Matched‐field replica model optimization and bottom property inversion using multitone signals in shallow water

Abstract

Matched‐field replica models based on an inaccurate knowledge of geoacoustic parameters such as bottom attenuation, shear, and interfacial sound‐speed discontinuities, can predict an incorrect number of propagating modes for a shallow‐water channel. The resulting degradation in the matched‐field ambiguity surface can be substantially reduced by optimizing over the modal‐sum‐limit used in the replica model. The use of this technique for multitone (70, 95, 145, and 195 Hz) source‐tow data recorded near San Diego during the first shallow water evaluation cell experiment (SWellEX‐1) significantly increased matched‐field correlation levels for all narrow‐band tones (and their average), with the maximum peak more frequently coinciding the correct source location than that observed with a previous nonoptimized model [Schey and Ryan, 2981A (1994)]. The predicted number of propagating modes was also reduced substantially. An inversion for bottom properties (attenuation, interfacial sound‐speed discontinuities, no shear) provided sediment attenuation estimates which agree well with Hamilton’s models and were an order of magnitude greater than that used in the nonoptimized model, which accounts for the reduction in the number of modes. A simulated model decomposition using the new attenuation verifies the number of modes predicted by the modal‐sum‐limit optimization.