Feasibility of remote underwater current measurement using acoustic doppler techniques

Based on available information on the concentration and size distribution of underwater scatterers, the acoustic scattering cross section of some typical scatterers is estimated. In particular, the scattering characteristics of suspended matter and temperature inhomogeneities are examined in detail. These results, in conjunction with the radar/sonar equation, are employed to ascertain the feasibility of a monostatic Doppler current velocity measuring system. It is shown that a pulsed Doppler technique should permit the measurement of local current velocities in a relatively small volume of water through the determination of frequency shifts between the incident and scattered acoustic radiation. As an example, at a frequency of 300 kHz for 100 W transmitted peak power, reasonably strong scattering (scattering cross section per unit volume per unit solid angle
d\sigma/d\Omega = 10^{-13} cm^{-1}
), and a pulse length resolution of 3 m it should be possible to detect returns from a range of 100 m with a 14 dB signal-to-noise ratio. In the calculations presented, both the ambient noise due to thermal agitation and transmission losses caused by absorption are taken into account.