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

Abstract

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.