A theoretical and experimental examination of seabed discrimination using the first and second backscatter from an echo sounder

Abstract

At normal incidence it is possible to detect a second echo from the seabed in addition to the first echo that is traditionally used by echo sounders. It can be shown that the second backscatter contains information about the seabed that is not contained in the first backscatter. The second echo requires treatment as a bistatic, on axis, geometry in order to understand the mechanisms that provide new information. The effective range of the source is three times the water depth, whereas the receiver is at the water surface for the case where the transducer is at the water surface. The bistatic geometry means that the receiver may be in the near field of the scattering from the seabed. This would mean that the second echo is sensitive to changes in the reflection coefficient of the sediment. Modeling the water surface roughness is achieved by expanding the beamwidth of the source to a level that is representative of the spreading at such an interface. Data from controlled tank experiments over a range of sediment types and water surface conditions are presented. Time-series data from an ocean experiment have been analyzed and show good agreement with ground truth data.