Pseudo-random Sweeping Method And Numerical Simulation For Seismic Response Based On A Vibrator

Up to now, the vibrator is the most successful non-explosive seismic source. The vibrator is different from the other seismic sources, such as bomb, hammer impulsion, and so on. The wave generated by vibrator can be controlled by onetime-variation function expression which we have already known. In other word, we take the wave generated by using a little energy but by long time sending to the subsurface replace the wave generated by using a large energy but in an instance. The principle of seismic exploration by using vibrator is: Apply the correlation technique to collecting the decentralization energy and compress a narrow-bandwidth impulsion then sending to the underground; during the seismic wave transmitting it will reflect at the interface of the rock formation or underground object surface so that the reflected wave is received and recorded by the detectors; the receiving signal transforms it into impulse signal with delayed information through correlation technology; Using the correlation signal to realize the detection of underground objects and complete the imaging of the underground information. The main problems of seismic exploration by using vibrator are: low seismic source energy capacity, narrow signal frequency band, and low resolution.This theme analyzes the scanning technology for vibrator in detail. The scanning type can be divided into two categories: linear sweep and nonlinear sweep. In order to increase the transmission of the high frequency components, increasing the resolution of vibrator exploration. The theme presents a pseudo-random sequence scanning technology. Pseudo-random sweep is a kind of non-linear sweep, the correction function of pseudo-random is similar to the impulse function. The biggest advantage of pseudo-random is that if processed properly the side lobe can be totally cleared in the correction function, so the resolution of vibrator will be improved greatly.As far as I know, there are many different kinds of pseudo-random sequences, Such as m sequence, M sequence, Gold sequence, R-S sequence, etc. Among all these sequences, however due to m sequence is binary pseudo-random sequence, having periodicity and possesses two electrical levels, therefore the generation of delay sequence and the calculation between the signal sequences will be improved greatly and simplified. In the other hand, m sequence is better than the other non-line sweep signals, because it has great characteristic of correction. So this theme is mainly researched on the m sequence. On a base of analyzing the m sequence's definition and correlation characteristic in detail, this theme gives out Matlab simulation of the m sequence. In order to apply the m sequence to the s vibrator sweep system, in this theme, the pseudo-random sequence scanning can be realized by controlled its phase, and its basic principle is using m sequence to modulate the sin function scanning signal, when the element in the sequence changed, turning 1 to 0 or 0 to 1 , accordingly, the phase of scanning signal at the same time to change between 0 andπ.Seismic signal reflects at the underground layer whose has different wave speed and then received by detectors, this record which in the detector was called as original seismic record. Its quality is not completely decisive by the character of scanning signal's auto-correlation function, otherwise it will effect the resolution of seismic record very much. In the theory and research method of seismic prospecting, construct earth-mediator model approach to the physical problem, then using differential equation mathematic model that satisfied the seismic wave transmission. Under the various excitation conditions, in a certain mediator model, adopting effective solving method to research transmission regular of seismic wave is an effective study means. This theme we adopting the finite-element method to make the numerical solution of seismic response, researching the change regularity for non-linear sweeping's seismic response. Individually using the Chirp signal linear sweeping, exponential sweeping and pseudo-random sweeping signal as the wave launched by vibrator, observing and analyzing emulation of seismic profile response, it will provide a support on manner and technique for high solution vibrator.