Characterization of seawater intrusion using 2D electrical imaging

Abstract

Abstract. We have investigated the potential of 2D electrical imaging for the characterization of seawater intrusion using field data from a site in Almeria, SE Spain. Numerical simulations have been run for several scenarios, with a hydrogeological model reflecting the local site conditions. The simulations showed that only the lower salt concentrations of the seawater-freshwater transition zone could be recovered, due to the loss of resolution with depth. We quantified this capability in terms of the cumulative sensitivity associated with the measurement setup and showed that the mismatch between the targeted and imaged parameter values occurs from a certain sensitivity threshold. Similarly, heterogeneity may only be determined accurately if located in an adequately sensitive area. At the field site, we identified seawater intrusion at the scale of a few kilometres down to a hundred metres. Borehole logs show a remarkable correlation with the image obtained from surface data but indicate that the electrically derived mass fraction of pure seawater could not be recovered due to the discrepancy between the in-situ and laboratory-derived petrophysical relationships.Surface-to-hole inversion results suggest that the laterally varying resolution pattern associated with such a setup dominates the image characteristics compared to the laterally more homogeneous resolution pattern of surface only inversion results, and hence surface-to-hole images are not easily interpretable in terms of larger-scale features. Our results indicate that electrical imaging can be used to constrain seawater intrusion models if image appraisal tools are appropriately used to quantify the spatial variation of sensitivity and resolution. The most crucial limitation is probably the apparent non stationarity of the petrophysical relationship during the imaging process.

Subsurface fluid distribution and possible seismic precursory signal at the Salse di Nirano mud volcanic field, Italy

“…5 shows that although the recovered conductive anomalies are located at depth, where ERT sensitivity and resolution is decreased, in particular the anomalies in the middle and in the NE part of the profile are still characterized by cumulative sensitivity values that allow a quantitative interpretation (e.g. Kemna 2000;Nguyen et al 2009). To explicitly prove that the conductive anomalies can be indeed recovered from the data, we inverted synthetic data computed from the obtained inversion result for the longitudinal profile.…”

Geophysical Methods for Monitoring Temperature Changes in Shallow Low Enthalpy Geothermal Systems

“…It is non-invasive since it only requires electrodes at the ground surface. However, surface measurements suffer from poor resolution at depth [45,113,114] even if Robert et al [51] successfully managed to follow a salt tracer in fractures at a depth of 20 to 30 m. Hermans et al [74] propose guidelines deduced from their study case to design surface arrays for monitoring studies. Using 62 electrodes with an electrode spacing a, they successfully imaged a heat plume 3.33a thick, 4a wide and at a depth of 4a.…”

“…Quantitative interpretation remains indeed difficult without additional information, generally in the form of a few additional in situ measurements or/and the use of additional geophysical methods (for instance GPR or induced polarization). Electrical resistivity tomography (ERT) has proven its efficiency to image and/or monitor spatial phenomena [43] such as salt water intrusions [44,45], variations in moisture content (e.g., [46,47]), biodegradation of hydrocarbons (e.g., [48,49]) and salt tracer experiments (e.g., [50,51] and references therein). An in-depth review of electrical properties of rocks can be found in Schön [52] or Revil et al [53], whereas a description of electrical methods can be found in Binley and Kemna [54].…”

Imaging Seawater Intrusion in Coastal Zone of Kavala (N. Greece) With Electrical Resistivity Tomography

“…Among them, the Electrical Resistivity Tomography (ERT) is considered as one the most efficient techniques in imaging such properties and processes associated to subsurface geoelectrical structure (Daily et al, 1992;Park, 1998;Muller et al, 2003;Singha and Gorelick, 2005;Mohnke et al, 2006;Nguyen et al, 2009). In order to eliminate limitations of resolution decrease with increasing depth of investigation, ERT techniques based on variations of borehole, surface-to-borehole and cross-hole measurements, have been proposed (Asch and Morrison, 1989;Daily and Owen, 1991;Slater et al, 2000;Zhou and Greenhalgh, 2000;Friedel et al, 2004;Marescot et al, 2002;Tsourlos et al, 2004Tsourlos et al, , 2005Wilkinson et al, 2008).…”