Resistivity imaging in steep and weathered terrains

In areas where tectonics and weathering have hydrogeological significance, electrical methods can assist in mapping the subsurface. In this work, resistivity imaging was used to map fractures, faults and quartz veins emplaced in strongly weathered volcanic rocks. The aim was to map geological units related to the formation of groundwater systems in the Rio Artiguas basin, Nicaragua. Eight 2D resistivity surveys selected from two sites with characteristic geological features are discussed in this article. The resistivity lines were carried out with a multi-electrode system together with different electrode geometries. The data inversion was done by applying the robust (L-i-norm) method and a finite element grid to accommodate the steep topography. The data interpretation included numerical modelling to assess the suitability of all used electrode arrays in relation to the expected geological settings. The results indicated a top high resistive layer consisting of colluviums with laterite (>50 Omega m), underlain by conductive clay weathered layers (10-50 Omega m). Below the conductive layer less weathered or fresh rock can be found with higher values (50-600 Omega m). The quartz veins and dykes stand out as the highest resistive bodies (100-4000 Omega m). The results indicated an overall weathering thickness of 10-30 m or more. In conclusion, the extensive resistivity imaging surveys allowed delineation of geological structures and weathering layers throughout the study area. Furthermore, the vertical extension of weathering was examined at all locations. A finite element grid in the inversion prevented distortions arising from topography regardless of the steep slope observed at the survey locations. The numerical modelling results revealed the strengths of different arrays for the particular cases of a fault and a quartz vein in irregular terrains