Mapping The Ocean Sound Speed At The ALOHA Cabled Observatory Using Reliable Acoustic Path Tomography

Abstract

We have investigated the feasibility of Reliable Acoustic Path (RAP) tomography using a mobile ship platform (R/V Kilo Moana) and the existing acoustic infrastructure at the ALOHA Cabled Observatory (ACO). Travel times of acoustic signals traveling along direct paths between the shipboard acoustic source and the bottom-mounted hydrophones were measured. Perturbations of the travel times relative to predicted travel times were obtained, based on the CTD cast closest in time to the experiment date. Stochastic linear inversion was employed to solve for sound speed perturbation fields using the travel time perturbation measurements. This provides a spatially-dependent sound speed field (a proxy of temperature) over a 60-km-diameter “teacup” volume of the ocean. This project is a continuation of previous RAP work which laid the foundation for the work described herein. The preliminary results from three RAP cruises since June 2017 showed non-physical range-dependence and ship-dependence of the travel time perturbations. Improvements to the ray tracing and corrections to data processing were made to solve these issues. This resulted in a reasonable range of travel time perturbation variability which yielded realistic sound speed spatial variability from the inversion process. The sensitivity of the travel time perturbation to the third empirical mode of the vertical sound speed perturbation structure was found to be higher than expected. The vertically-averaged sound speed was used to represent the model parameters as it had the overall highest resolution and corresponding low estimated error. These results demonstrate the practicality and utility of the RAP tomography at the ACO and general applicability for other observatories and seafloor geodesy.