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Todd Hefner

Senior Principal Physicist





Department Affiliation



2000-present and while at APL-UW

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.

Modeling and observations of sand ripple formation and evolution during TREX13

Penko, A., J. Calantoni, and B.T. Hefner, "Modeling and observations of sand ripple formation and evolution during TREX13," IEEE J. Ocean. Eng., 42, 260-267, doi:10.1109/JOE.2016.2622458, 2017

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1 Apr 2017

Ripples on the seafloor affect acoustic scattering and transmission loss, wave attenuation, and the amount of sediment transported in shallow water. Historically, seafloor roughness (a function of ripples, bedforms, sediment type, and size) is assumed to be spatially homogeneous and temporally static in hydrodynamic and acoustic models despite the often dynamic nature of the seafloor in the nearshore region. We present a spectral ripple model, Navy Seafloor Evolution Archetype (NSEA), which simulates the variations in seafloor roughness given measured or predicted wave conditions in sandy environments. NSEA simulates sand ripple formation and evolution based on bottom velocities either measured or predicted by a wave model. The time dependency is a function of equilibrium ripple geometries and the amount of sediment transport needed to reach an equilibrium state, which is dependent on the relict ripples. Spectral decay due to bioturbation is incorporated as a diffusive process. NSEA was validated with time series observations obtained in water depths of 7.5 and 20 m from April 20, 2013 to May 23, 2013 during the 2013 Target and Reverberation Experiment (TREX13) offshore of Panama City, FL, USA. The model predicted spectral ripple wavelengths that were in good agreement with observed spectral ripple wavelengths obtained using a fixed platform, high-frequency (2.25 MHz) sector scanning sonar. Likewise, the variations in the predicted normalized ripple heights and orientations were similar to the normalized spectral decay and orientations estimated from the sector scanning sonar imagery.

More Publications


Signal Processing and Generating Techniques for an Acoustical Navigation Beacon

Todd Hefner, Benjamin Dzikowicz


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