Acoustically characterizing detritus with a 420-kHz echosounder

Abstract

This paper presents the new application of hydroacoustically measuring and characterizing detritus (here defined as suspended decomposing plant material) for shallow-water environments such as bays, estuaries, or rivers. Detrital material was measured hydroacoustically using a 420-kHz digital echosounder, first in a controlled laboratory test and then in an open estuary. Controlled tests on both small matted masses and individual grass strands produced signals clearly visible (6–15 dB) above the background noise. Field tests were performed using ship-mounted downward looking sonar and echo integration software to determine the feasibility of using hydroacoustics to measure, characterize, and potentially quantify the detrital biomass. A vertical series of plankton nets, slightly downstream of the echosounder, and flow velocity measurements by a 1200-kHz acoustic doppler current profiler (ADCP) slightly upstream provided ground-truth information of local concentrations. Hydroacoustics proved successful in identifying variable material concentrations both spatially and temporally, with typical backscatter intensities of −70 db. Echo amplitude information from the ADCP showed a first-order correlation with backscatter detected by the echosounder and with net sample volumes.