The precision of travel time in ATOC experiments

Acoustic-ray travel-time measurements at 75 Hz are being used in the ATOC tomography experiment to infer important oceanographic variables such as heat content, currents, etc. The accuracy with which these measurements can be made is limited by the small-scale structure of the ocean (such as internal waves). Examined in detail, rayfronts show variations in time, depth (for a vertical array) and in pulse shape. At ranges of up to 5 Mm, coherence times of greater than 10 min, vertical coherence lengths of greater than 500 m, and rms pulse variability (spread and wander) of less than 50 ms were the norm. Comparison of these observations with predictions based on internal-wave effects will be described. Although the numbers above are preliminary estimates, the basic result is clear, basin scale measurements can be used to effectively monitor climate. Travel times can be measured by modal filtering as well as by ray beamforming. Methods to interpret these measurements will also be discussed. [Work supported by the Strategic Environmental Research and Development Program through ARPA.]