Characterization of a shallow aquifer in Acoculco geothermal area (Mexico) by applying Transient Electromagnetics

Abstract

We present a characterization of the aquifer located in the Acoculco geothermal area in Mexico using Transient Electromagnetic (TEM) data. A total of 65 TEM soundings were collected, spatially distributed across the survey area, using a 100 × 100 m² single-loop configuration. The acquired TEM data were interpreted using one-dimensional Occam and Marquardt inversion techniques. At all stations, a three-layer model was sufficient to explain the data.

Prior to interpretation, we analyzed model uncertainties by evaluating parameter importance and equivalent models. The second and third layers are well resolved. The relatively more resistive second layer was correlated with the shallow aquifer of the area, whereas the deeper, more conductive layer was associated with the clay cap resulting from hydrothermal alteration, which acts as an impermeable unit.

To visualize the subsurface structures, we generated nine profiles by interpolating the information derived by the inversion, creating thus quasi-2D models across the survey area. This allowed us to better delineate the aquifer's configuration, identify its lateral boundaries, and detect zones of heterogeneity due to high clay content. The aquifer's thickness ranges from 150 to 200 m.

We conclude that our results are consistent with observations from the exploratory well EAC-1 and that TEM is a highly effective technique for hydrogeophysical exploration in a cost-efficient manner.