Reliable acoustic path tomography at Aloha Cable Observatory

Abstract

Using a mobile ship platform (R/V Kilo Moana) with an acoustic source transmitting to a fixed bottom hydrophone at the ALOHA Cabled Observatory (ACO), we are investigating the feasibility of “RAP tomography” (Reliable Acoustic Path). This will allow the spatial mapping of the path integrated sound speed (temperature) over a 60 km diameter “teacup” volume of the ocean. This can be considered an extension of an inverted echosounder (from a vertical to near horizontal path) combined with the precise positioning and timing of seafloor geodesy, where noise for the latter, sound speed, is our signal. As a first step, transmissions were sent from a 4x4 array of transducers at 3.5 kHz when the vessel approached and departed from ACO. The measured travel times were compared with estimated travel times based on CTD measurements. As the next step, we use new instrumentation to ensure exact time of acoustic transmission and to test user-generated signals (LFM sweeps and M-sequences) to improve the time of arrival res...