SU622021A1 - Seismic prospecting method - Google Patents

Description

(54) METHOD OF SEISMIC EXPLORATION

one

The invention relates to the field of seismic exploration using continuous, quasi-harmonic oscillations modulated in frequency.

The known method of seismic exploration 1, which uses a cyclic change in the frequency of the excitation source to excite oscillations.

The excitation signal reflected on various seismic horizons is recorded by the geophone with a time delay relative to the excitation signal, the larger, the deeper the reflecting horizon is. With a continuous change in the frequency of vibration, this delay leads to the appearance of a difference in the frequencies of the excitation signal and the signal of the reflected wave. The coincidence of frequencies is observed only at the moments of intersection of the ascending and descending branches of the graphs of the frequencies of the excitation signals and the reflected wave. Before correlation processing, these areas are excluded from the record.

There is also known a method of seismic exploration 2, based on the excitation of continuous oscillations by a vibration source.

Reception, amplification, controlled frequency filtering and registration of waves, in which the pre-recorded signal of this geophone passes through a narrow-band tunable filter, the tuning frequency of which continuously changes according to the change in vibration frequency with a delay of twice the seismic reflecting horizon under investigation . Thereby, attenuation of both irregular noise and waves having a time delay other than the delay of the useful wave is achieved.

However, with this method, the field material is recorded in a narrow dynamic range, limited by the capabilities of the recording equipment, and not exceeding 70-80 dB, while the dynamic range of signals near the source reaches 120-140 dB. This limits the applicability of the method in the near zone from the vibrator.

Claims (2)

The controllable filter introduces phase distortions into the filtered signal, the more significant the more efficient the filter is. With subsequent correlation processing, this leads to distortions in the cross-correlation function, i.e., to a decrease in the recording resolution and, as a consequence, to a loss of information. The aim of the invention is to improve the accuracy of registration of useful signals by suppressing wave-to-noise and eliminating distortion introduced by the control filter of the working channel. The goal is achieved by the fact that the controlled frequency filtering is carried out before recording the signals, changing the frequency of tuning the band-delay filters of the working and reference channels synchronously with the frequency of the vibrator. The resulting phase distortions of the working channel are compensated by implementing similar filtering in ka. on the reference signal. The reference signal attenuated by the filtering is restored to its original value by amplification. The essence of the method is as follows. The signal from the working channel seismic receiver is subjected to compulsory notch filtering, in which the filter retention band is constantly tuned to the frequency of the oscillation source by a signal from a sensor, for example, a seismic receiver located on the vibrator operating element. With this filtering, the in-phase vibrator is weakened by a component of the seismic signal. The oscillations associated with the useful waves delayed by the propagation in the medium are not attenuated, since the filter's delay band does not coincide with the instantaneous frequency of the useful signal. The requirement of complete phase identity by the registration channel of the reference and operating signals leads to the fact that the introduction of any filtering, fundamentally introducing phase distortions into one of the correlated signals, entails the need to introduce similar changes into the other correlated signal. Therefore, in the registration channel of the reference signal, filtering is performed similar to that described. The weakening of the reference signal that occurs during filtering is compensated by the subsequent amplification of it. Thus, the phase characteristics of both channels will be the same, thereby eliminating the occurrence of distortion of the correlation function when processing the received information. Receiving information in the near zone from the source is of great practical importance, since in this zone such typical for seismic survey noises as head, exchange and multiple waves are typical. Using this information, it is possible to reliably isolate useful signals and minimize the blending components when processing field material. Claim method A seismic survey method based on the excitation of continuous oscillations by a vibration source, reception, amplification, controlled frequency filtering and recording of reflected seismic waves in the near zone from a vibrator, characterized in that in order to improve the accuracy of recording useful signals by suppressing waves - interference and elimination of distortions introduced by the controlled filter of the working channel; controlled frequency filtering is carried out before recording the signals, changing the frequency of the tuning - bandwidth the delay filters of the working and reference channels synchronously with the frequency of the vibrator Sources of information taken into account in the examination: 1. US patent number 3303458, cl. 340-155, 1974. 2. US Patent No. 3015086, cl. 340-15, 1973.