Advances in technology and signal processing for ocean acoustic tomography

Abstract

The acoustic source and receiver technologies used to make large-scale tomographic measurements have advanced significantly in recent years. High-efficiency, swept-frequency sources in the 200–300 band have been developed by Teledyne Webb Research. Ultra-low frequency sources in the 30–40 Hz band required for measurements in ice-covered regions have been developed by GeoSpectrum Technologies, Inc. Acoustic receiver technology has also improved. Early instruments were complicated devices because of the scientific requirements for precision timekeeping, measurement of the motion of the moored instruments, and the storage of large amounts of acoustic data. Modern developments in data acquisition systems and data storage have now made the required instrumentation more user friendly. Distributed Vertical Line Array (DVLA) receivers made up of distributed, self-recording Hydrophone Modules and a small number of central controllers now allow large vertical receiving arrays to be deployed. Of equal importance to the hardware developments, the processing of tomographic data has become much more routine. Estimator-correlator processing explicitly accounts for scattering in the receptions. The Viterbi algorithm is used for automated peak tracking to obtain time series of travel times. Finally, travel times (and other data) are now being used to directly constrain ocean circulation models to estimate the state of the ocean.