This paper together with (E.I. Thorsos, K.L. Williams, N.P. Chotiros, J.T. Christoff, K.W. Commander, C.F. Greenlaw, D. V. Holiday, D.R. Jackson, J.L. Lopes, D.E. McGehee, J.E. Piper, M.D. Richardson, and D. Tang. "An overview of SAX99: Acoustics measurements." IEEE Journal of Oceanic Engineering 26:4-25, 2001) provides environmental background and preliminary acoustic results from a highly successful high-frequency acoustics experiment (SAX99) conducted in shallow-water sandy sediments located off the Florida coast. The paper quantitatively demonstrates how sediment physical properties and microtopography (roughness) change rapidly in response to oceanographic (wave and bottom current) and biological (sediment redistribution) forcing. Quantification of these changes allows development of predictive models of the seafloor and prediction of high-frequency seafloor interactions. This paper presents such a complete quantification of the environment that, for the first time, no free parameters are left as ambiguities in high-frequency acoustic propagation and scattering model validation.

  Does it describe a new discovery or new methodology that’s useful to others?

The paper provides the first complete environmental characterization needed to develop, test, and validate high-frequency acoustic models that quantify scattering of acoustic energy from the seafloor, penetration of acoustic energy into the seafloor and propagation of acoustic energy through the seafloor. Especially important were the in situ measurements of sediment, sound, speed, and attenuation for a wide range of frequencies (50 Hz to 500 kHz); fine-scale characterization of sediment microstructure, and temporal characterization of seafloor roughness using photogrammetric, laser line scanner, and electrical resistivity techniques. We also report on several unique acoustic and radiological methods that were developed to obtain quantitative spatial and temporal characterization of seafloor physical properties. Perhaps the most important contribution was the integrated approach to environmental characterization.

  How did you become involved with this research?

The Naval Research Laboratory has a 20-year history of conducting high-frequency acoustic research with the Applied Physics Laboratory at the University of Washington. The work was supported by the Ocean Acoustics Program Office of the Office of Naval Research.

  Could you summarize the significance of your paper in layman’s terms?

Recent acoustic measurements suggest that predicting penetration of high-frequency acoustic energy into the sediment, especially at low grazing angles, is not as easy as simply applying Snell’s Law. Both seafloor roughness and heterogeneity in sediment physical properties contribute to greater acoustic penetration than expected. Acoustic scattering from 0.5 to 1.0 m wavelength ripples on the seafloor typically produced by storm events was found to dominate the acoustic penetration at low grazing angles. Predicting seafloor properties therefore becomes very important when acoustic methods are used to detect and identify objects below the seafloor, including the stratigraphic morphology or the location of pipelines and mines. Understanding frequency-dependent propagation is central to validation of models of acoustic propagation in sediments. The significance of this paper is further demonstrated in papers published in a later issue of IEEE Journal of Oceanic Engineering (27:341-601, 2002), where the essential and complete quantitative environmental characterization of this sandy environment was used for acoustic model development and validation.

Michael D. Richardson, Ph.D.
Marine Geosciences Division
Naval Research Laboratory
Stennis Space Center
MS, USA