Time‐lapse imaging of saline‐tracer transport in fractured rock using difference‐attenuation radar tomography

Abstract

[1] Accurate characterization of fractured-rock aquifer heterogeneity remains one of the most challenging and important problems in groundwater hydrology. We demonstrate a promising strategy to identify preferential flow paths in fractured rock using a combination of geophysical monitoring and conventional hydrogeologic tests. Cross-well differenceattenuation ground-penetrating radar was used to monitor saline-tracer migration in an experiment at the U.S. Geological Survey Fractured Rock Hydrology Research Site in Grafton County, New Hampshire. Radar data sets were collected every 10 min in three adjoining planes for 5 hours during each of 12 tracer tests. An innovative inversion method accounts for data acquisition times and temporal changes in attenuation during data collection. The inverse algorithm minimizes a combination of two functions. The first is the sum of weighted squared data residuals. Second is a measure of solution complexity based on an a priori space-time covariance function, subject to constraints that limit radarattenuation changes to regions of the tomograms traversed by high difference-attenuation ray paths. The time series of tomograms indicate relative tracer concentrations and tracer arrival times in the image planes; from these we infer the presence and location of a preferential flow path within a previously identified zone of transmissive fractures. These results provide new insights into solute channeling and the nature of aquifer heterogeneity at the site.

A salt tracer test monitored with surface ERT to detect preferential flow and transport paths in fractured/karstified limestones

“…In the past two decades, ERT has been used to monitor salt tracer tests or water infiltration through the vadose zone in relatively homogeneous or stratified hydrogeological systems either in the laboratory (Binley et al, 1996a;Slater et al, , 2002Koestel et al, 2008) or in test sites (Slater et al, 1997a;al Hagrey and Michaelsen, 1999;Day-Lewis et al, 2003;Singha and Gorelick, 2005;Cassiani et al, 2006;Müller et al, 2010). Table 1 presents a nonexhaustive list of surveys that were designed to monitor water infiltration or salt tracer propagation in the laboratory and in the field.…”

Applying compactness constraints to differential traveltime tomography

“…A general limitation of differential inversion is the requirement that all surveys have matching geometries, a requirement which is often difficult to fulfill in practice. A possible solution to this limitation would be to pose the imaging problem as a coupled joint inversion across multiple time steps [Day-Lewis et al, 2003], thus allowing the inclusion of compactness constraints on model perturbations without requiring explicit data differencing.…”

“…Since flow processes tend to localize in zones of high permeability, regularization operators favoring compact or connected anomalies seem reasonable. The infiltration of dense non-aqueous contaminants [Kueper et al, 1993] and the transport of saline tracers through permeable fractures [Day-Lewis et al, 2003] are two examples of compact flow features suitable for geophysical characterization.…”

Advancing Biogeochemical Research in the Field Hydrological Sciences: The CUAHSI Hydrological Measurement Facility – Biogeochemical Component

“…An even more intriguing possibility is the potential to directly image hyporheic flow distributions in streams using the attenuation characteristics of the GPR signal combined with conductive tracer tests. Characterizing the properties of materials using attenuation analysis is relatively common in crosswell studies (Chang et al 2002, Day-Lewis et al 2002, Day-Lewis et al 2003, Goldstein et al 2003, Grandjean et al 2000, Lane et al 2000, Peterson 2001, Zhou and Liu 2001. However, the development and evaluation of surface-based, reflection-attenuation analysis remains limited.…”