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WATER RESOURCES RESEARCH, VOL. 41, W05023, doi:10.1029/2004WR003460, 2005

Saline tracer visualized with three-dimensional electrical resistivity tomography: Field-scale spatial moment analysis

Kamini Singha

Department of Geological and Environmental Sciences, Stanford University, Stanford, California, USA


Steven M. Gorelick

Department of Geological and Environmental Sciences, Stanford University, Stanford, California, USA


Abstract

Cross-well electrical resistivity tomography (ERT) was used to monitor the migration of a saline tracer in a two-well pumping-injection experiment conducted at the Massachusetts Military Reservation in Cape Cod, Massachusetts. After injecting 2200 mg/L of sodium chloride for 9 hours, ERT data sets were collected from four wells every 6 hours for 20 days. More than 180,000 resistance measurements were collected during the tracer test. Each ERT data set was inverted to produce a sequence of 3-D snapshot maps that track the plume. In addition to the ERT experiment a pumping test and an infiltration test were conducted to estimate horizontal and vertical hydraulic conductivity values. Using modified moment analysis of the electrical conductivity tomograms, the mass, center of mass, and spatial variance of the imaged tracer plume were estimated. Although the tomograms provide valuable insights into field-scale tracer migration behavior and aquifer heterogeneity, standard tomographic inversion and application of Archie's law to convert electrical conductivities to solute concentration results in underestimation of tracer mass. Such underestimation is attributed to (1) reduced measurement sensitivity to electrical conductivity values with distance from the electrodes and (2) spatial smoothing (regularization) from tomographic inversion. The center of mass estimated from the ERT inversions coincided with that given by migration of the tracer plume using 3-D advective-dispersion simulation. The 3-D plumes seen using ERT exhibit greater apparent dispersion than the simulated plumes and greater temporal spreading than observed in field data of concentration breakthrough at the pumping well.

Received 2 July 2004; accepted 14 February 2005; published 24 May 2005.

Keywords: electrical resistivity tomography; spatial moments; tracer test.

Index Terms: 0915 Exploration Geophysics: Downhole methods; 1832 Hydrology: Groundwater transport; 1835 Hydrology: Hydrogeophysics; 3252 Mathematical Geophysics: Spatial analysis (0500); 5109 Physical Properties of Rocks: Magnetic and electrical properties (0925).

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Citation: Singha, K., and S. M. Gorelick (2005), Saline tracer visualized with three-dimensional electrical resistivity tomography: Field-scale spatial moment analysis, Water Resour. Res., 41, W05023, doi:10.1029/2004WR003460.