Title

Cross-Well Radar Attenuation-Difference Tomography to Monitor a Bromide Tracer Test

Publication Date

7-2004

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geophysics

Department

Geosciences

Major Advisor

Michael D. Knoll

Abstract

Time-lapse cross-well radar methods can be used to dynamically image the movement of water and contaminants in electrically resistive environments. Previous work has focused on monitoring the movement of saline tracers in fractured bedrock, the infiltration of water in the vadose zone, and the migration of engineered materials injected into wells or trenches for remediation purposes. Although a variety of time-lapse radar imaging methods have been demonstrated, the technology is still in its infancy. Questions remain about the capabilities and limitations of the methods in different environments as well as how such data can be used to characterize heterogeneous environments, identify preferential flow paths, and constrain flow and transport models.

The goal of this thesis is to test the hypothesis that time-lapse radar attenuation-difference tomography can be used to monitor a saline tracer moving through an unconsolidated, heterogeneous, fluvial aquifer. In August 2001, a team of researchers performed a tracer test and cross-well radar imaging experiment at the Boise Hydrogeophysical Research Site (BHRS) in an unconfmed aquifer comprised of coarse, fluvial deposits (dominantly cobbles and gravels with varying amounts of sandy matrix). The team injected a potassium bromide tracer into the shallow aquifer and monitored the movement of the plume across the wellfield using fluid conductivity measurements made in six wells and cross-well radar data collected across several planes using two sets of radar antennas with different center frequencies (100 and 250 MHz). I perform time-lapse imaging in two planes, B4-B1 and B4-B2, by analyzing the changes in trace energy of the cross-well radar level run and tomographic data. Attenuation-difference tomograms were constructed for the B4-Bl and B4-B2 planes using data collected at the same locations but at different times.

I compare the spatial and temporal position and concentration variations of the plume as indicated by the fluid conductivity data at well Al (central well) to those suggested by the radar attenuation-difference tomograms in the B4-Bl plane (which contains well AI) and find good correlation. The resolution and certainty of the attenuation-difference tomograms varies across the imaging planes. Resolution analysis shows that the regions of highest resolution include the regions through which the tracer plume travels. From the time-lapse level runs and attenuation-difference tomograms for the B4-Bl and B4-B2 planes, three preferential flow paths were identified between the injection well (B3) and the withdrawal well (B6); these flow paths are horizontal and located at 10.5-12 m, 12-13 m, and 13-15.5 m depth. I use the fluid conductivity data and attenuation-difference tomograms to identify breakthrough curve characteristics which describe the transport of the tracer plume through the central part of the BHRS; these characteristics include times, depths, and values of maximum changes in attenuation coefficients (inferred maximum tracer concentration). Finally, a method for converting the attenuation-difference tomograms to maps of bromide concentration in the imaging planes is demonstrated. The results of this study show that time-lapse cross-well radar methods can be used to: (a) monitor the movement of saline water in coarse, fluvial deposits; (b) identify preferential flow paths; and (c) estimate parameter values that can be used to constrain flow and transport models.

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