HR: 1340h
AN: S23B-1395    [Abstracts]
TI: Resolving Small Objects Using Seismic Traveltime Tomography
AU: * Loveday, D C
EM: lovedayd@vt.edu
AF: Virgina Tech, Department of Geosciences, 4044 Derring Hall, Blacksburg, VA 24061, United States
AU: Hole, J A
EM: hole@vt.edu
AF: Virgina Tech, Department of Geosciences, 4044 Derring Hall, Blacksburg, VA 24061, United States
AU: Imhof, M G
EM: matthias.imhof@exxonmobil.com
AF: ExxonMobil Corporation, P.O. Box 2189, Houston, TX 77252, United States
AB: It is often claimed that the first Fresnel zone associated with the dominant frequency represents the spatial resolution limit of traveltime tomography. To test this assertion, synthetic seismic data were generated for traveltime picking and inversion for a single, small velocity anomaly embedded in a homogeneous background velocity. A variety of traveltime picking techniques were tested and compared for their ability to detect the presence of objects smaller than a Fresnel zone. All picking methods produced accurate ray-theoretical (infinite- frequency) picks from noise-free seismic data for objects much smaller than the dominant-frequency Fresnel zone. All methods successfully detected the presence of objects smaller than a wavelength. Picking methods that focus on features along the onset of the first arrival were the most accurate, while cross-correlation with a known wavelet preformed the worst. The inversion of these traveltime picks always recovered the position and shape of the object. The relevant Fresnel zone limit for tomographic resolution is the maximum, not the dominant frequency in the data. For physically realizable causal signals, the maximum frequency is infinite. In practice, noise, instrument response, and sampling can lower the effective maximum frequency. Random noise at a range of signal-to-noise ratios was added to data for a small object. Pick times with different noise realizations are statistically centered on the noise-free pick, not the time that would be recorded in the absence of the object. Trace stacking prior to picking or the averaging of many picks improves the signal-to-noise ratio and can extract signal that is not detected on an individual pick. An averaging of traveltime picks also occurs during tomographic inversion. This inherent signal-to-noise improvement allows tomography to image objects that are undetectable in individual trace picks. Resolution is further improved by dense ray coverage. The resolution of tomography is limited not by the Fresnel zone associated with the dominant frequency, but by the accuracy of the traveltime picks.
DE: 0935 Seismic methods (3025, 7294)
DE: 6982 Tomography and imaging (7270, 8180)
DE: 7200 SEISMOLOGY
DE: 7270 Tomography (6982, 8180)
DE: 8180 Tomography (6982, 7270)
SC: Seismology [S]
MN: 2007 Fall Meeting