David E. Hannay
JASCO Res. Ltd., Sidney, BC V8L 3S1, Canada
N. Ross Chapman
Defence Res. Establishment Pacific, FMO Victoria, BC V0S 1B0, Canada
Geoacoustic models have been estimated for the upper oceanic crust by inversion of bottom reflection loss versus angle data. The data were obtained in the PACIFIC ECHO experiments at thin-sediment sites in the North Pacific where the age of the crust increased from 40 to 64 million years. The measurements were carried out using a horizontal line array and small explosive charges, and the reflection loss was determined from beamformed array data in order to resolve the specular component. The geoacoustic models consist of profiles of the density, and the compressional (P) and shear (S) wave speeds and attenuations of both the sediment and basalt crust layers. The inversion assumes either a simple homogeneous half-space basement, or a layered model with constant sound-speed gradients in the upper crust overlying a homogeneous basement. The results indicate that the P-wave speed near the sea floor increases with increasing age of the oceanic crust from values of 2700 to about 3300 m/s. These values are significantly lower than those for the sound speed of basalt at thick sediment sites of similar crustal age. Also, there is evidence of a shear wave critical angle at the oldest site. The effects of seafloor scattering and sediment shear wave resonances are discussed.