Inversion of multimegameter-range acoustic data for ocean temperature

Abstract

Ocean sound speed (temperature) derived from ray travel times obtained from multimegameter-range acoustic transmissions may be inaccurate when the reference ocean state is inadequate for linearized inversion. When the reference (e.g., the Levitus atlas) is significantly different than the ‘‘true’’ ocean, the ray paths defined by that reference inaccurately represent the true sampling. In addition, natural oceanic variations, such as the evolution of a summer mixed layer, can significantly change the ray sampling. The guiding principle for inversion is that ray travel times associated with the inverse solution must match the measured travel times. Inversion of travel times relative to an arbitrary mean can be inaccurate. Accurate inversions may be obtained by iterative methods or by a better choice of reference ocean, but the solution of this nonlinear problem is not always obvious. Inadequate depth resolution for the vertical structure in the inverse solution can make calculation of travel times difficult. A linearizing time-dependent reference ocean can reduce the solution uncertainties since the a priori model variances are less. Simulations employing the Levitus ocean atlas, so that the ‘‘true’’ ocean state is known, are used to define the problems of nonlinear inversion and to find accurate inversion methods.