HR: 1340h
AN: H23A-1102    [Abstracts]
TI: Effects of Finite Antenna Length on Crosshole GPR Tomography
AU: * Irving, J D
EM: jdirving@pangea.stanford.edu
AF: Geophysics Department, Stanford University, Mitchell Building, Stanford, CA 94305-2215 United States
AU: Knight, R J
EM: rknight@pangea.stanford.edu
AF: Geophysics Department, Stanford University, Mitchell Building, Stanford, CA 94305-2215 United States
AB: Over the past decade, crosshole ground-penetrating radar (GPR) tomography has become an important tool for the estimation of subsurface moisture content. In theory, in order to produce the highest resolution images possible with this technique, rays covering a wide range of angles between the boreholes are necessary. In practice, however, including high-angle ray data in crosshole GPR inversions often results in poor data fitting and tomographic images with obvious errors. The reasons for this discrepancy between theory and practice have not been adequately addressed in the literature, and usually high-angle rays are discarded in crosshole GPR inversions to avoid problems. Unfortunately, this leads to tomograms with poor horizontal resolution. Here, we investigate whether the failure to account for the finite length of borehole radar antennas plays some role in the difficulties encountered with high-angle ray data. Specifically, we look at whether, for large vertical offsets between the radar antennas, energy traveling between the antenna tips sometimes arrives before that traveling between their centers. This will result in geometrical inversion artifacts because standard inversion algorithms assume that all first arrival energy travels between the antenna centers. Through numerical modeling, we have found that, in both vadose and saturated zone environments, the velocity of a current pulse along a borehole GPR antenna can be significantly faster than the velocity through the earth between the boreholes. This results because the borehole antenna wire is not embedded directly in the earth, but rather surrounded immediately by the antenna insulation and borehole filling material. Consequently, for high source-receiver angles, energy propagating up the transmitter antenna, across to the receiver antenna tip, and up the receiver antenna, can sometimes arrive before that propagating directly between the antenna centers. Using our modeling results, we have simulated crosshole GPR with realistic, finite length antennas, and subsequently inverted the synthetic data with the standard assumption that rays join the centers of the antennas. We have found that not accounting for coupling between the tips of our borehole antennas can lead to significant errors in the resulting tomographic images of moisture content. We are currently working on an inversion algorithm that allows for the successful incorporation of high-angle rays into crosshole GPR inversions, and thus higher resolution moisture content estimates.
DE: 1832 Groundwater transport
DE: 1866 Soil moisture
DE: 1894 Instruments and techniques
DE: 0609 Antennas
DE: 0689 Wave propagation (4275)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting