HR: 1340h
AN: S23B-0258    [Abstracts]
TI: Application of the NEES T-Rex Vibrator for 3-component Crustal Reflection/Refraction Profiling: 2004 Test in the Basin and Range
AU: * Lerch, D W
EM: lerch@pangea.stanford.edu
AF: Dept. of Geophysics, Mitchell Building Stanford University, Stanford, CA 94305-2215 United States
AU: Klemperer, S L
EM: klemp@geo.stanford.edu
AF: Dept. of Geophysics, Mitchell Building Stanford University, Stanford, CA 94305-2215 United States
AU: Stokoe, K H
EM: k.stokoe@mail.utexas.edu
AF: Dept. of Civil Engineering, University of Texas at Autsin, Austin, TX 78712 United States
AU: Menq, F
EM: fymenq@mail.utexas.edu
AF: Dept. of Civil Engineering, University of Texas at Autsin, Austin, TX 78712 United States
AB: In September 2004, Stanford University and UT Austin collected crustal reflection/refraction data with the Network for Earthquake Engineering Simulation (NEES) tri-axial (T-Rex) vibrator in two separate surveys: (1) a 40 km crustal-scale profile in the Black Rock Desert and Black Rock Range, NV; and (2) a 16 km `high-resolution' basin imaging profile in Surprise Valley, CA. Both experiments were completed in the northwestern Basin and Range transition zone as part of a larger, 300 km wide-angle refraction line, recorded by an IRIS-PASSCAL transportable array. Our broad goals were to test the feasibility of the T-Rex in crustal-scale applications with existing IRIS-PASSCAL equipment, and to augment our refraction data with structural information. T-Rex, acquired by NEES-UT Austin in 2002, is a 64,000 lb buggy-mounted vibrator with a tri-axial mass, producing P, SV, and SH waves. In order to balance desired upper-crustal resolution with an attempt to record mid-crustal reflectivity and shear-wave data during the crustal-scale profiling, T-Rex swept at 300 m intervals for 5 minutes (1 min. P, 2 min. SV, 2 min. SH), and at 3 km intervals for 50 minutes (10 min. P, 20 min. SV, 20 min. SH). This source configuration afforded coverage of a relatively wide aperture in a modest amount of field time, while stacked sweeps recorded at 3 km intervals provided greater source effort than individual vibration points for the 1983 COCORP 40<sup>O</sup>N Nevada deep reflection profile which recorded strong Moho reflections. Crustal data were continuously recorded in four-hour windows at 125 sps on a combination of vertical (RefTek Texan) and 3-component (RefTek RT130) instruments forming a 40 km array spaced at 100 m, embedded within the larger wide-angle profile. The `hi-res' survey (250 sps, 40 m receiver spacing, 10 m source spacing) imaged the active Surprise Valley basin to constrain basin depth, fault geometry, and basin-fill P- and S-wave velocities. Our source effort (single 1 min. sweeps) is comparable with prior successful efforts to image upper-crustal basement structure (e.g., Virginia Tech's 1981 Atlantic Coastal Plain survey). Despite attenuation and poor coupling in the unconsolidated basin fill (Vp ~ 1.8 km/s) of Surprise Valley and the Black Rock Desert, coherent refracted energy on raw, single-sweep gathers is visible to 9 km offset, with reflections visible to ~ 3 sec (twtt). Our data indicate the NEES T-Rex vibrator is a viable source for upper crustal P- and S-wave surveys, and may produce useful mid-crustal data under favorable conditions.
DE: 0905 Continental structures (8109, 8110)
DE: 0935 Seismic methods (3025, 7294)
DE: 7205 Continental crust (1219)
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: Fall Meeting 2005