SU1513409A1 - Method of seismic survey with reflected waves - Google Patents

Abstract

The invention is intended for use in seismic surveys using the method of reflected waves for prospecting and exploration of mineral resources, in particular oil and gas deposits. The method allows studies in areas with a low signal intensity and a background of multiple high-intensity waves. The purpose of the invention is to increase the efficiency of seismic exploration by increasing the signal-to-noise ratio. To do this, in the method of seismic surveying by reflected waves, based on excitation, registration of oscillations in the aggregate of points located differently distant from the excitation point, located in the near-surface part of the geological section, as well as in internal points of the environment, sequential movement of the excitation points and recording along the profile and filtering the seismic records the registration is carried out for a time twice as long as the reflection time from the target horizon at the point of excitation in the near surface part and at N points vertically whether increments ΔH B range of depths H ₁ ≤H≤H ₁ + L from the surface, where: H ₁ = * 01CH _ist + λ, if (H _ist + λ) ≥H _ZMS, H _ZMS if (H _ist + λ) * 98 H _Sms , where H _ist , H _Sms is the depth of the source and the power of the SMS, λ = V _avg / F _pr , L = V _avg (1.6 F _in -F _n ) / (4F _in ^. F _n ), 98ХХ = V _avg / 4F _в , N = L / ΔХ + 1, if L / ΔХ≥9, L = 0,36V _av /, F _n , ΔХ = L / 9, N = 10, if L / ΔХ * 989 , F _pr , F _n , F _in - respectively, the prevailing frequency, the lower and upper limits of the frequency band of oscillations, V _cf - the average speed in the medium, register oscillations in the frequency range F _n ≤F ≤2F _in –F _n , and filtering seismic vibrations produced by reverse filter Om, which is calculated from the result of the selection of the incident waves on the vertical alignment recordings, the resulting time section is made up of the records obtained at the excitation point.

Description

151

The invention is intended for use in seismic exploration by the method of reflected waves for prospecting and exploration of minerals, especially oil and gas fields, and allows arbitrary studies in areas with low signal intensity and background of multiple wolf - high-intensity noise,

The purpose of the invention is to increase the efficiency of seismic exploration by improving the signal-to-noise ratio.

The method is implemented in the following sequence of operations. In the study area, at first, experimental observations were made in the range of 150 m under the low speed zone (GMS) in the absence of GMS under the surface and on the surface. As a result, information is received about the propagation speeds and wavelengths in the upper part of the section. information is obtained directly from field seismic records, for example, using a field computing complex (PEC).

The complex of observations includes three types: observations by the method of reflected waves in the near-surface part at the points of excitation, observations along the vertical, and the method of the common depth point (MCT). The general conditions for all three types are the same parameters of the oscillation source and filter during registration, the duration of registration is equal to twice the reflection time from the target horizon.

The oscillation excitation system is chosen in a standard way, taking into account the characteristics of useful waves, interference waves according to the experimental results. The parameters of the source (source) and their spatial arrangement are chosen such that the excited signal has the greatest intensity and the highest frequency composition, if possible.

For observations by the method of reflected waves at the points of excitation (PT), a continuous band is used; the adrenal group of seismic receivers consisting of sections, the signal from each of which is recorded by a separate channel of the seismic station. Registration by separate sections allows you to enter static corrections for the tracks for each section before they are summed up.

in one track. Area group parameters are calculated by a known technique.

The vertical position of the seismic receivers is installed on the PV (a beam of seismic receivers during exploration in a water basin or a probe in a borehole). This part of the observation system is designed to obtain a filter for suppressing multiple waves and transforming their energy into energy of useful single waves to increase the signal-to-energy ratio. interference on the resulting seismic records x.

Receiving a signal is considered sure if the signal-to-noise ratio is equal to and greater than 10. To build a filter, it is necessary to reliably select the falling wave, against the background of reflected waves, on the vertical recordings of recordings. The signal in this case is the incident wave, and the interference is reflected waves. From the practice of seismic prospecting it is known that the magnitude of the reflection coefficients from the horizons in geological section does not exceed 0.2-0.3, therefore, the incident wave is not less than 3-4 times higher than the reflected waves. In order for the incident wave to be confident, it is necessary to obtain an additional attenuation of the reflected waves by a factor of 3-4.

For a homogeneous group of seismic receivers, the directivity characteristic is expressed by the formula

sin (nj3 / 2) sin (/ 3/2) .x

 V

 1st

n is the number of seismic receivers in the group;

X is the distance between seismic receivers in the group, m;

f - frequency, Hz;

VK- apparent speed, m / s.

It follows from the formula that the necessary attenuation of the reflected waves by a factor of 3–4 is provided in the region of intermediate maxima on the basis of EE 0, with n: 5.

This region is located between the main maximum (frequencies from O to the first minimum f,; ",") and the first side maximum (f), which is equal in magnitude to the main maximum. Frequencies, for the main and

the first side of the maximum, which correspond to the values of the function; P is greater than 0.25, it is necessary to suppress, because at these frequencies there is practically no attenuation of reflected waves, which prevents the achievement of a positive effect.

The magnitude of the suppression region of the function P and the value of its boundary frequencies depend on the parameters of the group L, lx, h, The parameters indicated, as well as the limits of the bandpass filter when registering oscillations depending on the set values of the lower (fj) and upper (f) boundaries of the frequency range

Swans and Vcp are defined as follows.

When detecting incident waves on a reflected background

f -Yie .. MHW 2n-yx

f

 FOB MQICt

2UX

The number of observation points is determined by the formula

n -, +

Lh

one .

L

If it turns out that

then n is equal to 10, since the optimal suppression with the help of the function P at a constant base is reached at n 5:10, and Dx is taken equal to

-G- Substituted --- instead of them, semi-

chim

L 0.36 c.

n

For the vertical arrangement of seismic receivers we get

L Y Lilib E6l fjf)

4 f

AT

Dx

Have

4t

n -. +

Dx

one

if ---- 9,

Taking into account that the filter values in seismic stations are indicated at the level of the max function P together with the filter should provide four times suppression, we find the value of the function P as 0.25: 0.7 0.3571. Frequency corresponding to this value (0.7707 f, ct

level fu

min

08 P "

f C - 0.8 f,,.

This is the lower limit of the operating frequency range. To suppress reflected waves from the upper frequencies with a slope of not less than 18 dB / octave, it is necessary to fulfill the condition

f f 1 no6 max 6

I -.

Therefore, the upper limit of the working frequency range is equal. Taking into account the above formulas, we obtain

Oh

YiL

"AT

Define the base (p-1) 4, x

L considering that

  V p-uxfj, ь - - - ,. . IM

From here

t VtE. (1i6f6r fH.) 4 C.

otherwise n 10;

L 0.36 n

their -d-

The frequency band of the filter during registration must be determined by pressing

fb f 2f "- fu

And - J in And - Observation base vertically should be located under the PMS (and in its absence under the surface)

at a distance of at least X from the source. It is necessary to include in the incident wave all the elementary waves that have experienced reflections from the base layer of the SMF (or from the day surface). At a distance of less than 7 from the source, the process of forming an excited signal occurs, therefore, the registration of oscillations must be conducted outside this zone.

In water basins, where the water depth is not less than 9 + L, vertical observations can be made at each IW. Where drilling is required, monitoring at each well is not advisable. The diameter of the substantial reflection site at the surface part is 200-400 m, therefore, as the minimum distance between the observing wells 715

we can apply to 200-300 m (the optimal distance is selected by experimental sampling). For calculating the filter of seismic traces recorded at points in the vicinity of 100-150 m from the well, the recordings made in this well are used.

The source can be single 1 or group, explosive or nevryryviv-. The surface or norp TKeHHbiM, PeiHc oscillations both in the subsurface part and vertically are produced during the time I5 not less than two interrupting the registration time and correspondingly (from the target horizon. This ensures the registration of the full reflections of the falling wave from the target m The incident wave includes elementary waves reflected from all horizons (including the deepest), which have reached the sharpest reflecting boundaries in the near-surface part of the section, reflected from them and directed downwards.

To obtain information about the speeds, the MOSFUR observation system is used. The parameters of the CDPT (the number of these receivers and their step in the group, the step of removing the arrays of the PB, the multiplicity, etc.) are chosen in a standard way. on them there is no convergence effect I od o g raf o v o t ra o u n nykh ol oln,

The resulting time slice to is used to transfer the d-waves and correlator on the fragments, scan analysis sections to obtain information about the wave velocities of the fragment analysis filter sweat filter by the inverse filter. As a result, a set of temporary fragments of the analysis are obtained. from do-shniruyuschih multiple waves. Having times t and speeds of wave propagation, obtained by the method of seismic exploration by reflected waves, calculate the depths of the reflector x horizons at the main point of all the area profiles, on the basis of which sections and maps are constructed that illuminate the deep structure of the territory under study.

The positive effect of seismic exploration is achieved due to the fact that the method provides complete information (and the amplitude of J i-; phase spectra) about

20

five

A098

propagating into the medium (incident) wave to build the inverse filter, which eliminates the need for assumptions and guesses about the nature of the phase or amplitude spectrum of the signal, as is done in the known methods, in addition, the whole elementary multiple multiple waves are introduced into the incident wave, Testing at least one act of reflection from the sole of the PMR (or from the day surface) 5 therefore, a special inverse filter, which is clotted (the specified padding 1 Cpu in a single pulse, increases the signal-to-noise ratio not However, due to the suppression of multiple soils, HOj mainly due to the transformation of energy of multiple waves into single energy, the proposed method suppresses many waves and increases the sI1 ngshch-interference ratio as compared to the MOG and MOGT even in the absence of differences in effective , in each, tsykhs speeds and in the frequency composition of single and 1 circumferential waves. Clearing the cut from multiple waves and simultaneous compression of the signal leads to an increase in the rarefaction ability, and an increase in the signal-to-interference ratio to an increase in g Ubin seismic. In addition, the proposed method, in contrast to the MOG and MOGT for a temporary cut, does not require GGV-PP removal, especially as large as in the MOG and MOGT, therefore the resolution ability in the proposed method also increases due to the absence of the convergence effect. , hodographs of reflected waves. The double registration time ensures the completeness of the use of the energy of the multiple wolf to transform it into energy of one g multiple waves.

All records are processed with a special filter, after which the records on the vertical and vertical sections are used to make a time section, and the CDP records are used to obtain information on the speed. IH, Having the times and speeds of the waves, calculate the depths of the horizons

The positive effect is in reducing the number of search engines and

- exploration wells required for the exploration of areas for the presence of oil and gas deposits and j.TpyrHX minerals, since

thirty

15

0

50

1513409

An increase in the resolution and signal-to-noise ratio of the seismic record x leads to an increase in the reliability of depth constructions.

Claims (1)

Invention Formula gd Reflected seismic survey method based on excitation, recording oscillations in a set of points located differently from the excitation point, located in the subsurface part of the geological section, as well as at internal points of the medium, sequentially moving the excitation and recording points along the profile, filtering seismic records, determining the speed of waves according to the recordings, registered at various distances from the point of excitation, the calculation of the depths reflecting; their horizons along. t times and wave velocities, so that, in order to increase the efficiency of seismic exploration by improving the signal-to-noise ratio, additional recording is performed during a time twice as large as the reflection time from the target horizon, in the near-surface part of the willows at points along the vertical with a step Lx in the depth range H N g H, + L, oT day surface if n.,.-) "Ct H 35 5MS where is H "Zms. if (Hj, tt +) - 10 N and H -...- source depth and IHf t MS small power of velocity zone with CR f etc Vce. (I§ uifBl. Fe-fn vcf .. fe -L- + HH one, if a t ;; - in case - 9, SC THAT p ten; 36-f fflp, f and f, respectively, the prevailing frequency, the lower and upper limits of the frequency range of the oscillations; Vep is the average velocity in the medium; oscillations are recorded in the frequency range ff 2fg-f, and the seismic vibrations are filtered by a deconvolution function inverse to the sum of the records of the vertical paccf installation with the background of the ascending waves subtracted from them, obtained by adding the records in the direction of the incident the waves in the window from the moment of the first arrival of the waves to the point in time after which the incident waves become commensurate with irregular noise, the time slice tfl is composed of records obtained at the points Waiting for hesitation.