Geothermal systems in the Basin and Range Province of the western United States probably differ in many respects from geothermal systems already discovered in other parts of the world because of the unique tectonic setting. To investigate this, a study of the geothermal occurrences at Fly Ranch, approximately 100 miles north of Reno, Nevada, has been undertaken. Ample evidence for a geothermal system exists in this area, including the surface expression of heat flow in the form of hot springs, an extensive area of low electrical resistivity, and a high level of seismicity along faults bounding the thermal area. However, geophysical and geological studies have not yet provided evidence for a local heat source at depth. Additional detailed geophysical and geological studies, as well as drilling, must be completed before the geothermal system can be described fully

Crewpson, R. A.

Grose, L. T.

Keller, G. V.

English

NASA Technical Reports Server

THE COLORADO SCHOOL O F  MINES 
NEVADA GEOTHERMAL STUDY 
George V. Keller, L. Trowbridge Grose, and Robert A. Crewdson 
Colorado School of Mines 
Golden, Colorado 
Geothermal systems in the Basin and Range Province of the 
western United States probably differ in many respects f rom 
geothermal systems already discovered in other par ts  of the 
world because of the unique tectonic setting. 
a study of the geothermal occurrences a t  Fly Ranch, approxi- 
mately 100 miles north of Reno, Nevada, has been undertaken. 
Ample evidence for a geothermal system exists in this area,  
including the surface expression of heat flow in the form of hot 
springs, an extensive a rea  of low electrical resistivity, and a 
high level of seismicity along faults bounding the thermal a rea .  
However, geophysical and geological studies have not yet pro- 
vided evidence for a local heat source at depth. Additional 
detailed geophysical and geological studies, as well a s  drilling, 
must be completed before the geothermal system can be described 
To investigate this, 
fully. 
I. INTRODUCTION 
Large estimates of the potential capacity for producing geothermal power 
in the conterminous United States a r e  based on the assumption that the Basin 
and Range Province has many unrecognized geothermal systems. These sys- 
tems must differ in many respects f rom geothermal systems already discovered 
in other par ts  of the world because of the rather  unique tectonic setting of the 
Basin and Range Province. In other a reas ,  geothermal systems are usually 
closely associated with modern, easily recognized stages of volcanism, and the 
geothermal reservoirs  are formed directly in the  porous, pyroclastic rocks 
around these centers of volcanism. 
amount of modern volcanism is minor, raising a question as to whether o r  not 
enough heat is being supplied for the development of good geothermal systems. 
Also, if ieothermal reservoirs  a r e  present, they may exist in porous alluvial 
sedimentary rocks o r  in fractured crystalline rocks rather  than pyroclastics. 
At present, we cannot describe a Basin and Range Province geothermal system 
physically, so that geological and geophysical exploration programs can be 
designed specifically to find them. 
In the Basin and Range Province, the 
The Colorado School of Mines has received support from the National 
Science Foundation under Grant GI 43866 to do a definitive geological 
and geophysical study of a Basin and Range type geothermal system. 
73 
https://ntrs.nasa.gov/search.jsp?R=19750012765 2020-03-22T23:17:55+00:00Z
The study is to be carr ied out on a geothermal prospect which might be 
considered to be typical of geothermal systems expected to be present in the 
Basin and Range Province. Fortunately, Sun Oil Company agreed to share with 
us the results of an exploration program that they have carried out for geo- 
thermal energy in the Basin and Range Province for the past  decade. On the 
basis of their regional exploration results,  we selected the Black Rock Desert, 
and more specifically, the Fly Ranch - Gerlach hot springs complex, as a 
prospect with a high probability of developing into an economically viable geo- 
thermal reservoir .  The area l ies approximately 100 miles north of Reno, 
Nevada, as shown in Figure 1. 
area,  as indicated on Figure 1. 
Many thermal springs l ie  within the prospect 
The plan for the study is to make use of the best  available geological and 
geophysical techniques for evaluating the temperature, volume, and other 
pertinent characterist ics of any geothermal reservoir within the survey area.  
The effectiveness of the exploration is to be evaluated by drilling a test hole 
to verify the predictions made on the basis of exploration results. 
11. GENERAL GEOLOGY 
The study is to be centered about the Fly Ranch hot spring complex, 
which l ies in and around Hualapai Flat, about fifteen miles north of Gerlach, 
Nevada. Hualapai Flat is a 
topographic (and probably structural)  embayment of recent sediments into the 
Granite Range. It is separated by a low saddle from the extensive Black Rock 
Desert to the east .  It is bounded on the south and west by a granite massif 
(the Granite Range), and by thick volcanic piles on the northwest, which a r e  
in par t  continuous to the west with the volcanic rocks of the Modoc Plateau. 
A geologic map of the a rea  is  shown in Figure 2. 
The sequence of events recorded by the rocks of the a rea  are:  
Deposition and subsequent thermal metamorphism of late 
Paleozoic ( ?  ) volcanic and sedimentary rocks, 
Intrusion of Cretaceous granodiorite into the late Paleozoic ( ?  ) 
volcanic and sedimentary rocks, with profound erosion of the land 
surface. 
Extrusion and intrusion of Tertiary volcanic rocks. 
Quaternary lake cycles. 
Recent faulting. 
The late Paleozoic rocks a re ,  as a group, the oldest rocks exposed in 
northwestern Nevada. 
a sequence of metamorphosed flows, tuffs, breccias, and sedimentary rocks. 
They crop out as several  small hills roughly circling Hualapai Flat. 
They have not been studied and can only be described a s  
74 
The Granite Range massif is, on the basis of normative and modal 
quartz-orthoclase-plagioclase abundances, a granodiorite ( K / A r  age of 
91.6 million years;  Ref. 1). 
western Nevada with no major petrographic differences. 
block that r i s e s  5000 feet above the valley floor a t  Gerlach. 
contact with late Paleozoic ( ?  ) rocks in places. 
It resembles other intrusive rocks of north- 
It comprises a horst  
It is in intrusive 
The volcanic rocks bordering Hualapai Flat  a r e  of the Oligocene South 
Willow Formation (Ref. 2) .  The South Willow Formation is predominately 
intermediate to mafic volcanic flows and breccias.  
ably overlies pre-Tertiary rocks near Cottonwood Creek and is overlain just 
to the west by Miocene-Pliocene volcanic rocks which represent the eastern 
border of the Modoc Plateau volcanic rocks. The Calico Mountain Range is 
predominantly intermediate flows, tuffs, and sedimentary rocks. The base of 
the sequence is not exposed and the top is an erosional surface. 
rocks of the a rea  are several  intrusive rhyolite plugs in the northwestern corner 
of Hualapai Flat  which intrude the South Willow Formation. 
said to be post-South-Willow formation and probably late Miocene to Pliocene. 
The .formation unconform- 
The youngest 
They can only be 
Quaternary deposits consist of Pleistocene Lake Lahontan deposits and 
Recent playa, dune, and alluvial sediments. Sizeable buildups of sinter a r e  
present around several  hot springs and warm water wells. 
Pre-Tert iary s t ructure  i s  obscured by the extensive volcanic cover and 
lack of Mesozoic and older rocks. The a rea  marks the transition from a 
physiographic province where general Basin and Range faulting is obvious to 
the Modoc Plateau Province to the west, where continuous flows of basalt 
obscure the faulting to some extent. 
faults have been mapped across  Recent playa deposits with vertical offsets of 
up to ten feet. 
The a rea  is seismically active and several  
111. INFRARED SURVEY 
Airborne infrared scanning o r  mapping is potentially a powerful tool in 
prospecting for geothermal systems (Refs. 3, 4, and 5). The most impressive 
results have been obtained in neovolcanic areas  where extreme variations in 
temperature exist at the earth 's  surface. Infrared imaging may also be of use 
in exploration in a basin and range setting in two ways; by providing an inven- 
tory of both thermal and normal spring activity, and by serving as an indicator 
of the location of fault t races  a t  the ear th 's  surface. In an ar id  region such as 
northwestern Nevada, many springs a r e  intermittent, or  discharge below the 
ear th 's  surface. Such springs can alter the soil moisture sufficiently to affect 
the temperature o r  emissivity and cause a detectable change in thermodynamic 
temperature. 
water discharges to be sampled for geochemical studies. 
Infrared imagery can then be used to locate hidden springs and 
An infrared survey was flown in early October, 1974, with the a rea  being 
covered extending from the vicinity of Gerlach on the south to Soldiers Meadows 
on the north. The survey was flown by Earth Satellite Corporation, using a 
Daedalus scanner operating in the 8 to 14 micrometer wave-length band, and 
7s  
flown at an altitude of 7500 feet above the land surface. 
f rom the a rea  immediately around the Fly Ranch hot springs is  shown in 
Figure 3 .  
The imagery obtained 
IV. MICROSEISMICITY STUDY 
The author of Ref.  6 has reviewed evidence that there is a close associa- 
tion between locally intense microseismic activity and the occurrence of 
geothermal systems. 
described in several  papers (Refs. 7 ,  8, 9 and 10).  However, coverage of the 
Gerlach - Fly Ranch a rea  in these ear l ier  studies was poor, and so,  
Microgeophysics, Inc., of Golden, Colorado, was engaged to car ry  out a 
microseismicity survey of the Black Rock Desert a rea .  
five high-gain ( 3  to 6 mf, high-frequency (1 to 30 Hz) seismic a r rays  with a 
detection threshold below magnitude - 1. 0 were operated for a total of 30 days 
in May and June, 1974. 
The microseismicity of northwestern Nevada has been 
For  this purpose, 
Each a r r ay  operated with seven Sprengnether model MEQ-800-B portable 
seismic systems sited a t  separations of approximately 3 miles.  
geophones were used; these were connected to a recording system which 
recorded on smoked paper at a chart  speed of 120 mm/min. 
system had an integral timing system based on a precision clock synchronized 
daily with a WWVB signal. 
mined within *O. 0 5  seconds. Almost a l l  seismometer locations were on out- 
crops of crystalline o r  igneous rock. 
course of the survey, so that the same seven recording systems were used to 
form the five a r rays .  
Only vertical 
Each recording 
Arrival times for compressional waves were deter- 
Station sites were moved during the 
A total of approximately 420 local events and 11 1 teleseisms were identi- 
fied during the 30 days of operation. 
observed on a single day, Julian day 169, 1974, in a concentrated swarm that 
occurred along the boundary between the Black Rock Desert on the eas t  and 
the Granite Range block on the west. P r io r  to this day of swarm activity, the 
rate  of occurrence of local earthquakes was approximately 2 per  day. 
events outlined activity along a line separating the Granite Range block from the 
Black Rock Desert, and along an east-west line passing through Hualapai Flat. 
This may be seen in Figure 4, a map showing contours of the cumulative amount 
of strain energy released during the 25 days preceding the swarm, 
occurred immediately to the southeast of the alkali flat in the southern end of 
Hualapai Flat ,  a t  the site of maximum strain rate pr ior  to the swarm. 
All but 100 of the local events were 
These 
The swarm 
V. ELECTRICAL SURVEYS 
Electrical resistivity surveys are accepted as being the most direct  
approach to locating reservoirs  containing geothermal fluids (Ref .  11). An 
increase in rock temperature from a normal value of 20" to 60°C to an anoma- 
lous value of 200" to 300°C will evoke a five- to six-fold reduction in resistivity. 
A reservoir with sufficient volume to be of economic interest  will provide a 
very large target for electrical  prospecting techniques, unless the effect of 
76 
temperature is cancelled by compensating changes in water salinity o r  porosity. 
Such changes a r e  unlikely. 
Because of the size and large contrast in resistivity for a geothermal 
reservoir ,  any electrical  surveying technique with the capacity to reach to 
depths of 5000 to 10, 000 feet will be capable of detecting the reservoir .  We 
have used the dipole mapping technique for reconnaisance, and this i s  to be 
followed by more  definitive electromagnetic sounding surveys (Ref. 11 1 once 
the probable location of a geothermal reservoir has been determined. 
In the dipole mapping survey, the concept is  that a current field will be 
distorted by the presence of conductive masses  of rock such as a r e  associated 
with geothermal reservoirs .  This distortion is mapped by measuring electric 
field intensity at many points around a single bipole current source. Areas of 
unusually low electric field intensity a r e  normally assumed to be a reas  of 
unusually low resistivity. 
evaluation of dipole mapping surveys because in some cases ,  an a rea  of 
anomalously low electric field intensity may appear without there actually being 
any subsurface region of low resistivity present.  
fault-like boundary between a region with moderate resistivity values and 
another region with high resistivity values. 
such ''false" anomalies, it is necessary to provide multiple coverage of an a rea  
in which an anomaly has been found, using several  differently situated bipole 
sources. 
which i s  superior to theuse of multiple but randomly located sources.  
However, problems a r i se  in this straightforward 
This occurs commonly at a 
In order to avoid being misled by 
The rotating dipole method offers an approach to multiple coverage 
In the rotating dipole method, measurements of electric field intensity 
a r e  made a t  a receiver location a s  a function of the orientation of the source 
wire as the source wire is swung through a 360° rotation. 
rotates, the direction of current flow a t  the receiver site will rotate through 
all possible directions, and apparent resistivity values which a re  maximally 
and minimally affected by boundaries in actual resistivity will be measured. 
The field procedure consists of making only two sets of electric field measure- 
ments a t  a receiver si te,  one for each of two orientations of the source wire. 
Then, the fmo electric field vectors can be added in the proper proportions to 
determine the apparent resistivity for any orientation of the source bipole. 
As the source 
Rotating dipole surveys were carr ied out in the Fly Ranch - Gerlach a rea  
using five sets  of bipole sources.  
and powered with current s teps  with amplitudes ranging from 40 to 120 amperes.  
flhe current waveform was that of an asymmetrical square wave, with a repe- 
tition ra te  of 3 per minute. 
a t  320 receiver sites, at distances f rom the bipole source ranging up to 
5 miles. 
Each bipole source was one mile in length, 
Measurements of electric field strength were made 
Many different resistivity values can be computed from the data obtained 
in a rotating dipole survey, but computations made for various theoretical 
models suggest that the best  value is an average of the maximum and minimum 
apparent resistivities obtained on rotation. 
mum and minimum resistivit ies measured from one pair of bipole sources is 
shown in Figure 5 .  
ately south of the Hualapai Flat area, and over the epicenters of the majority 
A contour map of averaged maxi- 
The sources are located in the Black Rock Desert, immedi- 
77 
of the earthquakes detected during the microseismicity survey. 
extremely low resistivity is  present a t  the contact between the bedrock saddle 
and the Black Rock Desert sediments, with values of l e s s  than 1 ohm-meter 
being observed. 
apparent resistivities remain moderately low, being about 20 ohm-meters 
along the trend of the hot spring activity. 
An area  of 
Within the bedrock to the north, extending into Hualapai Flat ,  
. 
VI. SUMMARY 
Ample evidence exists for a geothermal system at Fly Ranch. Some of 
the most persuasive evidence i s  the surface expression of heat flow, repre-  
sented by hot springs at or  near the boiling point, and widespread areas  of 
warm water seepage, seen on the infrared imagery. That this is not some 
random surface manifestation of structurally controlled ground vE.ater flow 
along a fault system is the evidence for an extensive reservoir qt depth pro- 
vided by the electrical resistivity surveys. The sesimic activity along faults 
bounding the a rea  of low resistivity suggests that high temperatures at depth 
ha.ve raised the pressure of pore fluids above normal. 
geophysical and geological studies have not yet provided evidence for a local 
heat source at depth. 
On the other hand, the 
Additional detailed geophysical and geological studies must be carr ied out 
to evaluate the potential of the geothermal system properly. 
be necessary to dril l  at least  one tes t  well to determine the effectiveness of the 
exploration procedures. 
However, it will 
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1. 
2. 
3 .  
4. 
5 .  
Smith, J. G . ,  McKee, E .H . ,  Tatlock, D, B . ,  and Marvin, R . F . ,  
"Mesozoic Granitic Rocks in Northwestern Nevada: A Link Between the 
Sierra  Nevada and Idaho Batholiths, I f  Geol. SOC. Am. Bull., Vol. 82, 
pp, 2933 - 2944, 1971. 
Bonham, H. F., Geology and Mineral Resources of Washoe and Store 
Counties, Nevada. Nevada Bur. Mines Bull. 70, 140 pp., Reno, 1965. 
Hodder, D. T . ,  "Application of Remote Sensing to Geothermal Prospect- 
ing," Geothermics, Vol. 2, Pt. 1, pp. 368- 380, 1970, 
Hochstein, M. P. , and Dickinson, D. J . ,  "Infra-Red Remote Sensing of 
Thermal Ground in the Taupo Region, New Zealand, 
Vol. 2, Pt. 1, pp. 420 - 423, 1970. Geothermics, 
Gomez Valle, R . ,  Friedman, J. D.,  Gawarecki, .S. J. ,  and Banwell, C. J , ,  
"Photogeologic and Thermal Infrared Reconnaissance Surveys of the 
Los Negritos-Ixtlan De Los Hervores Geothermal Area, Michoacan, 
Mexico, Geothermics, Vol. 2, Pt. 1, pp. 381 - 398, 1970. 
78 
6 .  Ward, P. L. , "Microearthquakes: Prospecting Tool and Possible Hazard 
in the Develo othermal Resources, I '  , VOl. 1, 
Pt. 1, pp. 3 
7. Westphal, W. H. , and Lange, A, L.  , t tLocal Seismic Monitoring - 
FairGiew Peak Area, Nevada, I f  Bull. Seis,  SOC. Am. ,  Vol. 57, 
pp. 1279 - 1298. 
8. Stauder, W., and Ryall, A. , "Spatial Distribution and Source Mechanisms 
of Microearthquakes in Central Nevada, '' 
pp. 1317 - 1346, 1967. 
9. Oliver, J . ,  Ryall, A . ,  Brune, J. N. , and Slernmons, D. E . ,  1966, 
"Micro-Earthquake Activity Recorded by Portable Seismographs of High 
Sensitivity, Bull. Seis. SOC. Am., Vol. 56, pp. 899 - 924, 1966. 
Savage, W.  U . ,  "Microearthquake Clustering Near Fairview Peak, Nevada 
and in the Nevada Seismic Zone, I t  Geophys. Res . ,  Vol. 77, No. 35, 
pp. 7049 - 56, 1972. 
10. 
11. Keller, G.  V ,  , "Induction Methods in Prospecting for Hot Water, ' I  
Geothermics, Vol. 2, Pt. 1, pp. 318 - 332, 1970. 
79 
Fig. 1. 
Basin and Range Province in Nevada and California 
Contours of thermal spring temperatures in the 
. 
80 
Fig. 2. Reconnaissance geological map of the Black Rock Desert 
area of northwestern Nevada 
81 
ound the Fly Ranch 
82 ORIGINAL PAGE IS 
OF POOR 
BLACK ROCK DESERT 
FLY RANCH 
P 
A 
0 5 to 
c-------c------I 
KlLOMETERS 
CONTOURS IN 10' ERGSV2 
Fig. 4. 
Black Rock Desert 
Strain release for a twenty-five day period in the 
83 
9 
0 i 2 -
K i I ome ters 0 0 
0 
Contours in ohm-meters 0 
0 0 
0 
0 
Fig. 5 .  
western edge of the Black Rock D e s e r t  
Example of a rotating dipole resistivity survey at the 
84 


The Colorado School of Mines Nevada geothermal study

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