Time-lapse ERT monitoring of an injection/withdrawal experiment in a shallow unconfined aquifer

Abstract

To quantify performance of 3D time-lapse electrical resistivity tomography (ERT), a sequential injection/withdrawal experiment was designed for monitoring the pump-and-capture remediation of a conductive solute in an unconfined alluvial aquifer. Prior information is incorporated into the inversion procedure via regularization with respect to a reference model according to three protocols: (1) independent regularization involving a single reference model, (2) background regularization involving a reference model obtained via inversion of preinjection data, and (3) time-lapse regularization involving an evolving reference model obtained via inversion of data from previous experimental stages. Emplacement and sequential withdrawal of the solute is clearly imaged for all protocols. Time-lapse regularization results in greater amounts of model structure, while providing signifi-cant computational savings. ERT-estimated electrical conductivity is used to predict solute concentration and solute mass in the aquifer. At any experimental stage, we are able to estimate total solute mass in the aquifer with a maximum accuracy of 60%–85% depending on regularization protocol and survey geometry. We also estimate the withdrawn solute mass for every experimental stage (the change in mass between experimental stages). Withdrawn mass estimates are more reliable than total mass estimates and do not exhibit systematic underprediction or dependence on regularization protocol. Withdrawn mass estimates are accurate for changes in mass below [Formula: see text] of potassium bromide [Formula: see text] for horizontal and vertical dipole-dipole surveys, respectively. Estimating the withdrawn solute mass does not require background subtraction and, thus, does not require background data.

Time-lapse resistivity investigation of salinity changes at an ex-promontory land: a case study of Carey Island, Selangor, Malaysia

“…Image analysis, from spatially distributed resistances at specific time intervals, is through timelapse electrical-resistivity tomography (TLERT). TLERT's various applications have attracted many researchers: Barker and Moore (1998) for physical-model tests of groundwater flow and contamination; Kemna et al (2002), Cassiani et al (2006), and Oldenborger et al (2007) for tracertest study of aquifers; Olofsson and Lundmark (2009) for impact monitoring of roadside-soil de-icing-salt in saltwater investigation; and Ogilvy et al (2009) for study of near-coast saltwater intrusion; in each, TLERT images relate change in salinity to transport of solutes.…”

High-resolution Electrical Resistivity Tomography monitoring of a tracer test in a confined aquifer

“…By contrast 3D inversion produces volumetric images, fully reconstructing the 3D nature of the tracer plume, albeit with decreasing resolution at greater distances from the boreholes. This method has been used to monitor tracer tests on timescales of several hours to a few days, typically using fewer than 10 boreholes, each with some 10 -20 electrodes (Daily and Ramirez, 2000;Binley et al, 2002;Singha and Gorelick, 2005;Oldenborger et al, 2007a;Kuras et al, 2009). In many cases, quantitative estimates can be made of seepage velocities (Sandberg et al, 2002), spatial moments (Binley et al, 2002;Singha and Gorelick, 2005;Looms et al, 2008), hydraulic conductivity (Binley et al, 2002), and tracer mass and concentration (Singha and Gorelick, 2006;Oldenborger et al, 2007a).…”

Time-lapse resistivity analysis of Quaternary sediments in the Midlands of Ireland

“…Acceptable convergence was achieved after 5 iterations. From the time-lapse analysis perspective, the software package allows a joint inversion technique, where the inversion of the first measured resistivity data is used as a reference model to constrain the datasets collected at a later stage, this method has been used in previous research (Loke, 2001;Miller et al, 2008;Oldenborger et al, 2007). Miller et al (2008) compared three methods for time-lapse data inversion and found that using a reference model for the inversion of additional datasets minimised the occurrence of noiserelated time-lapse artefacts.…”

“…This technique can be carried out through short time periods (several days with readings taken every few hours) to evaluate the migration of contamination plumes (Radulescu et al, 2007), detection and monitoring of concentration of a conductive contaminant within aquifers (Cassiani et al, 2006;Chambers et al, 2004;Oldenborger et al, 2007), quantification of superficial water infiltration rates into the subsurface (Barker and Moore, 1998) and tracer test monitoring (Monego et al, 2010;Ward et al, 2010). Long term time-lapse resistivity surveys have been applied to monitor seasonal variations on seepage rates (Johansson and Dahlin, 1996;Sjödahl et al, 2008), monitoring salinity within aquifers in coastal areas (de Franco et al, 2009;Leroux and Dahlin, 2006;Ogilvy et al, 2009), safety assessment for storage of nuclear waste (Yaramanci, 2000), estimation of subsurface temperature variation (Morard et al, 2008), observing changes in liquid water saturation and temperature in frozen ground (Hauck, 2002) and monitoring permafrost active layer thickness variation (Kneisel, 2006).…”