Loop source transient electromagnetics in an urban noise environment: A case study in Santiago de Chile

Abstract

We have developed a transient electromagnetic (TEM) study in the noisy urban environment of the megacity Santiago de Chile. Our investigation characterizes the electrical conductivity structure of the Santiago Basin down to 300 m depth, providing key information about the sedimentary infill, hydrogeologic aspects, and geomorphological units. In total, 52 TEM soundings were recorded over roughly 900 km2. We identified different transient characteristics and noise patterns, spatially correlated to the investigation area. This step allowed classification of the soundings into three groups: highly distorted data, transients severely affected by cultural electromagnetic noise sources, and good-quality data with low noise levels. Conventional 1D inversion techniques were then used to derive resistivity depth models. The models were interpreted using sedimentary units, alluvial fan, static groundwater level information, and also were verified using borehole data. Based on 1D models, different minimum sedimentary thicknesses are observed, ranging between 50 and 300 m in depth. The average derived sedimentary thickness is approximately 210 m. In addition, seven soundings indicated the basement. These are mostly located toward the periphery of the Santiago Basin. We found a high conductive zone toward the north and a low-conductive area toward the south of the basin. The southern area is characterized by the absence of conductive layers at a depth between 100 and 150 m. This is likely related to the presence of a thick gravel layer and the absence of shallow clay layers acting as aquitards. The resistivity distribution and the sedimentary thickness obtained for this noisy and urban environment are key inputs for groundwater research as the water supply becomes more limited, specifically in central Chile. The overall consistency of the derived subsurface models highlights the suitability of the TEM method for investigating urban noise environments.