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Timothy Marston

Sr. Engineer





Department Affiliation



B.S. Electrical Engineering, Seattle Pacific University, 2004

M.S. Acoustics, Penn State, 2006

Ph. D. Acoustics, Penn State, 2009


2000-present and while at APL-UW

Circular synthetic aperture sonar imaging of simple objects illuminated by an evanescent wavefield

Plotnick, D.S., T.M. Marston, and P.L. Marston, "Circular synthetic aperture sonar imaging of simple objects illuminated by an evanescent wavefield," J. Acoust. Soc. Am., 140, 2839-2846, doi:10.1121/1.4964329, 2016.

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1 Oct 2016

This paper is motivated by the case where an underwater object located within the sediment is illuminated by a grazing acoustic beam below the critical angle. The included experimental work uses a liquid−liquid interface and vertically inverted geometry as a stand-in for the water−sediment boundary. In the super-critical regime sound in the water column refracts into the sediment before scattering. However, for sub-critical illumination a rapidly decaying evanescent wavefield is generated in the sediment near the water−sediment interface. For compact objects located in the sediment near the interface this can result in strong backscattering signals suitable for acoustic image reconstruction using synthetic aperture sonar techniques. Certain properties of the evanescent wavefield such as the vertical phase-locking behavior, the rapid amplitude decay with distance from the interface, and the low-pass filter effect have understandable ramifications for the image formation process and for characteristics of the reconstructed image. In particular, circular imaging techniques require correct placement of the imaging plane to properly focus an object; however, for backscattering (monostatic) evanescent image formation the imaging plane may be placed at the interface and the target will remain in focus regardless of burial depth. A laboratory experiment using simple scatterers is presented.

Volumetric acoustic imaging via circular multipass aperture synthesis

Marston, T.M., and J.L. Kennedy, "Volumetric acoustic imaging via circular multipass aperture synthesis," IEEE J. Ocean. Eng., 41, 852-867, doi:10.1109/JOE.2015.2502664, 2016.

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1 Oct 2016

In this paper, volumetric imaging via multipass circular synthetic aperture sonar (CSAS) is demonstrated using an autonomous underwater vehicle (AUV). A multidimensional aperture is synthesized by performing a series of circular scans at varying grazing angles around targets and coherently combining the backscattering information from the set of scans to form high-resolution volumetric images. A data-driven technique for precision alignment of the individual scans comprising the multipass set enables synthesis of a multidimensional array. To beamform in the vertical dimension using the irregular and undersampled multipass aperture, a compressive-sensing-based approach is adopted which is similar to methods used in analogous synthetic aperture radar tomography applications but modified to accommodate for the wider fractional bandwidth of the synthetic aperture sonar (SAS) system. The modification exploits a joint sparsity assumption in the vertical scattering profile at different subbands and adapts a standard joint sparse solving algorithm to the relevant case in which the sparsity profile is common between solution vectors but the sensing matrices are different. Results are shown for a variety of targets, including proud and obliquely buried unexploded ordnance, a 2-1 solid aluminum cylinder, and a steel oil drum.

Image-based automated change detection for synthetic aperture sonar by multistage coregistration and canonical correlation analysis

G-Michael, T., B. Marchand, J.D. Tucker, T.M. Marston, D.D. Sternlicht, and M.R. Azimi-Sadjadi, "Image-based automated change detection for synthetic aperture sonar by multistage coregistration and canonical correlation analysis," IEEE J. Ocean. Eng., 41, 592-612, doi:10.1109/JOE.2015.2465631, 2016.

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1 Jul 2016

In this paper, an automated change detection technique is presented that compares new and historical seafloor images created with sidescan synthetic aperture sonar (SAS) for changes occurring over time. The method consists of a four-stage process: a coarse navigational alignment that relates and approximates pixel locations of reference and repeat–pass data sets; fine-scale coregistration using the scale-invariant feature transform (SIFT) algorithm to match features between overlapping data sets; local coregistration that improves phase coherence; and finally, change detection utilizing a canonical correlation analysis (CCA) algorithm to detect changes. The method was tested using data collected with a high-frequency SAS in a sandy shallow-water environment. Successful results of this multistage change detection method are presented here, and the robustness of the techniques that exploit phase and amplitude levels of the backscattered signals is discussed. It is shown that the coherent nature of the SAS data can be exploited and utilized in this environment over time scales ranging from hours through several days. Robustness of the coregistration methods and analysis of scene coherence over time is characterized by analysis of repeat pass as well as synthetically modified data sets.

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Semiparametric statistical stripmap synthetic aperture focusing

Marston, T.M., and D.S. Plotnick, "Semiparametric statistical stripmap synthetic aperture focusing," IEEE Trans. Geosci. Remote Sens., 53, doi:10.1109/TGRS.2014.2353515, 2086-2095.

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1 Apr 2015

Autofocusing synthetic aperture imagery by maximizing a statistical quality metric such as contrast or sharpness is a well-documented approach in both synthetic aperture radar (SAR) literature and synthetic aperture sonar literature. It is most successfully applied in spotlight-mode SAR applications, where the assumption of spatial invariance of the corrupting phase function is strong and expressions for the gradients of various quality metrics with respect to standard error models have been calculated. Examples of application to stripmap imagery often involve sectioning images into small blocks, allowing spotlight algorithms to be patchwise applied. This paper formulates the gradient of the cost function in a manner that is consistent with the stripmap error model, inherently providing solutions that compensate for the spatial variance while simultaneously bypassing the need for subdividing an image or aperture. This paper formulates the stripmap gradient expression in conjunction with a computationally efficient imaging approach to rapidly achieve metric-maximizing solutions. Demonstrations are shown on a widebeam wideband rail-based system experiencing random jitter and on an unmanned-underwater-vehicle-mounted sonar system exhibiting artificially injected crabbing and sway errors. Results indicate that the algorithm is particularly effective at compensating for random and rapidly oscillating navigation and jitter errors, as well as sidelobes introduced by crabbing in arrayed systems.

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.

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