HR: 11:15h
AN: NS12A-04    [Abstracts]
TI: Sensitivity of electrical resistivity tomography data to electrode position
AU: * Oldenborger, G A
EM: greg@cgiss.boisestate.edu
AF: Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725 United States
AU: Routh, P S
EM: routh@cgiss.boisestate.edu
AF: Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725 United States
AU: Knoll, M D
EM: mknoll@cgiss.boisestate.edu
AF: Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725 United States
AB: The electrical resistivity tomography (ERT) method has proven to be a valuable geophysical tool for a variety of shallow subsurface imaging tasks. Limitations of tomographic imaging arise due to the difficulty of quantifying the reliability of tomographic images. A major source of uncertainty in tomographic inversion is data error. Near surface environmental and engineering geophysical surveys are often such that target size may be of the same order of scale as the experiment dimensions. It is at these overlapping scales that errors in electrode positions can significantly contaminate the ERT data. Electrode mislocation can result from borehole deviation, poor electrode emplacement or surveying mistakes. Position errors are particularly problematic as they do not show up in either reciprocal or repeatability tests but rather, propagate via forward modeling of the data. The dependence of the data on electrode mislocations is characterized by the sensitivity of electrical potential to both source and receiver positions. In the homogeneous case, the sensitivity of potential with respect to source/receiver position is purely geometrical and dependent on the electrode-electrode separation. The homogeneous sensitivity distributions illustrate the susceptibility to position errors for hypothetical survey geometries (cross-hole vs. common-hole) and data types (pole vs. dipole). In the heterogeneous case, the sensitivity of potential with respect to source/receiver electrode position is described by a scattering-type equation and, therefore, the sensitivity depends not only on electrode-electrode separation but also on the location and magnitude of contrasts in electrical conductivity. Accordingly, for surveys in which electrodes may be close to the target, sensitivity to source/receiver position can be high. Using synthetic experiments, we demonstrate that variations in the data due to errors in electrode position are comparable in magnitude to typical experimental noise levels and, in some cases, may overwhelm variations in the data due to changes in material properties. Furthermore, the statistical distribution of electrode position errors can be complicated and multi-modal such that bias may be introduced into the ERT data.
DE: 0699 General or miscellaneous
DE: 0915 Downhole methods
DE: 0925 Magnetic and electrical methods
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly