VELOCITY AND ATTENUATION PROFILES IN THE MONTEREY DEEP-SEA FAN

Data obtained during a marine refraction experiment are 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 m of water. Explosive sources, set to detonate at 1830 m, were launched at ranges between 3.5 and 37 km from the array. Estimates of velocity and attenuation as a function of depth are obtained from an analysis of the pressure time series generated by the explosive charges and received at the array. The sediment is modeled as a horizontally layered, laterally homogeneous medium. A least-squares solution is found for linear velocity gradients in each layer of the model. Velocity as a function of depth is obtained from these gradients. Attenuation coefficients in each layer of the model are found using a method of spectral ratios. The results presented here, valid for frequencies between 10 and 100 Hz, show an increase in attenuation to a depth of about 500 m. The method of spectral ratios and equations used to infer velocity structure from various parametrizations of the traveltime data are derived in appendices.