CN1220884C - Phased array seismic prospecting method - Google Patents

Abstract

The present invention discloses a phased array seismic prospecting method. The present invention adopts a plurality of controlled sources to be arranged in the collinear mode with uniform space; scanning signals emitted from the controlled sources form inphase superposition in a direction to strengthen the vibration signals by controlling the delaying or the phase difference of each controlled source; the orientation of seismic wave beams is realized by changing the delaying and the phase difference of the controlled sources to make orienting seismic prospecting possible. The present invention is adopted to provide an effective method for the seismic prospecting of steep slant substrates, the present invention realizes the orienting seismic prospecting, the present invention can increase the seismic prospecting depth and th eseismic prospecting resolution, and the present invention solves the problem that the combined earthquake does not need too many seismic sources.

Description

Phased Array Seismic Exploration Method

Technical field:

The invention relates to a shallow layer artificial seismic exploration method, which is especially suitable for the seismic exploration environment where multiple vibrators are constructed simultaneously.

Background technique:

Assuming that the seismic sources vibrate at the same time, the vibration signals generated by each seismic source propagate underground independently. Due to the distance between the seismic sources, there will inevitably be a wave path difference at any observation point underground, and the phase difference caused by the wave path difference will be Strengthen the signal in some areas and weaken the signal in some areas. In the existing combined seismic exploration of reflection method, several single seismic sources with small output force are combined together to simulate a seismic source with large output force to increase the depth of seismic exploration. Combined seismic exploration refers to several single seismic sources vibrating at the same time. Although the seismic waves generated by each seismic source can strengthen the seismic wave energy that propagates vertically and downward to the surface, it cannot generally strengthen the seismic wave energy in other directions. Therefore, it is not suitable for exploration of steeply inclined geological bodies, let alone directional exploration. Furthermore, the number of seismic sources used in combined earthquakes is limited, and generally cannot exceed four. Too many seismic sources will cause mutual interference of seismic waves during propagation, and the effect will be deteriorated. Therefore, by reasonably controlling the phase difference between the vibrators and compensating for the phase difference caused by the wave path difference, the seismic waves generated by each source in a certain direction can always be in the same phase, so that the seismic wave signal in this direction can be strengthened to the greatest extent. .

Invention content:

The purpose of the present invention is to provide a shallow seismic exploration method in which the number of seismic sources is not limited and the seismic wave energy can be strengthened in any direction in the underground three-dimensional space, aiming at the shortcomings and deficiencies of the existing combined seismic.

The present invention is realized in the following manner: the method includes the following sequence and steps, placing 1, 2, 3...n vibroseis at equal intervals to form a vibroseis array, and the detectors 4, 5, 6..The array and the vibroseis array are collinearly arranged at equal intervals. The track spacing and offset are determined according to the reflection seismic exploration method, the depth of the target layer is roughly determined according to the geological data, and the phase difference control parameters are designed according to the depth of the target layer. Or delay parameters, the source system and the receiving system are connected through the trigger unit, select the source at the center of the array as the reference source, use the reference source signal as the reference signal, and use the trigger signal of the reference source as the trigger standard of the receiving system. The vibrator is used as the trigger signal of the seismograph. When the scanning parameters of the vibrator are set, the vibroseis is started. By controlling the phase of the control signal of each vibroseis in real time or making each vibroseis vibrate with a delay in turn, each vibroseis generates The seismic wave forms a beam with a strong vibration signal in a certain direction underground, and the vibration signal in other directions is weakened or partially offset, so that the target layer signal is superimposed in phase when it reaches the geophone array, so that the signal received by the geophone array is stronger, and the signal-to-noise ratio higher. The seismograph receives the reflected signal, performs correlation processing with the set reference signal, and then uses the conventional seismic exploration processing software to obtain the seismic profile.

The beneficial effect of the present invention is: the signal-to-noise ratio of the received signal of the geophone is improved through the test, and the number of vibrators is not limited; the direction of the seismic wave can be manually selected according to the occurrence of the geological body, so as to improve the effect of seismic exploration , especially for the exploration of steeply inclined geological bodies, it fundamentally solves the problems that the combination of seismic sources cannot increase the number of seismic sources arbitrarily and cannot detect geological bodies with large dip angles.

Drawings and Description of Drawings

Figure 1 is a schematic diagram of the beamforming method of the phased array seismic exploration method

Description of drawings

1, 2, 3, n: Vibroseis, 4, 5, 6 geophones, 7 geological bodies, P far-field observation point

dVibrator spacing

Detailed ways:

Below in conjunction with the accompanying drawings for further detailed description, the method includes the following sequence and steps:

First of all, it is necessary to determine the source spacing d, which is restricted by various conditions. In order to avoid the occurrence of grating lobes, d<λ/2 should be satisfied; to improve the direction coefficient, the spacing d should be larger; to approximate the far-field model, the spacing d should be Get smaller. Various conditions restrict each other and need to be considered comprehensively. For the "electromagnetic high-power shallow seismic vibroseis system" disclosed in CN1643891, the distance d between adjacent vibroseis can be selected to be about 2-4 meters.

Then, place vibroseis 1, 2, 3... n at equal intervals to form a vibrator array, and the array of geophones 4, 5, 6... and the array of vibroseis are equidistant Collinear arrangement, determine the trace spacing and offset according to the reflection seismic exploration method, roughly determine the depth of the target layer according to the geological data, design the phase difference control parameters or delay parameters according to the depth of the target layer, the source system and the receiving system are connected through the trigger unit, The source at the center of the array is selected as the reference source, the reference signal is used as the reference signal, the trigger signal of the reference source is used as the trigger standard of the receiving system, and the nth vibroseis is selected as the trigger signal of the seismograph. When the vibroseis scanning parameters After setting up, start the vibrator, and control the phase of each vibroseis control signal in real time or make each vibroseis vibrate in sequence, so that the seismic waves generated by each vibroseis form a beam with a strong vibration signal in a certain direction underground. The vibration signals in other directions are weakened or partially canceled, so that the target layer signal is superimposed in phase when it reaches the geophone array, the seismograph receives the reflected signal, and performs correlation processing with the set reference signal, and then uses the conventional seismic exploration processing software to obtain the seismic profile .

Calculation method of phase difference control parameters:

If the phase-controlled seismic source works, the phase difference between adjacent seismic sources is controlled to be φ=(2π/λ)kd Sinθ (formula 1), and k is the sequence number 1, 2,...n of the seismic source in Fig. 1.

Then the signals generated by adjacent seismic sources in the direction of the observation point are superimposed in phase, and the vibration is the strongest. At this time, the direction θ of the observation point is the beam direction. The vibration signal sent by the vibrator is a frequency conversion signal, according to $\mathrm{\&lambda;}=\frac{v}{f}$ (Formula 2), substituting into Formula 1, then $\mathrm{\&phi;}=\frac{2\mathrm{\&pi;}\&Center\; Dot;f}{v}\&CenterDot;d\&Center\; Dot;\sin \mathrm{\&theta;}$ (Formula 3), where v is the seismic wave velocity, f is the instantaneous frequency of the seismic wave, and d is the source distance. It can be seen that the control phase difference φ satisfies formula 3, then θ is the beam direction.

Assuming that the underground medium is uniform and the direction of the observation point is θ, then the wave path difference between adjacent seismic sources is d Sinθ, and the phase difference generated by the wave path difference is: φ=(2π/λ)d Sinθ.

Among them, λ is the wavelength of the seismic wave in the underground medium, and d is the distance between adjacent seismic sources.

Calculation method of delay control parameters:

Generally speaking, by controlling the start-up time of each seismic source, the signals of each seismic source can reach the geophone at the same time. According to the reverse order of seismic source numbers, the time delay from No. 1 seismic source to No. n seismic source is sequentially t _k (k=1, 2,...n), where kd sinθ=v t _k (Formula 5), t _k is the kth The start-up time of the source is called the delay time here. After the source array and receiver array are determined, according to the depth of the target formation, the beam direction θ can be determined as shown in Figure 1. $\mathrm{tg\&theta;}=\frac{x}{2h}$ (Formula 6), where x is the distance from the center of the geophone array to the first seismic source, and h is the depth of the target formation. The delay time t _k (k=1, 2,...n) can be calculated by the formula 5, 6

The directional seismic beam can be formed by time-delay or real-time phase difference control method, and the direction of the seismic beam can be changed arbitrarily within 180° underground, thereby realizing the beam scanning effect. Therefore, in the detection of steeply inclined formations, by changing the beam direction, the receiver can receive the best signal effect, so as to obtain high-quality seismic exploration profiles.

When conducting field shallow seismic exploration, the vibroseis arrays are placed at equal intervals, the geophone arrays and the vibrator arrays are arranged in line, and the trace spacing and offset are determined according to the reflection seismic exploration method. According to the geological data, the target formation depth is roughly determined, and based on this, phase difference control parameters or delay parameters are designed. Due to the inaccuracy or errors in the wave velocity data in the survey area, the actual phase control parameters need to be fine-tuned through experiments. The source system and the receiving system are connected through the trigger unit. Among them, the start-up signal of source n in the array should be selected as the trigger signal of the seismograph. After the scanning parameters of the vibrator are set through the control system, the vibroseis system is started to work. The seismograph receives the reflected signal, performs cross-correlation processing with the reference signal, and then uses conventional seismic processing software to obtain the seismic profile.

Claims (1)

1, a kind of phased array method of seismic prospecting, it is characterized in that, this method comprises following order and step, equidistantly place vibroseis 1,2,3.....n it is individual, constitute the vibroseis array, wave detector 4,5,6...... being equidistant conllinear, array and vibroseis array arrange, determine track pitch and offset distance according to the reflection shooting method, roughly determine the destination layer degree of depth according to geologic information, according to destination layer depth design phase differential controlled variable or delay parameter, Seismic Source System and receiving system are got in touch by trigger element, the focus of choosing the array center place is the benchmark focus, with the benchmark source signal as the reference signal, with the trigger pip of benchmark focus trigger criteria as receiving system, choose the n vibroseis as seismographic trigger pip, when the vibroseis sweep parameter starts vibroseis after setting completed, by the phase place of each vibroseis control signal being controlled in real time or is made the vibrations of delaying time successively of each vibroseis, the seismic event that makes each focus produce forms the very strong wave beam of vibration signal in underground certain certain direction, the vibration signal of other directions weakens or partial offset, make when the destination layer signal arrives the wave detector array with superimposed, make the signal of wave detector array received stronger, signal to noise ratio (S/N ratio) is higher, seismograph receives reflected signal, make relevant treatment with set reference signal, re-using routinely, the seismic exploration process software can obtain seismic cross-section.