HR: 0830h
AN: H31B-0460    [PDF]
TI: Time Differential Electrical Resistivity for Water Resource Assessment: A Case Study in Cura‡ao, Netherlands Antilles
AU: * Coles, D
EM: dac@erl.mit.edu
AF: Earth Resources Laboratory; Department of Earth, Atmospheric, and Planetary Sciences; Massachusetts Institute of Technology, 42 Carleton St., E34-324, Cambridge, MA 02142 United States
AU: Vichabian, Y
EM: yerv@juno.com
AF: Earth Resources Laboratory; Department of Earth, Atmospheric, and Planetary Sciences; Massachusetts Institute of Technology, 42 Carleton St., E34-324, Cambridge, MA 02142 United States
AU: Sogade, J
EM: sogade@erl.mit.edu
AF: Earth Resources Laboratory; Department of Earth, Atmospheric, and Planetary Sciences; Massachusetts Institute of Technology, 42 Carleton St., E34-324, Cambridge, MA 02142 United States
AU: Spiertz, P
EM: mic.cur@attglobal.net
AF: MIC n.v. Consulting Engineers, Witteweg 16, Otrobanda, Curacao Netherlands Antilles
AU: Morgan, F D
EM: morgan@erl.mit.edu
AF: Earth Resources Laboratory; Department of Earth, Atmospheric, and Planetary Sciences; Massachusetts Institute of Technology, 42 Carleton St., E34-324, Cambridge, MA 02142 United States
AB: A method of groundwater prospecting is presented that capitalizes on changes in electrical resistivity arising from annual variability in subsurface fluid flow. According to Archie's Law, effective resistivity is a function of pore fluid resistivity, saturation, and porosity. For competent Earth materials, short-term temporal changes in near-surface porosity are negligible because changes in effective pressure are small with respect to the bulk moduli of the materials. Therefore, annual variability in effective resistivity is dependent only on fluid flow via changes in saturation and, to a lesser extent, pore fluid resistivity. Aquifer detection is but the first step; thereafter, it is necessary to estimate permeability, capacity, and, importantly, sustainability. Time differential resistivity is well matched to the task because it detects prospective aquifers and also illuminates their natural hydrodynamics. Explicitly, this method captures an undeveloped aquifer's seasonal volumetric variability, which is important as engineers reconcile monthly demand with monthly supply. The best scenario would be to detect a capacious, intermediate-resistivity (high pore fluid resistivity and high porosity) zone that is invariant from season to season (clays and ore bodies excepted). Less desirable, but still manageable, is the case where a large, porous formation undergoes significant seasonal resistivity variation; it behaves as a subterranean river with little lag-time between meteoric water input and groundwater throughput; carefully timed extraction and storage would be required in this case. A suite of thirty-eight electrical resistivity soundings were collected from the wet and dry seasons at Plantages PortoMari, an ecotourist plantation on the semiarid island of Cura‡ao, N.A. The data were analyzed with special attention paid to wet-to-dry season resistivity ratios. The results suggest two possible courses of action. (1) A hypothetical limestone terrace was detected whose estimated minimum water volume fluctuates between approximately 1200m$^{3}$ and 2300m$^{3}$; this terrace may be tapped with several boreholes descending to an elevation of 25m ASL. (2) A shallow subsurface dam and extraction fence (10m BGS) might be installed where the watershed corridor converges onto a flat plain whose surficial lithology is comprised of poorly sorted alluvia atop an impermeable clayey unit. Both extraction scenarios require storage facilities, as neither hydrogeologic setting indicates a persistent aquifer.
DE: 0910 Data processing
DE: 0925 Magnetic and electrical methods
DE: 1829 Groundwater hydrology
DE: 1836 Hydrologic budget (1655)
DE: 1875 Unsaturated zone
SC: Hydrology [H]
MN: 2003 Fall Meeting