Measurements of echo stability of an acoustic backscatter system

Acoustic backscatter from acoustic profiling instrumentation is commonly used as a proxy for suspended sediment concentrations. Past calibration procedures have used suspended beads or natural sediment to relate system output to sediment concentration. A new approach seeks to separate the instrumentation system constant from parameters that are dependent on the particles in the water column. Protocols are being developed for the standard-target calibration of an acoustic backscatter system (ABSS). These were initially applied to the Aquatec Acoustic Backscatter System, model AQUAscat 1000, with three transducers at the nominal operating frequencies of 1, 2.5, and 4 MHz. The standard target was a 10-mm-diameter sphere of tungsten carbide with 6% cobalt binder. In addition to measurements of sensitivity and angular response, the operating stability of the system was gauged over four successive nights, with total data collection periods between 16 and 20 hours. Pinging was performed at a constant 64-Hz rate for either 1200 or 1800 s during each hour. The nominal pulse duration was 6.7 μs at each frequency. Each series of 64 echo signals was averaged to form an average echo profile. A total of 1200 or 1800 echo profiles were stored during each hour. The resulting echo data have been processed by integrating the squared echo amplitudes in each profile over range intervals containing the target and over adjacent range intervals without the target, allowing an analysis of both signal and noise. Resulting energy-based measures of signal and noise are presented as time series together with their basic statistics. Enough data were collected to demonstrate that signal to noise ratios are high enough and the standard deviation of measurements recorded over several hours stable enough to allow a rigorous standard target calibration of this instrument in a test tank 3×4×4 m in dimension.