258 



UNITED STATES MINERAL RESOURCES 



quantitative importance cannot be evaluated without 

 making some estimate of the rate of exploitation of 

 the reservoir. If exploitation is very rapid, recharge 

 will have little significance, but if exploitation is 

 very slow, recharge will be significant; thus, estab- 

 lishment of a steady balance between extraction and 

 recharge (both heat and water) may permit the 

 geothermal system to be exploited indefinitely. 



The geothermal resources of the world, therefore, 

 lie somewhere between 2x10" cal (White's 1965 

 estimate of potential reserves to 3 km recoverable 

 as equivalent electrical energy using present tech- 

 nology) and 5-10x10'^ cal (calculated from Rex, 

 1972a). Of these geothermal resources, perhaps 

 5-10 percent are in the United States (White, 1965) . 

 The differences among the various resource esti- 

 mates will not be resolved until there is (a) better 

 knowledge of the distribution of geothermal energy 

 in the earth's crust, (b) clarification of the tech- 

 nological limitations upon geothermal resource ex- 

 ploitation, and (c) definition of uses. 



PROSPECTING TECHNIQUES 



The first step in reconnaissance geothermal ex- 

 ploration involves outlining broad regions where 

 the heat flow is significantly greater than 1.5xl0~^ 

 cal cm-- sec-^ (for example, Sass and others, 

 1971). Most of these regions of high heat flow are 

 in zones of young volcanism and tectonic activity, 

 and most are characterized by abundant hot springs. 



Techniques for identifying potentially economic 

 concentrations of geothermal energy within broad 

 regions of high heat flow are not well developed. 

 Important considerations include distribution of hot 

 springs, evaluation of volcanological and tectonic 

 setting, and chemical analysis of hot-spring fluids. 

 In particular, the content of silica (Fournier and 

 Rowe, 1966) and the ratios of sodium, potassium, 

 and calcium (Fournier and Truesdell, 1973) provide 

 information about the minimum subsurface tem- 

 perature to be expected. 



Several geophysical techniques have proved useful 

 in the final delineation of geothermal targets (Combs 

 and Muffler, 1973; Banwell, in press). Of these 

 techniques, perhaps the most unambiguous is the 

 direct measurement of temperature gradients at 

 depths of 25-100 meters (Combs, 1971; Burgassi 

 and others, 1970). Temperature measurements at 

 shallower depths, however, can be very misleading, 

 owing primarily to the effects of seasonal changes 

 in temperature and to the shallow movement of 

 ground water (Banwell, in press). Thermal infra- 

 red surveys detect surface temperature anomalies, 

 but these anomalies can be caused by many factors 



other than geothermal heat flow (Banwell, in press; 

 Watson and others, 1971). 



Several techniques that measure the electrical 

 conductivity at depth have had great success in geo- 

 thermal exploration. The conductivity at depth 

 varies directly with temperature, porosity, salinity 

 of interstitial fluid, and content of clays and zeo- 

 lites. All these factors tend to be higher within 

 good geothermal reservoirs than in the surrounding 

 ground, and consequently the electrical conductiv- 

 ity in these geothermal reservoirs is relatively high. 

 Electrical conductivity at depth can be measured 

 by electrical (galvanic) or electromagnetic (induc- 

 tive) methods. Among the electrical techniques, only 

 direct-current methods are reliable, owing to skin 

 effects attendant to alternating-current methods; 

 direct-current electrical arrays commonly used are 

 Wenner, Schlumberger, and dipole-bipole (Banwell 

 and Macdonald, 1965; Hatherton and others, 1966; 

 Risk and others, 1970; Macdonald and Muffler, 

 1972; Zohdy and others, 1973). Electromagnetic 

 methods (audiofrequency magnetotellurics, electro- 

 magnetic coil surveys, and loop-loop or wire-loop 

 induction surveys) have certain theoretical advan- 

 tages (Keller, 1970; Harthill, 1971) and are becom- 

 ing more widely used. 



Passive seismic methods are proving of use in 

 locating fractured and permeable zones in geother- 

 mal areas. Microearthquakes at relatively shallow 

 focal depths are concentrated along fracture or fault 

 zones in many geothermal areas (Ward, 1972 ; Ward 

 and Bjornsson, 1971; Hamilton and Muffler, 1972). 

 In addition, some geothermal areas appear to have 

 a high level of seismic ground noise (Whiteford, 

 1970) ; analysis of the areal distribution of this 

 noise may outline prospective zones of geothermal 

 production (Goforth and others, 1972). 



Several other geophysical techniques have proved 

 useful in special circumstances. Gravity surveys 

 may in some areas define positive anomalies that 

 are caused by alteration or metamorphism of sub- 

 surface rock (Hochstein and Hunt, 1970; Biehler, 

 1971), but in other areas they may define a nega- 

 tive anomaly that may represent an intrusive mass 

 at depth (Calif. Div. Mines and Geology, 1966). 

 Magnetic surveys in some areas define negative 

 anomalies that are caused by alteration of magne- 

 tite to pyrite (Studt, 1964), but in other areas they 

 define positive magnetic anomalies that are caused 

 by intrusions of magnetic igneous rock (Griscom 

 and Muffler, 1971). Active (explosion) seismology 

 is useful in defining subsurface geologic structure 

 (Hochstein and Hunt, 1970; Hayakawa, 1970), and 

 analysis of seismic attenuation across geothermal 



