Tomography (geophysical tool) at the service of archaeology: examples of habitat structures in Eastern Morocco

Geophysical approach is a reliable investigative tool, used before and during excavations. Geophysical prospecting permits the physical parameters of the subsoil to be mapped in large-scale reconnaissance surveys. One of the most commonly applied techniques in geophysical surveying is the electrical resistivity tomography (ERT). The at hand geophysical study was predetermined to help archaeologists in their planning of any future excavation and show if the previously cleared sites still contained considerable remains underneath. A two-dimensional (ERT) survey was undertaken in two archaeological sites located in the eastern part of Morocco (Guenfouda and Reggada-Jrawa), near Oujda city. The ERT survey was conducted by means of a Syscal Pro system with 24 active electrodes. The dipole-dipole array was selected for measurements, as it is well known, of its very high sensitivity to horizontal changes in resistivity. The Res2dinv resistivity inversion software was used to automatically invert the apparent resistivity acquired data and to yield a two-dimensional resistivity model. The survey at Guenfouda site was carried out using inter-electrodes with a spacing of 1m and 0.5m; the distance of profiles is 23m and 11.5m respectively. They reached depths of 5m and 2m respectively. The sections show strong lateral and vertical resistivity variations: a very resistant layer located just above the sub-soil covers a conductive layer and a resistant bed-rock. At Reggada-Jrawa site the survey was conducted using inter-electrodes with a spacing of 5m, 2m and 1m in order to reach depths of 25m, 10m and 5m respectively. The first meter below the subsurface shows the presence of structures with high resistivity having widths of 1m could correspond to residues of walls.