Toward acoustic tomography in a shallow estuary

There is presently much interest in understanding the hydrophysical parameters in a shallow estuary. The importance of predicting the currents in Hudson River was demonstrated during rescue and recovery of the USAirways aircraft crashed in the river on January 15, 2009. Usually hydrophysical parameters are measured by fixed location sensors that cannot provide high spatial resolution. Acoustic tomography, on the other hand, can map the hydrophysical parameters. Recently, Stevens Institute of Technology using their fully equipped underwater facility including two surface vessels has conducted the first feasibility test on acoustic tomography. The test was conducted in the Hudson River, a shallow estuary that is connected to New York Harbor. The test used an estimate of the differential time of flight of an acoustical signal in opposite directions between two nodes. One node was at a fixed location, while the other was on a research vessel drifting on the other side of the river. An application of a frequency sweep signal (30–90 kHz) and a cross-correlation technique allowed measurements of the difference in time of flight of the acoustic signals propagated in opposite directions that is proportional to the current in the river. The average current speed was estimated and correlated well with current measurements available from the Stevens Observing and Prediction System. [This work was supported by ONR Project #N00014-05-1-0632: Na.]