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$Refraction - IRIS$

SIGNALS AND SYSTEMS 2006 IRIS 5-YEAR PROPOSAL Passive Crustal Refraction Experiments Using IRIS/PASSCAL Facilities: New Surveys Show Variable Crustal Thickness in the Western Great Basin John N. Louie and Michelle Heimgartner • University of Nevada Utilizing commercial mine blasts and local earthquakes, as well as a dense array of portable seismographs, we have achieved long-range crustal refraction profiles across northern Nevada and the Sierra Nevada Mountains. In our most recent refraction experiment, the Idaho- Nevada-California (INC) transect, we used a dense spacing of 411 portable seismographs and 4.5-Hz geophones. The instruments were able to record events ranging from large mine blasts to small local earthquakes. Our instruments sensed blast first arrivals out to a distance of approximately 400 km. We have obtained 99% data recovery and clear refractions across the central Sierra Nevada and the northern Great Basin. The Northern Walker Lane refraction experiment, completed in 2002, confirms the presence of a thin crust ranging from 19-23 km thick in a 100 km region in the vicinity of Battle Mountain, Nevada. Pn crossover distances of less than 95 km from both the INC and Northern Walker Lane experiments support this observation. We also observe an unexpectedly deep crustal root under the northern Sierra Nevada, over 50 km in thickness and centered west of the topographic crest. In addition, we have created contoured crustal thickness maps based literature cited from previous compilations. These maps integrate our current experiments with past geophysical assessments of the Great Basin. Our seismic-refraction reconnaissance of the western Great Basin will contribute to the assessment of geothermal potential in poorly constrained areas. (left) Map indicating the location of the Idaho-Nevada-California (INC) Northern Walker Lane refraction experiments (Louie et al., 2004) across the western Great Basin. (right) Record section from the Barrick GoldStrike 100,000-lb mining blast on August 19, 2004: 100 sec long and beginning 8 sec after the blast. The “Texan” recorder transect extends from Fresno, CA (SW), to the Idaho Nevada border (NE), approximately 600 km. Louie, John N., Weston Thelen, Shane B. Smith, Jim B. Scott, Matthew Clark, and Satish Pullammanappallil, The northern Walker Lane refraction experiment: Pn arrivals and the northern Sierra Nevada root, Tectonophysics, 388, no. 1-4, 253-269, 2004. This material is based upon work supported by the U.S. Department of Energy under instruments numbered DE-FG07-02ID14311 and DE-FG36- 02ID14311, managed through the DOE Golden Field Office. Reftek RT-125 “Texan” recorders and field support provided by the NSF-funded IRIS/ PASSCAL Instrument Center at New Mexico Tech. 60
2006 IRIS 5-YEAR PROPOSAL SIGNALS AND SYSTEMS Signal Restoration Through Deconvolution Applied to Deep Mantle Seismic Probes Wolfgan Stefan, Ed Garnero, Rosemary Renaut • Arizona State University In this study we present a method of signal restoration to improve the signal-to-noise ratio, sharpen seismic arrival onset, and act as an empirical source deconvolution of specific seismic arrivals. Observed time series are modeled as a convolution of a simpler time series, and an invariant point spread function (PSF) that attempts to account for the earthquake source process. The method is used on the shear wave time window containing SKS and S, whereby using a Gaussian PSF produces more impulsive, narrower, signals in the wave train. The resulting restored time series facilitates more accurate and objective relative travel time estimation of the individual seismic arrivals. We demonstrate the accuracy of the reconstruction method on reflectivity synthetic seismograms. Clean and sharp reconstructions are obtained with real data, even for signals with relatively high noise content. Reconstructed signals are simpler, more impulsive, and narrower, which allows highlighting of some details of arrivals that are not readily apparent in raw waveforms. In particular, phases nearly coincident in time are separately identified after processing. This is demonstrated for broadband observations as well: two seismic wave pairs used to probe deep mantle and core-mantle boundary structure: (1) the Sab and Scd arrivals, which travel above and within, respectively, a 200-300 km thick higher than average shear wave velocity layer at the base of the mantle, observable in the 88° − 92° epicentral distance range; and (2) SKS and SPdiffKS (shown), which are core waves with the latter having short arcs of P wave diffraction, and are nearly identical in timing near 108° − 110° in distance. A Java/Matlab algorithm was developed for the signal restoration, which can be downloaded from the authors webpage (http://mathpost.la.asu.edu/ ̃stefan), along with example broadband data and synthetic seismograms. This method holds promise for facilitating waveform characterization and travel time measurements in large data sets. (a) SV component reflectivity synthetics, for a source depth of 500 km. Receiver distances are from 90 to 115 degrees, in one degree increments. (b) Deconvolved synthetics. Traces in both panels are aligned at the SKS onset detected in the deconvolved traces of panel (b) using an edge detection method. At a distance of 108 deg, the formation and subsequent move out of SPdiffKS can be seen in both plots, though it is first visible in the deconvolved traces: SKS remains rectangular until the formation of SPdiffKS initiates, which first broadens SKS, and then emerges as an additional rectangle. Stefan, W., Garnero, E. and Renaut, R. A., Signal restoration through deconvolution applied to deep mantle seismic probes, Geophys. J. Int., in review, 2005. NSF CMG-02223 61
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