FROM INVERSE RESISTIVITY MODEL TO RAW DATA: ASSESSING THE DETECTABILITY OF A THIN CLAY LAYER WITHIN THE ALLUVIAL AQUIFER OF THE SUNCERI TEST SITE (HONDURAS)

ERT surveys were undertaken at the Sunceri test site located in the city of San Pedro Sula (Honduras). Four deep pumping wells are located at the site, together with 26 shallow and 12 deep piezometers drilled through alluvial deposits. Piezometric levels, hydro-chemical features of groundwater and borehole logs indicate that a clay layer is located at a depth of approximately 24 m and separates an upper unconfined aquifer from a deeper confined aquifer. This layer is widespread across the entire area and shows a variable thickness (6 m on average) and an average resistivity of 17 Qm. The inverse resistivity sections reveal the presence of the layer. However, they fail to show that it is discontinuous across the area. In particular, the layer doesn't appear in an area affected by a severe drawdown of the piezometric level due to the interference between the cones of depression of the abstraction wells. Elsewhere, the inverse resistivity sections show a local overestimation of the layer thickness when compared to borehole logs. To explain these inconsistencies and assess the layer detectability it was necessary to look for the clay layer within the raw data. One dimensional resistivity soundings have been sorted from ERT apparent resistivity data. Then, inverse resistivity sections were compared to raw data resistivity soundings. Furthermore, to assess the influence of the shallow resistivity heterogeneities and the severe drawdown of the piezometric level on the apparent resistivity data, 2D synthetic dataset modelling and sensitivity analysis were performed. The results indicated that in the area affected by a severe drawdown of the piezometric level, the clay layer is undetectable where its thickness falls below 4.9 m. Elsewhere, the apparent thickness variation of the clay layer is due to the variation of the resistivity with depth and the occurrence of shallow lateral heterogeneities as saturated and unsaturated zones, depression cones and the drawdown of the piezometric level affecting the sensitivity and also the investigation depth along the profile.