Nonperturbative ocean acoustic tomography inversion of 1000-km pulse propagation in the Pacific Ocean

Abstract

A nonperturbative inversion was performed of acoustic tomography measurements made in the northeastern Pacific Ocean in July 1989, in which acoustic transmissions from a 250-Hz broadband source located near the sound-channel axis were recorded at a long vertical array of hydrophones 1000 km away. In contrast with a conventional inversion, this nonperturbative inversion does not assume that travel times are linearly related to the sound-speed deviations from a background sound-speed model. The inversion process involved three steps: (1) Measured pulse travel times and the source and receiver locations were used to determine the range average of the equivalent symmetric sound-slowness profile. That part of the inversion used only curve fitting and Abel transforms, and required independent (nontomographic) information only to help identify the pulse arrivals. (2) Under the assumption that the range dependence of sound speed was small, we used the reciprocal of the range-averaged sound-slowness profile to approximate the range average of the sound-speed profile. (3) Constraining the sound speed below the sound-channel axis to match climatological data and neglecting the range dependence of sound speed below the sound-channel axis allowed us to estimate the range average of the sound-speed profile above the sound-channel axis. This inversion was compared with the range average of sound speed calculated from CTD measurements made during the experiment over a 10-day period. The agreement was good between 50- and 300-m depths, but there were some disagreements near the surface and near the sound-channel axis.