Apr. 2012: Recent progress on the G2S project has led to this new website that shows the last 24 hours of global stratospheric wind evolution, and how that conspires with temperature to influence the potential for stratospheric ducting. The Cratio is the ratio of the effective sound speed in the stratosphere to that at the surface. Values of CRatio > 1 (warm colors) mean the ducting potential is great for broadband infrasound in the direction of the mean stratospheric wind (black vectors). Values less than 1 (cool colors) mean stratospheric ducting will be strongly frequency dependent (generally only happening at lower frequencies below 1 Hz). White squares are the IMS infrasonic arrays. The G2S system is installed on at least three institutions: UCSD, Univ. of Mississippi, and the Naval Research Lab (via Doug Drog, creator of 'G2S').

Apr. 22, 2012: There was a large "boom" felt in Northern California. The infrasound radiated from this event suggests the event was a mini-van sized meteor that exploded above the Earth's surface. Several of my arrays recorded this event. My analysis of SMIAR, a 9-element array near Temecula, and IS57 (PFO) shows a 15-minute long wavetrain coming from the northwest, pointing to central California. This event was also recorded by at least one OFIS sensor in Chico, California. That sensor provides a better recording of the source function since it was much closer to the event (1-5Hz, raw). Elizabeth Silber is analyzing this event in more detail.

Apr. 13, 2012: There was a large "boom" felt in San Diego. My preliminary analysis is presented here here.

Dec. 23, 2011: "Acoustic Ray Tracer, 2D" or "ART2D" has been released. Check out the link here.

Dec. 13, 2011: My USArray work has been published. Many thanks to JGR Solid Earth and two anonymous reviewers for helping this go to press so quickly. Click here for the paper.

Jan. 13, 2011: UCSD infrasound arrays are detecting microbaroms coming from the eastern Pacific Ocean. CLICK HERE FOR AN OCTOBER 2011 UPDATE or HERE FOR A JANUARY 2012 UPDATE. Each of the arrays comprise several infrasonic microphones separated by several hundred meters. Pressure signals generated just above the ocean surface, likely from beating due to interfering opposing swells, travel great distances through the atmosphere. These signals are detected by each of the sensors in the arrays. Array processing identifies the direction from which the signals originated (back azimuth). We project these back azimuths outward from these arrays to form an intersecting back azimuth density color map, where reds indicate high densities. The color map has been converted to signal-to-noise ratio. The black contour is the statistical 95% confidence region for a model where there is only one source to explain all the detections at the arrays. The OFIS CHIAR array provided results that were consistent with those of the other arrays. This is the first time that the OFIS technology has been used for infrasound source physics research. Click here for a movie.

Geophysics Project Scientist
Inst. of Geophysics and Planetary Physics
Scripps Institution of Oceanography
University of California, San Diego

Contact info

Research Summary

My current research is in the area of infrasonics, the study of subaudible sound. Specifically, I'm collaborating with Doug Drob (NRL) in making enhancements to the Ground-to-Space (G2S) atmospheric velocity modeling system, analyzing the Japan Tohoku earthquake's infrasonic radiation, analyzing the infrasonic events recorded by the Southern California Seismic Network and regional infrasonic arrays, and beginning to make progress on my recently funded NSF proposal to study infrasonic and gravity wave sources as well as the resulting energy propagation across the midwestern USArray microphones and seismometers. Although my position at SIO comes without institutional financial support (as all new SIO Researcher and Project Scientist positions are), I still make time to collaborate with others, especially with Mark Zumberge's Gravity Lab and Michael Hedlin's Laboratory for Atmospheric Acoustics. I invite you to check out my site, and if you have any questions, please feel free to ask.

Curriculum Vitae [PDF]

Education

2004: Stanford University, Ph.D., Passive-source Seismology (Thesis)
2000: Stanford University – M.S., Active-source Seismology
1998: University of Delaware – B.S. w/ Distinction, Geophysics, Cum Laude

Professional Experience

• 2010-12: Lead PI, Chico Microbarom Source Physics Experiment (OFIS array deployment and research)
• 2010-11: PI, Santa Margarita Microbarom Source Physics Experiment (SMIAR MB2000 array deployment and research)
• 2009: Lead PI, Evaluation of Line Averaging as a Means of Wind Noise Reduction (270 m long OFIS at Piñon Flat Observatory)
• 2009-11: Project Scientist, IGPP, Scripps, Univ. of California, San Diego
• 2007-08: Co-PI, six-element infrasound array "MRIAR" near San Diego, California
• 2005-06: Cecil & Ida Green Scholar, IGPP, Scripps, Univ. of California, San Diego
• 2004-09: Postdoctoral Researcher, IGPP, Scripps, Univ. of California, San Diego
• 2000-01: Lead Investigator, Snake River Plain Seismic Experiment
• 1998: Participant, Summer of Applied Geophysical Experience program

Recent Publications

Walker, K.T., Shelby, R., Hedlin, M., de Groot-Hedlin, C., and Vernon, F., 2011, Western U.S. Infrasonic Catalog: Illuminating infrasonic hot spots with the USArray, J. Geophys. Res., 116, B12305, doi:10.1029/2011JB008579.

de Groot-Hedlin, C., Hedlin, M., and Walker, K., 2011, Finite difference synthesis of infrasound propagation through a windy, viscous atmosphere: application to a bolide explosion detected by seismic networks, Geophys. J. Int., doi: 10.1111/j.1365-246X.2010.04925.x

Walker, K.T., Hedlin, M.A.H., de Groot-Hedlin, C., Vergoz, J., Le Pichon, A., and Drob, D.P., 2010, Source location of the 19 February 2008 Oregon bolide using seismic networks and infrasound arrays, J. Geophys. Res., 115, B12329, doi:10.1029/2010JB007863.

Hedlin, M.A.H., D. Drob, K.T. Walker, and C. de Groot-Hedlin, 2010, A study of acoustic propagation from a large bolide in the atmosphere with a dense seismic network, J. Geophys. Res., 115, B11312, doi:10.1029/2010JB007669.

Walker, K.T., and Hedlin, M.A.H., 2010, A review of wind noise reduction methodologies, in: Infrasound Monitoring for Atmospheric Studies, ed. by A. Le Pichon, E. Blanc, and A. Hauchecorne, Springer, p. 141-182, doi:10.1007/978-1-4020-9508-5.

Walker, K.T., and Shearer, P., 2009, Illuminating the near-sonic rupture velocities of the intracontinental Kokoxili Mw 7.8 and Denali Mw 7.9 strike-slip earthquakes with global P-wave back projection imaging, J. Geophys. Res., 114, B02304, doi:10.1029/2008JB005738.

Walker, K.T., Zumberge, M.A., Hedlin, M.A.H., and Shearer, P., 2008, Methods for determining infrasound phase velocity direction with an array of line sensors, J. Acoust. Soc. Am., 124, 2090-2099.

De-Groot Hedlin, C.D., Hedlin, M.A.H., Walker, K.T., Drob, D.P., and Zumberge, M.A., 2008, Study of infrasound propagation from the Shuttle Atlantis using a large seismic network, J. Acoust. Soc. Am., 123, 3829-3829.