RU2024891C1 - Method of seismic prospecting - Google Patents

Abstract

FIELD: civil-engineering survey. SUBSTANCE: during the seismic prospecting the linear source of seismic oscillations is disposed parallel to the non-longitudinal line of inspection at a definite distance. Temporary corrections for the velocity of detonation of the linear charge are determined on the basis of equality of the recording time of the sound wave in the check points. The distance between the source and the inspection line is chosen on the basis of securing the recording of the first phases of effective waves connected with the upper part of the cut in the periods of time less than the recording time of the sound and superficial waves. EFFECT: enhanced accuracy of measurements. 3 dwg

Description

 The invention relates to methods for studying the structure of the upper part of a geological section by seismic methods.

 A known method of studying the upper part of the section (VChR), based on special seismic observations using low-power excitation sources. The observations are carried out on relatively short receiving lines according to the type of soundings dispersed over a relatively rare network on the profile of standard seismic surveys (1). The sounding data are subsequently used to calculate corrections for the structure of the VChR.

 This method has the disadvantage that corrections for the structure of the HFR are calculated only at the sensing points, between these points the data on the structure of the HFR are interpolating in nature. Directly on the profile of standard CDP operations, the installation sites of receivers and sources may not coincide with the sensing points for the high frequency response. All this reduces the accuracy of calculating corrections for the structure of the VChR and, as a result, the accuracy of structural constructions according to seismic data decreases. There are also known methods of studying the structure of the HFR based on the interpretation of refracted waves recorded in the initial part of seismograms during standard CDP operations (2).

 However, this method has the following disadvantages. When using standard seismic streamers as receiving lines, the distance between the channels can be significant, which does not allow for reliable correlation of refracted waves. In addition, in the case of significant offsets between the registration channels and the source and relatively strong attenuation of the refracted waves, these waves cannot always be detected with a sufficiently high signal / noise ratio. If the grouping of receivers and sources is used in the standard work of an OGT MOV, then this can lead to a significant distortion of the kinematic parameters of the waves in the region of the first arrivals. For these reasons, the use of the initial part of the seismograms of standard works of the OGT MOV does not always allow obtaining reliable information about the structure of the upper part of the section.

 The purpose of the invention is to increase the accuracy and reliability of determining the parameters of the upper part of the section for calculating corrective corrections during standard work of an OGT MOV and obtaining information about the structure of the VChR.

где t₀ - нулевое время преломления волны до изучаемой границы; Δt - временной интервал между регистрацией преломленной и звуковой волнами; V_з и V_г ^_ скорости звуковой и преломленной волн, и построение временного разреза осуществляют с использованием временных поправок за скорость детонации линейного заряда, определяемых на основе равенства времени регистрации звуковой волны в контрольных точках, например, на первом и последнем каналах приемной расстановки, одинаково удаленных от линии возбуждения.This goal is achieved by the fact that according to the method of seismic exploration, based on the excitation and registration of waves associated with the upper part of the section, use a linear oscillation source (for example, a detonating cord) located parallel to the observation line at a distance that records the first phases of the useful waves associated with VChR, at times shorter than the time of registration of sound and surface waves, and the removal of X is calculated by the formula
X ≥

where t ₀ - zero time of refraction of the wave to the studied boundary; Δt is the time interval between the registration of the refracted and sound waves; V _s and V _g ^_ rate sound and refracted waves, and the construction time section is carried out using time corrections for the velocity of detonation of the linear charge, determined on the basis of equality of the sound wave registration time at the control points, for example, the first and last channels receiving arrangement equally remote from the excitation line.

In FIG. 1, in a plane perpendicular to the lines of excitation and reception, a beam diagram is shown for an element of a linear source and a receiving channel, explaining the essence of the method, where X is the distance between parallel lines of excitation and reception, h is the depth to the refracting boundary; V is the velocity in the overburden, V _g is the boundary velocity of the refracted wave, φ is the angle of inclination of the boundary, i = arcsinV / V _g .

 In FIG. 2,3 is an example implementation of the method.

, (1) где V_к -4 кажущаяся скорость целевой волны,
t_o =

cos i. При ограниченных Х и малых углах наклона φ преломляющей границы зависимости (1) может быть представлена в виде
t = t_o +

. (2) Так, при средних параметрах типовой зоны малых скоростей с V = 500 м/с, V_г = 1700 м/с и углах φ ≈ 1^о отличие во временах, подсчитанных для удаления Х = 70 м по формулам (1) и (2) не превышает точности дискретизации сейсмического сигнала (0,002 с). Перерасчет времени t₀ в вертикальные времена t_в =

производят по формуле
t_в =

t_o/cos i; i = arcsinV/V_г (3). В широком диапазоне реальных изменений V и V_г зависимость (3) может быть существенно упрощена:
t_в =

t_o , (4) где с = соsi для средних значений V и V_г на изучаемом участке профиля.The observed time of the refracted wave for an element of a flat refracting boundary is:
t = t _o +

, (1) where V _to -4 is the apparent speed of the target wave,
t _o =

cos i. For bounded X and small tilt angles φ of the refractive boundary of the dependence (1) can be represented as
t = t _o +

. (2) Thus, with average parameters of the typical low-velocity zone with V = 500 m / s, V _g = 1700 m / s and angles φ ≈ 1, ^the difference in the times calculated for the removal of X = 70 m using formulas (1) and (2) does not exceed the sampling accuracy of the seismic signal (0.002 s). Recalculation of time t ₀ in vertical times t _in =

produced by the formula
t _in =

t _o / cos i; i = arcsinV / V _g (3). In a wide range of real changes in V and V _g, dependence (3) can be significantly simplified:
t _in =

t _o , (4) where c = cosi for the average values of V and V _g in the studied section of the profile.

Δβ , где τ =

; B=v/v_г. Так, при изменении скорости V в пределах 300-500 м/с и V_г в пределах 1700-2500 м/с величина погрешности при применении формулы (4) не будет превышать величины дискретизации сигнала (0,002 с).When using dependence (4), the error values can be estimated from the relation
Δτ =

Δβ, where τ =

; B = v / v _g . So, with a change in speed V within 300-500 m / s and V _g within 1700-2500 m / s, the error value when applying formula (4) will not exceed the signal sampling value (0.002 s).

. (5) В качестве оценок скоростей могут быть приняты для звуковой волны V_з = 330 м/с, для t₀ и V_г - среднее значение нулевого времени и кажущейся скорости преломленной волны на изучаемой площади. Величина Δt выбирается равной 0,1-0,05 с и должна обеспечивать четкую, раздельную во времени регистрацию фазовых годографов преломленной и звуковой волн. Величина выбранного расстояния Х контролируется в любых точках, например на первом и последнем каналах приемной линии. Производят подрыв линейного источника и регистрацию сейсмических колебаний. Рассчитывают поправку за скорость детонации линейного заряда и вводят ее во времена наблюдения сейсмического волнового поля. Величина поправки рассчитывается по величине скорости "распрямления" годографа звуковой волны. Годограф должен быть параллелен линии наблюдения. Обычно скорость детонации для детонирующего шнура имеет порядок 6500 м/с. Ввод поправки за скорость детонации осуществляется по формуле
Δt_1j =

, (6) где l_j - расстояние от начального канала регистрации, расположенного напротив начала детонирующего шнура до j-того канала; V_д - скорость детонации.The method is implemented as follows. Linear sources of seismic vibrations are located on an area parallel to the observation line at a distance
X ≥

. (5) As estimates of speeds, they can be taken for a sound wave V _s = 330 m / s, for t ₀ and V _g - the average value of zero time and the apparent speed of the refracted wave in the studied area. The value Δt is chosen equal to 0.1-0.05 s and should provide a clear, time-separated registration of phase traveltime curves of refracted and sound waves. The value of the selected distance X is monitored at any point, for example, on the first and last channels of the receiving line. Undermine a linear source and record seismic vibrations. The correction for the linear charge detonation velocity is calculated and introduced during the observation of a seismic wave field. The correction value is calculated by the value of the "straightening" speed of the hodograph of the sound wave. The hodograph must be parallel to the line of observation. Typically, the detonation velocity for a detonating cord is of the order of 6500 m / s. The correction for the detonation velocity is entered according to the formula
Δt _1j =

, (6) where l _j is the distance from the initial registration channel located opposite the beginning of the detonating cord to the j-th channel; V _d - detonation velocity.

 With strict parallelism of the lines of excitation and reception, the hodograph of the sound wave (phase front in the seismogram) is straightforward, and after introducing corrections for the speed of detonation (reduction) it has an infinite apparent speed.

, (7) где S_tj - отклонение для j-го канала по годографу звуковой волны от годографа с бесконечной кажущейся скоростью;
V_з - скорость звуковой волны (330-340 м/с);
V_г - средняя граничная скорость преломленной волны для изучаемого разреза.The curvilinearity of the reduced hodograph of the sound wave indicates the non-parallelism of the excitation and reception lines. In this case, you need to enter corrective corrections according to:
Δt _2j = S _tj

, (7) where S _tj is the deviation for the j-th channel according to the travel time curve of a sound wave from the travel time curve with an infinite apparent speed;
V _s - the speed of the sound wave (330-340 m / s);
V _g - the average boundary velocity of the refracted wave for the studied section.

Typically, when the excitation and reception lines are not parallel within ± 3 m, which corresponds to deviations in the reduced hodograph of the sound wave of about ± 0.010 s, the correction Δt ₂ does not exceed the sampling step (± 0.002 s) in magnitude, and its input is not necessary. For values of S _tj ≥ 0.010 s, the introduction of the correction Δt ₂ makes sense.

Recalculation of the corrected observed hodograph in vertical times is carried out according to formulas (3) or (4). When using dependence (3), it is necessary to have information on the values of velocities V and V _g , for example, according to microseismic logs of wells, according to estimates on the longitudinal hodographs of the first wave arrivals recorded during observations of the OGT MOV, or to obtain it based on the use of an additional, more remote, parallel to the first linear source.

When using dependence (4) as the estimates of V and V _g in determining the coefficient C, their average values for the studied area are used. In a wide range of changes in V and V _g for typical sections of VChR in practice, it is enough to take the coefficient C constant. Based on the excitation of the detonating cord parallel to the receiving line, sound (3) and refracted (P) waves were recorded (Fig. 2). The distance is selected in accordance with formula (5) so that it provides a separate registration of these waves in time. After entering the kinematic correction for the detonation velocity according to dependence (6), introducing the corresponding correction based on dependence (7) and converting the observation times of the refracted wave using dependence (4), determine t _in wave П (Fig. 3) and use it in as corrections for the HDR.

Claims (1)

METHOD OF SEISMIC EXPLORATION, including excitation of seismic vibrations by a linear source, registration of refracted waves associated with the upper part of the section, processing of the obtained data and construction of a temporary section, characterized in that the linear source is parallel to the non-longitudinal observation line at a distance x, determined by the formula
x ≥

,
where t ₀ is the "zero" time of propagation of the refracted wave to the studied boundary;
Δ t is the time interval between the registration of refracted and sound waves;
V _s , V _g - the speed of sound and refracted waves,
and the construction of the time section is carried out taking into account time corrections for the detonation velocity of the linear charge, determined on the basis of the equality of the time of registration of the sound wave at control points equally distant from the excitation line, and correcting corrections for the non-parallel position of the receiving and explosive lines at other points of the profile based on non-parallelism hodographs of refracted and sound waves.

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