Noninvasive monitoring of DNAPL migration through a saturated porous medium using electrical impedance tomography

Abstract

Electrical impedance tomography was used to monitor the movement of a fluorinated hydrocarbon DNAPL through a saturated porous medium within a laboratory column. Impedance measurements were made using a horizontal plane of twelve electrodes positioned at regular intervals around the centre of the column. A 2D inversion algorithm, which incorporated the cylindrical geometry of the column, was used to reconstruct resistivity and phase images from the measured data. Differential time-lapse images of DNAPL movement past the plane of electrodes were generated by the cell-bycell subtraction of resistivity and phase baseline models from those associated with the DNAPL release stage of the experiment.The DNAPL pulse was clearly delineated as resistive anomalies in the differential time-lapse resistivity images. The spatial extent of the resistive anomalies indicated that, in addition to vertical migration, some lateral spreading of the DNAPL had occurred. Residual contamination could be detected after quasi-static conditions were re-established. Residual DNAPL saturation was estimated from the resistivity model data by applying Archie's second equation. Despite significant measured phase changes due to DNAPL contamination, the differential phase images revealed only weak anomalies associated with DNAPL flow; these anomalies could be seen only in the initial stages of the experiment during peak flow through the plane of electrodes.

Time-lapse resistivity investigations for imaging saltwater transport in glaciofluvial deposits

“…For the interpretation, the commercial software Res2Dinv (Loke 1996(Loke -2004) and the so-called robust inversion method were used. The minimization of a mixed L1-norm was done as an iteratively re-weighted least-squares algorithm, similar to that proposed by Farquharson and Oldenburg (1998).…”

“…Such models are often found to represent geological reality better and that is why the ''blocky method'' is often preferred. For estimating the changes between measurements made at two different times, the programme provides different options for the ''time-lapse inversion'', which define the constraints between successive time steps (Loke 1999;Chambers et al 2004). The aim is that the result should be more representative of actual changes in the underground and less affected by noise, due for example to changes in the contact between the electrodes and the ground.…”

Computation of Optimized Arrays for 3-D Electrical Imaging Surveys

“…The values can be calculated analytically as the potential values have a simple mathematical form. However for a non-infinite or nonhomogeneous medium where the potentials due to a current source do not have a simple mathematical form, the effective geometric factor (and associated sensitivity values) have to be calculated numerically (Chambers et al 2004). To calculate the potentials values, we use the finite-difference method (Dey & Morrison 1979;Loke & Barker 1996).…”

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

“…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).…”