Effects of lateral heterogeneity on 1D D.C. resistivity and transient electromagnetic soundings in Kuwait

Abstract

Aquifer storage and recovery (ASR) of treated wastewater is a viable sustainable water management option for Kuwait. A geophysical survey to characterize the target aquifer in the Dammam Formation was conducted to obtain one-dimensional (1D) resistivity using the D.C. resistivity (DCR) and transient electromagnetic (TEM) methods. For DCR, we implement a systematic approach to obtain a 1 D vertical profile using fixed-thickness and variable-thickness layer inversion techniques in succession. The optimal model has 6 layers above the half-space depth of 101 m, consisting of 3 surface layers down to 15 m depth and 3 intermediate layers, which correspond to the formations of the Kuwait Group overlying the Dammam Formation. Anomalies in the data which cannot be attributed to noise or error are not adequately fit by the best set of ID models. The possibility that lateral heterogeneity explains the variation in the data is explored using approximate 2D resistivity inversion. A comparison of the 1D vertical profile obtained from the approximate 2D image with the 1D layered model indicates that, in our case, 1D analysis provides a sufficient picture of the subsurface despite the evidence of possible lateral heterogeneities in the subsurface. Such heterogeneity is explained by the occurrence of gatch (caliche) in the Fars and Ghar formations of the Kuwait Group. The comparison between DCR and TEM indicates that the TEM data is not sensitive to a relatively resistive layer that is resolved by the 1D DCR inversion, or to the resistive heterogeneities that are indicated in the DCR data with respect to the best fit. We obtain the top of the Dammam formation - or the aquitard on top of the Dammam - as the model half-space depth at approximately 100 m below the surface in both data sets.