Date of Final Oral Examination (Defense)
Type of Culminating Activity
Master of Science in Geophysics
John Bradford, Ph.D.
Lee M. Liberty, M.S.
James P. McNamara, Ph.D.
The critical zone (CZ) is the earth’s layer where water, air, rock, and life meet. It is the zone with which humans interact most. The National Research council (2001) defines the CZ as a “heterogeneous, near surface environment in which complex interactions involving rock, soil, water, air, and living organisms regulate the natural habitat and determine the availability of life sustaining resources”. The CZ may extend roughly from the top of the vegetation canopy to the deepest part of the rock column where meteoric water circulates – this is often in the 10 – 30 m range. The upper 1-2 m of the CZ, the most weathered portion of the CZ depth profile, can be reached via soil pits or cores enabling detailed characterization. Weathering is the process in which a parent rock decays into mobile soil, through mechanical breakdown and chemical weakening. The maximum depth at which both, mechanical and chemical processes are present is referred to as weathering depth. Below 2 m, characterizing the CZ is a challenge because of the expense and logistical challenge of drilling boreholes, particularly in rugged, mountainous terrain. Geophysical methods are increasingly being used to probe deeper into the CZ and have proven to be a powerful tool. Fully characterizing the deep CZ is made even more challenging when fractures are present. Fractures may have a preferential orientation according to the local stress field which leads to both geophysical and hydraulic anisotropy. Because fractures can be hydraulically active, understanding fracture induced anisotropy retrieves information on the preferential distribution of water pathways in the subsurface. To test our ability to detect and characterize systems of deep CZ fractures with preferred orientation in a mountain watershed, I conducted a series of multi-azimuthal 2D electrical resistivity tomography and 2D seismic refraction surveys. I utilized the Dry Creek Experimental Watershed (DCEW) as a field laboratory – a previous outcrop study mapped fracture orientations throughout the watershed. I collected data, at three different sites, near or within the DCEW. For the anisotropy and fracture density case, I estimated fracture density as a function of depth at all sites, and determine that the depth at which most fractures close ranges from 13-27 m depth. I found significant P-wave anisotropy throughout the watershed with maximum values of 28%. Additionally, my results indicate that anisotropy continues to much greater depths. I infer that the observed geophysical anisotropy likely correlates with significant hydraulic anisotropy and has an important impact on deep water circulation in the DCEW. Additionally, I show that attempts to characterize this system with single azimuth data and an assumption of isotropy will lead to erroneous results – at my sites the error in estimated fracture density could be as high as 0.24. I conclude that geophysical investigations in similar terrains need to test for anisotropy and use appropriate models, particularly if the objective is quantitative estimation of hydraulic or other physical properties. General observations of the CZ were that weathering depth increased with decreasing elevations – this has been observed at other sites and is likely due to increased chemical reactivity at higher temperatures. At the low elevation site I observe a depth to bedrock at approximately 29 m depth, at the mid-elevation site I observe a depth to bedrock of approximately 23 m depth, at the high elevation site, I observe a depth to bedrock as little 5 m depth. I observed a significant increase in weathering depth on northwest facing aspects and speculate that this is a combination of lower evaporation rates, slow steady delivery of snow melt waters into the subsurface and the positive feedback of increased vegetation and root enhanced weathering.
Mendieta, Aida, "Seismic Refraction and Electrical Resistivity Tests for Fracture Induced Anisotropy in a Mountain Watershed" (2017). Boise State University Theses and Dissertations. 1351.