Jay J. Pulli, Zachary Upton, Robert Gibson, and Ted Farrell

BBN Technologies

Sponsored by The Defense Threat Reduction Agency
Arms Control Technology Division
Nuclear Treaties Branch

Contract Number DSWA01-97-C-0164


It is well known that acoustic energy that is trapped in the ocean's SOFAR channel can propagate for thousands of kilometers with little attenuation. When this energy encounters bathymetric features that intersect the SOFAR channel (e.g. islands, seamounts, and continental margins), it can be scattered and reflected back into the ocean. Historical data from many underwater explosions show large reflections arriving at receivers tens of minutes or hours after the direct arrival. If the sources of the bathymetric reflections can be identified, these reflected raypaths could then be used to improve the source localization. Spectral characteristics of the reflected signals are also similar to those of the direct arrivals and hence contain valuable information about the source. In some cases, such as when the direct source-receiver path is blocked by bathymetry, the only observable arrivals may be reflections.

Here we present a model for predicting long-range hydroacoustic reflections in the ocean. For a given source location, we use the program HydroCAM to predict ray paths, travel times and propagation losses to each potential scattering patch in the ocean. We then perform a second prediction where rays are computed from the receiver location to the scattering patches. These two results are combined to produce predictions of the bistatic ray path characteristics from the source to the scattering regions to the receiver location. We sort these calculations by time, starting with the direct arrival and extending in constant travel time ellipses out to a given time delay (e.g. 60 minutes past the direct arrival). For each travel time ellipse, we select the bathymetric features along that ellipse with slopes in the direction of propagation greater than a prescribed minimum. If these features intersect the SOFAR channel, their times and locations are stored and used to form an impulse response model of the ocean. Amplitudes are scaled by both propagation loss and an approximation to bathymetric scattering strength. This impulse response is then convolved with a model of the source time envelope to produce a synthetic waveform.

The model has been compared with recorded data for events in both the Pacific and Atlantic Oceans. For the Pacific, we modeled the signal from the IITRI underwater explosion, conducted off the coast of California on February 16, 1968. Recordings of this explosion from hydrophones off Midway Island show a prominent reflected arrival 20 minutes after the direct arrival. This corresponds with the model prediction of a reflection from the Aleutian Islands. For the Atlantic, we modeled the signal from the Chase 21 explosion off the coast of New Jersey in 1970 and recorded off Ascension Island. The reflected arrival seen in the data 9 minutes after the direct arrival corresponds with the model prediction of a reflection from the Guiana Plateau.

Key Words: Hydroacoustics, reflections, modeling, long-range propagation