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dc.contributor.authorLoveday, David Carlen_US
dc.date.accessioned2014-03-14T20:45:19Z
dc.date.available2014-03-14T20:45:19Z
dc.date.issued2007-08-24en_US
dc.identifier.otheretd-09122007-114833en_US
dc.identifier.urihttp://hdl.handle.net/10919/35032
dc.description.abstractIt is often claimed that the first Fresnel zone associated with the dominant frequency represents the spatial resolution limit of traveltime tomography. We show, however, that the relevant Fresnel limit for tomographic resolution is the maximum, not the dominant frequency in the data. For physically realizable causal wavelets, the maximum frequency is infinite. In practice, noise lowers the effective possible maximum frequency. To demonstrate these points, 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 that Fresnel zone associated with the dominant frequency. All methods produced accurate ray-theoretical (infinite-frequency) picks from noise-free seismic data for objects smaller than the dominant-frequency Fresnel zone. For the lowest dominant frequencies with Fresnel zones many times larger than the object, 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 less accurately. First-onset picking methods perform better because they take advantage of the highest frequencies in the data, whereas the correlation wavelet is typically in line with the dominant frequency. All methods successfully detected the presence of objects smaller than a wavelength. The inversion of the traveltime picks from the different picking methods always recovered the position and shape of the object. Random noise at a range of signal-to-noise ratios was then added to the seismic data and the data were repicked. 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. The resolution of tomography is limited not by the Fresnel zone associated with the dominant frequency, but by the accuracy of the traveltime picks. Resolution is further improved by dense ray coverage.en_US
dc.publisherVirginia Techen_US
dc.relation.haspartAppendix.pdfen_US
dc.relation.haspartLoveday_Thesis.pdfen_US
dc.rightsI hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to Virginia Tech or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report.en_US
dc.subjectFresnel zoneen_US
dc.subjecttomographyen_US
dc.subjecttraveltime picksen_US
dc.subjectdetection of small objectsen_US
dc.subjectspatial resolutionen_US
dc.subjectseismic refractionen_US
dc.titleResolving Small Objects Using Seismic Traveltime Tomographyen_US
dc.typeThesisen_US
dc.contributor.departmentGeosciencesen_US
thesis.degree.nameMaster of Scienceen_US
thesis.degree.levelmastersen_US
thesis.degree.grantorVirginia Polytechnic Institute and State Universityen_US
dc.contributor.committeechairHole, John A.en_US
dc.contributor.committeememberChapman, Martin C.en_US
dc.contributor.committeememberSnoke, J. Arthuren_US
dc.identifier.sourceurlhttp://scholar.lib.vt.edu/theses/available/etd-09122007-114833/en_US
dc.date.sdate2007-09-12en_US
dc.date.rdate2012-11-06
dc.date.adate2007-09-14en_US


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