Mean grain size mapping with single-beam echo sounders

Abstract

Echo energies of single-beam echo sounders are inverted for the sediment mean grain size via a combination of theoretical and empirical relationships. In situ measurements of the seafloor mass density have revealed the presence of a thin transition layer between the water and the sediment. Within this layer, which has a thickness of order 1 cm, the density continuously changes from the water value to the sediment bulk value. The associated impedance gradient affects the normal-incidence reflection coefficient at high frequencies, when the product of wave number and layer thickness is of order unity or higher. A mapping algorithm recognizing this gradient is applied to echo sounder data acquired in three different areas, and for five sounder frequencies between 12 and 200kHz. Compared with a scheme that relies on the Rayleigh reflection coefficient of a discrete interface, an overall improvement of several phi units in the grain size mapping is achieved by taking the gradient into account. A necessary condition to reach agreement between the acoustic and the ground truth grain size is that the thickness of the transition layer increases with a decreasing grain size.