The Study Of Seismic Signal Detecting System

The key to improve the resolution of seismic exploration is that the raw seismic data must have a large signal frequency range. Nowadays, the basic reason, which leads to low quality and uncertainty of the raw seismic data, is the receiving step (the seismic signal detecting system) of the seismic data acquisition. Here the seismic signal detecting system we mentioned above mainly means the geophone and the geophone-ground coupling system. Normally, we consider the signal received by the geophone could faithfully represent the actual vibration of the ground. But actually because of the limit of the geophone's capability and the existence of geophone-ground coupling phenomenon, this becomes different. First, the action of geophones in the seismic data acquisition is to covert the vibration signal to electrical signal, this is a energy covert process. In this process, because of the limit of the geophone's capability there must be the loose of the energy and distortion of the seismic signal. Second, the geophone-ground coupling phenomenon makes the difference between the actual vibration of the ground and the acquired seismic signal. If the geophone is not well planted, for example, in practical use the ground is usually the loose soil and sand, in this condition the geophone could not well coupled to the ground, so the signal may become distortion. Although, the geophone is well planted, the property of the geophone spike's material is very different from the ground soil's property and the shape of the spike will lead to the scattering of the seismic wave. This will actually affect the received seismic signal. So the seismic signal that we received is the reconstructed signal, the reconstructed degree is decided by the property of the geophone and the geophone-ground coupling system.To solve the upper problems, based on the development history, the recent research condition and existing problems, the research of this thesis is mainly about the following three aspects:(1) The property analysis and the practical study of the seismic geophones. Through the analysis of the current seismic geophones'theory and property, we can find that because the limit of materials, shape and structure, the magneto-electric geophone has many disadvantages such as low detecting precision and sensitive, high distortion of phase. Though the MEMS has some advantages, because the limit of his particular information, we can only to say that we just have a basic knowledge of it. From the mechanism-electricity covert theory, we could find that the MEMS could restrain the high energy low frequency signal, so its ability to resist the mechanism disturbance is not very good. The price of it is very high and the old acquisition accessorial equipment can not be used. In practical use, its performance is not as good as its producers say.Through the analysis of the piezoelectric geophones'theory and property, because of its structure and theory advantages, land piezoelectric geophone could realize the high sensitive, board frequency range, high accurate and zero phase detection. To testify the good performance of the land piezoelectric geophone in the practical use, we use the newly developing land piezoelectric geophone to do some experiments in Su Bei area and Da Gang area compared to the common geophone in the comparable condition. The result shows that the newly developing land piezoelectric geophone has larger dynamic and frequency range, higher resolution than the common geophone. And the profile that acquired by the piezoelectric geophone contains more information than that of common geophone. The common orientation axes of the piezoelectric geophone's profile are thinner that of common geophone.(2) The study of the geophone-ground coupling system models. Whether the geophone is well coupled to the ground, directly affects the quality of the acquired seismic data. But many people do not have clear view of it. The thesis analysis the current geophone-ground coupling models deeply. The model based on the wave and vibration theory describes the effects of soil's loose degree and the geophone's mass and shape to the couplings system. The model shows that to make the soil more solid and lower the geophone's mass could increase the system's renounce frequency, fine the phase of the system and make the useful frequency range lager. Although this model the effects of the geophone spike, actually in its theory it omits the effects spike's shape and material to the coupling system. One freedom and two freedom coupling system mainly use the mass-spring-damping model to describe the coupling system. This kind of model shows that we could improve the quality of the coupling system property through the increase the renounce frequency and damping of the coupling system. It could describe the property of coupling system qualitatively, and mainly use to describe the geophone-ground coupling phenomenon in the loose soil condition (plough, sand, swale). This model is just a approximate one, it omits the disturb of wave field caused by the geophone spike, and could not describe the geophone-ground coupling phenomenon in the well contacted condition.(3) The methods of how to improve the bad effects caused by the geophone-ground coupling system. The bad effect caused by the geophone-ground coupling system is an important aspect which affects the high resolution seismic data acquisition. In the study of two freedom coupling system in chapter 3, we find that the damping between the geophone spike and the ground has an important effect on the coupling system. And the best way to change the damping is to change shape of the spike. So we design some experiments to testify this idea. The result shows that we could decrease the random noise, increase the S/N ratio and fidelity of the useful signal by using spiral spike instead of common one, and it is also useful to large the frequency range.The practical use of SWDS (special seismic wave detection system) in the Sinkiang desert area shows that we could large the high S/N ratio frequency range and improve the resolution of the seismic data through increasing the damping between the geophone and ground and decrease the wave-impedance between the geophone spike and the ground soil. Because of high price of the SWDS we develop the match filter to solve this problem. The quality of data processed by the filter is nearly the same to the data gathered by the SWDS.The wave-impedance difference between the geophone spike and the ground medium is an important aspect in the geophone-ground coupling system which affects the energy and the frequency range of the gathered the data. To solve this problem we introduce the impedance transform method which was mainly used in micro-wave area. Through the theory study and numerical simulation, we find that using the impedance transformer could effectively decrease the reflectance between the geophone spike and the ground soil and increase the transmission energy of the seismic signal especially the high frequency part, and this would finally large the frequency range of the seismic signal.