Sub-Bottom Profiling and Geoacoustic Inversion Using a Ship-Towed Line Array

Bottom properties in littoral water can be very complex i.e., not horizontally stratified and are location dependent, as indicated by core samples and seismic surveys taken in some areas. Acoustic signal propagation cannot avoid interacting with the bottom in shallow water areas. Navy operations in littoral regions require some knowledge of the signal propagation loss, which determines the range that a distant target can be detected. Knowing the bottom, thus, becomes critical for Navy operations in shallow water. Unfortunately, there exist few direct measurements of the ocean bottom except in some isolated locations, since core sampling and seismic measurements of the bottom are generally expensive and time consuming. Existing data bases often yield incorrect predictions of propagation loss because the data entries are sparse and neglect the variation of the bottom properties along the propagation path. A recent effort at NRL is devoted to developing an acoustic method to invert for the bottom properties the geoacoustic inversion method using a horizontal line array towed behind a ship. A ship can cover a wide area in a reasonable time e.g., 24 km in 2 h. The towed array inversion method can provide snapshots of the bottom properties under the ship as it traverses an area, and in this way, build a data base for the Navy. This article summarizes recent advances made at NRL and experimental verifications of the method.