Title: The Borinquen Geothermal Area, Costa Rica: 1D and 3D inversion of resistivity data Geological/Geothermal interpretation

Abstract

This work consists of modelling and interpretation of resistivity data from the Borinquen
geothermal area in Costa Rica through 1D joint inversion of Time Domain Electromagnetic
(TDEM) and magnetotelluric (MT) data and 3D inversion of static shift corrected MT data.
A comparison of the results with gravity, geochemistry, geological investigation and well
log data is done.
A total of 97 co-located MT/TDEM sounding pairs were used to carry out the 1D resistivity
inversion. Resistivity cross-sections and depth slices were created based on the 1D and 3D
inversion results to compare both approaches. The subsurface resistivity structure shows a
clear image of the main elements associated with the different stages of alteration
mineralogy present in a typical high temperature geothermal system in a volcanic
environment. On top, a high resistivity zone is seen, reflecting unaltered rocks. Then, a
shallow lying conductive layer is found reflecting smectite alteration mineralogy. Below
the low resistivity cap, a high resistivity core is found, reflecting chlorite-epidote alteration.
Good correlation is observed between the subsurface resistivity structure and the alteration
mineralogy revealed in borehole cuttings.
A new constraint to the Cañas Dulces Caldera is suggested here by slightly modifying some
of the previously proposed boundaries and by suggesting the missing boundaries to the
north. Based on this work, the north and northeast boundary of the Cañas Dulces caldera is
proposed. It is suggested here that the Borinquen and Las Pailas geothermal areas are
mostly controlled by the north and northeast boundaries of the inferred Cañas Dulces and
San Vicente caldera structures and other secondary tectonic structures or fractures.
The results are important for a better understanding of the geothermal system and the
geological setting which is most essential for ICE (Instituto Costarricense de Electricidad,
Costa Rican Electricity Company) in developing future geothermal projects in the country.