General overview of geophysical studies at Cerro Prieto

Abstract

Geophysical investigations by the CFE in the Mexicali Valley near the Cerro Prieto volcano began nearly 20 years ago. Initially, gravity and seismic refraction methods were used for structural information related to faults and basement configuration. Geophysical emphasis shifted to d.c. resistivity surveys in the early 1970's after it was determined that a large area of low resistivity coincided with the thermal manifestations south of the volcano and the known high temperature zone at depth. Supplemented by ground magnetic and gravity measurements, the resistivity data are being interpreted to yield a detailed picture of the structure concealed by valley fill and to identify promising areas for future exploration. Under the DOE/CFE agreement, the LBL has been conducting various types of electrical, passive seismic and precision gravity surveys over the field. This work is less concerned with geothermal exploration than the CFE geophysical program. It involves applying exploration methods to reservoir-related questions; namely, can the reservoir area be delineated and can changes in reservoir characteristics be detected and monitored by means of surface measurements? As the second question involves a long time scale, repeated measurements and correlations to production data, only baseline information is presently being obtained. Dipole-dipole resistivity, magnetotelluric (MT) and self-potential (SP) surveys have been completed and results will be discussed. The large dipolar SP anomaly found over the field has encouraged further use of the method in the Mexicali Valley. Careful interpretation of dipole-dipole and Schlumberger data set yields a subsurface resistivity model which provides a surprising amount of useful detail. Permanent current electrodes have been emplaced and repeat measurements will be made at later times to determine whether changes in resistivity occurred within the reservoir area. Reference magnetic MT technique, developed recently at U.C. Berkeley, was applied at Cerro Prieto. Earlier tests have shown that in the presence of electro-magnetic noise, such as cultural noise, this technique yields more accurate subsurface information than conventional MT. Regional seismicity has been studied for many years. Recently, the CICESE has maintained a four station network in the Northern Mexicali Valley. The LBL passive seismic results show epicenters within the reservoir boundary associated with right-lateral strike-slip movement. V P/V S data from these events indicate a Poisson's ratio of 0.4 for the upper 2–3 km of the field. Although limited, P-wave velocity and attenuation data imply a local low near the western edge of the field superimposed upon a slight high for the entire production region. Beginning in the fall of 1978, continuous monitoring by the LBL and CICESE will provide more complete data on field characteristics. The first phase of precision gravity surveys at 60 monuments is nearly complete. Repeated gravity measurements coupled with repeated first-order leveling by the CFE will be needed to distinguish subsidence or tectonic induced gravity changes from the small changes (80–100 μgal) that might be associated with mass depletion due to fluid withdrawal. A five-station seismic array is being installed in shallow holes so that the long-term changes in seismic activity and relationship to subsidence and geothermal fluid production can be studied.