The U.S. Geological Survey (USGS) and Air Force Civil Engineering Center (AFCEC) have entered into a cooperative agreement to refine the hydrogeology in the Northeast AFRL and Arroyos groundwater areas of the Air Force Research Laboratory of Edwards Air Force Base. As part of these efforts, two electrical resistivity tomography (ERT) surveys- AFRL9 and AFRL10- were collected in the vicinity of the Mound Fault identified by Cyr and Miller (2023) to better determine the position of these faults.
Electrical resistivity tomography is a direct current geophysical method that is used to estimate the subsurface distribution of the electrical resistivity (measured in ohm-meters; ohm-m) of a material, and is based on the assumption that measured electric potentials (voltages) near current carrying electrodes are influenced by the electrical resistivities of the underlying material (Zohdy and others, 1974; Loke, 2000). ERT is a popular technique for subsurface investigations because it is based on simple physical principles and for its efficient data acquisition (Dahlin and Zhou, 2004).
A combination of the Dipole-Dipole and Strong Gradient arrays was used for this survey and combined to create an optimized dataset (Stummer and others, 2004). The Dipole-Dipole array type yields a high precision dataset, particularly of vertical structures, but can exhibit lower signal to noise ratios (Dahlin and Zhou, 2004; Binley and Kemna, 2005), while the Strong Gradient array provides more complete spatial coverage, and high signal to noise ratio with increased acquisition efficiency (Dahlin and Zhou, 2004; Dahlin and Zhou, 2006, Advanced Geosciences Inc., 2009).
Advanced Geosciences, Inc., 2009, Instruction Manual for EarthImager 2D, v. 2.4.0: Austin, TX, accessed at ftp://ftpcrustal.cr.usgs.gov/pub/Powers/EarthImager2D/.
Binley, A., and A. Kemna, 2005, DC resistivity and induced polarization methods, in Hydrogeophysics, edited by Y. Rubin and S. Hubbard, p. 129–156, Springer, Dordrecht, Netherlands, doi:10.1007/1-4020-3102-5_5.
Cyr, A.J., and Miller, D.M., 2023, Preliminary surficial geologic map of Leuhman Ridge and the surrounding area, Edwards Air Force Base and Air Force Research Laboratory, Kern and San Bernardino Counties, California: U.S. Geological Survey Open-File Report 2023–1014, 1 sheet, scale 1:18,000, pamphlet 15 p., https://doi.org/10.3133/ofr20231014.
Dahlin, T., and Zhou, B., 2004, A numerical comparison of 2D resistivity imaging with 10 electrode arrays: Geophysical Prospecting, v. 52, no. 5, p. 379-398, accessed June 30, 2021, at https://doi.org/10.1111/j.1365-2478.2004.00423.
Dahlin, T., and Zhou, B., 2005, Multiple-gradient array measurements for multichannel 2D resistivity imaging: Near Surface Geophysics, vol. 4, no. 2, p. 113-123.
Loke, M.H., 2000, Electrical imaging surveys for environmental and engineering studies - A practical guide to 2D and 3D surveys, short course training notes: Penang, Malaysia: accessed at https://www.semanticscholar.org/paper/Electrical-Imaging-Surveys-for-Environmental-and-Loke/efa943ffb44f5a217e2d22255f7e334778932248.
Stummer, P., Hansruedi, M. and Green, A.G., 2004, Experimental design: Electrical resistivity data sets that provide optimum subsurface information: Geophysics, Vol. 69, no 1, p. 120-139.
Zohdy, A.A.R., Eaton, G.P., and Mabey, D.R., 1974, Application of surface geophysics to ground-water investigations: U.S. Geological Survey Techniques of Water-Resources Investigations, book 2, chap. D1, 116 p.
