Karst and saltwater intrusion mapping using electric resistivity tomography: case study at the site of Cap Rhir, Southern Morocco

摘要

A 2D electrical resistivity method was used to survey the site of Cap Rhir, which is located on the Atlantic Coast, about 30 km north of Agadir, Southern Morocco. This site was proposed by the National Office of Electricity (ONE) for the construction of a large thermal plant for the provision of electrical power to the region. Nineteen 2D electrical resistivity tomography profiles with electrode spacings of 3 m and 5 m were acquired across the study area using the dipole-dipole electrode configuration. The apparent resistivity measurements were taken by the Sting R8 system (AGI) and the inversion of the data was accomplished by the EarthImager 2D software. The integration of the apparent resistivity inversion results, the surface geology and the borehole cores data indicate, in addition to the near surface topsoil cover, two prominent high and low resistivity areas on all the resistivity profiles. The high resistivity readings of up to 2000 Ohm.m correlate with the very compact reef limestone that crops out in the southern portion of the study area and with the more or less resistive block-faulted limestone that occurs in the northern portion of the study area. Nevertheless, the most important findings that might have a negative impact on the thermal plant project construction, local ecosystems and groundwater resources farther inland, are the low resistivity anomalies that dominate all the 2D resistivity profiles at relatively shallow depth. They show low resistivity values between 10 Ohm.m and 20 Ohm.m and have geometrical forms of alternating nearly spherical and elongate bodies. In accordance with the outcrops and the borehole cores, these features are likely to represent saltwater-filled buried karstic cavities connected by channels, caused by limestone dissolution. An alternative interpretation could be that excessive carbonate dissolution has resulted only in a nested network of large communicating pores filled with marine waters. Major faults, spanning-the entire study area, were imaged on the resistivity profiles, sharp cliffs were also identified and were interpreted as marks of ancient shorelines.