# Geo-Electrical Detection of Impermeable Membranes in the Subsurface
## 4 Laboratory Setup  
An experimental setup was created in the laboratory to test the ERT concept array for membrane detection. The setup consisted of a PE tank of 7.00 m long, 3.50 m wide and 2.50 m deep. The PE material ensures that the setup is electrically isolated from the surroundings. A PVC membrane was attached at one of the ends at the inside of the tank. A prototype tool was built, consisting of a pole with ring electrodes and a cross with pin electrodes (Figure 1d). The material between the electrodes was powder-coated for electrical insulation. The distance between the electrodes on the cross and on the pole was 5 cm. The diameter of the cross was 0.80 m. The cross contained 33 electrodes. The length of the pole was 1.00 m and the pole contained 17 electrodes. In the field, a measuring tool will be pushed vertically into the soil and will approach the membrane while measuring. The long side of the PE tank corresponds to the depth in the field. For practical reasons, such as the height of the roof, the laboratory setup was rotated 90 degrees relative to the field. Also for practical reasons, the tool was pulled away from the membrane in the laboratory, instead of pushed to the membrane as would be the case in the field. In this way, the prototype could be less robust and the concept could be tested. In order to achieve the pulling away from the membrane, the pole of the tool was connected to a crane which pulled the instrument through the tank via two pulleys (Figure 4, right. The tool was supported by a plastic  
Figure 3: Sensitivity study for two modes (see Figure 1c for electrode configuration) and their looking-ahead capabilities. The legend shows the largest distance between the electrodes (A and N for mode 1; A and M on the tool for mode 2, B and N are remote). The curves represent kernels. rail, because of its weight. Close to the membrane, measurement positions were spaced at 5 or 10 cm intervals. Further away from the membrane, the measurement interval was increased to 30 to 50 cm. The minimum distance between the membrane and the tool was 0.10 m, the maximum distance was 4.50 m. The remote electrodes consisted of four metal plates and they were placed in the four corners of the tank, on the right boundary (at +7.00 m). The measuring unit is an MPT DAS-1. The numerical simulation for the lab experiment is included in Section 5. In the first test, the tank was filled with tap water creating a homogeneous medium. The tool could move between positions with little effort, because of the absence of friction. Several tests were performed confirming that the tool was working properly. Results from the water filled tank are included in Section 7. For the second test, several slabs and cubes of clay were added to the tank filled with sand (Figure 4a) to study the effect of heterogeneity on the performance of the method. This setup reflects true field conditions with a heterogeneous subsurface to the closest. In general, the soil at the field site consists of sand, but cone penetration tests on the field site have shown that occasionally clay pockets are present. The steps for the installation of the final sand and clay tank were the following:  
1. Install the non-conductive supporting rail
2. Fill the tank with sand to about half the height (approximately 1 m), Figure 4a. 3. Install the clay slabs and cubes (Figure 4a)
4. Install the tool (Figure 4a)
5. Fill the rest of the tank with sand to a height of 2.0 m, to cover the tool and the clay slabs and cubes (Figure 4b)  
Figure 4: (**a**) Top view of the prototype of ERT tool with ring electrodes on the pole and pin electrodes on the cross. After filling the tank half full with sand, the clay slabs and cubes and the tool were installed. After that, the tank was filled up with sand and saturated with water. (**b**) photo of the laboratory setup with the tank, crane and pulleys. (**c**) Simulation of the tool and tank using COMSOL. The Y and Z coordinates are the same as the lab experiment.