Aubrey Espana Senior Physicist aespana@apl.washington.edu Phone 206-685-2311 |

Education

B.S. Physics, Washington State University, 2003

Ph.D. Physics, Washington State University, 2009

Publications |
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 |
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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 |
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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 |
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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. |

Fast model for target scattering in a homogeneous waveguide Kargl, S.G., K.L. Williams, and A.L. Espana, "Fast model for target scattering in a homogeneous waveguide," J. Acoust. Soc. Am., 132, 1909, doi:10.1121/1.4755005, 2012. |
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1 Sep 2012 |
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A fast ray model for propagation in a homogenous water column tracks time-of-flight wavepackets from sources to targets and then to receivers. The model uses image sources and receivers to account for interactions with the water column boundaries, where the layer of water lies between an upper semi-infinite halfspace of air and a lower semi-infinite halfspace of a homogenous sediment. The sediment can be either an attenuating fluid with a frequency-independent loss parameter or a fluid consistent with an effective density fluid model (i.e., a fluid limit to Biot's model for a fluid-saturated poroelastic medium). The target scattering process is computed via convolution of a free-field scattering form function with the spectrum of an incident acoustic field at the target location. A simulated or measured scattered free-field pressure from a complicate target can be reduced to a scattering form function, and this form function then can be used within model via interpolation. The fast ray-based model permits the generation of sets of realistic pings suitable for synthetic aperture sonar processing for proud and partially buried target. Results from simulations are compared to measurements where the targets are an inert unexploded ordnance and aluminum cylinder. |

Low- to mid-frequency scattering from submerged targets partially buried in the sediment at an oblique angle Zampolli, M., A.L. Espana, K.L. Williams, and P.L. Marston, "Low- to mid-frequency scattering from submerged targets partially buried in the sediment at an oblique angle," J. Acoust. Soc. Am., 131, 3393, doi:10.1121/1.4708804, 2012. |
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1 Apr 2012 |
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The scattering from elastic targets in the low- to mid-frequency regime is affected by the environment surrounding the target. For axisymmetric targets with the axis of symmetry parallel to the water-sediment boundary, previous work has dealt with the change in the target strength as a function of frequency and aspect angle in relation to the burial depth in the sediment. The present work deals with the extension of a finite element model, based on the decomposition of the acoustic and elastic fields into azimuthal Fourier modes, to the case of a target buried at a tilt angle. The interaction between the target and the sediment is represented by the model up to the first order of the scattering series, which means that the scattering of the incident field and of the target reflected field is taken into account, but the rescattering of the boundary reflected echo from the target is neglected. Model results up to 30 kHz are compared to experimental data for a 2 foot long aluminum cylinder of 1 foot diameter buried in sand at a tilt angle. |

Predicting the acoustic response of targets in an ocean environment based on modal analysis of finite element calculations España, A., K. Williams, M. Zampolli, and P. Marston, "Predicting the acoustic response of targets in an ocean environment based on modal analysis of finite element calculations," J. Acoust. Soc. Am., 131, 3394, doi:10.1121/1.4708806, 2012. |
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1 Apr 2012 |
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Low frequency sound is a viable means for the detection of elastic targets in contact with the ocean floor. The incoming sound, with wavelengths on the order of the target dimensions, can excite resonant modes of the target leading to enhancements in the scattered field. A hybrid model has been developed to predict the acoustic scattering from cylinders, pipes and unexploded ordnance (UXO) in proud or buried configurations in the ocean sediment. The model exploits the symmetry by decomposing the 3-D problem into a sum of 2-D independent Fourier modal sub-problems. This hybrid modeling technique has been shown to agree well with experimental measurements conducted in a pond [A.L. España et al., J. Acoust. Soc. Am. 130, 2330 (2011)]. Presently, these hybrid model results are used to examine the target response on a mode-by-mode basis. A modal map is generated by keeping track of the number of dominant modes contributing to the bright features observed in the acoustic template. For features that are predominantly due to one or two modes, simple analytical models can be used to predict their evolution as a function of target/sensor geometry within the ocean waveguide. |

Submerged target scattering: comparison of combined finite element/simplified acoustics models to data Williams, K., A. Espana, S. Kargl, and M. Zampolli, "Submerged target scattering: comparison of combined finite element/simplified acoustics models to data," J. Acoust. Soc. Am., 131, 3393, doi:10.1121/1.4708803, 2012. |
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1 Apr 2012 |
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The environment and the location of the target within that environment affect the scattering from elastic targets in ocean waveguides. Computational power is now realizable to compute the target scattering, in-situ, via finite elements. However, these calculations still require high cost computer facilities and in the end do not offer physical insight into processes involved. Here we compare two models, with different levels of simplification, to data acquired from an Aluminum target machined to replicate an Unexploded Ordnance (UXO). The first model treats the scattering using two-fluid Green's function propagators in combination with finite element calculations of the target scattering as placed within the waveguide. The second model uses free field, plane wave incidence, finite element results for the target scattering in conjunction with simple ray based propagation to account for the waveguide environment. The data/model comparisons are discussed in light of the physical insight they can help provide, the speed of the calculation and the level of fidelity they achieve. |

Measurements and modeling of the acoustic scattering from an aluminum pipe in the free field and in contact with a sand sediment Espana, A.L., K.L. Williams, S.G. Kargl, M. Zampolli, T.M. Marston, and P.L. Marston, "Measurements and modeling of the acoustic scattering from an aluminum pipe in the free field and in contact with a sand sediment," In Proceedings, MTS/IEEE OCEANS 2010, Seattle, 20-23 September, doi:10.1109/OCEANS.2010.5664603 (MTS/IEEE, 2010). |
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20 Sep 2010 |
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Recent experiments conducted in a fresh water pond investigated the monostatic scattering from an aluminum pipe (length-to-diameter ratio of 2) in the free field, as well as in a proud configuration on a flattened sand sediment. Synthetic aperture sonar (SAS) techniques are used to process the data. Absolute target strength is calculated over various spatial filter boundaries of the SAS images in order to isolate the specular and elastic responses of the pipe. A finite element (FE) model has been developed for the aluminum pipe in the free field, making use of the exact geometry associated with the pond experiment. The absolute target strength from these FE calculations is plotted in a similar manner to the experimental data, whereby the specular and elastic contributions are identified and compared to the data. |