U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings
of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993,
Water-Resources Investigations Report 94-4015
Velocity, Attenuation, and Dispersion Electromagnetic Tomography
in Fractured Rock
David L. Wright (U.S. Geological Survey, Denver, Colo.), Gary
R. Olhoeft (U.S. Geological Survey, Denver, Colo.), and Thomas P. Grover
(U.S. Geological Survey, Denver, Colo.)
Hole-to-hole electromagnetic tomography data have been acquired from
15 pairs of wells at the Mirror Lake site, Grafton County, New Hampshire,
using a high-speed borehole radar system designed and built by the U.S.
Geological Survey. Although tomography has good theoretical and mathematical
underpinnings, higher resolution, better data-acquisition methods, better
tomographic processing and better visualization of the processed data than
are currently available are needed if tomography is to achieve full potential
as a geophysical tool for hydrologists. A 3-dimensional display of two velocity
tomograms from data collected at the FSE well field at the Mirror Lake site
illustrates the potential for mapping fracture zones in three dimensions.
In addition, we show examples of velocity, attenuation, and dispersion tomograms
made from a single data set in the well pair FSE4-FSE1. Comparisons between
these three tomograms and with projected fractures from acoustic televiewer
logs indicate that velocity, attenuation, and dispersion tomograms, though
having some correlations with each other and with fractures, differ in shape.
Because tomograms respond to lithology as well as to fractures, it might
be necessary to use more than one type of tomogram to help discriminate
between the response to fractures and the response to lithology in tomograms,
unless the lithology is uniform.