Seal Cap Resistivity Structure of Los Humeros Geothermal Field from Direct Current and Transient Electromagnetic Soundings

Abstract

Geophysical methods provide important information in the exploration of geothermal resources.

In this study, we search for clues in the shallow resistivity structure of the presence of a known geothermal reservoir (Los Humeros, Mexico). We interpret about 410 vertical electric soundings (VES) and 230 transient electromagnetic (TEM) soundings, giving information usually down to depths of 1 km, although in some sites they reached 2km. The vertical structure of the resistivity generally consists of a resistive-conductive-resistive sequence. The most important feature is the conductive unit, known as the clay-cap, associated with hydrothermal alteration clays overlying the geothermal reservoir. This unit suffers from a widespread equivalence problem, where its resistivity and thickness cannot be determined independently. However, well temperatures and associated alteration clays help to constrain this problem. In the shallow resistive unit we found several zones where its resistivity showed abnormally low values, which could represent recharge zones where fractured rock permits the infiltration of meteoric water to reservoir depths. The conductive clay-cap not only occurs over the geothermal reservoir, but has a regional presence. However, over the reservoir this electric unit has a larger conductance and its top is shallower. The few locations where the resistivity soundings reached depths of the geothermal reservoir with well-resolved estimates give a mean resistivity of 118 ohm▪m, with no statistical difference between the producing and non-producing zones. This resistivity value falls within the range found in other geothermal zones in the world.