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| Deep Seismic Sounding (DSS) profile Western China is a showcase of complex geological and geophysical features, including sedimentary basins, regimes of continental collisional tectonics, and the thickest crust found on earth. To be able to accurately monitor western China for nuclear explosions we must first understand as much as possible about the crustal structure there. Here, we present results of a seismic refraction profile across western China (Wang et al., 2001). Seismic energy for this profile was provided by twelve chemical explosive shots fired in boreholes. The charge size ranged from 1500 to 4000 kg, sufficient to provide clear first arrivals to a maximum distance of 300 km. The distance between shotpoints ranged from 63 to 205 km, and the interval between portable seismographs was between 2 and 4 km. The profile was recorded along existing roads, and provided nearly straight profile segments.
During the experiments both P- and S-wave data were acquired, even for data recorded by single-component geophones. The reflection from the Moho was especially strong, and this made it possible for us to derive the crustal composition using the crustal Poisson抯 ratio or Vp/Vs ratio which could be obtained from the crustal P- and S-wave velocity structures.In the correlation of phases, reduction velocities of 6.0 km/s and 3.46 km/s were used for P- and S-waves, respectively. The time scales used for S-waves were multiplied by a factor of 0.58 in the S-wave record section so they would match the P-wave arrival times. Because digital filtering introduces a slight time shift, the unfiltered P-wave data was used for phase correlation and travel time picking. In order to improve the signal-to-noise ratio for phase correlation, the S-wave data was filtered with a bandpass of 0-6 Hz.
Based on the phase correlation, the first arrivals of the Pg phase were used to invert the upper crustal velocity structure using the finite-difference tomographic method of Hole (1992). The reflection phases were used to determine the one-dimensional crustal velocity structure using the X2-T2 method (Giese, Prodehl and Stein, 1976) and the Reflectivity method (Fuchs and Muller, 1971). With one-dimensional crustal velocity structures from each shotpoint, a crustal P-wave velocity structure was established using a 2-D dynamic raytracing program to model both kinetic and dynamic features of the observed seismic wave field (Cerveny, Molotkov, and Psencik, 1977; Cerveny and Psencik, 1984). The different phases on the record sections were all appropriately fitted for travel times and amplitudes.
It is clear that the transect shows three-layer stratification with P-wave velocities of 6.0-6.3 km/s, 6.3-6.6 km/s, and 6.9-7.0 km/s. Upper mantle (Pn) velocities of 7.7-7.8 km/s were found.
The accuracy of the final model is dependent on a number of factors, including the shotpoint interval, receiver density, and thickness of sediments in the shallow crust. Model accuracy primarily depends on the correct identification of the various phases and the density of rays penetrating a particular volume of the model, however. Perturbation of the models has shown that, depending on the uniformity of structure and the density of rays, the resolution of velocity and depth to interface may be accepted as better than 2% and 5%, respectively. By the relationship between the Poisson抯 ratio and the Vp/Vs ratio, the Poisson抯 ratio is determined to within an uncertainty of less than 2%. |
