Two- and three-dimensional resistivity models of the RMT data of the DeepEarthShape project (Chile) imaging the critical zone

In 2019, the DeepEarthShape project was launched to gain a deeper understanding of the interaction between geological, geochemical and biological processes controlling the weathering in the first tens to hundred meters of the subsurface. The elongated Chilean Coastal Range was selected as the ideal study area to investigate the effects of vegetation, precipitation and erosion on the transformation of intact bedrock into regolith within the so-called critical zone (CZ). This area encompasses several climate zones, from dry to humid, within a similar geological setting. We have carried out a Radio-Magnetotelluric (RMT) survey using a horizontal magnetic dipole (HMD) transmitter to image the lateral extent of the near-surface layers and the CZ by means of the subsurface electrical conductivity distribution at Santa Gracia and Nahuelbuta in Chile - two sites of the DeepEarthShape project. We inverted the controlled source Radio-Magnetotellurics data using the finite element code GoFEM and the finite difference ModEM algorithm to obtain the first resistivity models. The models show boundaries between the conductive weathering front of the soil layer at the surface, the regolith layer and the intact bedrock down to a depth of ≈ 100 m . The depth of the CZ is consistent with borehole logs and drill cores, as well as with the results of a seismic study. We interpret electrical conductivity structures below the CZ, in particular the conductive channels as pathways for fluids, which might be linked to topography and meteoric waters. In particular, the results from Nahuelbuta illustrate that geophysical imaging prior to drilling activity is essential and protects against misinterpretation. A comparison of our conductivity models from the two locations with different climate and precipitation rates supports the assumption that the extent of the CZ is related to both properties.

Graphical abstract