Abstract
In this paper, we have investigate looking ahead capabilities of electrical resistivity tomography (ERT) for a soil penetrating tool with electrodes. In our case study, the desired detection resolution (10 to 20 cm at a depth of at least 6 m) was much higher than what can be achieved from classical surface ERT measurements. Therefore, we designed a logging-type tool, that can be pushed into the ground. Our target was a buried PVC membrane which acts as an electrical insulator. In this phase, we performed numerical simulations and laboratory measurements. The methodology is based on a two-step approach. First, we calculate the background resistivity of the tool’s path is determined by inversion of near-looking electrode configurations. Next, the theoretical response (kernel) of the far-looking configurations is calculated for different membrane positions. The root mean square (RMS) error between the kernel and the measurements is minimized to detect the membrane. If the membrane is within sensing reach, the RMS has a minimum for the kernel corresponding to the true membrane position. If no minimum in RMS is found, the membrane is not within sensing reach and the tool can be pushed closer to the membrane. The laboratory tests comprised measurements in a tank filled with either water or saturated sand or saturated sand with clay slabs and chunks. The laboratory results were successful in pinpointing the position of the membrane with an accuracy of 10 to 20 cm, depending on the dimension of the tool and the distance from the membrane.
Highlights
Infrastructure works in low-lying countries with high water tables occasionally require measures to prevent groundwater from flooding the structure
In the case at hand, the road has been constructed in a large trench which is lined with an impermeable membrane in the 1980′s
Small variations in measured data originate from the uncertainties in the laboratory, e.g., from small errors in the actual positions of the tool during the experiment, uncertainties in the dimensions of the tool, the finite size of the electrodes and noise in the data
Summary
Infrastructure works in low-lying countries with high water tables occasionally require measures to prevent groundwater from flooding the structure. In the case at hand, the road has been constructed in a large trench which is lined with an impermeable membrane in the 1980′s. For safety and to avoid flooding during excavation and construction, it is important to know the exact position of the membrane. The goal was to locate the membrane with an accuracy of 10 to 20 cm at a depth of 6 to 10 m. The approximate location of the membrane is available from archive design drawings. The tool needs to be kept at a “safe” distance of approximately 1 m from the theoretical (design) position of the membrane. Puncturing the membrane is to be avoided at all costs, because flooding of the road would result in serious economic damage
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