A Direct Method for the Estimation of Sediment Sound Speed with a Horizontal Array in Shallow Water

In this paper, a fast approach for estimating sediment sound speed in a shallow-water environment is developed. Under certain assumptions, this algorithm recovers the seabed sound-speed profile using pressure field measurements at low frequencies. The geometry of the problem involves measuring the pressure at horizontally placed hydrophones in the water column. The Deift-Trubowitz integral equation is then solved. This work introduces two methods for this task. The first is a modified Born approximation that builds upon a standard first-order approximation; the second is based on interpolating the integrand. It is shown with synthetic data that these methods work well with successful sound-speed estimation and identification of sharp discontinuities in sound speed. Although the methods are stable and effective with noise-free data, problems arise when noise contaminates the acoustic field. Regularization approaches, reducing the disruptive effect of singular points and smoothing a measured reflection coefficient, are developed to remedy this problem, leading to improved results in noisy environments. In addition to providing sound-speed estimates, the method also computes sediment thickness. This feature is of particular interest, since it makes the method suitable as a preprocessing step providing useful information to other inversion methods. Sensitivity analyses demonstrate that some assumptions required for the approach implementation are not restrictive.