Signal Design And System Development Of Pulse-Coded Impact Seismic Source

Pulse-coded impact sources based on swept impact seismic technique can effectively increase the signal-to-noise ratio and resolution of seismic prospecting compared with the conventional single-pulse method. The swept impact method can also result in higher efficiency and is appropriate for shallow-reflection seismic prospecting and other environmental projects. In association with the national major project 863"Key techniques and equipments of metallic mineral deposit"(serial number:2007AA060801), this paper carried out the signal design and system development of pulse-coded impact source, which provided solid theoretical and practical foundations for improving and perfecting the pulse-coded impact source. It also realized the swept impact method and provided reference basis for the pulse-coded impact source being widely used in practical seismic prospecting.To put it briefly, this paper explained the basic concept of controlled sources seismic prospecting and the theory of pulse-coded seismic record. It devised the linear swept impact sequences suitable for pulse-coded impact source. A detailed account of the hardware and software structure for pulse-coded impact source system had been described with pulse-coded signal being real-time simulated. At last, swept impact experiments had been conducted indoor and outdoor using the autonomous developed pulse-coded impact source system in combination with distributed seismic data acquisition system. Minute descriptions of the whole process of the experiments had been given with the seismic data being quantitatively analyzed, which verified the operating stabilization, the uniform of the swept impacts, as well as the feasibleness and superiority of the technique.The major contents of this paper are as follows.This paper analysed the limitations of the existing vibrators based on Vibroseis technique and the random sources based on Mini-Sosie technique. It combined the advantages of the two seismic prospecting techniques and made key researched on the theory of swept impact seismic technique and the design of coded swept signals. It can be known from the comparison and analysis of the advantages and disadvantages of other forms of seismic sources that designing and realizing the pulse-coded impact source are of great theoretical and practical meanings.The paper made a systematic study of the pulse-coded impact source around the respects of theory, hardware and software design and experiments.In the theoretical respect, the coding seismic techniques of controlled sources in seismic surveys had been summarized. It also introduced the basic ideology of seismic prospecting using controlled seismic sources and the correlation technique and the sweeping technique, revealing the thought of impact sources using low-power and long-time impacts equivalent to the high-power and instantaneous excitation of pulse sources. Through theoretical derivations, it gave expression of linear time intervals of swept impacts, providing the theoretical basis for software design. The form of linear sweeping is simple and effective which is easy to realize in mechanical in comparison with the forms of controlled sources producing continuous sweeping signals and random impacts.Special explanations had been focused on the design of hardware and software of impact source system. Due to the requirement of high time precision pulse-coded signals, and for the sake of realizing the uniform of swept impacts, descriptions of overall structure of impact source had been made with a structure block diagram at first, and then it elaborated every unit models in the block diagram. The impact unit of the source system is based on the theory of electromagnet coil, and each component's function had been introduced fulfilling the design of a portable, disassemble, stable and reliable impact unit. The core component of the drive unit is the SSR(solid state relay), and it described some of physical and electrical characteristics of the SSR. Joined with power models, the drive unit can transmit the pulse-coded signals precisely, driving the impact unit to accomplish stable and uniform swept impact. The paper focused on the hardware and software design and real-time simulations of the pulse-coded signal generation unit. The MCU P89LPC936 was used to generate linear pulse-coded swept signals. By means of man-machine interactive interface, swept parameters can be set and the operating process of impact source could be monitored. In software design, it simulated the generation of linear pulse-coded swept signals, completing the entire design and development of pulse-coded impact source system.In the indoor and outdoor experiments of pulse-coded impact source, schemes with different swept parameters had been worked out to accomplish the purposes of the experiments. The single impact experiment gradually reduced the scope of the pulse width, and finally the optimum pulse width 260ms was set for the impact source to operate efficiently, which laid solid foundations for outdoor experiments. And the comparisons between experiments data in different coupling conditions obviously reflected the importance of good coupling condition between the base plate of the impact unit and the ground. In the outdoor experiments, by laying down seismic paths and combining with distributed seismic data acquisition system, a whole process of impact source seismic prospecting had been accomplished. The Seg-2 data generated by the data acquisition system was preliminarily processed in Matlab, and the seismic records were decoded with correlation noise being suppressed effectively, providing original data for further data process like removing surface waves.The main achievements that the paper acquired were:1. In terms of theory, simple and efficient linear-coded swept impact signals were designed by combining Vibroseis and Mini-Sosie seismic techniques, and the mathematical expression of time intervals had been given. The Ricker wavelet was used to replace the seismic wavelet for plotting coded swept signals, and it was researched in algebraic and geometric respects.2. Every component of the pulse-coded impact source were designed and developed at the very core of high requirement of time precision. Under the control of MCU and man-machine interactive interface, it can generate high time precision pulse-coded signals. The photoelectric coupling features of the solid state relay can realize the isolation between strong electrical signals and weak electronic signals with the stable and uniform swept impacts generated from the impact unit driven by the drive unit.3. The minimum pulse width of 260ms had been found in the indoor experiments, which made the impact unit operate stably. The computation and analysis of experiment data preliminarily verified the uniform of swept impacts and the high precision of time intervals, which provided prerequisite conditions for generating high quality seismic signals. A whole outdoor seismic prospecting experiment had been accomplished by cooperating with distributed seismic data acquisition system, and sectional drawings of single-pulse and decoded seismic record were made. The achievements and the solid theoretical and practical foundations for further improvement of pulse-coded impact sources verified the feasibility and efficiency of the whole seismic source scheme.