Joint Use of GPR and ERI to Image the Subsoil Structure in a Sandy Coastal Environment

In the present work we show a study carried out in a coastal sandy environment in NW Spain with the main aim of imaging the shallow subsoil structure. The goals of the study included the detection of the water table and bedrock, but also a description of the aeolian dunes and progradating sand layers. With these objectives two independent but complementary geophysical techniques were jointly used: ground-penetrating radar (GPR) and electrical resistivity imaging (ERI). Regarding GPR, shielded antennas of 200 & 250 MHz from two different manufacturers were used in the study; it also allowed us to make a comparative study between them. To obtain a more detailed image of the sand layers structure it was also used a shielded 500 MHz antenna. 100 & 200 MHz unshielded antennas were also used to apply the CMP method in order to obtain a precise estimation of the velocity of the electromagnetic waves into the subsoil. ERI was carried out in order to obtain independent estimations of both water table and bedrock depths, as well as the resistivity values of the groundwater water that are useful to determine the occurrence -or not- of fresh water on marine water. Although the resolution of ERI is low comparing with GPR, the penetration depth is greater. The results show that the bedrock is clearly identified in ERI profiles at about 8-9 m depth, whereas the water table is located at 5-6 m depth in good agreement with the GPR profiles. Resistivity values in the most external ERI profiles show that the water table is constituted by fresh water. In addition to this, the high resolution of both 200-250 and 500 Mhz antennae has allowed to obtain a clear image of the internal structure of the sand deposits. This information has been used to infer the dynamic of progradation of the coastal system.