The overarching theme of my research is solving and understanding "inverse problems" to learn about geophysical phenomena. Geophysics is Earth science, and these inverse problems are all about how to get useful information about the Earth from indirect measurements. In many scientific disciplines, we wish to learn about a quantity that we cannot measure directly. In seismology, ocean acoustics, planetary physics, we often wish to learn about the structure or composition of some interior (of the Earth, of the ocean, etc) but we can only take measurements at the surface or at some other boundary. What's a scientist to do?
We have mathematical tools available to us which take notional interior properties as input, and then predict the corresponding surface measurements as the output. But that's the opposite direction from what we need; we want to take the measurements and use them to estimate the interior properties. So we must "invert" those mathematical tools, in a process that is challenging but fun. For a given geophysical problem we must design a lens through which we can shine the surface measurements and see some projection of the interior properties.
The trouble with this process (which handily provides jobs and graduate degrees for some of us), is that this lens is generally far from perfect. The lens is often "foggy" and blurs things so that you don't get perfect resolution of the desired quantity at the end. And there is always some uncertainty on the measurements ("10cm plus or minus 3" for example), which when projected through the lens can become really warped and difficult to interpret. All this makes these problems fascinating puzzles to work on. The areas I'm currently applying these techniques are:
NPAL
TCTD CHAIN AND PHILSEA'09 CRUISE
This past spring I was in the Philippine Sea with
APL-UW's North Pacific Acoustic Laboratory (NPAL)
group, on a test-cruise in preparation for a full
experiment in the same location in 2010. A major focus
of this group is the estimation of ocean sound-speeds
and temperatures from receptions of sound sent through
the water. A new instrument in our experiments is hoped
to contribute to a better understanding of ocean
processes, in turn improving understanding of "where
the sound goes". The instrument is a new, 800m long
version of a "towed CTD chain" (TCTD). It's an
oceanographic cable that contains with 88 sensors of
conductivity, temperature, and pressure spread along
its length. This is exciting because traditional,
individual CTD casts give geographic point
measurements, whereas this newest version of the towed
instrument would give ~5m x ~5m resolution in a
500m-deep, 2D slice of the ocean for as far as it is
towed.
While my "inversion" interests in this research group are still about estimating the soundspeed (and temperature) field in the water from acoustic receptions, a big limitation of that work is in correctly modeling where the sound goes in the ocean -- complicated oceanic phenomena such as "internal waves" and "spice" are generally not modeled in such work, feeding additional uncertainties into the estimations. By using an instrument like this to directly study the statistics of those ocean processes, we can then learn something about their effects on the estimations.
Unfortunately there were a number of technical troubles with this new system during this cruise; we hope for better things to come from it in the future. Members and colleagues of the APL-UW ocean acoustic community can use your passwords to access the part of this site containing TCTD notes, data, cruise report, test results, and pictures. And anyone can access the other section of the site with the TCTD deployment/recovery pictures, miscellaneous other technical pictures (including acoustic sources and TCTD) and fun "personal interest" pictures from the cruise.
While my "inversion" interests in this research group are still about estimating the soundspeed (and temperature) field in the water from acoustic receptions, a big limitation of that work is in correctly modeling where the sound goes in the ocean -- complicated oceanic phenomena such as "internal waves" and "spice" are generally not modeled in such work, feeding additional uncertainties into the estimations. By using an instrument like this to directly study the statistics of those ocean processes, we can then learn something about their effects on the estimations.
Unfortunately there were a number of technical troubles with this new system during this cruise; we hope for better things to come from it in the future. Members and colleagues of the APL-UW ocean acoustic community can use your passwords to access the part of this site containing TCTD notes, data, cruise report, test results, and pictures. And anyone can access the other section of the site with the TCTD deployment/recovery pictures, miscellaneous other technical pictures (including acoustic sources and TCTD) and fun "personal interest" pictures from the cruise.
MY PhD
RESEARCH
The subject of my PhD is the study of the ocean bottom by analyzing sound waves (from sonar for example) that travel into it and bounce off of it. It's a bit like when you sing in a room with your eyes closed and you can tell by listening whether you're in a tile-walled bathroom or in a wood-walled sauna. Except in this case the "wall" is the seafloor, perhaps a quarter-mile below underwater, and we want to discern the composition and features of not only the seafloor surface, but what lies underneath it as well. I study seismology math, or "theoretical seismology", because seismologists do essentially this same work - they analyze surface recordings of earthquake waves which bounced and bent in layers inside the Earth, and from them estimate the composition and features inside the Earth. My research focus is theoretical rather than with measurements in the field - this math is all about estimating quantities that are measured indirectly, and my niche for my PhD thesis relates to improving the quality of those estimates in the ocean bottom problem.
The subject of my PhD is the study of the ocean bottom by analyzing sound waves (from sonar for example) that travel into it and bounce off of it. It's a bit like when you sing in a room with your eyes closed and you can tell by listening whether you're in a tile-walled bathroom or in a wood-walled sauna. Except in this case the "wall" is the seafloor, perhaps a quarter-mile below underwater, and we want to discern the composition and features of not only the seafloor surface, but what lies underneath it as well. I study seismology math, or "theoretical seismology", because seismologists do essentially this same work - they analyze surface recordings of earthquake waves which bounced and bent in layers inside the Earth, and from them estimate the composition and features inside the Earth. My research focus is theoretical rather than with measurements in the field - this math is all about estimating quantities that are measured indirectly, and my niche for my PhD thesis relates to improving the quality of those estimates in the ocean bottom problem.
A SIDE INTEREST
Gravity inversion of icy moons
My geophysical inversion interests tie into my interests in planetary physics as well. A growing number of the icy moons in our solar system (like some moons of Jupiter and Saturn) are found to likely have buried oceans on them, and acoustic inversion is one of the ways these oceans could be realistically explored. That's pretty far off in the future though. In the meantime, existing spacecraft missions allow for investigation of possible large-scale (like continent sized) structures in those oceans or in the surface ice: As spacecraft like Cassini currently around Saturn, and Galileo previously around Jupiter, speed past one of these icy moons, they have dips in their trajectory due to gravitational tugs from density differences inside the moon. The spacecraft is radioing Earth while this happens, and each gravitational tug results in a Doppler shift on that radio signal. So you can work your way backward from measurements of the radio Doppler signal and invert for large-scale density structure in the moon's interior. With colleagues at Caltech-JPL, I have been writing proposals to improve on certain aspects of those inversions.
(Image courtesy NASA/Caltech-JPL)