CiteSeerX — A NATIONAL PROGRAM

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TECHNICAL SERIES 45-029-72

GEOTHERMAL SCIENCE AND TECHNOLOGY A NATIONAL PROGRAM

by

Carl F. Austin, Ward H. Austin, Jr., and

G. W. Leonard

~BSTRACT. The major portion of the geothermal prospect called the

Coso Thermal Area lies within the instrumented test ranges of the Naval Weapons Center, China Lake, Calif. In developing plans for scientific utilization of the Coso Thermal Area, the state-of-the-art of geothermal science and technology was reviewed. The review indicated that the·

development of geothermal deposits for the purpose of generating elec- tricity, providing heat, and obtaining raw materials was a technology in its infancy, with critical aspects subject to uncertainty. This study has resulted in a proposal for a national geothermal science and technology advancement program which will be accomplished by gathering scientific and engineering data from five selected sites representing each of the five principal types of geothermal deposits that are known or hypothesized.

Naval Weapons Center

CHINA LAKE. CALIFORNIA • SEPTEMBER 1971

DISTRIBUTION LIMITED TO U.S: GOVERNMENT AGENCIES ONLY; TEST AND EVALUATION; 9 SEPTEMBER 1971. OTHER REQUESTS FOR THIS DOCUMENT MUST BE REFERRED TO THE NAVAL WEAPONS CENTER.

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Introduction . . . . Geothermals Defined

Granitic Stock Systems Basaltic Magma Systems Metamorphic Zone Systems

CONTENTS

Wet Geothermal Gradient Systems Dry Geothermal Gradient Systems The National Program

Program Goals .

Goal Number 1 - A Computer Model Granitic Stock Systems

Recommended Deposits for Exploration Plan for Accomplishing Goal No. 1 . Goals Related to Model Utilization Energy Conversion Technology

Time and Cost Estimates to Implement a National Geothermal Program . .

Phase I. Detailed Survey . . . . . . • . Phase II. Detailed Studies of Selected Granitic

Stock Type Deposit (Coso Thermal Area) . . . Phase III. Five Year Core Drill and Testing Program

for Selected Granitic Stock Type Deposit (Coso Thermal Area) . . . • . . . • . . . • . Phase IV. Modeling of Selected Granitic Stock Type

Deposit With Associated Testing Program of Pro- duction, Injection, Environmental Control, etc.

Phase V. Basaltic Magma System Research Program Phase VI. Hetamorphic Zone Geothermal System Phase VII. Wet Geothermal Gradient System Phase VIII. Dry Geothermal Gradient System .

Phase IX. Hodeling and Test Programs Related to Areas Outside of Selected Granitic Stock Deposit . . Phase X. Cost Estimate to Develop Hot Water Energy

Conversion (Delegated to Industry).

Cost Summary . . . . Conclusions and Recommendations

Appendix: Location of Thermal Springs and Wells

Bibliography . . .

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INTRODUCTION

The increasing national concern over the ecologic impact of electric power generation with both fossil fuel and nuclear driven plants, and the increasing problem of low cost fossil fuel and nuclear fuel depletion, has swung the national interest toward the development of geothermal resources. Geothermal resource development is significantly hampered by the fact that geothermal science and technology are generally in their infancy. Conflicting theories regarding sources of heat and the mecha- nisms of heat transfer fail to yield adequate guidance for exploration and development programs. Drilling for geothermal steam in the United States has been generally unsuccessful and has been marred by blowouts or other accidents. A significant improvement in geothermal state-of- the-art is not expected in the near future unless a comprehensive_ joint government-industry research program is undertaken for the purp·ose of providing the needed basic knowledge for the major types of geothermal deposits found in the United States.

The United States government oHns all of a geothermal prospect in eastern California called the Coso Thermal Area. Figure 1 shows the location of this deposit in relation to major population centers and other potential geothermal sites in California and Nevada. Three fourths of this deposit (of the locally active heat cells) is located within the confines of the Naval Weapons Center (~•c) and the remaining one fourth is on public domain administered by the Bureau of Land Management.

Figure 2 shows that portion of the Coso Thermal Area on ~C instrumented test ranges .

The Naval Weapons Center believes the Coso Thermal Area may be of national importance and should be kept as a unit so that:

1. The actual potential of the area as a geothermal resource can be identified.

2. Fundamental technical dat-a, which will be of value in establish- ing regulations and procedures for the exploitation of other geothermal areas throughout the United States, can be obtained from the Coso area and given Hide distribution.

3. The programs of study and investigation to develop the funda- mental technical data can evolve as cooperative efforts betHeen the Department of Interior, National Science Foundation, Atomic Energy Com- mission, and other interested state and federal government agencies as well as public utility and private pO>ler companies.

4. If the area is found to be of economic value as a power source, plans can be developed for commercial exploitation of the available power.

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The Center is interested in multiple use of the property under its responsibility, and believes that this can be accomplished through careful planning and management so that the principal mission of the Naval Weapons Center will not be impaired nor will the usefulness of its uniquely instrumented ranges be degraded for weapon evaluation.

The intent of this paper is to propose a national geothermal program and to show how the Coso Thermal Area could be used to support the

national program. Preliminary cost estimates for a national geothermal technology program are presented.

GEOTHERMALS DEFINED

A geothermal deposit can be defined as any geologic environment with sufficient heat content to enable the extraction of energy such as electric power from geothermal steam or fluids. With present day tech- nology these deposits consist of areas of steam emission, hot water emission, fumarolic- and volcanic-type gas emissions (other than from active volcanoes or contemporary lava flows), mineral springs of any temperature, mineral deposition indicating young-to-recent liquid and gas leakage of the preceding types, and young molten rock intrusions at modest depths. To date, most industrial exploitation attempts have been carried out in areas of apparent igneous intrusive activity. Dominant features are doming and arcuate patterns related to magma chamber emplacement and subsidence. Expanding the industrial approach of the past few years, which has been limited to the development of only one type of deposit (granitic stock), a total of five fundamental geothermal deposit types can be defined. These are:

1. Granitic stock1· heat sources 2.

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Wet geothermal gradient4 heat sources 5. Dry geothermal gradient heat sources

Figures 3 through 7 illustrate these deposit types schematically.

1stock ~ an underground accumulation of molten rock of less than L~Q square J.lP.ies cross section.

2Metamorphic - involving the processes of recrystallization under heat and pressure.

3 Magma - molten rock.

4Geothermal gradient the rate of temperature rise with increasing depth in the earth.

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FIG. 3. Generalized Block Diagram of a Graniti~ Stock Heat Source Type of Geothermal Deposit Showing the Main Magmatic Chamber (a) (Batholith), Smaller Stocks and Apophyses (b) Which Result in Localized Geothermal Cells, and Hydrothermal Alteration Which Provides Seal to Entrap Hot Fluids (c).

(Not to scale.)

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FIG. 4. Generalized Block Diagram of a Metamorphic Zone Heat Source Type of Geothermal. (a) Deeply folded sediments undergoing recrystallization. (b) Fault zone acting as conduit. (c) Deep magmatic zone.

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FIG. 5. Generalized Block Diagram of a Basaltic Magma Heat Source Type of Geothermal Deposit.

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FIG. 7. Generalized Cross Section of a Dry Geothermal Gradient Heat Source Type of Deposit \Vith Superimposed Depth-Temperature Curve.

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GRANITIC STOCK SYSTEMS

A granitic stock type of geothermal system is by definition a deposit for which the heat source can be shown (or hypothesized) to be an intru- sion at relatively shallow depths and with a composition that is not basaltic (i.e., generally acidic to intermediate in compositions). The surface evidence may be magmatic leakage in the form of perlitic domes as at the Salton Sea and Coso, as flows and ash deposits as at the Geysers and Wairakei (see Fig. 8 location map) or typical volcanic fluid leakage as at Larderello, Italy. The location of the controlling or primary magma system can be readily observed on high altitude photographs in the form of closed arcuate patterns in the form of ellipses comprised of fracture, alteration, intrusion and collapse patterns. These primary magmatic chamber patterns range from 25 to 30 miles in length and 15 to 20 miles in width. Within the controlling primary pattern are smaller patterns generally 4 to 6 miles in diameter. These smaller circular to elliptic patterns are believed to represent the surface expression of underlying stocks and apophyses of stocks, and mark the active geothermal cells suitable for exploration. One of the prime reasons granitic stocks seem to provide the best possibilities for economic development is the rela- tive abundance of hydrothermal alteration which provides a seal or cap rock under which high temperatures and pressure can accumulate. Figure 9a shows an unretouched photograph of a portion of the granitic stock geothermal at the Coso Thermal Area, while the same area, with the structural pattern included is shown in Fig. 9b.

The economic recovery of electric energy and of by-product chemicals from granitic-stock-type geothermal deposits has been underway since 1905 when the Italians began successful operation of a lm< pressure 40 horse- power generator on geothermal steam at Larderello, Italy. This same overall area now has over 400 megm<atts of electric power generating capacity.

The economic potential of granitic-stock-type plants has been further demonstrated at the Big Geysers in California where geothermal drilling, started in 1928, culminated in commercial operation of a generating plant in the late 1950's. The operations at the Big Geysers have an anticipated goal of more than 1000 megawatts of generating capacity.

Other geothermal generating plants of this type that are now on line include: the Wairakei, New Zealand site with a capacity of 170 megawatts, the Pauzhetsk and Kunashir plants on the Kamchatka peninsula of Russia with a 29 megawatt capacity and the Matsukawa, Onikobe, Otake and Hachimanti plants in Japan with a total capacity of 50 mega>Tatts. In the Hestern United States alone, the authors have personally examined over 250 granitic-stock heat-source-type geothermal deposits, and this

5Rocks high in silica content are called "acidic¹¹ in composition.

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Thermal. Area. View 9b shows the major structural features indicating the heat source locations. (Photogeology by Ward Austin, 18 July·l971.)

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number is less than one-half of the presently knmm prospects in the eleven western states. Outside of the western states (but still within the United States) this type of deposit appears widespread in Alaska and also appears to be present in the southeastern United States. Figures 10 through 12 show thermal springs distribution in the United States.

BASALTIC MAGMA SYSTEMS Basaltic Magma Type

Basaltic-magma-type geothermal systems have developed l<ithin both the alkaline-olivine basalt associations of the ocean basins and continents and within regions of quartz diabase-tholeite flood basalts on the conti- nents. In general, the evidence in hand suggests that small basaltic fields that have resulted from deep fracturing (Amboy and Siberian Craters area of California as an example) have not had an adequate heat transfer ability and have not resulted in local hot rock or hot fluid accumulations with any degree of persistence beyond the time of actual vulcanism.

In the case of extremely recent to ongoing vulcanism associated heat sources, the problems of reservoir mechanics become very severe.

For areas such as Island Park in Idaho, the geology appears _favorable as the source is large, and it appears well capped and to have had high aqueous extrusions indicating a more than adequate fluid content for the purposes of heat transfer.

Basaltic systems represent the potential heat source for vast regions of the world. To date, other than for shallow unsuccessful drilling in porous rocks in Hawaii, the concept of basaltic-magma-type geothermals is completely untested. An additional problem in purely basaltic areas is their limited water content and hence the reduced amount of hydro- thermal activity and resultant failure to form a seal or cap rock under which high temperatures and high pressure can develop. Figures 13 and 14 show basaltic magma heat source distribution in Hawaii and Idaho respectively (some of the Idaho hot springs are related to basaltic activity), and Fig. 15 shows details of the Island Park Caldera which is basaltic or basaltic/differentiate mixed in nature.

METAHORPHIC ZONE SYSTEMS

Metamorphic zone geothermal systems are defined as those geothermals whose energy transfer media are the result of either regional or local metamorphic processes which need not have any closely associated igneous intrusive activity. Broad areas of regional metamorphism can be demon- strated for past geologic time, but by their very nature (deep burial), the present day location of deep metamorphic zones must be based on less

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(See appendix for names of springs.)

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FIG. 11. (from Waring) Eastern Part of the Conterminous United States Shm-ling Location of Thermal Springs. Only the springs in Arkansas appear to be granitic stock heat source type deposits on the strength of present evidence. (See appendix for names of springs.)

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The springs of the Alaskan panhandle are granitic heat-source type as are those studied to date in detail in the interior. The island arc areas are generally basaltic heat-source type deposits. (See appendix for names of springs.)

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FIG. 13. (from Waring) Hawaii Showing Location of Thermal Springs and Thermal Wells. All sites are considered basaltic magma heat source type. (See appendix for names of sites.)

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FIG. 15. The Island Park Caldera of Idaho.

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than direct observation. The production of hot fluids from deeply folded sediments or from the margins of mountain fold belts would be considered to represent production from a metamorphic zone. To date no metamorphic geothermal systems have been delineated by industry or explored by drill- ing in the United States with the possible exception of the Glenblair- Fort Bragg area where an industrial drilling operation in the mid 1960's was defeated by the flow of plastic rock prior to reaching geothermally significant depths. An example of a former metamorphic zone would be the great Rhodesian copper deposits which are believed by some geochem- ists to represent the passage of metamorphic fluids in former geologic times.

WET GEOTHERMAL GRADIENT SYSTEMS

The wet geothermal gradient systems are those where deeply circu- lating ground water is heated by the normal geothermal gradient, to reemerge as hot water. This type of geothermal can develop in deeply folded sediments and in deep fault zones (with attendant frictional heating). Examples of deep burial of sediments with resultant heating of the enclosed fluids can be seen in the oil field brines of the Gulf Coast and the hot springs of New York, Virginia, and Georgia. No wet geothermal gradient type of deposits have been developed other than for shallow drilling in support of spas or resorts.

DRY GEOTHERMAL GRADIENT SYSTEMS

A dry geothermal gradient system is one where rocks of limited porosity and permeability are heated, but there is no fluid for heat transfer. A randomly located drill hole on the northern Atlantic coastal plain would be in either a wet or a dry geothermal gradient system, depend- ing upon the availability of pore space fluids at depth. No dry gee-

thermals have been developed to date though some deep petroleum tests and possibly some deep drill holes in mining areas might qualify for this type of deposit.

THE NATIONAL PROGRAM

In order to achieve a rapid rate of geothermal development for the purposes of offsetting and perhaps reducing present trends in atmospheric pollution, nine specific goals related to finding and safely develop- ing and producing energy from the five fundamental ·types of geothermal deposits must be achieved in as little time as possible. All of the information gained in achieving these goals must become public knowledge rather than be proprietary. This program outlines the broad approaches which will attain the nine major goals outlined below. Certainly there are many more detailed steps, but the general approaches are shown.

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PROGRAM GOALS

The potentials and the problems of geothermal deposits must be considered in the light of the present state-of-the-art. Such deposits can and do produce relatively pollution-free electricity but do so with certain potential problems. Fresh water may be a by-product as at the Geysers in California. On the other hand, steam production may be accompanied by corrosive brines or by dense metal bearing brines as at Niland, California. To quote an official at Larderello which has a capacity of over 400 mega,•atts from the district, they have three prod- ucts: electricity, borax, and rust, with the borax bearing fluids being both corrosive and incompatible with local agriculture and the rust representing chemical and pov1er plant corrosion.

At tvTO localities in the '"estern United States, geothermal develop- ment has resulted in the cratering of public roads and in one instance a passenger in a motor vehicle was scalded fatally. At one western geothermal location, arsenic in effluents has been a serious problem.

Any attempts at developing geothermal deposits must consider in detail the effects of the loss of control of a well on the surrounding region and in the case of areas with live streams, of the effects on downstream water users of dumping large amounts of corrosive and poisonous brines as the result of an accident.

A national program of geothermal development should have the follow- ing specific goals which are intended to place enough knm,ledge in the public arena to enable the rapid, safe, and efficient development of this nation's abundant geothermal resources. These major goals are:

1. Develop computer models of the five principal types of. geothermal deposit. These models should be designed to handle more than one type of host or reservoir rock and are intended to yield predictions on host rock and contained fluid properties with depth, lateral dispersion from the heat source, and Hith time including paleoclimatic variation.

2. Develop production techniques which minimize or can adequately solve the problems of in-the-well and in-the-host precipitation of solid minerals.

3. Develop production techniques involving both fluid extraction and fluid injection but which do not yield destructive seismic activity as a by-product.

4. Based upon computer models developed for a given type of deposit, develop reinjection programs for waste brine disposal which do not lead to pollution problems in adjacent or overlying culinary or low salinity

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5. Develop production drilling and field management concepts based upon successful computer modeling with the model programs used to ensure the best use of contained geothermal fluids and the maximum energy and metals recovery from the deposit.

6. Develop stimulation techniques for deposits or portions of deposits in which fluid-based heat transfer within the deposit is absent or too limited for commercial utilization.

7. Develop energy conversion techniques for the conversion of energy in hot water into electric energy.

8. Develop operating environmental criteria for the various major life zones in which geothermal deposits occur in the United States.

9. Develop exploration and identification techniques for the various types of geothermals. In particular, for the case of granitic stock heat source geothermals, develop a temporal identification methodology to enable the recognition in the field of heating, active, and declining geothermal systems. For all types of geothermal systems, establish the effects of varying paleoclimates upon the surface chemical and remote sensing signatures (aerial photos, infrared photos, etc.)

GOAL NUMBER 1 - A COMPUTER MODEL

To achieve a workable computer model of each of the five deposit types will require the selection of a "type example" which is undeveloped

(preferably on public domain), and which is in a geographic environment amenable to effluent control in the event of accident. The question of what constitutes a type deposit will no doubt be argued vehemently by proponents of each of the many geothermal concepts '"hich have appeared in print. Once a type deposit is selected, it should be set aside from normal commercial development until such time as the various goals delineated have been reached or have been deemed as beyond the foresee- able state-of-the-art. Five deposits are desired.

GRANITIC STOCK SYSTEMS

The three most famous geothermal deposits in the world and the three most successful in terms of steadily increasing production levels are Larderello, Italy; Wairakei, New Zealand; and Big Geysers, California.

These three deposits are compared for the purpose of showing a common- ality of structural setting. In addition the geology of seven other areas is discussed in some detail - indicating the similarities between their geology and that of the three main producers. These are the Coso Hot Springs, Naval Weapons Center, China Lake, California; Niland,

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Imperial Valley, California; Cerro Prieto, Mexico (55 miles south of Niland); Long Valley (Casa Diablo), California (125 miles north of the Coso Hot Springs); Mono Lake, California; Beowawe, Nevada; and Island Park, Idaho (see Fig. 1, 16, 17, and 18 for locations). The geologic evidence in hand indicates that nine of these areas are related to granitic magma. The Island Park, Idaho area is included to demonstrate that structural details that are related to subsidence and doming are common to both basaltic (basic) and granitic igneous extrusive activity.

Comparison will be by discussion of the following points:

1. Dimensions of the apparent batholithic magma chambers through the use of aerial photos (primary thermal cell).

2. Dimensions of local surface hot areas (local thermal cell) apparently related to smaller intrusions or apophyses which rise above the main magma chamber.

3. Stratigraphy of the prospect area with a discussion of the host (reservoir) rock and the seal (cap) rock.

4. Local seismic activity which may be related to movement of magma, collapse into the partially depleted magma chamber or impending eruption.

· 5. Developed area in relation to the local thermal cell of item 2.

6. Maximum steam and hot water production as an indicator of the ground water flow.

7. Rainfall as an indication of the recharge of the ground water system. Note that apparently 2 to 3 times as much water (hot or steam) can be produced from a local thermal cell as is indicated currently by ground water and rainfall analysis.

8. Chemical analyses of waters associated with the producing or prospect areas.

9. Mineral deposition in the producing or prospect areas.

Magma Chambers

In early 1963 one of the writers, while consulting for a private company, pointed out from aerial photos and direct aerial observations that the 23 x 12 mile Long Valley - Casa Diablo Hot Springs area might be a volcanic caldera collapse area giving an indication of the position of a partially depleted magma chamber. McNitt, Mining Geologist,

California Division of Mines and Geology, Special Report 75 in late 1963 published an account of his interpretation of the Long Valley depression

(shmm on Fig. 19). McNitt states the following:

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REVILL.AQIGEDO ISLANDS (To Stilt• of Coli~•)

SPOUtlna: spr1np 1,3, 5,ti, 7,1,t,~ c-~

Bollin• Sprln•• 2 4, 10.11 Hot pools 12. 15 Mud Vo~noe• 13, 11 Hot w.ter wen 17

TEXAS

Solfatllric volcano •

GULF IOF MEXICO

FIG. 18. (from Waring) Mexico Showing Location of Thermal Springs and Principal Volcanoes. (See appendix for names of sites.)

'--- ^~ '---' ·..__._; '----' '----' ^~

FIG. 19. (from Tocher, et al) Generalized Geologic Map of Owens Valley Region.

27

"The gravity data shown in Fig, 20 (attached) indicate that Long Valley is a structural depression as well as a physiographic basin, and is bounded on all sides by steep faults. The structural depression is also elliptical in shape, 23 miles long and 12 miles wide. The Cenozoic deposits in the depression increase gradually

from a thickness of less than 5,000 feet on the east. (Pakiser, 1961, p. 253) Pakiser has interpreted Long Valley to be "a volcanic- tectonic depression caused by subsidence along faults, following extrusion of magma from a chamber at depth." . . . • . . McNitt con- tinues with, "An aeromagnetic survey of the region disclosed a sharp magnetic high having a relief of from 2500 to 5000 gammas, located over the center of Long Valley (Pakiser, 1961, p. 252). This mag- netic anomaly roughly corresponds to a local positive gravity anomaly which has a relief of about 10 milligals, as compared with a 60

milligal negative anomaly over the major part of the Long Valley depression. The calculated depth to the upper surface of the mag- netic body is about 3000 feet below the valley floor, which places

the top of the body in the upper part of the Cenozoic section as determined by gravity methods. This mass of dense and magnetic material may represent the buried volcanic or intrusive rock which is the heat source for the various thermal springs in Long Valley."

(Cas a Diablo included).

The writers note at this point that if this magnetic mass is the heat source then the Long Valley area and Casa Diablo are at an advanced age in the geothermal cycle, A molten heat source ••auld be expected to give a magnetic low, not a high. At the Geysers an airborne survey for a private company showed a magnetic low. Note that Casa Diablo is still very active seismically so there still may be molten magma remaining in the partially depleted chamber and collapse may still be going on. At Mono Lake 25 miles north of Long Valley, the Mono Lake basin appears to have had a history similar to Long Valley. The Mono Lake Depression is 20 x 12 miles. Very few seismic epicenters are reported in the Mono Lake area and this may well indicate that the Mono Lake Depression is at an advanced age in the geothermal cycle and is essentially dead with only some residual heat left. The GRI-Getty-Southern California Edison 6500 foot deep proposed test scheduled for August 1971 will drill out under the center of the lake from a shoreline location in Sec 17-TIN-R27E.

It will be interesting to see what temperatures are encountered, if steam or hot water are found, and if they are, if there is a rapid decline in productivity because of the possible relatively lower tem- perature of the heat source.

Continuing with McNitt's writeup he states,

"Rinehart and Ross (written communication, 1962) are of the opinion that the thermal activity at Casa Diablo Hot Springs, as well as in most of the Long Valley area, appears to be localized along steeply dipping to vertical faults that trend north to north- west. The writer (McNitt) believes that arcuate faults (underlined

28

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0 Yl 1 .2 3 o4 5 MILES DATUM IS MEAN SEA LEVEL

EXPLANATION Cenowie clastic deposita

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Pleistocene Bishop tuff of Gilbert (1938)

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Cenozoic rhyolite and quartz latite

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Gilbert (1941)

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INDEX TO FIGURE SHOWING GEOLOGY SOURCES

FIG. 20. (from McNitt) Gravity Anomaly of the Long Valley Structural Depression.

by Austins) trending to the northwest from Casa Diablo Hot Springs are suggested by the configuration of the contact between the Tertiary rhyolite and younger Pleistocene units. Moreover, inspec- tion of aerial photographs strongly suggests that collapse structures enclosed by arcuate faults are common in the Long Valley depression.

If this is correct, then there is a striking similarity between the geologic structure of the Casa Diablo thermal area and the graben structure at The Geysers in Sonoma County."

In September 1966, Mr. Roger Chapman of the California Division of Mines and Geology published a Gravity Map of the Geysers Area in Mineral Infor- mation, Vol. 19, No. 9. This map, Fig. 21, shows an elliptical 20 milligal negative anomaly which is 21 miles long and 17 miles wide. The writers in preparation for this report assembled a photo index mosaic of the Geysers Area and found a major elliptical photo anomaly 21 miles long and 15 miles wide: In the case of The Geysers - the long axis of both the aerial photo and gravity ellipses runs North-South at an angle of 45 degrees to the NW trend of the Franciscan metasediments and volcanics as shown in Fig. 22 and 23. Fracturing, faulting and alteration are just in the initial stages along the edges of the Geysers "Magma Chamber."

Therefore the Geysers are at an early stage and probably are still heat- ing up in contrast to Long Valley (Casa Diablo) and Mono Lake which appear to be in late stages of development. Figures 24 through 27 illustrate specific features at the Big Geyser area.

As an additional demonstration of the pattern which should be looked for in aerial photo exploration for geothermal prospects, the Island Park Caldera of rhyolite and basalt in Eastern Idaho is cited. This area is described by Warren Hamilton in the USGS Prof. Paper 504c 1963, and is shown in detail in Fig. 15.

Hamilton states that, "The Island of the Snake River Lava Plain, is an elliptical collapse structure 18 x 23 miles in diam- eter that "ras dropped from the center of a shield volcano composed of rhyolite ash flows. The western semicircle of the Caldera margin is a single scarp in the northtvest and a ·composite scarp in

the southwest. (Note by writers- Scarp is 1200 feet high).

Rhyolite domes and lava flows were extruded along the western rim during and after the period of collapse. The eastern semicircle of the Caldera scarp has been covered completely by rhyolite ash flows, domes, and lava flows that ,;rere extruded along it. The caldera is filled in upward succession by rhyolite ash flows, interbedded rhyolite ash flows, olivine-basalt lava flows, and flows of olivine basalt alone. Rhyolite domes protrude through the basalt .•. The Island Park Caldera is part of the Snake River- Yellowstone province of intense Pliocene and Quaternary volcanism

of olivine basalt and rhyolite. In this province, as in other bimodal volcanic provinces, rhyolite and basalt erupted from vents interspersed in both time and space, and simul-taneous eruptions of

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FIG. 22. Unretouched Composite Photomosaic of The Big Geysers Geothermal Area.

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^{FIG. 23.} Photomosaic of Fig. 22 With

Batholith (Corresponding to a Gravity Stock Centers Called Local Geothermal Austin, 12 July 1971.)

33

the Arcuate Patterns Outlining the

Low~ ' and the Smaller Granitic Cells. (Photogeology by Ward

FIG. 24. Looking North Across Big Sulfur Creek Toward The Geysers Power Plant and Showing Roads to Well Sites. Note the two prominent elliptic patterns marked, one of which is an alteration pattern. The pattern which shows the edge of the main underlying batholith is also marked.

34

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FIG. 25. A Closeup of the Elliptical Alteration Pattern of Fig. 24 Showing the Three Power Plant Locations (Arrows). This elliptic pattern is considered the surface expression of the under- lying granitic stock heat source for this local geothermal cell.

35

FIG. 26. Looking Northwest Down Sulfur Creek Past a Large Elliptical Pattern Southwest of

the Elliptical Alteration Pattern Shown in Detail in Fig. 25. Again note the border structures which mark the western edge of the main batholith.

36

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FIG. 27. A Closeup of the Nearer Elliptic Pattern at the Bottom of Fig. 26.

37

both from the same or nearby vents are known to have occurred. In the Island Park Caldera, the eruptive sequence and geometry suggest that the large magma chamber contained liquid rhyolite overlying liquid olivine basalt.

Several kinds of evidence indicate that the rhyolite of this and other bimodal volcanic provinces has formed by differentiation of basaltic magma."

In his description of other recognized collapsed calderas, Hamilton refers to the Lake Taupo collapse depression just south of the Wairakei Geothermal field (15 x 20 miles) as shown in Fig. 28 (Grange, 1937), and an oval collapse depression 14 x 28 miles in the volcanic San Juan Moun- tains of Colorado (Leudke and Burbank, 1962).

It seems apparent therefore that the means for recognlzlng the limits of a geothermal area (i.e., the outline of the parent magma chamber and the outlines of local cells) have been available for some time though unused. The problem has been that most workers have concentrated on hot springs and the local area immediately surrounding the springs. No aerial photo work has been done, or if it has, it has been in too limited an area to outline the limits of the magma chamber area of interest. Although conveptional scale aerial photography can be utilized in mosaic form to outline 20 x 15 mile or larger features as has been done in this study, the photo anomalies in many cases are so. subtle that a torn-edge-type mosaic disturbs and can confuse the interpreter. Ideal photos would be high altitude at a scale of 1:250,000 or about 1 inch= 4 miles. This would enable an entire anomaly to be seen on one photo. Such photos

could be taken using a high altitude photo reconnaissance plane with a short focal length lens. When Skylab is operative, unclassified satel- lite photographs can be taken which will be especially valuable in deter- mining-the limits of magma chambers related to geothermal deposits and

their associated hot springs, as shown by the Gemini III photo S-65-18741 in Fig. 29a and b at an average scale 1:1,500,000 or about 1 inch= 24 miles. As has been noted by other interpreters of satellite photography,

the advantage of the extreme high altitude is the elimin'ation of cultural details which can confuse or obscure the surface geological picture and an accentuation of subtle surface anomalies which on conventional scale photos might be missed. As an example, in the highly developed Imperial Valley, culture obscures the aerial photo pattems which appear to out- line the limits of the magma chamber supplying the heat for the Niland geothermal steam and brine field, and only the very experienced photo interpreter can readily see the pattem. On satellite photos such as Fig. 29 even untrained observers can see the elliptical patterns. Lacking vertical satellite photography, a photomosaic has been used to develop

the elliptic geothermal cell pattems south of the Salton Sea. These are shown in Fig. 30a and b. Figure 31 shows gravity and magnetic data for this area.

38

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New Zealand's Main Thermal Area

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(a) (b)

FIG. 29. NASA Satellite Photography From Gemini III of the Cerro Prieto and Salton Sea Areas Showing Prominent Elliptic Patterns Which Outline Centers of Geothermal Activity.

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(b)

See Gemini III

~~- Photo S-65-1874 (Fig. 29)