portrait in the old family 1964.5 Mustang convertible
Hi, I'm Andy. I have been a computational geophysicist at the Applied Physics Laboratory (APL-UW) of the University of Washington for the past 16 years, focusing on Earth science topics with computational physics themes such as inverse problems and wave propagation in random media, as well as parameter estimation and tracking filters, signal processing, and data analysis – often implemented on high-throughput computing platforms. In my research I use sound, electricity, and gravity as remote sensing tools.

For example, in long-range ocean acoustic scattering projects my collaborators and I experimentally explore how ocean dynamical variations affect fluctuations in acoustic signals that propagate through that ocean. In "geoacoustic" inversion, I solve for properties of the ocean bottom from ocean acoustic measurements. In a controlled-source electromagnetic inversion project I solve for time-varying profiles of salinity in estuarine waters given electrical measurements on a newly developed riverbed instrument that we are working to commercialize. In a planetary science project, my collaborators and I are using physics and inversion tools to inform mission planning for the next NASA spacecraft to Jupiter's moon Europa. My PhD was in theoretical and computational concerns in the geophysical inversion of physical properties of the seafloor from acoustic receptions in the ocean water column. I have been on two extended (>1mo) ocean acoustic experiment cruises.


LONG-RANGE OCEAN ACOUSTIC SCATTERING (with Rex Andrew, APL-UW)
In 2009 and 2010 I participated in a major ocean acoustics experiment in the Philippine Sea with APL-UW's North Pacific Acoustic Laboratory (NPAL) group. One of the key topics our research group is interested in is how oceanic sound propagation is affected by internal waves (waves down deep in the water) and by ocean "spice" (blobs of water with a different soundspeed but same density as their surroundings, so they don't behave like waves). Both these phenomena cause variations in soundspeed of the water and thus acoustic transmissions through it, and acousticians would like to understand them better. Our group is also interested in the estimation of ocean sound-speeds and temperatures from receptions of sound that we send through the water. Find out more about our use of these different methodologies in our research on my NPAL Ocean Acoustics page.


RADIO SCIENCE GRAVITY INVERSION FOR ICY MOON INTERNAL STRUCTURE (with Steve Vance, JPL-Caltech & James Roberts, JH-APL)
The nature of an icy satellite's interior relates fundamentally to its composition, thermal structure, formation and evolution history, and prospects for supporting life. Gravity measurements via radio Doppler information during spacecraft flybys are an important tool used to infer gross interior structure of these moons. Liquid water and ice layers have previously been inferred for the interiors of Jupiter's icy satellites Europa, Ganymede, and Callisto on the basis of magnetic field measurements by the Galileo probe, and on Europa and Callisto induced magnetic field signatures measured by the Galileo probe provided strong evidence for an ionic aqueous ocean. We apply geophysical inverse theory tools to assess the icy moon's interior density anomaly distribution that could be estimated from radio Doppler measurements, to support the search for mass anomalies in the ice shell (meteorites or diapiric upwellings) or near the H2O/rock interface (seamounts).


GEOPHYSICAL INVERSE THEORY MATERIALS
Over the years I've been pulling together various geophysical inversion materials onto a geophysical inverse theory resources webpage to share with others. This began with my TA'ing a graduate-level geophysical inverse theory course and then continued with my guest-teaching a number of lectures in a later year. Contents include recommended reading lists, links to web resources, a few Matlab scripts, and my lecture notes. Students, researchers, and professors alike may find something useful or interesting in here.

ELECTROMAGNETIC INVERSION OF ESTUARINE SALINITY STRUCTURE USING THE SIGMA PROFILER INSTRUMENT (with Tom Sanford, APL-UW)
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The Sigma Profiler is an instrument for remotely observing estuarine salinity profiles via electromagnetic measurements. Electromagnetic (EM) waves are attenuated in seawater as a function of frequency, and conductivity structure (closely related to salinity structure) in the water can be inferred by combining measurements of EM waves at different frequencies on a distant electric field receiver. Geophysical inversion methods are applied to estimate the estuarine salinity profile from the EM measurements. Using inverse theory techniques, we take advantage of statistical rigor and let the data determine the structure of the conductivity profile and quantify the uncertainty and resolution of the salinity profile.


GEOACOUSTIC INVERSION OF SEABED PROPERTIES (my PhD topic at UW)
The Navy’s engineering interest in improving sonar performance in shallow waters is dependent upon an understanding of features and properties in the ocean subfloor which interact with the sonar acoustics. This need in turn drives the geophysical science goal of understanding the nonlinear inversion of ocean bottom properties from hydrophone-based measurements given an acoustic source in the water, within the context of typical naval equipment and frequencies. This problem is closely related to terrestrial seismic inversion and certainly to marine seismic inversion, but at different frequencies and size scales than typically used in seismology and oil exploration work. This work studies the geoacoustic inverse problem at a theoretical and computational level, with goals of obtaining the most information possible out of the measured data without imposing preconceived notions of what the solution should be, and of providing tools to plan new geoacoustic experiments that seek to obtain the most informative data possible for the problem.