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Aubrey Espana

Principal Physicist





Department Affiliation



B.S. Physics, Washington State University, 2003

Ph.D. Physics, Washington State University, 2009


2000-present and while at APL-UW

Scattering from objects at a water–sediment interface: Experiment, high-speed and high-fidelity models, and physical insight

Kargl, S.G., A.L. España, K.L. Williams, J.L. Kennedy, and J.L. Lopes, "Scattering from objects at a water–sediment interface: Experiment, high-speed and high-fidelity models, and physical insight," IEEE J. Ocean. Eng., 40, 632-642, doi:10.1109/JOE.2014.2356934, 2015.

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1 Jul 2015

In March 2010, a series of measurements were conducted to collect synthetic aperture sonar (SAS) data from objects placed on a water-sediment interface. The processed data were compared to two models that included the scattering of an acoustic field from an object on a water-sediment interface. In one model, finite-element (FE) methods were used to predict the scattered pressure near the outer surface of the target, and then this local target response was propagated via a Helmholtz integral to distant observation points. Due to the computational burden of the FE model and Helmholtz integral, a second model utilizing a fast ray model for propagation was developed to track time-of-flight wave packets, which propagate to and subsequently scatter from an object. Rays were associated with image sources and receivers, which account for interactions with the water-sediment interface. Within the ray model, target scattering is reduced to a convolution of a free-field scattering amplitude and an incident acoustic field at the target location. A simulated or measured scattered free-field pressure from a complicated target can be reduced to a (complex) scattering amplitude, and this amplitude then can be used within the ray model via interpolation. The ray model permits the rapid generation of realistic pings suitable for SAS processing and the analysis of acoustic color templates. Results from FE/Helmholtz calculations and FE/ray model calculations are compared to measurements, where the target is a solid aluminum replica of an inert 100-mm unexploded ordnance (UXO).

High frequency backscattering by a solid cylinder with axis tilted relative to a nearby horizontal surface

Plotnick, D.S., P.L. Marston, K.L. Williams, and A.L. España, "High frequency backscattering by a solid cylinder with axis tilted relative to a nearby horizontal surface," J. Acoust. Soc. Am., 137, 470-480, doi:10.1121/1.4904490, 2015.

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1 Jan 2015

The backscattering spectrum versus azimuthal angle, also called the "acoustic color" or "acoustic template," of solid cylinders located in the free water column have been previously studied. For cylinders lying proud on horizontal sand sediment, there has been progress in understanding the backscattering spectrum as a function of grazing angle and the viewing angle relative to the cylinder's axis. Significant changes in the proud backscattering spectrum versus the freefield case are associated with the interference of several multipaths involving the target and the surface. If the cylinder's axis has a vertical tilt such that one end is partially buried in the sand, the multipath structure is changed, thus modifying the resulting spectrum. Some of the changes in the template can be approximately modeled using a combination of geometrical and physical acoustics. The resulting analysis gives a simple approximation relating certain changes in the template with the vertical tilt of the cylinder. This includes a splitting in the azimuthal angle at which broadside multipath features are observed. A similar approximation also applies to a metallic cylinder adjacent to a flat free surface and was confirmed in tank experiments.

Acoustic scattering from a water-filled cylindrical shell: Measurements, modeling, and interpretation

España, A.L., K.L. Williams, D.S. Plotnick, and P.L. Marston, "Acoustic scattering from a water-filled cylindrical shell: Measurements, modeling, and interpretation," J. Acoust. Soc. Am., 136, 109-121, doi:10.1121/1.4881923, 2014.

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1 Jul 2014

Understanding the physics governing the interaction of sound with targets in an underwater environment is essential to improving existing target detection and classification algorithms. To illustrate techniques for identifying the key physics, an examination is made of the acoustic scattering from a water-filled cylindrical shell. Experiments were conducted that measured the acoustic scattering from a water-filled cylindrical shell in the free field, as well as proud on a sand-water interface. Two modeling techniques are employed to examine these acoustic scattering measurements. The first is a hybrid 2-D/3-D finite element (FE) model, whereby the scattering in close proximity to the target is handled via a 2-D axisymmetric FE model, and the subsequent 3-D propagation to the far field is determined via a Helmholtz integral. This model is characterized by the decomposition of the fluid pressure and its derivative in a series of azimuthal Fourier modes. The second is an analytical solution for an infinitely long cylindrical shell, coupled with a simple approximation that converts the results to an analogous finite length form function. Examining these model results on a mode-by-mode basis offers easy visualization of the mode dynamics and helps distinguish the different physics driving the target response.

More Publications

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