# Geo-Electrical Detection of Impermeable Membranes in the Subsurface
## 2 Principles of ERT
Alternative to ERT, is the use of logging tools (i.e., Lateral Log [8; 9]). Those tools though are optimized to measure in the very close vicinity of the logging tool. For our case of finding the position of the PVC membrane and a safe excavation depth, we performed several classical ERT lines with electrodes on the surface, parallel to the roadsoide slope. We used a cable with 2.00 m spacing between the electrodes and a total number of 81 electrodes. The array configurations were gradient and dipole-dipole with a total number of 4617 measurements. The measurements were inverted to resistivity profiles (Figure 2). The overall root-mean-square (RMS) error of the inversions was around 8.5%. This rather high RMS is probably due to the urban environment (highway, artifacts, lampposts), which causes electrical noise. The inversion result (Figure 2) shows a simple resistivity structure of the subsurface, with a zone of 8 m unsaturated sand, followed by 6 m of saturated sand with some clay pockets. The membrane is visible at 14 to 16 m depth, but the resolution of ERT in those depths is about 1 m (or more) in the vertical scale. This result is insufficient for our case. Still, this investigation was useful for obtaining information about the background resistivity and information about the degree of heterogeneity. In order to increase the resolution, the electrodes need to be closer to the target. We therefore need a tool that penetrates the soil and is able to look ahead to check whether the target (PVC membrane) is near. Our idea of looking ahead is inspired by two similar techniques known as borehole logging and Tunnelling Boring Machines (TBM). During borehole logging, a clay-mud is required providing both stability to the borehole and good electrical contact [10]. The very first logging tools utilized 4 electrodes arrays only. In these cases, current may be confined within the more conductive mud, especially when mud with higher salinity, and therefore missing the desired target. To overcome this problem, new logging tools were developed that utilize the simultaneous use of an array of electrodes,  
Figure 2: Two surface ERT lines aimed at detecting the membrane from the surface. The lines were situated parallel to the road on the slope of the roadside. The membrane shows as a very high resistive target (more than 1000 Ohm-m) at a depth of 14–16 m. The vertical resolution is about 1 m, i.e., not sufficient to pinpoint the positioning with the desired resolution. achieving focused currents into the formation and avoiding the effect of the mud. Since the initial development of focused arrays for borehole applications, many unique methods for focusing have been manufactured for commercial use (Laterolog LLS, Laterolog LLD, HRLA Dual Laterolog etc., [8; 9]). In our case, however, boreholes are not allowed. Another application with looking ahead capabilities originates from tunnelling. Operators needed predictions of soil conditions ahead of the cutter face. Literature on the details of these methods is limited to the PhD thesis of [PERSON] [11]. There are two commercial systems available, called the Bore-tunneling Electrical Ahead Monitoring (BEAM) system [12] and the BEAM4 [13]. These systems were developed over the last two decades to be fully integrated into TBMs. Both systems use electrical resistivity and induced polarization (IP) methods to characterize the material ahead of the tunnel face. They make clever use of the cutterheads of the TBM as electrodes. The system jumps between two different modes, one that uses the focusing electrodes to image further ahead and one scan mode to look in the vicinity of the TBM. The TBM methods, however, are patent-pending and therefore scientific literature is not available apart from [12; 13].