Technique Study Of Seismic Velocity Change Observation System Using Small Scale Active Source

Knowing the inner earth structure and its variation in the medium has magnificent meaning to understand the seismogenic process, stress change in the subsurface, as well as oil and gas flow monitoring. Changes of the medium inside the earth can be reflected by seismic velocity change. Currently, using active sources to monitor underground medium has become an important study field. As base lines in observation are different, our study of seismic velocity change in the medium using active sources can be divided into three scales, from large to small. Compared to the large scale sources, it is more strict in the precision of seismic velocity change monitoring for small ones.Based on former study, this article aims at increasing precision of velocity change monitoring in the medium. We analyzed factors that influence the precision of seismic velocity change measurement, from which we construct a set of small scale seismic velocity change observation system with high precision. We also carried out an continuously observation experiment at Xi Bozi station in Yan Qing district, Beijing which last for about one year. Results of velocity change at the station with high precision are acquired, and several key techniques are done.Studies show that the main factors which influence the precision of the seismic velocity measurement are as follows:(1) Energy of the source and its repeatability; (2) Trigger precision of the sampling in time; (3) SNR of the wave forms; (4) Analysis of the wave forms; (5) Performance of the acquisition equipment; (6) Deployment and operation of the observation system.A set of small scale observation system for seismic velocity change monitoring has been constructed, using giant magneto striction transducer and high speed data acquisition equipment. The system is stable enough for the high precision observation of small scale seismic velocity change.Through analyzing and processing of the continuous observation data, we get the following conclusion:(1) Giant magneto striction transducer is a reliable repeat source, its correlation coefficient can be up to 0.999 and above; (2) By increasing the sample rate to 10M, the trigger error in time has greatly reduced; (3) SNR of the wave forms can be greatly increased by stacking, filtering and pre-amplifying; (4) The precision of seismic velocity change measurement can be up to 10-6 using Back and Front correlation; (5) The results of the seismic velocity change show good correlation to the temperature and atmosphere change.