Long Range Acoustic Correlation Current Profiler

A new low frequency, long range acoustic correlation current profiller (ACCP) has recently been built and tested. Results indicate that water profiles were successfully measured to depths of approximately U00 meters and seabed returns were recorded to depths of approximately 5000 meters. The ACCP water profiles agree with both the Doppler results and theoretical predictions for signal-to-noise ratio and variance. relatively wide beam acoustic transducer of angular width 8 and frequency f.,. The returning pressure waves are monitored in time at differing spatial locations. The signal return from one receiver at a time, t, will be highly correlated with the return from another receiver at a time e+ P, provided that the scatters have moved exactly one-half the spatial vector difference between the two receivers in a time T,. This effect is referred to as "waveform invariance" fl). Equivalently, "waveform invariance" implies that the time domain return from a series of stationary scatterers depends not on the exact location of the transmitter and receiver, but rather only on the midpoint of the line segment which joins the transmitter and receiver. The principle of waveform invariant geometry assumes only that the vertical distance to the scatterers is large compared to the transducer element separation. For the correlation sonar, the transducer separations are typically of the same order of magnitude as the wavelength of sound, and therefore this approximation applies in almost every conceivable case. Another interesting feature of waveform invariance is the complete lack of any dependence of the horizontal velocity on the speed of sound in the water.