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Hardrock Seismic Reflection Through Cover: Defining Controls on Mineralization via Reflection Attribute Analysis
AuthorGray, Kyle T.
AdvisorLouie, John N.
Geological Sciences and Engineering
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This study attempts to modify oil and gas industry seismic processing and interpretation techniques for use in Carlin-type deposit (CTD) gold exploration. Magmatic and deformation overprints on the Nevada carbonate platform-slope setting present challenges in seismic interpretation when compared to conventional seismic data, which is more commonly imaged in petroliferous basins with low levels of deformation. Barrick Gold Corporation provided 2D seismic reflection data for this case study, which assesses the viability of certain seismic practices when applied to hardrock seismic data collected in NE Nevada. Initial seismic interpretations of the pre-stack depth migrated (PSDM) sections located first-order structures and enhanced the geological model. This study uses derivatives of the PSDM, called seismic attributes, in an attempt to improve interpretability. Seismic attributes can reveal structural and stratigraphic features that are not apparent in the conventional PSDM amplitude data. Attribute analysis in this study leverages correlations made from a seismic response database of ~500 petrophysical drill core samples. These petrophysical measurements indicate that the ore zone exhibits a porosity, acoustic impedance, decarbonatization relationship that is distinguishable from unaltered rock. Down-hole geophysical data suggest an even larger contrast between altered and unaltered limestone. Given sufficient data quality, these observations make attribute analysis for detection of CTD alteration viable. An exhaustive calculation of attributes applied to one 2D reflection profile, which transects the Goldrush CTD resource, suggests that energy- and frequency-based attributes best highlight the ore zone, which is expressed as a chaotic zone of reduced amplitude within one 2D profile. RMS amplitude and instantaneous amplitude identify broad zones of low amplitude whereas an average frequency attribute highlights possible high-frequency attenuation effects in the vicinity of the ore-zone. The sweetness and frequency washout attributes combine frequency and amplitude attributes to more effectively highlight the ore zone. However, the erratic response of sweetness and frequency washout suggest that they may be negatively affected by noise. One structural model is also presented, which used the instantaneous phase attribute to better visualize possible thrust faulting.