Velocity and Attenuation Profiles in the Monterey Deep-Sea Fan.
Data obtained during a refraction experiment is used to estimate velocity and attenuation profiles in an area of thick sediments (2.5-3 km) on the Monterey Deep -Sea Fan. A 20 element vertical hydrophone array was deployed at mid-depth in 2800 meters of water. Explosive sources, set for a depth of 1820 meters, were detonated at ranges between 3.5 km to 37 kilometers from the array. Estimates of velocity as a function of depth, and attenuation as a function of frequency and depth are obtained from an analysis of the pressure time series generated by the explosive charges and received at the array. To find the velocity profile, the sediment is modeled as a horizontally layered, laterally homogeneous medium. A least squares solution is found for the velocity gradients in each layer of the model. Velocity as a function of depth is obtained by integrating these gradients. A second approach to inferring velocity structure utilizes linear programming. Errors in the data are easily incorporated into the problem using this formulation. The method takes upper and lower bounds on the input data and gives as a solution upper and lower bounds on the velocity profile. All velocity profiles that are consistent with the data lie within these bounds. A method of spectral ratios is used to estimate attenuation in the sediment as a function of frequency and depth. The sediment is again modeled as a layered medium, with each layer having a separate attenuation coefficient. Solving for each of the coefficients gives attenuation as a function of depth.
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- Geophysics; Physics: Acoustics; Geology