Geoelectrical constraints on radar probing of shallow water-saturated zones within karstified carbonates in semi-arid environments |
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| Authors: | Damayanti Mukherjee Essam Heggy Shuhab D Khan |
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| Institution: | 1. Department of Earth and Atmospheric Sciences, 4800 Calhoun Road/312 Science & Research Building–1, The University of Houston, Houston, TX 77204–5007, United States;2. Institut de Physique du Globe de Paris, 4 Avenue de Neptune, Saint Maur des Fossés, 94100, France;1. EDYTEM (UMR 5204 CNRS-Université de Savoie), Campus Scientifique, 73376 Le Bourget du lac Cedex, France;2. ISTeP (UMR 7193 CNRS-UPMC), Université Pierre et Marie Curie, Case 117, 4, Place Jussieu, 75252 Paris Cedex 05, France;1. The State Key Laboratory of Oil & Gas Reservoir Geology and Exploitation Engineering, SWPU, Chengdu, Sichuan, China;2. Post-doctoral Research Station, SWPU and Postdoctoral Innovation and Practice Base, Sichuan College of Architectural Technology, Deyang, Sichuan, China;1. Planetary Science Directorate, Southwest Research Institute, Boulder, CO, USA;2. Space Science and Engineering Division, Southwest Research Institute, San Antonio, TX, USA;3. Consulting Geophysicist, Houston, TX, USA.;1. Arkansas Center for Space and Planetary Sciences, University of Arkansas Fayetteville, AR 72701, USA;2. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA;1. Department of Physics and Geology, University of Perugia, Via A. Pascoli snc, 06123 Perugia, Italy;2. INGV, Osservatorio Etneo-Sezione di Catania, Catania, Italy |
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| Abstract: | Shallow carbonates are of utmost importance as potential sources of groundwater in karstified semi-arid terrains. Ground-Penetrating Radar (GPR) is being increasingly used as a prominent mapping tool in such environments. However, its potential in exploring and identifying shallow water-saturated zones (WSZs) in carbonates is constrained by the geoelectrical properties of carbonate soils as a function of moisture content. We report results of a case study that includes laboratory geoelectrical characterization and their comparison to in situ GPR attenuation measurements performed on Cretaceous Edwards Formation rudist mounds in central Texas, which we hypothesize as analogs for water-bearing formations in semi-arid karstified carbonate terrains. Dielectric measurements on field-collected rock samples carried out in the laboratory under controlled conditions of moisture saturation suggest that real and imaginary parts of dielectric constants of rocks with higher porosity and/or permeability have steeper dependence on pore moisture content; they produce better dielectric contrasts but allow shallower penetration. Our analyses suggest that within carbonates, dielectric contrasts improve with decrease in sounding frequency and/or increase in moisture content; and the relationship between dielectric permittivity and moisture content may be represented by 3rd order polynomial equations. GPR surveys using a wide-band 400 MHz antenna reveal subsurface mound morphologies with heights of ～ 1–2 m and basal diameters of ～ 8–10 m resembling outcrop analogs. Each mound appears to be composed of smaller amalgamated lithounits that seem geoelectrically similar. Amplitudes decays of the backscattered radar signal correlate to moisture distribution. Measuring the differences in signal attenuation allows differentiation between saturated and non-saturated zones. Velocity analyses of GPR profiles enable estimation of moisture distribution in the vicinity of the mounds. Optimal delineation and production of high-resolution GPR data up to a depth of ～ 10 m were observed for a sounding frequency of ～ 250 MHz with moisture content of ～ 5% by weight. Below this moisture level, the dielectric contrast is insufficient to uniquely identify water-saturated zones from the surrounding geoelectrical context, and above it, the radar signal is substantially attenuated leading to a total inefficiency of the method. |
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