Comparison of Velocity and Attenuation between the Nicobar and Bengal Deep Sea Fans

摘要

Compressional wave velocity and attenuation measurements from a deep source, deep receiver seismic refraction experiment from the Nicobar Fan, Indian Ocean, are presented and compared to a similar set of measurements in the Bengal Fan. The two deep sea fans consist of thick sections of turbidites derived from the same sediment sources but have slightly different depositional histories. Travel time inversion of the data into a velocity depth model uses the lau-zeta linear inversion scheme. We present a revised reparameterization of the travel time data and improved error analysis for the inversion process. Although the velocity structures with depth are morphologically similar, there exist discrete differences between the two stations. In the Nicobar Fan, the initial velocity (assumed) is 1.513 km/s, with an initial gradient of 2.32/s. The velocity increases smoothly with depth, while the velocity gradient decreases rapidly to a constant 0.81/s. The Bengal Fan station showed the same trend, although the velocity gradient at depth was only 0.67/s. The attenution profiles are similar only in gross structure, with the Nicobar Fan values of 1/Q being a factor of 3 to 4 less than the Bengal station. Differences in velocity and attenuation structure can be attributed to a number of causes, including porosity and sedimentation rates (which are related to the distance from the sediment source) and the presence of a normal fault near the Nicobar Fan site. This fault may act as a conduit for water removal in the presence of overburden pressure.