Dipole-dipole resistivity monitoring at the Cerro Prieto geothermal field
Two 20 km-long dipole-dipole lines with permanently emplaced electronics at 1-km spacings were established over the field area; one of these lines is remeasured annually. Resistivity measurements are taken using a 25 kW generator capable of up to 80A output and a microprocessor-controlled signal-averaging receiver; this high power-low noise system is capable of highly accurate measurements even at large transmitter-receiver separations. Standard error calculations for collected data indicate errors less than 5% for all points, but 95% confidence intervals show error limits about 2 to 4 times higher. Data indicate little change of apparent resistivity within the upper 300 m over the field. However, apparent resistivity increases are observed over the producing zone at depths of 1 km and greater. Large zones of decreasing apparent resistivity are observed flanking the zone of increases on both sides. To explain the resistivity changes observed, simple two-dimensional reservoir simulations were performed in which cooler, less saline recharge water enters the reservoir from above through a leaky caprock and laterally through a more permeable vertical boundary. The calculated magnitude of a resistivity change after 3 years of simulated production fits the observed data, but the anomaly shapes differ. It is concluded that the rapidly moving hydraulic front produces a salinity change large enough to explain the resistivity increase, but that our recharge assumptions were probably oversimplified.
- Research Organization:
- Lawrence Berkeley Lab., CA (USA)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 5937139
- Report Number(s):
- LBL-15831; CONF-8309104-2; ON: DE83016000
- Resource Relation:
- Conference: International Society of Exploration Geophysicists meeting, Las Vegas, NV, USA, 11 Sep 1983
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
CERRO PRIETO GEOTHERMAL FIELD
RESISTIVITY SURVEYS
RECHARGE
SIMULATION
TWO-DIMENSIONAL CALCULATIONS
ELECTRICAL SURVEYS
GEOPHYSICAL SURVEYS
GEOTHERMAL FIELDS
SURVEYS
Geothermal Legacy
150301* - Geothermal Exploration & Exploration Technology- Geophysical Techniques & Surveys