HR: 09:00h
AN: H21A-05    [PDF]
TI: Suitability of Archie's Law For Interpreting Electrical Resistivity Data
AU: * Singha, K
EM: ksingha@stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford University, Building 320, Geology Corner, Stanford, CA 94305 United States
AU: Gorelick, S M
EM: gorelick@pangea.stanford.edu
AF: Dept. of Geological and Environmental Sciences, Stanford University, Building 320, Geology Corner, Stanford, CA 94305 United States
AB: Electrical resistivity tomography (ERT) is examined as a method to provide spatially continuous images of saline tracer concentrations during transport through unconsolidated fluid-saturated media. It is frequently accepted that there exists a quantitative relationship between the electrical conductivity of dilute electrolytes in pore water and bulk electrical conductivity of the subsurface measured using resistivity methods. The assumed relationship is typically Archie's Law. We tested the applicability of Archie's Law to field-scale data collected over a 10 m by 14 m area. A 20-day weak-dipole tracer test was conducted, in which 2 g/L NaCl were introduced into the upper 30 m of the saturated zone in a coarse sand and gravel aquifer. Cross-well ERT data were collected at 4 geophysical monitoring wells and inverted in 3-D. Fluid electrical conductivity was measured directly from a multilevel sampler. The change in the direct measurements of fluid electrical conductivity exceeded the change in bulk conductivity values in the tomograms by an order of magnitude. The estimated Archie formation factor from the field data was not constant with time, due largely to smoothing during the image reconstruction process. We illustrate by modeling synthetic cases over the field site that the ERT response is difficult to match to measured fluid conductivities due to the variability in the effects of regularization, which change in both space and time. Analysis of both the field data and synthetic cases suggest that Archie's Law cannot be used to directly scale ERT conductivities to fluid conductivities.
DE: 0915 Downhole methods
DE: 0925 Magnetic and electrical methods
DE: 1832 Groundwater transport
DE: 5109 Magnetic and electrical properties
SC: Hydrology [H]
MN: 2003 Fall Meeting