Study On Seismic Beam-forming Method Based On Phased-array Vibrator System

Phased-array vibrator system (PAVS) is able to generate directional seismicwave, which is helpful to improve reflected wave signal-to-noise ratio (SNR)and resolution. It is possible to increase exploration depth using PAVS. Sincethe direction of seismic wave can be controlled, PAVS will be efficient fordipping reflectors' seismic exploration. Based on the project 'Seismic imagingtechnique of PAVS for underground survey', which is a project of ChineseHi-tech Research and Development Program ? project 863 (serial number:2002AA135240), the technique of directional seismic wave is studied in thispaper. It will be beneficial to popularize PAVS seismic method.To be brief, it illustrates why PAVS can form directional seismic wave intheory, and how to detect directional seismic wave signal in PAVS seism. Inthese conditions, seismic records in geophone groups for PAVS weresynthesized, and quantitative analysis shows the influence on SNR andresolution. Except for numeric simulation, it also showed seismic field testresults for single, combination array and PAVS. According to comparisons oftime profiles, the directional seismic wave exploration method is feasible andbest among three different sources.Main contents are following:It analyzes disadvantages of routine controllable vibrator seismicexploration: low SNR and resolution. By analyzing its beam direction,combination array source can form fixed-direction seismic wave, which isvertical to the earth's surface. According to the layout of observation system,geophone group receives seismic wave in non-main-beam exactly. Therefore, itmakes us to think whether source array can concentrate seismic wave on anydirection. This idea is called "phased array technique" in RADAR system. Inreview of the evolution of controllable vibrator and application of phased-arraymethod in seismic exploration and active monitoring for earth's lithosphererecently, it is reasonable and important to study phased-array method andinstruments.In general, this paper studied seismic beam-forming based on PAVS intheory, simulation, and field test.The first part is why and how PAVS can form directional wave. Amathematic model of PAVS is set at first. By model analysis, it is proved thatPAVS can generate directional wave when it uses monofrequency sweep-signal.Furthermore, we conclude that main-beam direction is not related to seismicwave's frequency;however, main-beam width is relevant to instantaneousfrequency. At last, it shows that PAVS can form directional wave when adjacentvibrators work one by one with equal time interval. This part is a theory basis ofPAVS seismic method.The second part is how PAVS improves reflected wave signal SNR andresolution by numerical simulation.Owing to directional seismic wave signal based on PAVS is different fromthat in single and combination sources, detection method for PAVS seismicsignal is studied specially in the paper. There are many methods such ascross-correlation, high-order spectrum and wavelet transform arithmetic, whichwere used in detecting seismic signal. Because of high efficiency in time andspace domain, cross-correlation is applied widely in seismic signal process. It isvery important to select reference signal for PAVS in cross-correlation detectiontechnique. That is not only relates to whether reflected signal can be detectedrightly, but also seismic signal's SNR. Reference signal expression incross-correlation detection is introduced by concept of equivalent source.Including some strong noises, synthetic seismograms for single, combinationarray and PAVS are compared by numerical simulation. According to thedetection results for three sources, it concludes that the technique ofcross-correlation for PAVS is feasible.By numerical simulation, synthesized seismogram of PAVS for one layerand multi-layer horizontal media model is shown. Based on PAVS' directioncoefficient, this simulation method studies how PAVS influenced reflectedwaves, which come from different seismometers or different reflectors. Toexplain the effect of PAVS seismic exploration, we synthesized seismograms insame media model and noise condition for single source, n -source arraycombination and n -unit PAVS. It shows that SNR of reflected waves in PAVS'profiles is the best one. Moreover, we studied how some factors includingtime-delay, distance between adjacent vibrators and number of vibratorsinfluenced PAVS seismic synthetic records and introduced some rules andmethods of determine PAVS' parameters.According to profiles, SNR of reflected waves in PAVS' syntheticseismograms is the best one among three sources in figures. But this qualitativeanalysis is not enough, quantitative results will be necessary. The purpose ofusing PAVS is to improve the quality of seismic wave signal, including SNRand resolution. Therefore, it analyzed how much PAVS can improve signal'sSNR, and then explained the relation between SNR and resolution. Accordingto the definition of resolution, there are two main factors including seismicsignal bandwidth and SNR, which restrict seismic wave signal resolution. Whennoise in tested field is so strong, SNR is far less than 1 in single source model,SNR is more important for resolution. That is why PAVS can increaseresolution in strong noise condition. At last, we computed how much resolutionof PAVS increases than combination array sources model. According to boththeory and simulated results, PAVS plays an important role in high SNR andhigh resolution seismic signal acquisition.The last part is to prove that directional seismic wave based on PAVS ispossible by field test. We know, only simulated results is not enough, it isnecessary to do some field experiments with PAVS. Using hammer, singlesource, combination source array and PAVS, we got seismic records andprofiles in same tested field. Comparing these results, directional seismic waveexploration method is the best one in given tested conditions.In short, there are some achievements in this paper.1. To prove that PAVS with multi-frequency sweep signal can formdirectional seismic wave in theory, and two methods are proposed toform directional seismic wave, including real time phase control anddelay time interval control.2. To bring forward detecting directional seismic wave signal bycross-correlation technique.3. Having simulated directional seismic signal for PAVS and explainedhow PAVS improved SNR and resolution quantificationally in givenhorizontal multi-layer model.4. To prove feasibility and superiority by some field experiment for thedirectional seismic wave exploration method based on PAVS.