MEASUREMENTS OF ACOUSTIC SCATTERING FROM ROCKY OUTCROPS IN MONTEREY BAY

Abstract

Acoustic measurements were made off Asilomar Beach using a 200kHz split-beam echosounder. The scattering cross section was estimated using the acoustic data collected and the sonar equation. Two methods were used to estimate the grazing angle from the split-beam angle data; a flat bottom assumption and estimated bottom slopes from split-beam angle data. Scattering strength measurements demonstrated an increase as grazing angle increased and spatial variability due to bottom roughness. Scattering strength for 40 degree grazing angle ranged between -37 and -32 dB for flat bottom assumptions and -30 to -27 dB for estimated bottom slopes. The flat bottom assumption achieved an angular range of 40-65 degrees while the estimated bottom slope achieved an angular range of 10-65 degrees. Both cases were fit to Lambert's Law, with the estimated bottom slope having a Lambert parameter between -24 and -22 dB, compared to the flat bottom assumption Lambert parameters of -29 to -27 dB. Estimated bottom slopes were then qualitatively compared to high-resolution bathymetric data and were found to be in agreement. This indicates that a split-beam echosounder can be used as a remote sensing tool to estimate bottom slope features. The findings presented indicate that seafloor slope estimates from a split-beam transducer can be integrated into bottom scattering strength methods to provide improved measurements compared to flat bottom assumptions when making measurements in very rough areas.