Chapter 20: Refraction Nonuniqueness Studies at Levee Sites Using the Refraction-tomography and JARS Methods
Julian Ivanov, Richard D. Miller, Jianghai Xia, Joseph B. Dunbar, Shelby Peterie, 2010. "Refraction Nonuniqueness Studies at Levee Sites Using the Refraction-tomography and JARS Methods", Advances in Near-surface Seismology and Ground-penetrating Radar, Richard D. Miller, John H. Bradford, Klaus Holliger
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The utility of two varied approaches to first-arrival time analysis of seismic data acquired at several unique levee sites is demonstrated by solving the inverse refraction-traveltime problem (IRTP). These data were evaluated using conventional refraction tomography and joint analysis of refractions with surface waves (JARS). The JARS approach uses a reference model, derived from surface-wave-calculated shear-wave velocity estimates, as a constraint in reducing refraction nonuniqueness. At those levee sites, conventional refraction-tomography and JARS methods provided different solutions, equally matching the observed data. This observation suggests both approaches are equally possible from a numerical perspective. The JARS images reveal horizontal layering patterns, laterally uniform velocity trends, mild velocity variations, and channel-like features consistent with geologic expectations. In addition, the JARS approach demonstrated the capability for imaging low-velocity layers/zones, something not seen using conventional refraction or refraction-tomography techniques. As a result of these qualitative observations, without ground truth to support an earth model (e.g., from wells), the JARS approach can be viewed as an additional method for finding solutions to the IRTP. However, from all evidence in those studies, the JARS approach represents a possible solution and an example of the potential adverse affect of nonuniqueness. These empirical results support the understanding that for a given refraction data set, significantly different and equally possible velocity-model solutions can exist, resolving which is truly best using invasive ground truth.