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Todd Hefner

Senior Principal Physicist

Email

hefner@apl.washington.edu

Phone

206-616-7558

Department Affiliation

Acoustics

Publications

2000-present and while at APL-UW

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.

Underwater acoustic navigation using a beacon with a spiral wave front

Dzikowicz, B.R., B.T. Hefner, and R.A. Leasko, "Underwater acoustic navigation using a beacon with a spiral wave front," IEEE J. Ocean. Eng., 40, 177-186, doi:10.1109/JOE.2013.2293962, 2015.

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1 Jan 2015

In this paper, a method for performing underwater acoustic navigation using a spiral wave-front beacon is examined. A transducer designed to emit a signal whose phase changes by 360°in one revolution can be used in conjunction with a reference signal to determine the aspect of a remote receiver relative to the beacon. Experiments are conducted comparing spiral wave-front beacon navigation to Global Positioning System (GPS) onboard an unmanned surface vehicle. The advantages and disadvantages of several outgoing signals and processing techniques are compared. The most successful technique involves the use of a phased array projector utilizing a broadband signal. Aspect is determined by using a weighted mean over frequencies. Sources of error for each of the techniques are also examined.

From the pole to the equator: Utilizing a screw dislocation in an acoustic wavefront

Hefner, B.T., "From the pole to the equator: Utilizing a screw dislocation in an acoustic wavefront," Proc., 2nd International Conference and Exhibition on Underwater Acoustics, 22-27 June, Rhodes, Greece, 239, 2014.

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22 Jun 2014

A screw dislocation in a wavefront is characterized by a phase dependence about the dislocation axis that varies as exp(–i m φ), where m is an integer and φ is the angle about the axis. This talk discusses two sources which generate an acoustic field with a screw dislocation but for very different applications. The first is the helicoidal wave transducer which generates a beam with a screw dislocation along its axis [Hefner and Marston, J. Acoust. Soc. Am. 106, 3313 (1999)]. At the axis, the phase is indeterminate and as a result there is a corresponding null in the pressure magnitude. The screw dislocation is found to exist in both the far- and near-fields of the transducer. This null then clearly indicates the axis of the beam at all distances and has the potential to be used as an aid in the alignment of objects in sonar experiments or other similar applications. This beam is also shown to carry angular momentum. The second source utlizes a screw dislocation but far from the null axis. It generates a wavefront in the x-y plane that has a phase which is proportional to the azimuthal angle about the source (m = 1). This transducer is combined with an omnidirectional, reference source to produce a spiral wavefront beacon. The phase difference between these sources contains information about a distant receiver's azimuthal angle relative to the beacon and can be used for underwater navigation [Hefner and Dzikowicz, J. Acoust. Soc. Am. 129, 3630 (2011)]. Navigation using this beacon has been demonstrated experimentally and propagation models have been developed to assess the performance of the beacon for the general case of propagation in a horizontally stratified waveguide [Hefner and Dzikowicz, J. Acoust. Soc. Am. 131, 1978 (2012)]. This talk discusses both of these unique sources and their applications as well as the underlying physics which connects them.

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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