Analysis and modeling of the acoustic tomography signal transmission from Davidson Seamount to Sur Ridge : the forward problem

Abstract

The repeated transmissions of a tomography signal from an autonomous sound source placed on Davidson Seamount was continuously monitored by a bottom- lying, cabled-to-shore receiver on Sur Ridge. To address the signal stability, resolvability and identifiability criteria that determine the applicability of ocean tomography along this path, the data recorded from July 1998 to January 1999 were first processed to obtain the multipath pulse arrival structure and its variability in time. The processed signals showed strong arrivals that were both stable and resolvable. In order to identify the resolved arrivals, acoustic propagation modeling was performed using ray theory in conjunction with measured sound speed and high-resolution bathymetric data. A comparison of the predicted and measured arrival structures show that the observed arrivals were clearly identifiable and were made up of eigenray groups (i.e., eigenray tubes) instead of individual eigenrays. Since the eigenrays within each group were found to have almost identical trajectories through the ocean, the common passage along which the ray group integrates the ocean variability was unambiguous. Consistent with previous CalCOFI observations, the extracted ray group travel time series exhibited dominant oscillations with semidiurnal, diurnal, 8-day, 18-day and 26- day periods, respectively. Using spectral estimation techniques, the travel time variances of these dominant oscillations were quantified. These travel time variances represent direct measurements of the variances of spatially averaged ocean temperatures. Therefore, they are useful for establishing the solution and noise variances for the construction of the inverse solution.