U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 3 of 3--Subsurface Contamination From Point Sources, Water-Resources Investigations Report 99-4018C

Borehole Radar Tomography using Saline Tracer Injections to Image Fluid Flow in Fractured Rock

ABSTRACT

Cross-hole radar tomography surveys using saline tracer injections have been developed and tested at the U.S. Geological Survey Mirror Lake fractured-rock field-research site in Grafton County, New Hampshire to delineate transmissive fracture zones and image fluid flow at a scale of a meter to about 100 meters (m). High concentration (20-50 gram per liter) sodium chloride tracers injected into transmissive fractures increase the electromagnetic (EM) attenuation observed in cross-hole radar scans as compared to the observed background attenuation. The observed differences in EM attenuation are inverted to produce attenuation-difference tomograms.

Attenuation-difference tomograms acquired under 'steady-state' injection and pumping conditions were used to delineate the locations and orientations of transmissive fracture zones between boreholes. Time-lapse radar-tomography methods can monitor slug-injection saline tracer tests and image fluid flow paths in fractured-rock. Time-lapse cross-hole radar data sets were acquired at Mirror Lake by using controlled injections of fixed volumes of a high-concentration saline tracer, while scanning small sections of the tomographic image. Equivalent time data from each scanned section are sorted, analyzed to determine attenuation differences, and inverted. Time-lapse attenuation-difference tomograms delineate transmissive zones, identify variability within transmissive zones, and provide kinematic information that images ground-water flow and transport at a scale of a meter to 100 m.

The location of transmissive zones within the tomographic image plane and tracer travel times to the image plane are provided by steady-state and time-lapse attenuation-difference tomograms. Results of steady-state and time-lapse tomography surveys were used to help construct and calibrate ground-water flow and transport models in the FSE well field.

Time-lapse tomography would be more useful if time-dependent attenuation changes could be used to estimate tracer specific conductance and concentration at the image plane. A simple effective medium method of analyzing time-dependent attenuation changes to estimate tracer specific conductance was tested on radar data acquired over a meter-scale fractured granite block. Preliminary results illustrate the importance of estimating secondary porosity accurately, and suggest that robust analysis may require (1) accounting for effects of EM scattering on attenuation, and (2) modifying field methods to include several steady-state injection tests using different tracer concentrations.