence of the propagation velocity of the harmonics of the signals on the frequency. As a result, the premises of the vibrational seismic survey method are violated, based on the notion that the recorded signal is similar in shape to the radiated one. Due to the frequency-dependent attenuation and velocity dispersion, the recorded and emitted signals are significantly different, which leads to a decrease in the geological efficiency of vibroseismic exploration.

 The technical problem solved by the invention is to improve the quality and information content of seismic material by providing optimal conditions for the seismic signals for various levels of depth.

The problem is solved as follows:
In a multi-level vibroseismic exploration method, including excitation of seismic vibrations by a vibration source, their registration, processing and correlation of the reference signal with the vibrogram, the depth levels and the corresponding time windows on the vibrograms to be studied are selected, the frequency-dependent attenuation and velocity dispersion characteristics are selected for each level, they calculate the characteristics of filters describing the distortion of seismic signals in the medium, filter the sig emitted by the vibrator with these filters al, after which the calculated tentative correlogram and determining filter parameters optimal for each of the levels of the maximum intensity of the waves to be researched and then carried vybrogrammy final correlation with the reference signal after pre-processing optimum for each level of depth filters.

 Significant differences of the invention are as follows.

 On vibration programs, the depth levels and the corresponding time windows are selected for study. Depth levels are selected on the basis of the tasks in accordance with the objectives of the study, which reduces the cost of labor and computer time for calculating correlograms.

 For each of the levels, the values of the characteristics of the frequency-dependent attenuation and velocity dispersion are selected, the characteristics of the filters describing the distortion of seismic signals in the medium are calculated from them, the signal emitted by the vibrator is filtered with these filters, then the test correlograms are calculated and the filter parameters are determined that are optimal for each of the levels , according to the maximum intensity of the waves to be studied. This allows you to determine the characteristics of the environment for all the depth levels to be studied, which are necessary for calculating the filter parameters used to precede the correlation of the processing of sweep signals.

 The final correlation of the vibrogram with the reference signal is carried out after its preliminary processing by filters that are optimal for each depth level, which allows one to obtain the maximum amplitude of the cross-correlation function for the waves to be studied in all depth intervals. This ensures high quality and informativeness of seismic material for all studied depth levels.

 From the studied scientific, technical and patent literature, the author does not know the technical solution with the listed set of distinctive features, this gives reason to conclude that the claimed object meets the criteria of the invention.

 In FIG. Figures 1 and 2 give examples of distortions in the shape and spectrum of the reflected reference (sweep) signal: a) the original reference signal, b) the amplitude spectrum of the original reference signal, c) the reference signal reflected from the boundary, d) the amplitude spectrum of the reflected signal.

 In FIG. 3 and 4 are examples of changes in the reflected signal in correlograms.

 In FIG. 5, the relative intensity of the signal with the attenuation decrement in the medium δ = 0.02 (a) and 0.05 (b).

Здесь A(t) - амплитуда сигнала; f_н, f_к - начальная и конечная частота; ΔF - полоса частот; T - длительность сигнала; Φ(t) - функция, определяющая фазу сигнала.The rationale for the implementation of the described method is based on the following theoretical provisions. In vibroseismic exploration, the excitation of elastic vibrations is carried out using a vibrator that sends reference signals (in the foreign literature, sweep) during the time interval T to the medium. The most common in practice are linear frequency-modulated signals. In general, they are written as
S (t) = A (t) cos {2π [f _n + (f _to -f _n ) t / 2T] t + Φ (t)}, (1)
The spectrum of this signal is determined by the direct Fourier transform

Here A (t) is the signal amplitude; f _n, f _k - initial and final frequency; ΔF is the frequency band; T is the signal duration; Φ (t) is a function that determines the phase of the signal.

где F_BK(t) - функция взаимной корреляции (ФBK); S_Σ(t) - результирующая запись - виброграмма.A mandatory procedure for processing records obtained using vibration sources (vibrograms) is to calculate the cross-correlation function of the vibrogram and the reference signal. The result is records - correlograms similar to those from pulsed sources. The cross-correlation function takes its maximum value when the shift of the vibrogram is equal to the travel time of one of the waves, i.e. when the oscillations of one of the waves contained in the field recording coincide in time with the oscillations of the sweep signal. The cross-correlation function is calculated according to the formula:

where F _BK (t) is the cross-correlation function (ФBK); S _Σ (t) - resulting record - vibrogram.

где ψ(l,f) - спектральная характеристика среды; ψ_и(f) - спектр импульса при 1 = 0.The practical use of the above formulas is based on the assumption that the shape of the reference signal is constant for all studied medium interfaces and all section intervals. Therefore, when calculating correlograms, the reference signal that is invariable for all time intervals is used. However, signals propagating in real environments experience significant distortion. In general terms, a pulse after passing a path of length l is represented by the following Fourier integral:

where ψ (l, f) is the spectral characteristic of the medium; ψ _and (f) is the momentum spectrum at 1 = 0.

Здесь δ декремент поглощения.The spectral characteristic of the medium can be written as
ψ (l, f) = Φ (l, f) e ^{iX (l, f)} . (5)
A. G. Averbukh [4] showed that in media with frequency-dependent damping and the accompanying velocity dispersion, the function ψ (l, f) is determined by the following relation

Here δ is the absorption decrement.

Формула (7) позволяет количественно оценить искажения формы опорного сигнала после прохождения им некоторого пути 1 в разрезе. Эти искажения увеличиваются с ростом увеличения пути пробега волны. Поэтому использование в формуле (4) исходной формы опорного сигнала вместо истинной искаженной (см. формулу 7) ведет к снижению эффективности вибросейсморазведки, особенно для глубоких горизонтов.Substituting (2) and (6) in (4), we obtain

Formula (7) allows you to quantify the distortion of the shape of the reference signal after it passes a certain path 1 in the context. These distortions increase with increasing increase in the wave path. Therefore, the use of the initial form of the reference signal in formula (4) instead of the true distorted one (see formula 7) leads to a decrease in the efficiency of vibroseismic exploration, especially for deep horizons.

To ensure optimal conditions for the selection of signals for all depth levels and, consequently, to increase the geological efficiency of vibroseismic exploration, it is proposed to calculate the reference signal in calculating correlograms in accordance with distortions determined by the frequency-dependent properties of the medium and velocity dispersion. It is proposed to determine the environmental characteristics necessary for the optimal change in the reference signal by selecting δ V (f ₀ ) in formula (3) according to the criterion for the maximum intensity of reflections on test correlograms calculated for different levels.

 The effectiveness of the proposed method is proved by the results of seismic-geological modeling of the section with frequency-dependent attenuation. A five-layer seismic-geological model of the section with horizontal interfaces and homogeneous layers was analyzed. Reflection coefficients for all boundaries were chosen equal and amounted to 0.5. The depths of the interfaces corresponding to the midpoints of the four selected depth levels were 1 km, 2 km, 3 km, and 4 km. The first level corresponded to a depth level of 0.5 km - 1.5 km, the second 1.5 km - 2.5 km, etc. The attenuation decrement in the medium ranged from 0.02 to 0.05, which is typical for sedimentary rocks [4]. The reference LFM signal with a duration of 1 s, an initial frequency of 10 Hz, and a final frequency of 50 Hz was chosen as the initial one. In FIG. 1 and 2, examples of distortion of the shape and amplitude spectrum of the original reference signal after its reflection from the boundary located at a depth of 1 km at values of δ = 0.02 and δ = 0.05, respectively, are given. Comparing the waveform before and after the passage of the medium, significant distortions can be noted, especially in the tail, which are manifested in a significant attenuation of the intensity of the high-frequency components of the signal, a shift of the same extremes in time, especially noticeable in FIG. 2. The consequence of these distortions is a decrease in the efficiency of vibroseismic exploration, which is shown by the example of a change in the maximum amplitudes of the reflections in the correlograms (Fig. 3.4). As follows from the analysis of FIG. 3 and FIG. 4 neglecting distortions in the shape of the reference signal in the medium when calculating correlograms leads to a significant decrease in the intensity of the extracted seismic signals, especially for deep-seated levels. So, with increasing depth from 1 km to 4 km at δ = 0.02, the amplitude of the main maximum decreased 1.55 times (see Fig. 3), and at δ = 0.05 this decrease was 6, 5 times (Fig. 4). In the case of taking into account the effect of frequency-dependent attenuation and velocity dispersion during correlation, a significant increase in the intensity of the reflected seismic signals in the correlograms is observed (Fig. 5). The relative gain in intensity varies from 1.1 for a border with a depth of 1 km to 11 for a border with a depth of 4 km, which significantly increases the quality and information content of seismic material by increasing the signal-to-noise ratio.

Literature
1. A.S. USSR N 185503, publ. 10/09/67, BI N 21 - analogue.

 2. RF patent N2126983, publ. 02/27/99, BI N 6 - analogue.

 3. RF patent N2143713, publ. 12/27/99, BI N 36 - prototype.

 4. Averbukh A.G. Study of the composition and properties of rocks during seismic exploration. - M .: Nedra, 1982. - 232 p.

Claims (1)

 A method of multi-level vibroseismic exploration, including excitation of seismic vibrations by a vibration source, their registration, processing and correlation of the reference signal with the vibrogram, characterized in that the depth levels and the corresponding time windows on the vibrograms to be studied are selected, the values of frequency-dependent attenuation characteristics are selected for each level, and velocity dispersion, they calculate the characteristics of filters describing the distortion of seismic signals in the medium, filtered with these filters emitted signal by the vibrator, after which test correlograms are calculated and filter parameters are determined that are optimal for each level from the maximum intensity of the waves to be emitted, then the vibrogram is finally correlated with the reference signal after preliminary processing by filters that are optimal for each depth level.

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