On the Interpretation of Measurements of Acoustic Backscatter from Dredged-Material Plumes
David R. Palmer
During the Mobile Alabama Field Data Collection Project (MFDCP), a barge repeatedly released dredging material at an ocean dumpsite near Mobile Bay while a research ship tracked the resulting underwater plumes. This ship was equipped with high-frequency sonar systems and equipment for measuring water properties and collecting sediment and water samples. Ogushwitz has presented an analysis of the relationship between particle concentration in the plumes, as measured from water sampling, and sonar echo strength. To explain the great variability in echo strength measurements, Ogushwitz listed a number of possible sources of variability which we group into four categories: experimental design, plume characteristics, ambient ocean conditions, and instrumentation effects. Ogushwitz argued that one of these sources of variability, the tumbling of the irregularly shaped particles that comprised the plumes, could result in up to 7 dB variability in echo strength. The argument is based on the short-wavelength or geometrical acoustics result that the backscattered intensity is proportional to the geometrical area of the target particle as seen by the incident sonar beam. This argument is somewhat inconsistent, however, since it is known that the scattering took place in the long-wavelength or Rayleigh region. New analytic techniques have been developed since the publication of Ogushwitz’s results that allow us to obtain a more accurate determination of the maximum variability in echo strength that can be attributed to particle shape. In this paper we develop a formalism for applying these techniques to MFDCP. We find tumbling of irregularly shaped particles in the plume can only lead to a variability in the backscattered intensity of from 0.9 dB below the intensity for scattering from spheres to 3.8 dB above the intensity for spheres. We also use this formalism to discuss several of the sources of variability in Ogushwitz’s list and their significance for estimating particle concentration from backscattered intensity. The discussion is framed in terms of the insonified volume determined by the sonar characteristics and defined such that at any specific time the received pressure field is the sum of the fields scattered by particles in this volume. The random distribution of the particles in the insonified volume leads to a Rayleigh distribution for the backscattered intensity. Ping-to-ping variability of the mean concentration of particles in the insonified volume can be used to characterize plume type. For dredging material plumes this variability makes it very difficult, if not impossible, to obtain quality estimates of particle concentration based on knowledge of the acoustic intensity. In addition to this inverse scattering problem, we discuss the value of the images of plumes obtained from single-ping data or data averaged over a few pings. Despite their qualitative nature these images have several valuable uses. We point out that quantitative plume images would have additional uses and that there does not seem to be any obstacle in developing, for a given experimental situation, a formalism for creating them.