Detection of objects in sea sediments and estimation of bottom parameters

Abstract

An important objective of the SITAR project, abbreviation for "Seafloor imaging and Toxicity: Assessment of Risks caused by buried waste," is to investigate various acoustic techniques for detection and classification of objects partially or completely buried in the sediments of the bottom. In this paper we report some results from in situ measurements in the Stockholm archipelago in fall 2003. A ROV is running over the bottom with an echo sounder transmitting pulses into the bottom. The pulses are short transients, with centre frequencies of 5 kHz, 10 kHz and 20 kHz. The echoes are received by hydrophones located at the same position as the transmitter, i.e. in the monostatic configuration. A partially buried object exists at the bottom. The first part of this paper will focus on detection of this target, and its appearance in the received signals for different transmitted pulse forms. The second part will focus on the appearance of the bottom in the data and estimation of bottom parameters. As a first approach the acoustic impedance of the bottom is estimated from the ratio of received to transmitted energy at the transmitter/receiver location, but corrected for geometric spreading loss in the water. It is observed a reduction of the higher frequencies in the spectra of the received signals when compared with the spectrum of the transmitted pulse. An attempt is made to explain this fact using theoretical modeling, and the results are used to obtain a roughness estimate as well as an improved acoustic impedance estimate of the bottom.