SU890297A1

SU890297A1 - Method of vibrational seismic prospecting

Info

Publication number: SU890297A1
Application number: SU802913121A
Authority: SU
Inventors: Шаукет Камилевич Абдрашитов; Юрий Григорьевич Щерба; Виктор Борисович Попов
Priority date: 1980-04-15
Filing date: 1980-04-15
Publication date: 1981-12-15

Description

Изобретение относится к сейсморазведке, использующей непрерывные, квазигармонические колебания, модулированные по частоте.The invention relates to seismic exploration using continuous, quasi-harmonic, modulated frequency.

Известен способ сейсмической разведки, использующий для возбуждения колебаний циклическое изменение частоты источника возбуждения. Возбуждаемый сигнал, отраженный на различ-. ных сейсмических горизонтах, регистрируют сейсмоприемником с задержкой по времени относительно возбуждаемого сигнала, тем большей, чем глу6же находится отражающий горизонт. При непрерывном изменении частоты вибрации эта задержка приводит к появлению разницы частот сигнала возбуждения и сигнала отраженной волны, что используется для преобразования непрерывных сейсмических колебаний в импульсную форму при корреляционной обработке^ 1].A known method of seismic exploration, using to excite oscillations, a cyclic change in the frequency of the excitation source. The excited signal reflected on the different-. seismic horizons, recorded by the seismic receiver with a time delay relative to the excited signal, the greater the deeper the reflecting horizon. With a continuous change in the vibration frequency, this delay leads to the appearance of a frequency difference between the excitation signal and the reflected wave signal, which is used to convert continuous seismic vibrations into a pulsed form during correlation processing ^ 1].

Функция автокорреляции представляет собой симметричный затухающий в обе стороны импульс, максимум энергии которого сосредоточен в центральном экстремуме. Степень затухания боковых экстремумов зависит от полосы частот и длительности исходного квазигармонического сигнала. При ограниченном по спектру и длительности возбуждаемом квазигармоническом сигнале автокорреляционная функция имеет значительный уровень боковых экстремумов .The autocorrelation function is a symmetrical pulse decaying in both directions, the maximum energy of which is concentrated in the central extremum. The degree of attenuation of lateral extrema depends on the frequency band and the duration of the initial quasi-harmonic signal. When the quasiharmonic signal is limited in spectrum and duration, the autocorrelation function has a significant level of lateral extrema.

Однако наличие боковых экстремумов (в особенности'от интенсивных волн-помех) значительно снижает разрешающую способность результирующей записи и зачастую делает невозможным выделение слабых волн от более глубоких отражающих границ.However, the presence of lateral extrema (in particular, from intense interference waves) significantly reduces the resolution of the resulting recording and often makes it impossible to distinguish weak waves from deeper reflecting boundaries.

Известен также способ сейсморазведки, основанный на возбуждении непрерывных колебаний вибрационным источником, приеме, усилении, управляемой частотной фильтрации и регистрации непрерывных сейсмических волн, в котором сигнал, поступивший от сейсмоприемника рабочего канала, подвергают управляемой режекторной фильтрации, при которой полоса задержания фильтра постоянно настраивается на частоту работы источника колебаний сигналом, поступающим с датчика, например «сейсмоприемника·., расположенного на рабочем органе вибратора. При такой фильтрации ослабляется синфазная с работой вибратора составляющая сейсмического сигнала. Колебания, связанные с полезными волнами, задержанными по времени в результате распространения в среде, не ослабляются, так как полоса задержания фильтра не совпадает с мгновенной частотой полезного сигнала.There is also a known method of seismic exploration based on the excitation of continuous oscillations by a vibration source, receiving, amplifying, controlled frequency filtering and recording continuous seismic waves, in which the signal received from the working channel seismic receiver is subjected to controlled notch filtering, in which the filter delay band is constantly tuned to the frequency the operation of the oscillation source by a signal coming from a sensor, for example, a “geophones ·.” located on the working body of the vibrator. With this filtering, the in-phase component of the seismic signal is weakened in-phase with the vibrator. Oscillations associated with useful waves delayed in time as a result of propagation in the medium are not attenuated, since the filter delay band does not coincide with the instantaneous frequency of the useful signal.

Недостатки этого способа следующие. Остронаправленная характеристика применяемого режекторного фильтра позволяет проводить подавление и уменьшение мешающих колебаний только вблизи вибратора, где они имеют минимальный фазовый и временной сдвиг относительно исходного возбуждаемого колебания . Колебания вблизи вибратора имеют большой динамический диапазон, поэтому процесс фильтрации производят до регистрации, что не всегда целесообразно.Но кроме исходных колебаний, возбуждаемых вибратором, в начальной части сейсмической записи присутствуют интенсивные колебания, связанные с волнами, образовавшимися в верхней части разреза, - отраженными, преломленными поверхностями и т.п. Указанный способ не уменьшает уровня этих колебаний, так как они не синфазны с колебаниями вибратора. А всем этим колебаниям после корреляционной обработки сопутствуют свои боковые экстремумы на функции взаимной корреляции, которые являются помехой и значительно понижают разрешающую способность вибрационной сейсморазведки .The disadvantages of this method are as follows. The oversized directional characteristic of the used notch filter allows the suppression and reduction of interfering vibrations only near the vibrator, where they have a minimum phase and temporal shift relative to the initial excited oscillation. Oscillations near the vibrator have a large dynamic range, so the filtering process is carried out before registration, which is not always advisable. But in addition to the initial vibrations excited by the vibrator, in the initial part of the seismic recording there are intense vibrations associated with the waves formed in the upper part of the section - reflected, refracted surfaces, etc. The specified method does not reduce the level of these oscillations, since they are not in phase with the vibrations of the vibrator. And all these vibrations after correlation processing are accompanied by their lateral extrema on the cross-correlation functions, which are a hindrance and significantly reduce the resolution of vibrational seismic exploration.

Целью изобретения¹ является снижение уровня помех путем осуществления программного регулирования амплитуд непрерывных волн в зависимости от их задержки относительно исходного возбуждаемого колебания.The aim of the invention ¹ is to reduce the level of interference by implementing programmed regulation of the amplitudes of continuous waves depending on their delay relative to the initial excited oscillation.

Поставленная цель достигается тем, что согласно способу вибрационной сейсморазведки, основанном на возбуждении непрерывных колебаний;.вибра ционным источником, приеме, усилении управляемой частотной фильтрации, перестраиваемой синхронно с частотой вибрации, и регистрации непрерывных сейсмических волн, фильтрацию непрерывного вибросейсмического сигнала выполняют с амлитудно-частотной характеристикой полосы подавления сигналов, имеющей форму, обратную форме зависимости амплитуд сейсмических волн от их задержки относительно возбуждаемого колебания, -в масштабе частот, определяемом скоростью изменения частоты вибрации, а частоту полосы подавления перестраивают сигналом, поступающим с датчика, например, сейсмоприемника, установленного на рабочем органе вибратора, и задержанным относительно начала возбуждения колебаний на время подхода к сейсмоприемнику канала максимальной по амплитуде волны, например, волны в первых вступлениях.This goal is achieved by the fact that according to the method of vibrational seismic exploration, based on the excitation of continuous oscillations; a vibration source, reception, amplification of controlled frequency filtering, tunable synchronously with the frequency of vibration, and registration of continuous seismic waves, filtering a continuous vibroseismic signal is performed from the amplitude-frequency characteristic of the signal suppression band, having the form inverse to the form of the dependence of the amplitudes of seismic waves on their delay relative to the excitation vibration, on a frequency scale determined by the rate of change of the vibration frequency, and the frequency of the suppression band is tuned by a signal from a sensor, for example, a geophone installed on the working body of the vibrator, and delayed relative to the start of excitation of oscillations for the time of approaching the channel’s geophone with maximum amplitude waves, for example, waves in the first arrivals.

На фиг. 1 и 2 приведены графики, поясняющие предлагаемый способ.In FIG. 1 and 2 are graphs explaining the proposed method.

Сущность способа заключается в следующем.The essence of the method is as follows.

Сложный вибросейсмический сигнал подвергают управляемой частотной фильтрации с амплитудно-частотной характеристикой, при которой уровень подавления различных волн, слагающих сложный вибросейсмический сигнал, выбирают в зависимости от рассогласования этих волн по частоте с частотой волны, имеющей максимальную амплитуду. Причем на частоту этой волны настраивают максимум полосы подавления фильтра, для чего задерживают на необходимое время сигнал настройки, поступающий с датчика, например, сейсмоприемника, установленного на рабочем органе вибратора. При этом для определения параметров фильтра используют зависимости амплитуд волн от их задержек по времени и рассогласования частот, однозначно связанных между собой градиентом изменения частоты возбуждаемого колебания. Связь между зависимостью амплитуд сейсмических волн от времени, градиентом изменения частоты возбуждаемого колебания и зависимостью амплитуды сейсмических волн от рассогласования по частоте, показана на фиг. 1. Кривая 1 показывает изменение амплитуд сейсмических волн от времени A(t), кривая 2 - градиент изменения частоты возбуждаемого колебания, кривые 3 изменение амплитуд сейсмических волн от величины рассогласования их по частоте где f_Maf- текущая частота исходного колебания. Линии, параллельные кривой 2, представляют собой графики изменения частоты регулярных волн, а точки пересечения их . с осью времен — задержку этих волн относительно исходного колебания, значения, найденные по кривой 1 в этих точках·, определяют амплитуды колебаний каждой из волн. Из сопоставления кривых 1 и 2 видно, что отличие волн, составляющих сложный вибросейсмический сигнал, по амплитуде и частоте колебаний относительно исходного колебания сохраняется постоянным и определяется задержкой волн относительно исходного колебания. Построив зависимость амплитуды колебаний волн (с различными задержками) относительно значений их мгновенных частот для любого времени, например t, получают амплитудно-частотную характеристику волн, слагающих сложный вибросейсмический сигнал, которая и является искомой характеристикой фильтра, например, режекторного. Осуществление такой фильтрации с перемещением полосы подавления фильтра соответственно значению частоты исходного колебания (кривая 2). приводит к программному регулированию амплитуд волн по закону, определяемому кривой 1.A complex vibroseismic signal is subjected to controlled frequency filtering with an amplitude-frequency characteristic, in which the level of suppression of various waves composing a complex vibroseismic signal is selected depending on the frequency mismatch of these waves with the wave frequency having the maximum amplitude. Moreover, the maximum frequency of the filter suppression bandwidth is tuned to the frequency of this wave, for which the setup signal coming from a sensor, for example, a geophones mounted on a working body of the vibrator, is delayed for the necessary time. In this case, to determine the filter parameters, the dependences of the wave amplitudes on their time delays and frequency mismatch are used, which are uniquely related to each other by the gradient of the frequency variation of the excited oscillation. The relationship between the time dependence of the amplitudes of the seismic waves, the gradient of the frequency of the excited oscillation, and the dependence of the amplitude of the seismic waves on the frequency mismatch is shown in FIG. 1. Curve 1 shows the change in the amplitudes of the seismic waves as a function of time A (t), curve 2 shows the gradient of the frequency of the excited oscillations, curves 3 shows the amplitudes of the seismic waves as a _{function of} their frequency mismatch, where f _Maf is the current frequency of the initial oscillation. Lines parallel to curve 2 are graphs of changes in the frequency of regular waves, and their intersection points. with the time axis — the delay of these waves relative to the initial oscillation, the values found from curve 1 at these points · determine the oscillation amplitudes of each of the waves. A comparison of curves 1 and 2 shows that the difference in the waves that make up the complex vibroseismic signal in terms of amplitude and frequency of oscillations relative to the initial oscillation is kept constant and is determined by the delay of the waves relative to the initial oscillation. Having built the dependence of the amplitude of the wave oscillations (with various delays) relative to the values of their instantaneous frequencies for any time, for example t, we obtain the amplitude-frequency characteristic of the waves composing a complex vibroseismic signal, which is the desired characteristic of a filter, for example, a notch filter. The implementation of such filtering with the movement of the filter suppression band according to the value of the frequency of the initial oscillation (curve 2). leads to programmed regulation of wave amplitudes according to the law defined by curve 1.

Такое программное регулирование амплитуд позволяет уменьшить мгновенный динамический диапазон регистрируемых непрерывных вибросейсмических волн и понизить уровень боковых экстремумов взаимнокорреляционной функции от подавляемых волн-помех на результирующей сейсмической записи.Such programmed amplitude control allows one to reduce the instantaneous dynamic range of recorded continuous vibroseismic waves and lower the level of lateral extrema of the cross-correlation function of suppressed interference waves on the resulting seismic record.

Указанный положительный эффект получают следующим образом. Амплитуды сейсмических волн закономерно уменьшаются в зависимости от времени их регистрации. Закон изменения амплитуды A(t) зависит от конкретных сейсмогеологических условий и может выражаться, например кривой 1, как показано на фиг. 2. Кривая 2 представляет возможный характер затухания функции взаимной корреляции между сигналом вибрации и квазигармоническим колебанием, зарегистрированным на времени t. Из со* поставления кривой 1 и 2 видно, что боковые экстремумы достигают значительного уровня, который может приближаться к уровню полезного сигнала и в некоторых случаях превышать его. Подвергая вибросейсмический сигнал до корреляции управляемой частотной фильтрации с амплитудно-частотной характеристикой, обратной закону изменения сейсмического сигнала от времени регистрации * (кривая 1), осуществляют тем самым программную регулировку, приводящую амплитуды непрерывных волн к кривой 3. Боковые экстремумы после корреляции определяются уровнем амплитуд центральных экстремумов, выраженных также кривой 3. А уровень боковых экстремумов от сигнала на времени определяется кривой 4. Из сопоставления зависимостей 3 и 4 видно, что соотношение полезный сигнал - помеха (под помехой подразумеваются боковые, экстремумы) значительно возросло по сравнению с соотношением сигнал помеха, определяемым кривыми 1 и 2. После корреляционной обработки амплитуды волн (кривая 3) можно восстановить до уровня, определяемого кривой 1. Подъем уровня боковых экстремумов происходит по той же зависимости и, в частности, для сигнала на времени уровень, определяемый кривой 4, поднимается до уровня, определяемого кривой 5. Из сопоставления зависимостей 1,2 и 5 видно, что происходит значительное увеличение соотношения сигнал - помеха. При этом понижается уровень амплитуды регистрируемого непрерывного вибросигнала, так как амплитуды всех волн нормируются к амплитудам волн, имеющих меньшую интенсивность.The specified positive effect is obtained as follows. The amplitudes of seismic waves naturally decrease depending on the time of their registration. The law of amplitude change A (t) depends on specific seismic and geological conditions and can be expressed, for example, by curve 1, as shown in FIG. 2. Curve 2 represents the possible character of the damping of the cross-correlation function between the vibration signal and the quasi-harmonic oscillation recorded at time t. It can be seen from the comparison of curve 1 and 2 that the lateral extrema reach a significant level, which can approach the level of the useful signal and, in some cases, exceed it. Subjecting the vibroseismic signal to the correlation of the controlled frequency filtering with the amplitude-frequency characteristic inverse to the law of change of the seismic signal from the recording time * (curve 1), program adjustment is carried out thereby bringing the amplitudes of the continuous waves to curve 3. The lateral extrema after the correlation are determined by the level of central amplitudes extrema, also expressed by curve 3. And the level of lateral extrema of the signal over time is determined by curve 4. From a comparison of dependencies 3 and 4 it can be seen then the ratio of useful signal to interference (interference means lateral, extrema) has increased significantly compared to the ratio of interference signal determined by curves 1 and 2. After correlation processing of the wave amplitudes (curve 3), you can restore to the level determined by curve 1. Raising the level of the side extrema occurs according to the same dependence and, in particular, for a signal over time, the level determined by curve 4 rises to the level determined by curve 5. From a comparison of dependencies 1,2 and 5 it can be seen that increase the signal - noise ratio. In this case, the amplitude level of the recorded continuous vibration signal decreases, since the amplitudes of all waves are normalized to the amplitudes of waves having a lower intensity.

Claims

This invention relates to a seismic survey using continuous, quasi-harmonic, frequency modulated oscillations. The known method of seismic prospecting uses a cyclic change in the frequency of the excitation source to excite oscillations. An excited signal reflected on different seismic horizons is recorded by a seismic receiver with a time delay relative to the excited signal, the greater the depth the reflecting horizon is. With a continuous change in the vibration frequency, this delay leads to the appearance of a difference in the frequencies of the excitation signal and the reflected wave signal, which is used to convert continuous seismic vibrations into a pulsed form during correlation processing. The autocorrelation function is a symmetrical damping on both sides of the pulse, the maximum energy of which is concentrated at the central extremum. The degree of attenuation of lateral extremes depends on the frequency band and the duration of the original quasi-harmonic signal. With the excitation of a quasiharmonic signal limited in terms of the spectrum and duration, the autocorrelation function has a significant level of lateral extremes. However, the presence of lateral extremes (especially from intense wave-interference) significantly reduces the resolution of the resulting record and often makes it impossible to separate weak waves from deeper reflecting boundaries. A seismic survey method is also known, based on the excitation of continuous oscillations by a vibrational source, reception, amplification, controlled frequency filtering, and registration of continuous seismic waves. 3 in which the signal received from the working channel seismic receiver is controlled controlled rejection filtering, in which the filter retention band is continuously tuned to the frequency of the source of the codec signal coming from the sensor, for example, the seismic receiver located on the vibrator working element. With such filtering, the component of the seismic signal that is in phase with the operation of the vibrator is weakened. The oscillations associated with the useful waves, which are delayed by time as a result of 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 disadvantages of this method are as follows. The sharp characteristic of the notch filter used allows suppression and reduction of disturbing vibrations only near the vibrator, where they have a minimum phase and time shift relative to the original excited oscillation. The oscillations near the vibrator have a large dynamic range, therefore, the filtering process is performed prior to recording that; It is not always advisable. But, apart from the initial oscillations excited by the vibrator, there are intense oscillations in the initial part of the seismic record associated with the waves formed in the upper part of the section — reflected, refracted surfaces, etc. This method does not reduce the level of these oscillations, since they are not in-phase with vibrator oscillations. After correlation processing, all these oscillations accompany their side extremums on the cross-correlation functions, which are a hindrance and significantly lower the resolution of vibrational seismic prospecting. . The aim of the invention is to reduce the level of interference by programmatically adjusting the amplitudes of continuous waves as a function of and the delay relative to the initial excitation oscillation. The goal is achieved by the fact that, according to the method of vibration imaging, based on the excitation of continuous oscillations: by a vibration source, receiving, amplifying controlled frequency filtering tuned synchronously with the frequency of vibration, and recording continuous seismic waves, filtering a continuous vibration seismic signal is performed with an amplitude frequency response characteristic of the suppression band of signals having a shape, the shape of the amplitude of seismic waves as a function of their delay relative to the excited oscillation, in terms of frequencies determined by the rate of change and frequency of vibration, and the frequency of the suppression band is rearranged by a signal coming from a sensor, for example, a seismic receiver installed on the vibrator's operating rrg, and delayed relative to the onset of oscillation at the time of approach to the seismic receiver of the channel with the maximum wave amplitude, for example, waves in the first steps x FIG. Figures 1 and 2 are graphs explaining the proposed method. The essence of the method is as follows. A complex vibroseis signal is subjected to controlled frequency filtering with an amplitude-frequency characteristic, at which the level of suppression of various waves composing a complex vibroseismic signal is chosen depending on the inconsistency of these waves in frequency with the frequency of the wave having the maximum amplitude. Moreover, the maximum of the suppression band of the filter is tuned to the frequency of this wave, for which the tuning signal coming from the sensor, for example, a geophone mounted on the working element of the vibrator, is delayed for the required time. In this case, to determine the filter parameters, the amplitudes of the waves are used as a function of their time delays and frequency mismatch, which are uniquely related to each other by the gradient of the change in the frequency of the excited oscillation. The relationship between the dependence of the amplitudes of seismic waves on time, the gradient of the change in the frequency of the excited oscillations, and the dependence of the amplitudes of seismic waves on the frequency mismatch is shown in FIG. 1. Curve 1 shows the change in the amplitudes of seismic waves from time A (t), curve 2 shows the gradient of change in frequency 5 of the excited oscillation, curves 3 changes the amplitudes of seismic waves from the magnitude of their error in frequency (t, rJtAf), where f is the current frequency of the original oscillation. The lines parallel to curve 2 are graphs of the frequency variation of regular waves, and their intersection points with the time axis — the delay of these waves relative to the original oscillation; the values found from the curve at these points determine the oscillation amplitudes of each wave. From a comparison of curves 1 and 2, it can be seen that the difference between the waves that make up a complex vibroseismic signal, in amplitude and frequency of oscillations relative to the initial oscillation, is kept constant and is determined by the delay of the waves relative to the initial oscillation. By plotting the amplitude of the oscillations of the waves (with different delays) relative to their instantaneous frequencies for any time, for example, t, the amplitude-frequency characteristic of the waves composing the complex vibroseismic signal is obtained for the torus and is the desired characteristic of the filter, for example, notch. Carrying out such filtering with moving the filter suppression band according to the frequency value of the original oscillation (curve -2). leads to software controlling wave amplitudes according to the law defined by curve 1. Such software amplitude control reduces the instantaneous dynamic range of recorded continuous vibration seismic waves and lowers the level of the side extremes of the mutual correlation function from the suppressed wolfpom on the resulting seismic recording. in the following way. The amplitudes of seismic waves are regularly reduced depending on the time of their registration. The law of variation of the amplitude A (t) depends on the specific seismic and geological conditions and can be expressed, for example, by curve 1, as shown in FIG. 2. Curve 2 represents the possible attenuation of the cross-correlation function between the vibration signal and the quasi-harmonic oscillation recorded at time t. From cov 7, supply of curve 1 and 2, it can be seen that the side extremes reach a significant level, which can approach the level of the useful signal and in some cases exceed it. Subjecting the vibration seismic signal to the correlation of the controlled frequency filtering with the amplitude-frequency characteristic, inverse to the law of change of the seismic signal from the recording time (curve 1), thereby programmatically adjusting the amplitudes of the continuous waves to curve 3. Side extremes after correlation are determined by the level of the amplitudes of the central extremes, also expressed by the curve 3. And the level of the lateral extremes from the signal at time t.determined by the curve 4. From the comparison of dependences 3 and 4, o that the useful signal-to-noise ratio (by interference is meant side, extremes) increased significantly compared to the signal-to-noise ratio defined by curves 1 and 2. After correlation processing, the wave amplitudes (curve 3) can be restored to the level determined by the curve 1. The rise of the level of lateral extremes occurs along the same dependence and, in particular, for a signal at time t. the level determined by curve 4 rises to the level determined by curve 5. From the comparison of dependences, 2 and 5, it is seen that there is a significant increase in the signal-to-noise ratio. At the same time, the amplitude level of the recorded continuous vibration signal decreases, since the amplitudes of all waves are normalized to the amplitudes of waves having a lower intensity. The invention of the method of vibration seismic survey, based on the excitation of continuous vibrations by a vibration source, reception, amplification of controlled frequency filtering, tunable synchronously with the frequency of vibration, and registration of continuous seismic waves, in order to reduce the level of interference by programmatically adjusting the amplitudes continuous waves, depending on their delay relative to the original oscillation. filtering of a continuous vibration seismic signal is performed with the amplitude-frequency characteristic of the signal suppression band having a form inverse to the shape of the amplitude of seismic waves as a function of their delay relative to the excited oscillation at the frequency scale, determined by the rate of change of the vibration frequency, and the frequency of the suppression band is tuned the signal coming from the sensor, for example, a seismic receiver mounted on the working body of the vibrator of the rator, and delayed relative to the beginning of the excitation of oscillations at time m approach to the channel's seismic receiver with a maximum wave amplitude, for example, waves in the first arrivals. Sources of information taken into account in the examination 1. US patent number 3015036, cl. 340-15, published. 1969.

2. USSR author's certificate number 622021, cl. From 01 V 1/00, 1978 (prototype).