Seismic Tomography Of Shizigou In The Western Qaidam Basin

Shizigou in the western Qaidam basin is a potential area for the accumulation of oil and gas. But the geological structure is complicated, the surface erosion is serious and the topography is highly undulated, which seriously affect the geophysical surveys in Shizigou region. Considering the merits of passive source seismic methods such as abundant information in seismic waveform, less limits on the surface observational condition and low exploration cost, Institute of Geomechanics of Chinese Academy of Geological Sciences, financially supported by National Oil and Gas Office, deployed a portable seismic array in Shizigou in the western Qaidam basin for the first time. Using the seismic data recorded by these portable stations, we attempted to reconstruct the velocity structure in Shizigou with seismic tomography, to provide information for the investigation of the geological structure and characteristics in Shizigou, and to test the validity of the methods in delinating small size of structure.We processed the data recorded continuously by the stations from October, 2004 to August, 2007 in Shizigou in the western Qaidam basin. This paper employed the double difference earthquake location algorithm to relocate micro-earthquakes, both teleseismic and local seismic tomographies to infer the three-dimension velocity structure and receiver function method to study trially along three profiles in the study region. These results do reveal some geological structure and characteristics and provided some useful information for investigation and evaluation of oil and gas resources in Shizigou.The double difference earthquake location algorithm proposed recently is a relative location method. This paper summarized its basic principle and introduced to the relocating of the micro-earthquakes in Shizigou in the western Qaidam basin. The relocated results show that the hypocenters are spatially related to the fault zone in the study region. Besides, the activities of oil pumping and fluid injection through the wells have certain effects on inducing the micro-earthquakes. The root mean square residual is about 0.36s, and the averaged relative error is estimated at about 0.15m in both horizontal and vertical directions. These accuracy results of double difference earthquake location disclose the seismic activity, which is beneficial to investigate the geological structure in Shizigou.Distant earthquakes and local earthquakes were extracted respectively from the seismic data and their first arrivaltimes were picked up. In this paper, we used ACH teleseismic tomography and double difference local seismic tomography methods respectively to inverse the three dimensional velocity structure in Shizigou. The tomographic results reveal that teleseismic tomography can unravel some background velocity structure and local seismic tomography can partically recover some local structure features in Shizigou. A NW-trending low velocity anomaly dips to northeast. This low velocity anomaly is surrounded by some high velocity anomalies from the north, northeast and east. These high velocity anomalies are in the same strike and coverage as the Shizigou anticline. The images of P wave velocity (V_P), S wave velocity (V_S) and V_P/V_S ratio show the spatial distribution of velocity anomalies in the study area. In the southwest of the study region, the low velocity anomaly extends to ~6km depth toward northeast with a dip angle of～40°. This low velocity anomaly correlates well with the hypocenters of the micro-earthquakes. The images of S wave velocity (V_S) and V_P/V_S ratio display a low velocity anomaly toward southwest at depths about 1.5km to 4km.The receiver function images outline the velocity structure beneath the stations and the structural characteristics of the inhomogeneous bodies along three profiles. The results reveal that there are continuous P-to-S conversion interfaces at～1.5km and～3.5km beneath some stations respectively. We guessed the interfaces at～1.5km and～3.5km depths might be the important stratigraphical boundaries in the study area.In summary, this thesis presents my study on the employing seismic tomography and receiver function method to reconstruct the shallow velocity structure in Shizigou in the western Qaidam basin. We achieved certain practical experience on resolving the small size of structure as the Shizigou structure. The results suggest that seismic tomography could be used to delineate the strike and the extents of structures in the study area and receiver function method could display some characteristics of conversion interface beneath the stations.