Current tomography in coastal regions

Abstract

Reciprocal acoustic transmissions have proved successful for tomography in deep water. However, relatively little effort has been focused on shallow-water environments where sound propagation and scattering takes place often over very short ranges in both the ocean (due to volume and surface) and the seabed. In these regions the effect of depth variations on sound speed is negligible and the salinity and temperature are dominant causes of volume fluctuations. Nevertheless, the methodology can still be utilized to determine the current. In a well-calibrated high-frequency (0.6–18 kHz) acoustic experiment, the range averaged horizontal current in two different directions is determined based on the differential travel times recorded from three reciprocal source–receiver stations. These tripod stations were arranged in a triangular configuration. Data were obtained in nearly isovelocity condition with slight variations in the salinity profile during several tide cycles. The feasibility of obtaining range averaged current profile in very shallow-water regions for long-term measurement is demonstrated. It is also shown that the current can be obtained accurately by utilizing not only the refracted (direct) path, but also the surface reflected arrivals during relatively calm surface conditions. The accuracy of this approach depends on the choice of center frequency and bandwidth of the signal. [Work supported by ONR and Sea Grant.]