An Evaluation of the WOTAN Technique of Inferring Oceanic Winds from Underwater Ambient Sound

Abstract

The potential of the WOTAN technique to estimate oceanic winds from underwater ambient sound is thoroughly evaluated. Anemometer winds and sound spectrum levels at 11 frequencies in the range 3–25 kHz from the FASINEX Experiment are used to establish both the frequency and wind speed dependencies of ambient sound. These relationships are then tested using independent data from four other deployments, and found to hold in the deep ocean in the OCEAN STORMS but not in shallow coastal waters. The OCEAN STORMS ambient-sound wind speed estimates are within ±0.5 m s−1 of anemometer values for wind speeds between 4 and 15 m s−1. Causes of differences, including disequilibrium of the surface wave field, are discussed and it is argued that they are no larger than expected. The procedure for processing ambient-sound data is developed. It includes temperature dependent calibration detection of shipping and precipitation contamination, and standardization of measurements to 1 m depth. The latter procedure allows data from different depths and sound speed profiles to be compared. The potential for using the technique on remote platforms is assessed. On-board processing and subsequent despiking and interpolation would result in a continuous wind record. For time scales of 12 hours or longer the results would be very similar to those obtained with an anemometer. Over shorter time scales there may be some important differences.