Title: Resistivity methods with application to Planillas geothermal field, Mexico

Abstract

The depth of penetration of SchJumberger soundings is controlled by the shortest
distance between the current electrode and the potential electrode (S-P). This
together with the application of the finite potential electrodes separation over a
resistivity distnbution with high contrasts between layers and lateral resistivity
variations near the surface at the sounding site, causes converging and constant shifts
of the different segments of the apparent resistivity curve. These shifts, if not correctly
treated, will lead the interpretation astray. Therefore the theory of one-dimensional
interpretation of Scblumberger soundings is presented in this report in some details in
order to understand the reasons for these shifts and to make an appropriate use of two
computer programs for one-dimensional interpretation. A two-dimensional
interpretation program, which takes the topography into account, is also discussed.
SchJumberger soundings from PlanilJas Geothermal Field, Mexico were interpreted
one- and two-dimensionally. This resulted in the delineation of a low resist ivity
anomaly (resistivity less than 40 Om). At 1.500 m a.s.1. the anomaly presents a NW-SE
trend. It continues at greater depths (1.000 m a.s.l.) with the same NW-SE trend
together with a mixed N-S trend having lower resistivity values (less than 20 Om).
The geothermal fluids associated with Primavera Geothermal Area are stored in
extensional strike slip faults with NW-SE orientation, within the lower Cordilleran
volcanics (andesites), reactivated and opened by a vertical stress field. Primavera
Geothermal Area is located 7 km north of PlanilJas Geothermal Field. The same
geological formations, i.e. lower Cordilleran volcanics are located beneath Planillas
Geothermal Field. Two-dimensional interpretation of a profile perpendicular to the
low resistivity anomaly is characterized by a less than 15 Om anomaly, 3.5 km wide in
the southwestern slopes of Cerros Las PlanilJas, flanked by two relatively resistive
structures, reaching towards the surface. At the NE flank there are steam vents and
near the SW flank hot springs are found, at a relatively low elevation. Inside this main
anomaly there is a narrower anomaly, 1.2 km wide having a resistivity of 7 Om. This
suggests a possible vertical flow of geothermal fluids in the middle of the low
resistivity anomaly probably convecting along an open fault oriented NW-SE within
the lower Cordilleran volcanics and leaking laterally SW and NE into the upper
Cordilleran volcanics (lithic tuffs).