SU1695242A1 - Seismic prospecting method - Google Patents

Description

one

(21) 4649687/25 (22) 13.02.89 (46) 30.11.91. Bul №44

(71) Volga branch of the Institute of Geology and the development of combustible minerals

(72) V.N. Andreev, V.D. Karpov, N.M. Yakovlev, H, R. Sivkov, Yu.V. Fourkalyuk and V.N. Yakovlev

(53) 550.834 (088.8)

(56) USSR Copyright Certificate No. 1038895, cl. G 01 V 1 / 00,1982.

USSR Copyright Certificate № 1305615, cl. G 01 V1 / 00, 1985.

(54) METHOD OF SEISMIC EXPLORATION

(57) The invention relates to seismic exploration and can be used in prospecting and exploration of minerals. The aim of the invention is to increase the efficiency due to the unification of field work technology. The goal is achieved by the fact that the excitation of oscillations is performed by surface low-frequency and borehole high-frequency electrospark sources, the boundaries of the frequency intervals during registration are chosen from the amplitude spectra of forward waves, they accumulate signals at selected intervals of frequencies from single effects of sources, with the number 4 of accumulations in each part At a separate interval, it is chosen from the basis of the alignment of the spectrum of the useful signal, taking into account the attenuation of the seismic wave and the achievement of the required signal-to-noise ratio. The positive effect of the invention is achieved by obtaining a broadband balanced at low and high frequency components of the spectrum of seismic signals of reflected waves, 1 sludge.

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The invention relates to seismic exploration and can be used in prospecting and exploration of minerals.

The purpose of the invention is to increase efficiency by scaling up useful technology.

The drawing shows graphs explaining the method, where: curve 1 is the amplitude-frequency spectrum of a direct wave when excited by a surface low-frequency electrospark source; curve 2 is the direct wave spectrum when excited by a borehole high-frequency source; curve 3 is the amplitude-frequency spectrum of the total direct wave after the accumulation of the effects of different-frequency sources, which ensures the alignment of the spectrum of the reflected wave with regard to its attenuation; curve 4 is the amplitude-frequency spectrum of the reflected wave as a result of accumulation of the effects of different-frequency sources, which ensures the alignment of the spectrum and the achievement of the required ratio of useful signal / interference.

The method is carried out as follows.

Observations according to the MOV-OGT technique are carried out under combined excitation of seismic waves by surface low-frequency and borehole high-frequency electrospark sources. To adjust the spectrum and get neo

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Traversable energy of the useful signal is accumulated by single effects of sources. Sources can work alternately and simultaneously. For the in-phase summation of seismic records from surface and borehole radiators, they are interconnected by each other by introducing an appropriate time shift. Signals are conducted by single wide-band pressure receivers, immersed under the sole of the ZMS low velocity zone, into the well drilled along the observation profile. The registration of the oscillations is carried out by digital seismic in a wide range, alternately in the allocated frequency intervals. Before starting work, the required number of accumulations of signals in each of the selected frequency intervals is determined to form a uniform spectrum of the useful signal in a wide range of frequencies and to achieve the required signal-to-noise ratio for seismic records of reflected waves. The number of accumulations is determined by the amplitude-frequency spectra of forward waves from low- and high-frequency sources, taking into account the frequency-dependent attenuation of the seismic wave at the time of the registration of the deepest reflection. Straight waves are obtained from the single actions of each of the sources, taken by a pressure receiver located in the well in the near zone of the source of excitation. The calculated amplitude-frequency spectra of direct waves from different-frequency sources are shown together on the same graph (curves 1, 2). The greatest extremum is indicated by the level of amplitude amplification for alignment of the forward wave spectrum. Further, attenuation in the spectrum of high-frequency components due to the frequency-dependent attenuation of seismic wave during propagation in the medium is taken into account. The calculation is made for deep reflection. The dependence of the attenuation coefficient on the frequency a (f) is established, for which the VSP data on the well located in the MOP area of work are attracted.

When exponentially approximating the attenuation of a seismic wave with the recovery depth of the spectral amplitudes of the reflected wave recorded at time t, is achieved by multiplying them by the value of exp a (f) vt, and when normalized by the level by multiplying by the ratio exp, (f) vt / exp (f) vt, where V is the average velocity of wave propagation, established according to seismic trolley data or

VSP, and (f) is the dependence of the attenuation coefficient on the frequency for the first frequency interval. Taking into account the attenuation, the spectrum of the reflected wave is aligned, for this the spectrum

the direct wave in the near zone of the excitation center should be an increasing function.

Along with the alignment of the spectrum, it is necessary to obtain a sufficient amplitude of the reflected wave A (C / P). To do this, the procedure of accumulating signals on the alignment of the spectrum in the experiment must be repeated K times, until the required signal-to-noise ratio is obtained on the seismogram of reflected waves. As a result, the number of signals accumulated by frequency intervals is determined by the expression

20

- Amax expoi (f) vt .. ni-1.4AG x tTLTT K

where Amax and Ai are the amplitudes of the direct and interval waves, respectively.

Rational technology of signal accumulation assumes at first to accumulate single effects of sources in the full frequency range (fmm-fmax), C reaching the estimated number of accumulations for the first frequency interval, the nc filters switch to the recording range f and continue accumulating the effects for subsequent intervals. After reaching the estimated number of accumulations for the second frequency interval P2 (with

considering u) the filters switch to the range and continue to accumulate the effects in the remaining second and third intervals up to the number of pz, then set the following

range of fa-fmax and complete the accumulation of effects in the interval with the achievement of the estimated number of accumulations.

Use for excitation of seismic waves of the same type of electric spark sources simplifies the organization and technology of field observations,

Claims (1)

Claims of Invention A method of seismic exploration based on the excitation of oscillations by low-frequency and high-frequency electrospark sources, a broadband reception of pressure receivers submerged beneath the low-velocity zone, in-phase summation and digital recording of signals in different frequency intervals, whose boundaries are chosen from the amplitude spectra of forward waves recorded in the near zone of the excitation center, characterized in that, in order to increase efficiency by unifying field technology, an electric spark is used as a low-frequency source source signals from high frequency band BUT tsh the sources are accumulated in separate frequency intervals, while the accumulations are brought to alignment of the amplitude-frequency spectrum of the reflected wave and to achieve the necessary signal-to-noise ratio. Jfafik