IDENTIFICATION AND INVESTIGATION OF SHALLOW PALEOCHANNELS IN THE CHAMELECON VALLEY (HONDURAS): 1D VS 2D ELECTRICAL RESISTIVITY SURVEYS

Electrical Resistivity Tomography (ERT) and resistivity depth sounding surveys were undertaken at two sites located along the Chamelecòn Valley (Honduras). The objective of the surveys was to determine the occurrence, geometry and lithological properties of the Chamelecòn River’s shallow paleochannels. Following the interpretation of aerial photograms which indicated the likely presence of paleochannels, two sites were selected for the surveys. Resistivity depth soundings were used to achieve electro-stratigraphic sections obtained by interpolating 1D inverted data that would assist with assessing the occurrence of paleochannels at a large scale. The presence of paleochannels at the two sites was assessed through interpretation of the electro-stratigraphic sections. At Site 2, the abundant 1D data, the small distance between the soundings and the shallow target, allowed determining the course of the paleoriver as shown by resistivity and electrical transmissivity maps. The results from the 1D data, although affected by strong heterogeneities, are congruent with the 2D inverted models. ERT surveys confirmed the presence of paleochannels and allowed achieving detailed imaging of the alluvial geological bodies. Paleochannel deposits show resistivity values ranging between 70 and 100 ohm m at Site 1, while resistivity values range between 90 and 120 ohm m at Site 2. The latter resistivity values were affected by the presence of coarser deposits. Resistivity values suggest that at both sites paleochannel deposits have a clay content that is lower than 3 %. Other alluvial deposits in the area show resistivity values that range between 19 and 70 ohm m at Site 1, while resistivity values range between 30 and 90 ohm m at Site 2. These lower values suggest that clay content can be as high as 29 %. The presence of paleochannels assessed by means of 1D surveys and the lithological characterisation assessed by means of 2D surveys were confirmed by logs from boreholes drilled at the sites.