Recently, close range, high resolution sonars and acoustic Doppler current meters have created a need for understanding underwater acoustic volume reverberation at frequencies greater than 500 KHz. The cause of signal dropout in Doppler current meters must also be determined. At the high frequencies and close ranges involved, traditional volume reverberation results are inappropriate. Several scattering models are described which predict volume reverbelation at frequencies up to 10MHz, and ranges down to 12 inches. A coherent scattering explanation of signal dropout has also been developed. Numerical results for representative conditions in the world's oceans are given. Sensor parameters for maximum return signal are described. Suggestions are offered for applying the analytical results to the improvement of Doppler current meters and the development of acoustic fish counters, pollutant monitors, ship's speed logs, and oceanic thermal fluctuation spectral analysis.
The work described in this paper was performed to meet the objective of studying the operating principles of Doppler acoustic current meters and high frequency, short range sonars in general. The goal of the analysis has been both to predict and offer suggestions for improved performance.
The two basic concepts required for understanding the operation of a Doppler current meter are
the Doppler frequency shift of sound backscattered from a moving object.
acoustic backscattering from fluid Inhomogeneities
If sufficient volume reverberation occurs, the magnitude of the backscattered signal is not of interest for current meter applications. However, the acoustic inhomogeneities which scatter the incident sound are usually minuscule particulate matter or, possibly, thermal fluctuations about ambient temperature. These inhomogeneities produce only a small perturbation in the average fluid parameters. This implies the return backscattered signal is only a small fraction of the transmitted power. To predict whether current meter operation is possible in very clear water and to optimize such operation, knowledge of the backscattered signal level is necessary. A number of scattering models suitable for this purpose have been developed.
Related to the weak return is the phenomenon of signal fading or dropout. Operationally, such dropouts have proven to be a problem. Analysis of the backscattering process has yielded a possible explanation for this phenomenon in terms of coherent effects. Several cures are suggested.
Doppler current meters are not new devices. Although a considerable history for Doppler current meters exists, most of the references are empirical accounts of data measurements. Virtually nothing has been available which models the scattering process in a manner useful to equipment designers and users.