Resistivity survey in Abasseya

As part of our scope of work for this season we performed an electrical resistivity survey towards the west side of the Abasseya B building. The first challenge was to build our own instrument which proved to be not that difficult by using four bolts as electrodes attached to a circuitry including a small battery, a switch and two multitesters (one for voltage and one for amperage), all of them mounted in a PVC frame. Actually, we built two types of systems: Pole-Pole array and Wenner array, which would give us different depths of investigation. The surveyed area was a square of 19.5 mts x 15 mts located around 15 mts from the first step of the west access. This area was covered with the Pole – Pole array by surveying 10 lines of 13 readings each. The results were plotted in a colour coded grid map. A high resistivity area was identified towards the north-east corner of the surveying area. The measurements were checked by surveying a small grid of 4 mts x 3 mts with the Wenner array within the high resistivity area. The readings were confirmed and a trench of 3 mts x 3 mts was opened to find out what was causing the high resistivity. Several scattered partially preserved redbricks and a small but complete sandstone block among fragments of pottery, redbrick and sandstone were found in the trench at a depth between 20 cm and 40 cm from surface. All this material was surrounded by clean quartzite sand. We have concluded that this homemade resistivity survey device is enough to provide subsurface data that would allow us to identify possible structures. Further improvement has to be made on the sensibility of the signal and its relation to the actual depth of investigation. So far both type of arrays have provided similar results, although the Wenner array is easier to deploy. This is something important to consider on extensive survey areas.

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Resistivity map

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