Inverse methods and results from the 1981 Ocean Acoustic Tomography Experiment

Abstract

Ocean acoustic tomography was proposed in 1978 by Munk and Wunsch as a possible technique for monitoring the evolution of temperature, density, and current fields over large regions. In 1981, the Ocean Tomography Group deployed four 224 Hz acoustic sources and five receivers in an array which fit within a box 300 km. on a side centered on 26°N, 70°W (southwest of Bermuda). The experiment was intended both to demonstrate the practicality of tomography as an observation tool and to extend the understanding of mesoscale evolution in the low-energy region far from the strong Gulf Stream recirculation. The propagation of 224 Hz sound energy in the ocean can be described as a set of rays traveling from source to receiver, with each ray taking a different path through the ocean in a vertical plane connecting the source and receiver. The sources transmitted a phase-coded signal which was processed at the receiver to produce a pulse at the time of arrival of the signal. Rays can be distinguished by their different pulse travel times, and these travel times change in response to variations in sound speed and current in the ocean through which the rays passed. In order to reconstruct the ocean variations from the observed travel time changes, it is necessary to specify models for both the variations and their effect on the travel times. The dependence of travel time on the oceanic sound speed and current fields can be calculated using ray paths traced by computer. The vertical structure of the sound speed and current fields in the ocean were modelled as a combination of Empirical Orthogonal Functions (EOFs) from MODE. The horizontal structure was continuous, but was constrained to have a gaussian covariance with a 100 km. e- folding scale. The resulting estimator closely resembles objective mapping as used in meteorology and physical oceanography. The tomographic system has at present only been used to estimate sound speed structure for comparison with the traditional measurements, especially the first two NOAA CTD surveys, but the method provides means for estimating density, temperature or velocity fields, and these will be produced in the future. The sound speed estimates made using the tomographic system match the traditional measurements to within the associated error bars, and there are several possibilities for improving the signal to noise ratio of the data. Given high-precision data, tomographic systems can resolve ocean structures at small scales, such as in the Gulf Stream, or at large scales, over entire ocean basins. Work is in progress to evaluate the usefulness of tomography as an observation tool in these applications.