HR: 0830h
AN: NS51A-07    [Abstracts]
TI: Three-dimensional sensitivity distribution of low-induction-number frequency-domain electromagnetic instruments
AU: * Callegary, J B
EM: jcallega@usgs.gov
AF: U.S. Geological Survey, 520 N. Park Ave., Tucson, AZ 85719 United States
AU: Ferre, T P
EM: ty@hwr.arizona.edu
AF: Hydrology and Water Resources The University of Arizona, 1133 E. North Campus Drive Harshbarger Bldg PO Box 210011, Tucson, AZ 85721 United States
AU: Groom, R W
EM: rgroom@petroseikon.com
AF: PetRos EiKon Inc., 222 Snidercroft Rd., Concord, ON L4K 2K1 Canada
AB: Low-induction-number frequency-domain (LIN FEM) instruments operate in three dimensions, but analyses have traditionally been done only in one. For proper site-selection and planning of field surveys, it is critical that users be aware of the complex variability of instrument sensitivity in three dimensions and be able to predict the manner in which the electrical conductivity (EC) of a given environment affects the spatial distribution of sensitivity. The objectives of this study were to examine the three-dimensional sensitivity of LIN FEM instruments and to describe the implications of this for field surveys. Simulations were carried out to map the local sensitivity in three-dimensional, homogeneous-half-space environments with EC of the ground varying from 1 to 100 mS/m. Local variations in instrument sensitivity were complex and included regions having opposite polarities. The largest variations in sensitivity were within a few meters of the instrument. Changing the orientation of the transmitter and receiver dipoles from horizontal to vertical resulted in significant differences in the local sensitivity distribution. The results of this study can be used to identify potential sources of error in field applications and design. For example, gross errors in interpretation of target location can occur if it is assumed that instrument sensitivity is uniform throughout the sample volume or even if it is assumed that sensitivity drops off monotonically as a function of distance from the instrument. In heterogeneous environments, there also could be near-surface effects. High near-surface EC can make it difficult to measure EC at greater depths. Temporal variations in water content of upper layers can, by changing EC, make exploration depth variable with time. Addressing the three-dimensional sensitivity distribution will result in better assessment of the suitability of the method to the target or process under investigation.
DE: 0694 Instrumentation and techniques
DE: 1866 Soil moisture
DE: 1875 Unsaturated zone
DE: 1894 Instruments and techniques
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly