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See detailCovariance-constrained difference inversion of time-lapse electrical resistivity tomography data
Hermans, Thomas ULg; Kemna, Andreas; Nguyen, Frédéric ULg

in Geophysics (in press)

Hydrogeophysics has become a major field of research in the past two decades and time-lapse electrical resistivity tomography (ERT) is one of the most popular techniques to monitor passive and active ... [more ▼]

Hydrogeophysics has become a major field of research in the past two decades and time-lapse electrical resistivity tomography (ERT) is one of the most popular techniques to monitor passive and active processes in shallow subsurface reservoirs. Time-lapse inversion schemes have been developed to refine inversion results, but they mostly still rely on a spatial regularization procedure based on the standard smoothness constraint. In this paper, we propose to apply a covariance-based regularization operator in the time-lapse ERT inverse problem. We first illustrate the method for surface and cross-hole ERT with two synthetic cases and compare the results with the smoothness-constrained inversion (SCI). The examples show that the covariance–constrained inversions (CCI) better image the anomaly both in terms of shape and amplitude. Although more important in low-sensitivity zones, improvements are observed everywhere in the tomograms. Those synthetic examples also show that an error made in the range or in the type of the variogram model has a limited impact on the resulting image, which still remains better than SCI. The method is then applied to cross-borehole ERT field data from a heat tracing experiment, where the comparison with direct measurements shows a strong improvement of the breakthrough curves retrieved from ERT. The proposed method could be extended to the time dimension which would allow the use of CCI in 4D inversion schemes. [less ▲]

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See detailData-driven selection of the minimum-gradient support parameter in time-lapse focused electrical imaging
Nguyen, Frédéric ULg; Kemna, Andreas; Robert, Tanguy et al

in Geophysics (2016), 81(1), 1-5

We have considered the problem of the choice of the minimum-gradient support (MGS) parameter in focused inversion for time-lapse (TL) electric resistivity tomography. Most existing approaches have relied ... [more ▼]

We have considered the problem of the choice of the minimum-gradient support (MGS) parameter in focused inversion for time-lapse (TL) electric resistivity tomography. Most existing approaches have relied either on an arbitrary choice of this parameter or one based on the prior information, such as the expected contrast in the TL image. We have decided to select the MGS parameter using a line search based on the value of the TL data root-mean-square misfit at the first iteration of the nonlinear inversion procedure. The latter was based on a Gauss-Newton scheme minimizing a regularized objective function in which the regularization functional was defined by the MGS functional. The regularization parameter was optimized to achieve a certain target level, following the Occam principles. We have validated our approach on a synthetic benchmark using a complex and heterogeneous model and determined its effectiveness on electric tomography TL data collected during a salt tracer experiment in fractured limestone. Our results have determined that the approach was successful in retrieving the focused anomaly and did not rely on prior information. [less ▲]

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See detailA shallow geothermal experiment in a sandy aquifer monitored using electric resistivity tomography
Hermans, Thomas ULg; Vandenbohede, Alexander; Lebbe, Luc et al

in Geophysics (2012), 77(1), 11-21

Groundwater resources are increasingly used around the world for geothermal exploitation systems. To monitor such systems and to estimate their governing parameters, we rely mainly on borehole ... [more ▼]

Groundwater resources are increasingly used around the world for geothermal exploitation systems. To monitor such systems and to estimate their governing parameters, we rely mainly on borehole observations of the temperature field at a few locations. Bulk electrical resistivity variations can bring important information on temperature changes in aquifers. In this paper, we demonstrate the ability of surface electrical resistivity tomography to monitor spatially temperature variations in a sandy aquifer during a thermal injection test. Heated water (48°C) was injected for 70 hours at the rate of 87 l/h in a 10.5°C aquifer. Temperature changes derived from time-lapse electrical images are in agreement with laboratory water electrical conductivity-temperature measurements. In parallel, a coupled hydrogeological saturated flow and heat transport model was calibrated on geophysical data for the conceptual model, and on hydrogeological and temperature data for the parameters. The resistivity images showed an upper flow of heated water along the well above the injection screens and lead to a new conceptualization of the hydrogeological source term. The comparison between the temperature models derived from resistivity images and from the simulations is satisfactory. Quantitatively, resistivity changes allowed estimating temperature changes within the aquifer, and qualitatively, the heated plume evolution was successfully monitored. This work demonstrates the ability of electrical resistivity tomography to study heat and storage experiments in shallow aquifers. These results could potentially lead to a number of practical applications, such as the monitoring or the design of shallow geothermal systems. [less ▲]

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