Inversion for water column sound speed profile from acoustic travel times using empirical orthogonal functions.

Abstract

An acoustic propagation experiment was conducted in the western continental shelf of India (off Kollam, Kerala) in water depth of ∼71 m with seafloor consisting of hard sandy sediments. The multipath arrival times are obtained from peaks in acoustic impulse response measurements made on a single hydrophone for two source-receiver ranges of 245 m and 320 m. The arrival times are used for inverting the water column sound speed profile (SSP) utilizing the empirical orthogonal functions (EOFs), which can completely describe large datasets. The EOFs are generated from a seasonal dataset consisting of 12 SSPs collected once every month of the year at the same location. Inversion is formulated as an optimization problem and solved by employing the method of Differential Evolution Algorithm. A ray-theory based forward propagation model is implemented to model multipath arrival times with candidate SSPs, reconstructed from the EOFs as input for the two source receiver ranges. The objective function measures mismatch between the observed and modeled travel time estimates. The SSP estimated from modeled arrival times with EOFs as search space is found to agree reasonably well with in situ SSP for the two ranges.