Magnetotelluric Models for the Characterization of the Geothermal System of the Azufral Volcano

Abstract

A magnetotelluric study (MT) was carried out in the Azufral volcano (AZV) which is located on in the western Cordillera of Andes, south-west of Colombia in the department of Nariño, to 11 km west of the municipality of Túquerres. This Volcano hosts a geothermal system of high enthalpy and priority within the national context, established by reconnaissance and pre-feasibility work carried out throughout the Colombian territory, for this reason it was decided to perform 1D and 2D modeling of the resistive structure of the subsoil to characterize the geothermal system of AZV. Therefore, layers models based on the Occam inversion process were defined, elaborating interpolated horizontal views obtained after static shift correction for each of the MT stations. In the same way, robust processing was carried out by applying the cascade decimation algorithm, impedance tensor analysis to estimate the directionality and dimensionality of the subsoil and correction of the distortion tensor to eliminate the galvanic effects in the data of according to the methodologies of Groom & Bailey and McNeice & Jones. The resistive structure of the area is characterized by zones of conductive anomalies between 1-10 ohms-m, intermediate between 10-100 ohm-m and resistive zones with values more than 100 ohms-m observable in the 1D and 2D modeling. The areas of conductive anomalies extend laterally through the study area, with a thickness of 1.2 km here associated with clayey layers that would form the seal of the geothermal system. The superficial resistive anomalies were associated to unaltered pyroclastic deposits and the deep ones to the basement, which corresponds to the Diabasic group in the area. Other contributions of the models obtained to the knowledge of the subsoil structure are the intermediate resistivity contrasts generated by the Cali - Patía and Guachucal regional faults, and conductivities anomalies near the surface in the outcrop of lavas of the Azufral and Cumbal - Chiles volcanoes attributable to fracturing.