HR: 08:45h
AN: NS21A-04    [Abstracts]
TI: A Deterministic Approach to Analyzing Audiomagnetotelluric Models and Borehole Data in a Hydrological Environment
AU: * Pellerin, L
EM: pellerin@ak.net
AF: Green Engineering, Inc, 2215 Curtis St, Berkeley, CA 94702, United States
AU: McPhee, D K
EM: dmcphee@usgs.gov
AF: US Geological Survey, MS 989, 345 Middlefield Rd., Menlo Park, CA 94025, United States
AB: We present a three-step deterministic approach to understand the relationship between borehole data and surface geophysics using audiomagnetotelluric (AMT) data. Traditionally, geoscientists have used borehole data as ground truth, but it is not clear how representative a point measurement is for laterally extensive or regional areas. Furthermore, it is unclear when and where it is valid to compare borehole and surface data. Geophysics is used to site wells, but the borehole data are often used to calibrate the geophysics, and it is the relationship between the two data sets that we quantitatively investigate here. As part of a hydrological study of the Basin and Range province, an arid, mountainous, sparsely populated region of the western United States, many AMT surveys were conducted. AMT soundings were typically collected along profiles at stations spaced roughly 200-400 m. The resulting two-dimensional resistivity models successfully imaged subsurface faults and structures down to roughly 500 m depth. These faults are a primary structural control on the hydrogeology of many valleys in this region. Borehole data, including both lithological and geophysical logs, were available from several water monitoring and testing wells close to our AMT stations. Wells were located between 10m and 1.6 km from our AMT profiles, and extended down to 600 m below the surface. Although borehole data, whether lithological or geophysical logs, have excellent vertical resolution they are essentially point source data, and there are many reasons that the borehole data may not faithfully represent the survey area. The borehole can be unfortunately sited so that it is located in an anomalous area, or problems with instrumentation can cause inaccuracies with the logs. In addition, there is a great deal of borehole data that has been poorly archived and may be difficult to decipher or use. Our approach to quantitatively compare the AMT and borehole data involves three steps: 1) One-dimensional forward modeling based on borehole data; 2) Inverse modeling of AMT data using various starting models; and 3) A resolution/sensitivity analysis based on comparison of forward and inverse models. Our results fall into three categories: 1) The AMT and borehole data are consistent and image similar subsurface electrical structure; 2) There is a discrepancy that can be explained; and 3) There is a strong discrepancy that cannot be readily explained, but one result is clearly preferred.
DE: 0699 General or miscellaneous
DE: 0915 Downhole methods
DE: 0925 Magnetic and electrical methods (5109)
DE: 1835 Hydrogeophysics
SC: Near-Surface Geophysics [NS]
MN: 2009 Joint Assembly