Inference of biological and physical parameters in an internal wave using multiple-frequency, acoustic-scattering data

Abstract

High-frequency sound (>10 kHz) is scattered in the ocean by many different processes. In the water column, marine organisms are often assumed to be the primary source of acoustic backscatter. Recent field experiments and theoretical work suggest that the temperature and salinity microstructure in some oceanic regions could cause acoustic scattering at levels comparable to that caused by marine life. Theoretical acoustic-scattering models predict that the scattering spectra for microstructure and organisms are distinguishable from each other over certain frequency ranges. A method that uses multiple-frequency acoustic data to exploit these differences has been developed, making it possible to discriminate between biological and physical sources of scattering under some conditions. This method has been applied to data collected in an internal wave in the Gulf of Maine. For regions of the internal wave in which the dominant source of scattering is either biological or physical in origin, it is possible to combine the acoustic-scattering data and temperature and salinity profiles with acoustic-scattering models to perform a least-squares inversion. Using this approach, it is possible to estimate the dissipation rate of turbulent kinetic energy for some regions of the internal wave, and the length and numerical abundance of the dominant biological scatterer, euphausiids, in others.

Distribution, patchiness, and behavior of Antarctic zooplankton, assessed using multi-frequency acoustic techniques

“…Again, this is consistent with previous acoustic studies of euphausiids, where density estimates from acoustic systems have exceeded those from nets by one to three orders of magnitude (e.g., in the Antarctic, Zhou et al, 1994; in the Irish Sea, Mitson et al, 1996; in the Gulf of Maine, Warren et al, 2003).…”

“…This process of inferring the abundance and distribution of zooplankton in a quantitative sense from acoustic measurements, however, is not straightforward (Stanton et al, 1994;Wiebe et al, 1996). Scattering in the water column can result from both physical oceanic processes (e.g., microstructure; Warren et al, 2003) and the biota, where scattering from the latter is a complex function of the taxonomic composition of animals present, and the associated variability in their size, shape, physical properties, and behavior. Accurate inference of organismal parameters such as abundance from acoustic measurements thus requires a comprehensive understanding of the scattering processes involved.…”

“…Target strength is a critical quantity in making abundance estimates from acoustic data, and the Greene et al (1991) model marked a substantial improvement over earlier target strength models developed during the BIOMASS program (see review in Miller and Hampton, 1989), but these discrepancies have yet to be fully reconciled. Furthermore, developments made in zooplankton acoustics elsewhere involving the use of multi-frequency acoustic data to estimate simultaneously the abundance and size of animals (e.g., Warren et al, 2003) have as yet not been applied in the Southern Ocean beyond individual test-case krill aggregations. Thus while the field of Antarctic krill acoustics has achieved a reasonable level of sophistication, there still exist opportunities for improvement.…”

“…Aside from the artifactual scattering excised from the acoustic data collected by the up-looking transducer described above, we have little information on the possibility of scattering from non-biological sources. Work in the Gulf of Maine has demonstrated that small-scale variations in the temperature and salinity structure of the water column (i.e., microstructure) may at times scatter sound at levels comparable to that from zooplankton (Warren et al, 2003).…”

Continued Analysis of High-Frequency Broadband Acoustic Scattering from Non-Linear Internal Waves during SW06

“…More recently, there has also been significant effort directed towards the quantitative use of narrowband acoustic scattering techniques for investigating small-scale physical processes, such as oceanic microstructure (e.g. Goodman, 1990;Seim et al, 1995;Lavery et al, 2003;Ross and Lueck, 2003;Warren et al, 2003). Acoustic scattering techniques provide a rapid, high-resolution, synoptic, remote sensing alternative to more traditional sampling strategies.…”