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Cerro Prieto Geothermal Field, CFE's Geophysical Studies

by
C. Davenport, L. Fonseca, I. Puente Cruz and A. de La Pena

 

ABSTRACT

The Cerro Prieto Geothermal Field, which is currently producing 180 megawatts, is one of the largest producing geothermal fields in the world. Since initial production in 1973 until 1980, the field has generated in excess of four billion kilowatt hours of electricity, which represents a savings of 8 million barrels of petroleum. Cerro Prieto is projected to produce 620 megawatts by the year 1984.

 

The Mexican government, through the Federal Electricity Commission (CFE), has been conducting surface and drill hole geophysical surveys at the Cerro Prieto field and in the Mexicali Valley since 1962. The initial studies consisted of seismic refraction profiles and a gravity survey in the area of the geothermal field. Since that time, various geophysical techniques, including resistivity, magnetics, magnetotellurics, self potential and seismic reflection surveys, have been used in order to delineate the unconsolidated-consolidated sediment boundary, define buried structures, map the basement, and delineate reservoir boundaries. We have found the most useful of the geophysical techniques to date to be seismic reflection, refraction, gravity and magnetics.

CFE is currently conducting approximately 3500 square kilometers of gravity surveying per year, and has completed approximately 400 kilometers of seismic reflection/refraction profiling. The seismic profiles, programmed to cross the major structures and over gravity anomalies, are obtained using a 256 trace GEOCOR II System.

A regional geologic model and cross sections of the geothermal field have been developed and are continually updated by the integration of the results of the geophysical surveys and the exploration wells. The Cerro Prieto Geothermal Field is located within the San Andreas tectonic system, and two fault systems within the immediate area of the geothermal field have been defined. The principal system, parallel to the San Andreas and Imperial faults, is called the Cerro Prieto system. Perpendicular to this system are faults designated as the Volcano system. Faults of the Cerro Prieto system generally have strike-slip displacement and those of the Volcano system generally occur en echelon, downthrown to the southeast.

Due to the complex geologic setting and the depositional environment, the geology has been grouped into the following three units. The basement, Unit C, consists of upper Cretaceous age granitic and metasedimentary rocks with seismic velocities ranging from 5,000 to 5,700 m/sec. Unit C is unconformably overlain by an irregular sequence of Tertiary age continental deltaic sediments, Unit B. Permeable zones in this unit contain the hot aquifers. Unit B has seismic velocities ranging from 3,100 m/sec to 4,300 m/sec. The higher velocities are believed to correlate with zones of low grade metamorphism. Estimated thickness of Unit B is between 2,000 and 2,500 meters. The uppermost sequence, Unit A, is a nonuniform sequence of Quaternary age unconsolidated and semiconsolidated continental sediments. This unit has seismic velocities between 1,750 and 2,750 m/sec and a thickness ranging from 600 to 2,500 meters.

INTRODUCTION

During the past fifteen years, numerous detailed investigations and technical studies have been completed on the Cerro Prieto geothermal field. This work along with eight years of experience gained with the commercial operation of the Cerro Prieto Geothermoelectric Plant has demonstrated the benefits of utilizing geothermal energy to generate electric power. The plant currently has an operational capacity of 180 megawatts. A thorough examination of the results obtained from the operation of Cerro Prieto has led to the decision by the Federal Electricity Commission of Mexico (CFE) to design an accelerated program of construction of new geothermoelectric facilities at Cerro Prieto, such that by the year 1984 the Cerro Prieto system will have an installed capacity of 620 megawatts.

Simultaneously with this planned construction program, CFE is conducting geothermal exploration programs in the Mexicali Valley in order to estimate the geothermal potential of thermal anomalies and to select and classify those anomalies which could be used for the commercial generation of electric power. Based on the exploration work carried out to date, CFE estimates that there is sufficient steam in the Mexicali Valley to support a geothermoelectric generating capacity of 1000 megawatts.

This paper presents a review of the geophysical exploration conducted to date and outlines some of the postulated structural conditions which are related to the geothermal environment in the Mexicali Valley (Figure 1). The relationship of some types of geophysical anomalies to potential geothermal areas is also discussed. The Mexican government has invested a great deal of effort in the enlargement of the Cerro Prieto geothermal field, and the Mexicans attach much professional pride to this effort and the successful operation of the field.

The material presented in this paper is based on previous exploration studies and information obtained from numerous wells drilled in the Cerro Prieto field. All of this information has been used to create a general geological model of the geothermal field.

The objective of the exploration program was to locate potential areas for the drilling of multiple-use geothermal wells up to depths of 1200 meters. ("Multiple-use" refers here to the practice of using the hot water for hydroponic gardening and prawn farming, in addition to the use of steam for power generation.) Possible geothermal areas within the Mexicali valley were identified, initially, through temperature anomalies observed in a number of irrigation wells and regional geochemical anomalies. These data were used to design gravity and magnetic surveys which were followed-up by seismic reflection and refraction surveys where appropriate.

It should also be mentioned that some of the exploration targets were selected because of their geographic position in relation to the geothermal fields within the Imperial Valley, their common characteristics with these fields, and observed gravity highs.

The final results of the exploration programs conducted to date resulted in the extension of the production zone to the east, and the delineation of three areas that are currently of high exploration interest: Tulicheck, Riito and Aeropuerto.

GENERAL GEOLOGY

The Mexicali Valley is part of the physiographic province of the Gulf of California. Topographically, the valley is characterized by a flat surface above which the dormant Cerro Prieto volcano rises to an elevation of 225 meters. Geologically, the valley is part of the delta system of the Colorado River and contains semiconsolidated clastic Quaternary sediments of deltaic and piedmont origin. These sediments are differentiated only by their form of deposition and have been deposited intermittently on well-consolidated Cenozoic sediments. These Cenozoic sediments, in turn, discordantly overlie metasediments and granitic rocks of Upper Cretaceous age. It is within permeable zones of the Cenozoic sediments that aquifers containing heated water are found.

Structurally, the valley is characterized by a series of grabens and horsts associated with the NW-SE striking San Jacinto, Cerro Prieto, Imperial and San Andreas Faults. These faults, and others in the general area, are part of the San Andreas System, which penetrates the Gulf as a suite of en echelon faults . This system, in turn, is the result of movement between the Pacific and American Plates. Movement of the plates has produced stress in the crust and the mantle and, as a consequence, displacement of large blocks of material. This displacement process has formed deep trenches such as the Wagner Basin, which joins the Cerro Prieto Fault with the Mexicali Valley.

Due to the tectonic setting, the Mexicali Valley is an active seismic zone characterized by shallow earthquake foci. The valley also exhibits crustal thinning and igneous intrusions. These conditions cause some zones in the valley to be areas of high thermal flow.

The Cerro Prieto Fault, with a NW-SE strike, extends from the Gulf of California to the Cerro Prieto Volcano. The fault’s relationship to the geothermal field has been intensely studied. The results of this study, indicate that, contrary to earlier belief, the fault does not act as a supplier of fluids, but acts as a boundary to the geothermal field.

To the north of the principal production field, another fault, the Morelia, has been delineated by geophysical studies (Figure 7). This fault has a NE-SW orientation and is also considered to act as a boundary to the geothermal field. The Morelia fault forms part of a fault system called the Volcano System. This system has an overall NE-SW orientation, and contains three fault zones: Delta, Patzcuaro and Hidalgo.

The Volcano System, at right angles to the orientation of the regional NW-SE tectonics, presents clear evidence of en echelon displacement, which is characteristic of transform fault zones such as the Mexicali Valley. If the intersections of the faults of the Volcano System are a dispersion center, it may be that these faults are the principal conductors of fluids.

The basis for the geological model for this paper is primarily one of crustal opening and continental thinning, the effects of which have produced the present Gulf of California. This geological model can be summarized as follows: in Pliocene time, the Baja California peninsula was formed as part of a continental granitic mass by complex forces within the crust and mantle; the movement of the Pacific and American Plates initiated the separation of this mass from the continent (Anderson, D., 1971); during this distention process, fracturing and the displacement of large blocks of material produced a thinning of the ocean floor and the injection of magma.

In the Mexicali-Imperial Valley, a series of geological phenomena which appear to favor the development of potential commercial geothermal environments have been observed. These include high seismicity, marginal basement thinning, transform faults with vertical and horizontal displacements, magmatic intrusion, volcanoes, and a variety of zones of thermal flow. These conditions are the motivating force behind the exploration programs in the Imperial and Mexicali Valleys, programs which have resulted in discoveries of eight known geothermal resource areas (KGRA) in the Imperial Valley (Salton Sea, Heber, East Mesa, North Brawley, East Brawley, Glamis, Dunes and Border), some of which have recently started commercial operations (Figure 8).

In the Mexicali Valley, where commercial operations started in 1973 at Cerro Prieto, recent exploration extended the Cerro Prieto geothermal field towards the east and, additionally, delineated three new prospective areas, Tulicheck, Riito and Aeropuerto.

METHODS USED IN GEOPHYSICAL EXPLORATION

The Cerro Prieto geothermal field has been an exciting field for the application of diverse geothermal exploration methods. The techniques applied have been utilized to delineate the configuration of the contact of the unconsolidated sediments (UA) with the more consolidated sediments containing the overheated aquifers (UB), to define the subsurface structure of the basement (UC) and to delineate the boundaries of the geothermal resource (Figure 9).

Between the lithologic units UA, UB and UC, there exist contrasts in seismic wave velocities and densities. A magnetic susceptibility contrast exists between units UB and UC. However, a correlation of available well log data does not show any well defined electrical resistivity contrasts between any of the units.

Based on CFE’s exploration history at the Cerro Prieto geothermal field and elsewhere in the Mexicali Valley, the geophysical methods which have produced the most useful results have been found to be, in order of importance, seismic reflection, seismic refraction, gravity and magnetics. Table 1 presents a chronological summary of geophysical techniques applied at Cerro Prieto.

Initial exploration studies in the Cerro Prieto geothermal zone commenced in 1960. Geological and geochemical surveys were used to locate the first well which produced a water-vapor mix. Two other wells, 450 and 700 meters deep, were also drilled, both of which showed very high temperatures. Based on these wells, CFE decided to conduct detailed geological, geochemical and geophysical exploration studies. A refraction survey was conducted in 1962 and the data indicated good velocity contrasts in the subsurface layers (Figure 10). The results of this survey were used to select the sites for several deep wells. One of these wells, M-3, penetrated the granite basement at a depth of 2547 meters with production coming from a zone between 700 and 900 meters in depth; a temperature inversion occurs below this depth.

Following well M-3, well M-4, 19 kilometers to the NW of M-3 and on the other side of a fault located in the refraction survey, was drilled to a depth of 2000 meters without any indications of anomalous temperatures.

A local gravity survey was performed in the Cerro Prieto geothermal field in 1968. This survey had as its objective the detection of variations in thickness of sedimentary fill in zones immediately around the well field. The results of this work, correlated with the earlier refraction survey, stimulated interest in drilling outside of the known production area. Wells M-51 and M-53 were drilled based on the gravity survey results. The good production results obtained from these wells indicated future possibilities for the expansion of the producing field (Figure 11).

In order to investigate known heat anomalies, an aeromagnetic survey was performed in 1971 to determine if these heat anomalies were related to basement structure. The information obtained from this survey showed a series of igneous intrusives located close to the Cerro Prieto and San Andreas faults, and provided information on the basement morphology and the valley edges.

From 1972 until 1975, electrical resistivity methods were used to study both the geothermal field and the Mexicali Valley. Schlumberger, dipole-dipole and Wenner arrays were employed and these surveys detected a series of resistivity minima associated with the Cerro Prieto and Imperial Fault traces. This created interest in determining the possible relationship of resistivity minima with zones of high temperature. Similar associations occur in the Broadlands, New Zealand geothermal zone, and in various geothermal fields in Italy. Initially, it was thought that liquid-dominant geothermal well fields could possibly be identified by their low resistivities due to high temperatures, porosities and salinities. However, resistivities are observed to be very high in the Cerro Prieto geothermal field, thus the suspected correlation of resistivity minima due to zones of high temperature does not appear to be a useful exploration tool in the Mexicali Valley.

Based on the experience obtained up to 1975, it was decided in 1977 to continue the exploration programs employing gravity, magnetic and resistivity surveys. The only difference was the introduction of the self potential method. These surveys confirmed the areas of major sedimentary thickness and principal structures crossing the Cerro Prieto geothermal field. Additionally, the results of the self potential method indicated a dipole anomaly in the area of production (Figure 12). This anomaly was attributed to the combination of fluid flow (electrokinetic effect) along a fault or fracture and temperature gradient (thermoelectric effect). A similar self potential anomaly was observed in other fields such as East Mesa and Tulicheck (Corwin R., et al, 1979).

In the electrical resistivity surveying, AB/2 spacings of up to 5000 meters with the Schlumberger array were used. This technique was used to increase the coverage of the previous years programs. However, in view of the ambiguous results obtained, CFE abandoned the Schlumberger method in 1978.

In gravity surveying, some models were made using the Talwani method. Utilizing this method, it was possible to quantify, in a general form, the thickness of the sediments (Figure 13).

A seismicity study of the Mexicali Valley was started in 1977 by CICESE*. The objectives of this study were to obtain data on earthquake patterns and mechanisms. The study produced information on seismicity, tectonic patterns and regional forces (Figure 14). Seismicity surveys have been performed in the Cerro Prieto geothermal field and the Mexicali valley to investigate the relationship of earthquake swarms, aftershocks, and attenuation of compressional waves (Q.), shear waves and subsequent wave trains to geothermal activity.

In 1977 a Mexican-American cooperative program was initiated with the United States Government, Department of Energy, through Lawrence Berkeley Laboratories. (LBL) . This program included the application of geophysical techniques with the principal objective of obtaining data concerning changes which occur with time in the geothermal field due to production. In this respect, surveys have been made since 1977 using the dipole-dipole resistivity method. The results of these surveys show better definition of deep structure in the eastern part of the geothermal field (Figure 15). Geophysical models of the data indicate the production zone is associated with a resistive body which dips towards the east at an angle between 30 and 50 degrees. This resistive body may possibly underlie the eastern part of the production zone. A conductive zone, narrow and steeply inclined, can be modeled lying immediately to the east of the resistive body. It may be possible to associate this conductive zone with a recharge zone or with faulting (Wilt, M., 1979). However, the salinity (up to 3000 ppm) and temperature of the aquifers in the field mask the electric properties of the subsurface materials, making the interpretation of the data somewhat ambiguous.

The application of magnetotellurics (MT) was programmed in 1978 complement the resistivity survey. This technique, which is a passive method based on the measurement of natural, low frequency electromagnetic oscillations, has the advantage of permitting greater exploration depths than those obtained in the resistivity surveying. This type of survey, combined with a remote magnetic reference, can yield usable results in the presence of interference (Gamble, T., et al, 1978). The system used was designed by LBL with the express purpose of obtaining deep resistivity data and to experiment with magnetotelluric methods in noisy areas.

The preliminary results of the magnetotelluric survey showed a marked similarity with the results obtained from the electrical resistivity surveys of the dipole-dipole and Schlumberger arrays. The results of the magnetotelluric survey gave better information on the deeper strata which in the electrical method are masked by shallow saline layers of low resistivity. The magnetotelluric method was, therefore, used to verify the consistency of the resistivity models developed from other methods (Figure 16).

Precision gravity surveying has been performed by LBL since 1978. These surveys were programmed to observe differences in gravity on the order of microgals. These differences were believed to originate from changes in the well field, perhaps due to the removal of the fluid, densification of the permeable rocks and formation of gaseous phases due to pressure reductions. Slight ground subsidence originating from the extraction of fluids has been observed at some producing geothermal areas, including the Cerro Prieto geothermal field.

Observed gravity values have been computed for each station and these are compared with the previous values. However, these differences appear to be within the resolution of measurement. High precision topographic surveying has also been performed each year at each gravity station. These surveys indicate minute elevation changes, also perhaps within the resolution of measurement. Therefore, it is not possible to interpret the observed changes in gravity values as originating from either a change in mass or a change in elevation. The results of the initial survey were used to produce a detailed Bouguer anomaly map over the Cerro Prieto geothermal field.

Seismic monitoring investigations have been performed by LBL in the Cerro Prieto region since 1977. This work was designed to investigate the seismicity of the region and characteristics of seismic wave propagation in the Cerro Prieto geothermal field. The principal objective of this study was to determine the level and behavior of microseismic activity in the Cerro Prieto area and to determine the velocity and attenuation of the P and S waves in the production zone (Majer, E., 1979). Initial results indicate a complex structure associated with the field. The microseismic activity appears to be lower in the production zone. However, high levels of artificially generated noise limit the quantity of usable velocity and attenuation data.

Analysis of the limited data suggest anomalies in velocity and attenuation of the P wave for the production zone, and also suggest high values of V p /VS , which may imply a shallow, fluid saturated zone.

Microseismic activity in the Cerro Prieto geothermal field has been studied by both LBL and CICESE. The interpretations of these groups have differed in some aspects and the dimensions of the monitoring zones are different; however, the studies are considered to be complementary.

Gravity studies done in the Imperial Valley (Biehler, S., 1971-72) indicate the shape and regional inclination of the sediments. From these studies, sediment thickness and principal geologic structures that cross the valley can be inferred. In some cases the gravity maxima show close correlation with high temperature zones (Figure 17). This is believed to be due to an increase in the density of the sediments produced by high temperatures of the formation waters. In the convection process these waters deposited silica and carbonates in the intergranular voids of the sediments, increasing the sediment densities and, consequently, increasing the gravitational field. This interpretation of gravity maxima, in corr- elation with data obtained from magnetic surveys, led to the discovery of eight geothermal regions, Salton Sea, Heber, East Mesa, North Mesa, East Brawley, Glamis, Dunes and Border, each of which is located on a gravity maximum. Only the Salton Sea region had surface thermal manifestations.

The Cerro Prieto geothermal field is located over a gravity maximum and similar gravity maxima were noted in the SW and NE margins of the valley (Figures 18, 19 and 20). Some of these maxima represent the basement structure. Other gravity maxima observed towards the center of the valley are located near large faults and are associated with observed thermal anomalies. This association led to the programming of seismic reflection and refraction profiles, with the objectives of delineating the contact between the unconsolidated and consolidated sediments, the geologic structure, the sedimentary thicknesses and the seismic velocities of the sediments and basement.

CFE, since 1978, has performed 3500 square kilometers of gravity and magnetic surveying each year, and to date has completed approximately 400 line kilometers of seismic reflection and seismic refraction profiling. The seismic lines are perpendicular to the regional structures and over gravity anomalies. For the seismic surveying, the GEOCOR II system, utilizing 256 traces, four phones per group, with 80 foot group intervals, has been used to acquire common depth point data. Each line has been shot in the reflection and the refraction mode, and each mode is processed separately (Figure 21).

Using the seismic surveys, the thickness and morphology of the stratigraphic units have been determined and the geologic structures which cross the field have been detailed. By correlating the seismic data with the well log data, it is possible to project the boundaries of the Cerro Prieto geothermal field to the east of the actual production zone. This projection corresponds with the location of a gravity maximum, which has the form of a dome (Figure 20). The location of this gravity maximum is in an area where, based on the seismic results, the basement is interpreted to be over 5000 meters deep, therefore implying that the gravity maximum is associated with a high temperature zone rather than shallow basement structure.

The seismic velocities of the granitic basement and the metasediments range from 5000 to 5700 meters per second. The consolidated sediments, which contain the overheated aquifers, exhibit seismic velocities ranging from 3100 to 4300 meters per second. (The higher velocities may correspond to zones of low grade metamorphism.) The calculated thickness of the consolidated sediments varies from 2000 to 2500 meters. The unconsolidated sediments exhibit seismic velocities ranging from 1750 to 2750 meters per second and have a calculated thickness of up to 2500 meters.

In addition to the normal data acquired from the seismic surveys, an attenuation phenomenon was noted over the Cerro Prieto geothermal field production zone. On seismic reflection profiles A-Al and D-DI (Figures 22 and 23), zones of poor reflections are noted below zones of good reflections. This attenuation phenomenon has also been observed at the East Mesa geothermal field (Howard, J., et al, 1978). At East Mesa it was originally believed that the zones of poor reflections were associated with extensive fracturing of brittle rocks in areas of high temperature. However, open fractures are not considered to be associated with the hydrothermally altered zone at Cerro Prieto. This attenuation phenomenon, which is referred to as the Reflection Attenuation Zone (RAZ) may be related to closely-spaced faults which disrupt the continuity of reflections, or to the lessening of acoustic impedances by the destruction of porosity due to hydrothermal alteration. Although reflection attenuation zones can be produced by a variety of geological and structural environments, their relationship to geothermal areas appears to offer one exploration approach for delineating geothermal zones.

CONCLUSIONS

The Federal Electricity Commission of Mexico has endeavored to apply geophysical techniques with the objective of locating potential geothermal production zones. Some of these techniques, such as seismic reflection and seismic refraction, gravity and magnetics, have direct practical applications. Other techniques, such as electrical resistivity, self potential and magnetotellurics, are considered to be applicable under certain circumstances.

The correlation of the results of these geophysical techniques with the known geology and thermal anomalies has resulted in the detection of three potential geothermal areas in the Mexicali Valley, namely, Tulicheck, Riito and Aeropuerto.

BIBLIOGRAPHY

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Corwin, R.F. , Morrison, H.F. , Diaz C. , S. , and Rodriguez D.J., 1979, Self-Potential Studies at the Cerro Prieto geothermal field, in proceedings, First Symposium on the Cerro Prieto geothermal field, Baja California, Mexico September 197,8: Berkeley, Lawrence Berkeley Laboratory, LBL-7098 P. 204-210.

De la Peña L.A. y Puente C.I. 1979. "El campo Geotérmico de Cerro Prieto". 92nd meeting, San Diego. Geological Society of America. Coordinadora Ejecutiva de Cerro Prieto, Comisión Federal de Electricidad.

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Elders, W.A., R.W. Rex, T. Robinson -and S. Diehler, 1972, Crustal spreading in southern California; science, 178.

Elders, W.A., J.R. Hoagland, E.R. Olson, S.D. McDowell and P- Collier, 1978, A Comprehensive Study of sample from geothermal reservoirs: Petrology and light stable Isotope Geochemistry in the Cerro Prieto Geothermal Field, Baja California, Mexico UCR/IGPP 78/26.

Fonseca L.H.L.., Razo M.A. y Palma G.O. , 1980 Estudios de Gravimetria y magnetometria en el Valle de Mexicali, B.C. In-forme Pieliminar 2-80. Comisión Federal de Electricidad. Depto. de Geotérmia Ofna. de Exploraciones.

Fonseca, H.L. y Razo M-A., 1979 "Estudios Gravimetricos, Magnetometricos y de sismica de Reflexión en el Campo Geotérmico de Cerro Prieto. 2o. Simposio sobre el Campo Geotérmico de Cerro -Prieto. Comisión Federal de Electricidad.

Gamble, T.D., Goubau, W.M., Goldstein, N.E. Miracky, R., Stark, M., and Clarke, J., 1980. Magnetotelluric Studies at Cerro Prieto, Second Symposium on the Cerro Prieto geothermal field, Baja California, Mexico. October 1979.

Gastil, G.R., Allison, E.C., y Phillips, R.P., 1971 Reconnaissance geologic map of the State of Baja California: Prepared by the Students and Staff of the Universidad Autónoma de Baja California and San Diego State College.

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Griscom, A., and L.J.P. Muffer, 1971, Salton Sea Aeromagnetic Map, U.S.G.C., Washington D.C. Map GP-754.

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Heartherton, T., W.J.P. MacDonald, and G.E.K. Thompson, 1966, Geophysical Methods in Geothermal Prospecting in New Zeland. Bull. Volcanol., v. 29. pp-485-497.

Howard J. , Apps. J.A. , et al 1978, "Geothermal Resource. and Reservoir Investigations of U.S. Bureau of Reclamation Lease holds at East Mesa, Imperial Valley, California., October, 1978, Lawrence Berkeley Laboratory, University of California, Berkeley, California.

Kovack, R.L., Allen, C.R., and Press F., 1962. "Geophysical. Investigations in the Colorado Delta Region"; jour Geophys, Res. v. 67, p-2485-2871.

Lambert, W-D. and Darling, F.W. ,Values of Theoretical Gravity On the International Ellipsoid", Bull, Geodesique, No. 32, October, November, December, 1971.

Lomnitz, C. , F. Mooser, C. Allen, J.N. Brune and W. Thatcher, 1970, Seismicity of the Gulf of California Region, Mexico, Preliminary Results; Geofis. Inst. 10,37.

Meidav, T. and Howard J.H., 1979 ,An Update of Tectonics and Geothermal Resource Magnitude of the Salton sea Geothermal Resour cell, Geothermal Resource Council, Transactions, vol. 3, September, 1979.

Meidav, T. y Rex W.R. 1970, "Geophysical Investigations for Geothermal Energy Sources. Imperial Valley, California’’. Phase I:1968 Field Project.

Nettleton, L.L. , 1976, Gravity and Magnetics in Oil Prospecting, New York, McGraw-Hill.

Puente, C.I, 1978, Geology of the Cerro Prieto Geothermal Field (abs.): Abstracts, lst- Simposium on the Cerro Prieto Geothermal Field, Baja California, Mexico: Lawrence Berkeley Laboratory Rept. LBL-7098 ABS, p.6.

Rex, R.W- E-A. Babcock, S. Biehler, J. Combs, T.B. Coplen, W.A. Elders, R.B. Fugerson, Z. Garfunkel, T. Meidav and P.F. Robinson, 1971, Cooperative Geological, Geophysical-Geochemical Investigations of Geothermal Resource in the Imperial Valley Area of California. ‘UCR/IGPP.

Reyes, Z.C-A., 1979. Estudio de microsismicidad del Sistema de Fallas transformadas Imperial Cerro Prieto. Informe Técnico GE079-01. Centro de Investigación Cientifica y de Educación Superior de Ensenada.

Reyes, Z.C.A., 1980 Reporte Preliminar del Sismo "Victoria" Baja California Norte del 8 de junio de 1980 (ML=6.7) informe Técnico GE080-02. C.I.C.E.S.E., Centro de investigación Cientifica y de Educación Superior de Ensenada. junio, 1980.

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Velasco Hernandez, J., y Martinez Dermadez, J.J. , 1963 "Levantamiento Gravimétrico en la Zona Geotérmica de Mexicali, B.C."; Consejo de Recursos Naturales no Renovablet, 24 p. inedit.

Wilt, M.J., and Goldstein, N.E. , 1980. Resistivity Monitoring at Cerro Prieto, Second. Symposium on the Cerro Prieto Geothermal Field, Baja California, Mexico. October, 1979.

Table 1. Chronological Summary of Geophysical Techniques Applied at Cerro Prieto
Year Type of Survey Activity
1962 Seismic Refraction Four lines in Cerro Prieto area.
1963 Gravity 340 gravity stations
1972 Aeromagnetic Survey In Cerro Prieto area.
1972 Electrical Resistivity Vertical Electrical Soundings and Dipole-Dipole survey.
1973 Aeromagnetic Survey Colorado River Delta area
1973 Electrical Resistivity 133 vertical electrical soundings and dipole-dipole survey.
1977 Microseismic Survey Install five stations in Cerro Prieto field.
1977 Self Potential Two lines over Cerro Prieto geothermal field
1978 Electrical Resistivity 114 vertical electrical soundings.
1978 Seismicity Install 12 station network
1978 Magnetotellurics Seven soundings
1978 Electrical Resistivity Two long dipole-dipole lines
1978 Self Potential Five lines over Cerro Prieto geothermal field.
1978 Precision Gravity 60 stations occupied
1978 Gravity and Magnetics Data taken along five lines
1978 Seismic Reflection and Refraction 180 kilometers of profile
1979 Precision Gravity Reoccupy previous stations.
1979 Electrical Resistivity & Self Potential six self potential lines, 110 vertical electrical soundings
1979 Seismicity Install five semi-permanent stations.
1980 Seismic Reflection and Refraction 220 kilometers of profile
1980 Magnetotellurics Ten soundings
1980 Resistivity Monitoring One line over Cerro Prieto geothermal field.
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