Study On Time-domain Seismic Multibeam-forming Method

In seismic exploration, Signal-to-noise(SNR) has always been an important content of the concern of the data acquisition and processing stages. High SNR data is good for the processing and interpretation of seismic data. However, under the strong noise, complex mining environment, or deep seismic exploration, the seismic data has a low SNR, which seriously affectes the post-seismic data processing and interpretation. The SNR of target direction can be significantly enhanced with time-domain seismic beam-forming method through forming the seismic beams. Thereby, the quality and efficiency of seismic exploration could be improved.Time-domain seismic beam-forming method can be realized based on the source or the receiver. When based on the source, the method needs many vibrators or large quantities of explosive source. At the same time, the inconsistency between sources effects the formation of the seismic beams easily. What is more, using single delay parameters can only improve the signal of single target direction. If we want to strengthen the signal of the other direction, many different delay parameters would be need, which makes the work efficiency very low. These factors restricted the promotion and application of seismic beamforming methods based on source.Seismic beamforming method based on receiver is very good to solve the problems existing in the seismic beamforming method based on source. It is carried out by using data processing to form the seismic beams with the seismic data which is acquired with single source. So it does not need special data acquisition method, and the seismic signal of arbitrary direction can be strengthened via choosing arbitrary delay parameter.However, no matter the seismic beam-forming method is carried out based on the source or the receiver, single time delay is used to strengthen the signal of signal direction each time, which can not get the whole improved signal simultaneously. Moreover, as a preprocessing method, this time-domain seismic single beam-forming method(including based on source and receiver) will produce sets of data consisting of many different direction strengthen signal, which increases the storage cost of seismic data and the workload of data processing and is against the seismic interpretation. Therefore, we have proposed a method of time-domain seismic multibeam-forming method.This paper focuses on the theory, numerical simulation, methods and the application in synthetic data and field data, and expands research contents for time-domain seismic multibeam-forming methods,the main research work are as follows:In the aspect of theory, this paper expounds the theoretical foundation of time-domain seismic multibeam-forming. Firstly, by studying the basic principles of the phased array forming a single beam, we know that it is the key to maintain constant phase difference and excitation delay time between adjacent sources for the seismic beam-forming. Then, according to the principle of seismic wave field stack, we develop an idea that the multibeam can be formed with many single beams. In addition, we propose that the multibeam formation could be based on source and receiver, make a comparison with the two methods, and choose the latter method to form seismic multibeam. Finally, we made a further analysis of the beam width and the main beam direction from the source number, source spacing, delay parameters, seismic wave velocity and other aspects, and analyze the characteristics of multibeam stack. This part is the basis of the paper.In the aspect of numerical simulation, we simulate seismic multibeam propagation rule in different medium by the method of wave equation finite difference numerical simulation. The source of the setting of the initial condition is the key of time domain seismic single beam numerical simulation through the analysis of finite difference forward modeling theory. According to the theory of tim-domain seismic multibeam-forming, it is very difficult to form the seismic multibeam directly by setting the initial conditions, so we use the stack of many single beams to achieve the multibeam numerical simulation. Finally, we apply the method to the homogeneous medium model, the layered medium model and multiple orebody model, and simulate the seismic multibeam propagation rule in simple and complex media successfully. The results show that the time-domain seismic multibeam-forming method can strengthen the seismic wave of more than one target simultaneously. it has been proved that the SNR of seismic record can be improved by the method of time-domain seismic multibeam-forming in theory.In the aspect of methods, in view of the problem of distortion wave field in time-domain single beam-forming influencing multibeam-forming, we propose two multibeam-forming methods: local correlation weighted time-domain seismic multibeam-forming method and time-domain adaptive weighted seismic multibeam-forming method.Local correlation weighted time-domain seismic multibeam-forming method is proposed for the problem that the distortion of the wave field in time domain single beamforming disturbs multibeam-forming. First, from the principle of seismic beamforming, it reflects that different delay signals stack in time domain. Taking three signals for example, it is known that effective signal enhancement is the result of different signal in-phase stack, while the distortion signal is the result of delay parameter is not sufficient to compensate for the phase difference between the different signals. According to the distortion analysis, we use local maximum correlation coefficient to generate the weight to extract the main beam direction signal. Based on time-domain seismic multibeam-forming principle, we propose two local correlation weighting method: the weight is used before or after the beam-forming. The weight can be obtained by a series of process of maximum correlation coefficient spectrum between local raw center records and seismic beamforming recorded. The process mainly includes delay filtering, single-channel threshold filtering, multi-channel median filtering, global threshold filtering and encoding. In order to further expand local correlation weighted time-domain eismic multibeam-forming method, we develop a random noise suppression method based on it. We derived a multibeam-forming method with a single seismic record. This method mainly uses angle domain segmentation, the extraction and stacks of main beam signals, to form the multibeam record.Time-domain adaptive weighted multibeam-forming method is developed because of the problems(complex operations and process, many process parameters and other issues) existing in the local correlation weighted time-domain seismic multibeam-forming method. By introducing local time window parameters, the common receivert data could be divided into different time window. So it can avoid the extraction of the seismic main beam direction signal via seismic single beam-forming in each time window. In additon, the combination of cross-correlation algorithm and improved simulated annealing algorithm eliminate the delay parameters in seismic beam-forming process. With the optimal weight estimation, the SNR has been improved greatly. Compared to the local correlation weighted time-domain multibeam-forming method, most of the parameters in time-domain adaptive weighted multibeam-forming method are depended on seismic data. Therefore, it is simple and easy to implement with less variable parameters. But it will cost large computation. So the further improvement of this method is to improve the calculation speed.In the synthetic and field data applications, different time-domain seismic multibeam-forming methods are applied to different SNR data. The results shows that: local correlation weighted time-domain seismic multibeam-forming method is good for improving the SNR of raw data and seismic section; the noise suppression of time-domain seismic multibeam method can effectively improve the continuity of the event in the single-shot record, and a greater degree of random noise suppression is implemented; time-domain adaptive weighted seismic multibeam-forming method has the ability to simultaneously strengthen the whole signal.In summary, this paper proved the effectiveness of time-domain seismic multibeam-forming method in seismic exploration from the perspective of theory and simulation, and put forward different time-domain seismic multibeam-forming methods. The result of synthetic data and field data shows that these methods make a great contribution to improving the SNR of seismic data.