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Todd Hefner

Chair - Acoustics Department & Senior Principal Physicist

Email

hefner@apl.washington.edu

Phone

206-616-7558

Department Affiliation

Acoustics

Publications

2000-present and while at APL-UW

Overview of midfrequency reverberation data acquired during the Target and Reverberation Experiment 2013

Yang, J., D. Tang, B.T. Hefner, K.L. Williams, and J.R. Preston, "Overview of midfrequency reverberation data acquired during the Target and Reverberation Experiment 2013," IEEE J. Oceanic Eng., 43, 563 - 585, doi:10.1109/JOE.2018.2802578, 2018.

More Info

1 Jul 2018

The Target and Reverberation EXperiment 2013 (TREX13) included a comprehensive reverberation field project in the frequency band of 2–10 kHz, and was carried out off the coast of Panama City, FL, USA, from April 21 to May 17, 2013. A spatially fixed transmit and receive acoustic system was used to measure reverberation over time under diverse environmental conditions, allowing study of reverberation level (RL) dependence on bottom composition, sea surface conditions, and water column properties. Extensive in situ measurements, including a multibeam bathymetric survey, chirp sonar subbottom profiling, gravity/diver cores, sediment sound speed and attenuation, interface roughness, wind-generated sea surface waves, and water column properties, were made to support studies of environmental effects on RL. Beamformed RL data are categorized to facilitate studies emphasizing physical mechanisms of 1) bottom reverberation; 2) sea surface impact; and 3) biological impact. This paper is an overview of RL over the entire sea trial, intending to summarize major observations and provide both a road map and suitable data sets for follow-up efforts on model/data comparisons. Emphasis is placed on the dependence of RL on local geoacoustic properties and sea surface conditions.

A time-domain model for seafloor scattering

Tang, D., and D. Jackson, "A time-domain model for seafloor scattering," J. Acoust. Soc. Am., 142, 2968-2978, doi:10.1121/1.5009932, 2017.

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1 Nov 2017

Bottom scattering is important for a number of underwater applications: it is a source of noise in target detection and a source of information for sediment classification and geoacoustic inversion. While current models can predict the effective interface scattering strength for layered sediments, these models cannot directly compute the ensemble averaged mean-square pressure. A model for bottom scattering due to a point source is introduced which provides a full-wave solution for mean-square scattered pressure as a function of time under first-order perturbation theory. Examples of backscatter time series from various types of seafloors will be shown, and the advantages and limitations of this model will be discussed.

Characterization of seafloor roughness to support modeling of midfrequency reverberation

Hefner, B.T., "Characterization of seafloor roughness to support modeling of midfrequency reverberation," IEEE J. Ocean. Eng., 42, 1110-1124, doi:10.1109/JOE.2017.2702005, 2017.

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1 Oct 2017

A seafloor laser scanner was deployed in the Gulf of Mexico during the 2013 Target and Reverberation Experiment (TREX13). This system collected digital elevation maps at 14 locations along the main reverberation track, and these measurements provide roughness power spectra for modeling seafloor acoustic scattering. The spectra were divided into two regimes according to the mid and high-frequency acoustic measurements made during the experiment. For the wave numbers corresponding to the midfrequency regime (2–4 kHz), the spectra could be approximated using the mean spectral exponent derived from the all of the spectra. With this spectral exponent, the best fit spectral strengths were found to be negatively correlated to the backscatter levels measured at 400 kHz using a multibeam echosounder (MBES). While the scattering mechanisms at 400 kHz are not influenced by the roughness at these low wave numbers, this correlation may be indirectly related to the bioturbation and the spatial variation of the shell content. A more pronounced correlation was found for the high wave numbers, where again a single spectral exponent could be used to a good approximation. In this case, the spectral strengths were also linearly related to the MBES backscattering level but with a positive correlation. For these wave numbers, the roughness is largely influenced by the shell content, which is also the dominant scattering mechanism at 400 kHz. The correlations between the roughness and the MBES measurements provide a means to approximate the seafloor roughness parameters in both wave number regimes throughout the experiment site. For the low wave number spectrum, an alternative approach is also proposed, which uses the spectral parameters for the mean spectrum to approximate the roughness throughout the TREX13 site.

More Publications

Inventions

Signal Processing and Generating Techniques for an Acoustical Navigation Beacon

Todd Hefner, Benjamin Dzikowicz

Disclosure

15 Jan 2011

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center
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