Title

Characterization of an Active Fault Zone Using the Three-Dimensional Reflection Seismic Method

Publication Date

3-2006

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geophysics

Department

Geosciences

Major Advisor

John Bradford

Abstract

Three-dimensional reflection seismic is often too costly and/or labor intensive for most investigations and has seldom been applied to near-surface studies. As part of a project to research Life at Interfaces and the Biocomplexity of Extreme Environments, I have collected, processed, and interpreted three-dimensional reflection seismic data to determine the near-surface geology and infer the hydraulic architecture for the Borax Lake Hydrothermal System, located in southeast Oregon, USA. This site makes an ideal study area because it has a linear, probably fault controlled, set of active hot springs.

I applied a typical near-surface processing scheme to the data. From the image of the three-dimensional data, I have interpreted a fault zone that consists of two oppositely dipping normal faults. The extensive, east-dipping Borax Lake Fault may act as the primary control on upflowing geothermal water from depth. The smaller, west-dipping West Fault has significantly less offset and is associated with a step-over region in the linear trend of the hot springs at the surface. Based on comparison to the stratigraphy in two wells drilled 2-4 km to the south, both faults are presumed to cut through silt sediments that rest on a well-lithified sedimentary rock.

The results of this analysis combined with additional geophysical data suggest that the hydraulic architecture of the fault zone consists of deformation bands and/or slip surfaces of low- to moderate-permeability interspersed with spatially-discrete, high- permeability flow paths that facilitate flow parallel to fault plane. The 50 meter step-over in the surface expression of the hot springs is attributed to the interaction of the two faults. Because of the approximate offset of 400 m associated with the Borax Lake fault, the geothermal water is presumed to follow the Borax Lake fault from the reservoir at depth and then travel along both faults in the near-surface sediments. This model of fault hydraulic architecture agrees well with the model for lithified sedimentary rocks, but diverges from the model for weakly-lithified sediments where faulting is believed to decrease permeability. Even though the surface sediments in the vicinity of the hot springs are poorly-lithified, it is possible that precipitates from the geothermal water cement the sediments to create a hydraulic architecture similar to that of well-lithified sediments with increased permeability due to faulting. This study demonstrates how three-dimensional reflection seismic can be used in conjunction with other geophysical data to constrain subsurface fault architecture and infer the permeability structure of the associated geothermal hot springs.

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