Low-frequency measurements of bottom reflectivity at a thin sediment site in the North Pacific

Abstract

Bottom reflectivity measurements were made in the North Pacific, during Pacific Echo I, using a low-frequency, towed source. A MK VI source transmitted 50-s FM sweep signals (5 to 15 Hz) to a vertical array of 16 hydrophones. The FM ramps were deconvolved and beamformed to separate direct and bottom arrivals in time and angle. In addition, ray theory predictions of the arrival structure, scaled by spreading loss, were beamformed. A comparison between the time integrated energy in the measured and theoretical bottom arrivals, led to a measure of the bottom reflectivity. FM ramps were processed along the source ship track to vary the bottom grazing angle from 10 to 75 deg. Reflectivity as a function of bottom grazing angle, smoothed over angle to reduce variance, will be presented. Theoretical computations of reflectivity from a simulated bottom that includes a sediment layer with sound-speed gradient and an elastic basement, with shear and compressional speed gradients and attenuations, will be presented. Theory and measurements are compared to infer the sediment thickness and the wave speed profiles in the upper 200 m of the oceanic crust.