Measurements of high-frequency acoustic scattering from glacially-eroded rock outcrops

TL;DR

This study measures high-frequency acoustic backscattering from glacially-eroded rock outcrops using synthetic aperture sonar, comparing results with theoretical models and providing calibration methods for accurate scattering strength estimation.

Contribution

It introduces a method to estimate scattering strength and calibration parameters from SAS data, validating models for smooth and rough rock surfaces in a glacial environment.

Findings

Smooth surfaces match small-slope and perturbation theory predictions.

Rough surfaces follow Lambertian model with -30 to -20 dB scattering.

Perturbation and small-slope models overestimate scattering for rough surfaces.

Abstract

Measurements of acoustic backscattering from glacially-eroded rock outcrops were made off the coast of Sandefjord, Norway using a high-frequency synthetic aperture sonar (SAS) system. A method by which scattering strength can be estimated from data collected by a SAS system is detailed, as well as a method to estimate an effective calibration parameter for the system. Scattering strength measurements from very smooth areas of the rock outcrops agree with predictions from both the small-slope approximation and perturbation theory, and range between -33 and -26 dB at 20$^\circ$ grazing angle. Scattering strength measurements from very rough areas of the rock outcrops agree with the sine-squared shape of the empirical Lambertian model and fall between -30 and -20 dB at 20$^\circ$ grazing angle. Both perturbation theory and the small-slope approximation are expected to be inaccurate for the very rough area, and overestimate scattering strength by 8 dB or more for all measurements of very rough surfaces. Supporting characterization of the environment was performed in the form of geoacoustic and roughness parameter estimates.