Sediment Classification Using the Chirp Sonar

Abstract

ABSTRACT The chirp sonar is a quantitative seismic profiler used to generate high resolution reflection profiles of the seabed and estimates of the acoustic properties of sediments. Since the FM acoustic pulse is generated by a D/A converter with a wide dynamic range and a transmitter with highly linear components, the energy, amplitude, and phase characteristics of the acoustic pulse can be precisely controlled. This precision provides the high repeatability and signal definition required for sediment classification. Chirp sonar data is processed to produce acoustic impedance and attenuation profiles which are used to classify ocean sediments and to predict physical properties. These sediment classification procedures together with ground truth core data are useful for building maps of the horizontal and vertical distribution of sediment properties such as acoustic impedance, acoustic attenuation, grain size, bulk density, and sound speed. Such maps are needed for EEZ resource development, geotechnical and hazard surveys, environmental site investigations and geological studies. INTRODUCTION The development of a towed instrument which remotely measures the properties of ocean sediments would provide a cost effective means of generating vertical and horizontal maps of sediment properties. Presently, the most commonly used method for determining sediment properties is coring or drilling from a stationary vessel; this method provides only point samples of the seafloor, requires a dedicated survey vessel and consumes significant personnel resources, thereby making mapping of seafloor properties over large areas, such as the EEZ, cost prohibitive. A sediment classification sonar that provides real time estimates of sediment properties could be used to rapidly survey offshore and nearshore areas to support a wide range of scientific and engineering investigations. civil engineers could utilize the data from sediment property maps to estimate the costs of removing sediments from harbor channels and to calculate the bearing capacities for offshore structures, bridge and port construction. These maps of physical and acoustic properties would aid in identifying offshore hazards and estimating the amount of sediment gas. Geologists could use the maps for studying Holocene sedimentation and offshore mineral resources. Maps of acoustic attenuation and impedance could be used in underwater acoustic models to improve the prediction of sound propagation losses over large distances. To develop the capability of remote acoustic classification of seafloor sediments, the Office of Naval Research sponsored the development of the chirp sonar, a digital, quantitative FM sonar that measures the impulse response of the seafloor at normal incidence. Development of the sonar was completed in 1990 and the technology was transferred to industry and the U.S. Navy. Since 1990, algorithms have been developed to provide real time estimates of acoustic attenuation and impedance as a function of subbottom depth. These acoustic parameters can be empirically related to physical properties such as grain size, porosity, rigidity, and bulk density. This paper presents some results of recent experiments in which the chirp sonar was used to predict sediment type. ACOUSTIC IMPEDANCE AND ATTENUATION Two acoustic properties are estimated from chirp sonar reflection data: acoustic reflectivity and compressional wave attenuation.