NSF/RA/N--74-159-7
CONFERENCE ON RESEARCH
FOR THE DEVELOPMENT OF
GEOTHERMAL ENERGY RESOURCES
September 23 - 25, 1974
Pasaden a, California
Organized by
Jet Propulsion Laboratory /
Cblifornia Institute of Technology
Sponsore d by
National Science Foundation
RANN-Research Applied to National Needs
Division of Advanced Energy
Research and Technology
Grant No. AG-545
DISTRIBUTION OF THIS DOCUMENT UNUMITE
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
.
I.
Geothermal systems in the Basin and Range Province of the
western United States probably differ in many respects f r o m
geothermal systems already discovered in other p a r t s of the
world because of the unique tectonic setting. To investigate this,
-a study of the geothermal occurrences a t Fly Ranch, approximately 100 miles north of Reno, Nevada, has been undertaken.
Ample evidence for a geothermal system exists in this a r e a ,
including the surface expression of heat flow in the f o r m of hot
springs, an extensive a r e a of low electrical resistivity, and a
high level of seismicity along faults bounding the thermal a r e a .
However, geophysical and geological studies have not yet p r o vided evidence for a local heat source a t depth. Additional
detailed geophysical and geological studies, as well a s drilling,
m u s t be completed before the geothermal s y s t e m can be described
fully.
INTRODUCTION
L a r g e estimates of the potential capacity for producing geothermal power
in the conterminous United States are based on the assumption that the Basin
and Range Province has many unrecognized geothermal systems. These s y s tems m u s t differ in many r e s p e c t s f r o m geothermal systems already discovered
in other p a r t s of the world because of the r a t h e r unique tectonic setting of the
Basin and Range Province. In other a r e a s , geothermal systems are usually
closely associated with modern, easily recognized stages of volcanism, and the
geothermal r e s e r v o i r s a r e formed directly in t h e porous, pyroclastic rocks
around these c e n t e r s of volcanism. In the Basin and Range Province, the
amount of modern volcanism i s minor, raising a question a s to whether o r not
enough heat i s being supplied for the development of good geothermal systems.
Also, if geothermal r e s e r v o i r s a r e present, they may exist in porous alluvial
sedimentary rocks o r in fractured crystalline rocks r a t h e r than pyroclastics.
At present, we cannot describe a Basin and Range Province geothermal s y s t e m
physically, so that geological and geophysical exploration programs can be
designed specifically to find them.
The Colorado School of Mines has received support f r o m 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
DISCLAIMER
This report was prepared as an account of work sponsored by an
agency of the United States Government. Neither the United States
Government nor any agency Thereof, nor any of their employees,
makes any warranty, express or implied, or assumes any legal
liability or responsibility for the accuracy, completeness, or
usefulness of any information, apparatus, product, or process
disclosed, or represents that its use would not infringe privately
owned rights. Reference herein to any specific commercial product,
process, or service by trade name, trademark, manufacturer, or
otherwise does not necessarily constitute or imply its endorsement,
recommendation, or favoring by the United States Government or any
agency thereof. The views and opinions of authors expressed herein
do not necessarily state or reflect those of the United States
Government or any agency thereof.
DISCLAIMER
Portions of this document may be illegible in
electronic image products. Images are produced
from the best available original document.
The study i s to be c a r r i e d out on a geothermal prospect which might be
considered to be typical of geothermal systems expected to be p r e s e n t in the
Basin and Range Province. Fortunately, Sun Oil Company agreed to s h a r e with
us the results of an exploration p r o g r a m that they have c a r r i e d out f o r geothermal energy in the Basin and Range Province for the p a s t decade. On the
basis of their regional exploration r e s u l t s , we selected the Black Rock Desert,
and m o r e specifically, the F l y Ranch Gerlach hot springs complex, as a
prospect with a high probability of developing into a n economically viable geot h e r m a l - r e s e r v o i r . The area l i e s approximately 100 m i l e s north of Reno,
Nevada, a s shown in F i g u r e 1. Many t h e r m a l springs lie within the prospect
a r e a , as indicated on Figure 1.
-
The plan for the study i s to make u s e of the b e s t available geological and
geophysical techniques -for evaluating the temperature, volume, and other
pertinent characteristics of any geothermal r e s e r v o i r within the survey area.
The effectiveness of the exploration i s to be evaluated by drilling a t e s t hole
to verify the predictions made on the b a s i s of exploration results.
11.
GENERAL GEOLOGY
The study is to be centered about the F l y Ranch hot spring complex,
which l i e s in and around Hualapai’Flat, about fifteen m i l e s north of Gerlach,
Nevada. A geologic map of the a r e a is shown in F i g u r e 2. Hualapai Flat i s a
topographic (and probably s t r u c t u r a l ) embayrnent of r e c e n t sediments into the
Granite Range. It is separated by a low saddle f r o m the extensive Black Rock
D e s e r t to the east. It i s bounded on the-south and w e s t by a granite massif
(the Granite Range), and by thick volcanic piles on the northwest, which a r e
in p a r t continuous to the west with the volcanic rocks of the Modoc Plateau.
The sequence of events recorded by the rocks of the a r e a a r e :
(1)
Deposition and subsequent thermal metamorphism of late
Paleozoic ( ? ) volcanic and sedimentary rocks.
(2)
Intrusion of Cretaceous granodiorite into the late Paleozoic ( ? )
volcanic and sedimentary rocks, with profound erosion of the land
surface.
(3)
Extrusion and intrusion of T e r t i a r y volcanic rocks.
(4)
Quaternary lake cycles.
(5)
Recent faulting.
The late Paleozoic r o c k s are, as a group, the oldest rocks exposed in
northwestern Nevada. They have not been studied and can only be described as
a sequence of metamorphosed flows, tuffs, breccias, -and sedimentary rocks.
They c r o p out as s e v e r a l s m a l l hills roughly circling Hualapai Flat.
74
d
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 y e a r s ; Ref. 1). It resembles other intrusive rocks of northwestern Nevada with no major petrographic differences. It comprises a h o r s t
block that r i s e s 5000 feet above the valley floor at Gerlach. It is in intrusive
contact with late Paleozoic ( ? ) rocks in places.
The volcanic rocks bordering Hualapai F l a t a r e of the Oligocene South
Willow Formation (Ref. 2). The South Willow Formation i s predominately
intermediate to mafic volcanic flows and breccias. The .formation unconformably overlies p r e - T e r t i a r y rocks near Cottonwood Creek and i s overlain j u s t
to the west by Miocene-Pliocene volcanic rocks which r e p r e s e n t the e a s t e r n
border of the Modoc Plateau volcanic rocks. The Calico Mountain Range is
predominantly intermediate flows, tuffs, and sedimentary rocks. The base of
the sequence i s not exposed and the top is an erosional surface. The youngest
rocks of the a r e a a r e several intrusive rhyolite plugs in the northwestern c o r n e r
of Hualapai.Flat which intrude the South Willow Formation. They can only be
said to be post-South-Willow formation and probably late Miocene to Pliocene.
Quaternary deposits consist of Pleistocene Lake Lahontan deposits and
Recent playa, dune, and alluvial sediments. Sizeable buildups of sinter a r e
.
p r e s e n t around several hot springs and w a r m water wells.
P r e - T e r t i a r y s t r u c t u r e i s obscured by the extensive volcanic cover and
lack of Mesozoic and older rocks. The a r e a m a r k s the transition f r o m a
physiographic province where general Basin and Range faulting i s obvious to
the Modoc Plateau Province to the west, where continuous flows of basalt .
obscure the faulting to some extent. The a r e a is seismically active and s e v e r a l
faults have been mapped a c r o s s Recent playa deposits with vertical offsets of
up to ten feet.
111.
INFRARED SURVEY
Airborne infrared scanning o r mapping i s potentially a powerful tool in
prospecting for geothermal systems (Refs. 3, 4, and 5). The m o s t i m p r e s s i v e
r e s u l t s have been obtained in neovolcanic a r e a s where extreme variations in
temperature exist at the e a r t h ' s surface. Infrared imaging may also be of u s e
in exploration in a basin and ratlge setting in two ways; by providing an invent o r y of bo€h thermal and normal spring activity, and by serving as an indicator
of the location of fault t r a c e s a t the e a r t h ' s surface. In an a r i d region such a s
northwestern Nevada, many springs a r e intermittent, o r discharge below the
e a r t h ' s surface. Such springs can a l t e r the soil moisture sufficiently to affect
the temperature o r emissivity and cause a detectable change in thermodynamic
temperature. Infrared imagery can then be used to locate hidden springs and
water discharges to be sampled f o r geochemical studies..
An infrared survey was flown in e a r l y October, 1974, with the a r e a -being
covered extending f r o m the.vtcinity of Gerlach on the south to Soldiers Meadows
on the'north. The survey was flown by E a r t h Satellite Corporation, using a
Daedalus scanner operating in the 8 to 14 micrometer wave-length band, and
75
L
flown at an altitude of 7500 feet above the land surface. The imagery obtained
f r o m the a r e a immediately around the F l y Ranch hot springs i s shown in
Figure 3.
IV.
MICROSEISMICITY STUDY
The author of Ref. 6 has reviewed evidence that t h e r e i s a close association between locally intense m i c r o s e i s m i c activity and the occurrence of
geothermal systems. The microseismicity of northwestern Nevada has been
described in several papers (Refs. 7, 8, 9 and 10). However, coverage of the
Gerlach - F l y Ranch a r e a in these e a r l i e r studies was poor, and so,
Microgeophysics, Inc., of Golden, Colorado, was engaged to c a r r y out a
microseismicity survey of the Black Rock D e s e r t area. F o r this purpose,
five high-gain ( 3 t o 6 m ) , high-frequency (1 to 30 Hz) s e i s m i c a r r a y s with a
detection threshold below magnitude 1. 0 w e r e operated for a total of 30 days
in May and June, 1974.
-
Each a r r a y operated with seven Sprengnether model MEQ-800-33 portable
s e i s m i c s y s t e m s sited at separations of approximately 3 m i l e s . Only vertical
geophones w e r e used; these w e r e connected to a recording s y s t e m which
recorded on smoked paper at a c h a r t speed of 120 m m / m i n . Each recording
s y s t e m had an integral timing system'based on a precision clock synchronized
daily with a WWVB signal. A r r i v a l times for compressional waves w e r e d e t e r mined within *O. 05 seconds. Almost all s e i s m o m e t e r locations w e r e on outcrops of crystalline o r igneous rock. Station s i t e s w e r e moved during the
course of the survey, s o that the s a m e seven recording s y s t e m s w e r e used to
f o r m the five a r r a y s .
.
,
A total of approximately 420 local events and 111 t e l e s e i s m s were'identi- ,.
fied during the 30 days of operation. All but 100 of the local events w e r e
observed on a single day, Julian day 169, 1974, in a concentrated s w a r m that
occurred along the boundary between the Black Rock D e s e r t on the e a s t and
the Granite Range block on the west. P r i o r to this day of s w a r m activity, the
r a t e of occurrence of local earthquakes was approximately 2 p e r day. These
events outlined activity along a line separating the Granite Range block f r o m 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 s t r a i n energy r e l e a s e d during the 25 days preceding the s w a r m , The s w a r m
occurred immediately to the southeast of the alkali flat in the southern end of
Hualapai F l a t , at the site of maximum s t r a i n rate p r i o r to the swarm.
V.
ELECTRICAL SURVEYS
Electrical resistivity surveys are accepted as being the m o s t d i r e c t
approach to locating r e s e r v o i r s containing geothermal fluids (Ref. 11). An
increase in rock temperature f r o m a normal value of 20" to 60°C to a n anomalous value of 200" t o 300°C will evoke a five- to six-fold reduction in resistivity.
A r e s e r v o i r with sufficient volume to be of economic i n t e r e s t will provide a
v e r y l a r g e t a r g e t for electrical prospecting techniques, unless the effect of
76
temperature i s cancelled by compensating changes in water salinity o r porosity.
Such changes a r e unlikely.
Because of the size and l a r g e contrast in resistivity for a geothermal
r e s e r v o i r , any electrical surveying technique with the capacity to r e a c h to
depths of 5000 to 10,000 feet will be capable of detecting the r e s e r v o i r . We
have used the dipole mapping technique f o r reconnaisance, and this is to be
followed by m o r e definitive electromagnetic sounding surveys (Ref. 11) once
the probable location of a geothermal r e s e r v o i r has been determined.
In the dipole mapping survey, the concept i s that a c u r r e n t field will be
distorted by the presence of conductive m a s s e s of rock such a s a r e associated
with geothermal r e s e r v o i r s . This distortion is mapped by measuring e l e c t r i c
field intensity a t many points around a single bipole c u r r e n t source. A r e a s of
unusually low e l e c t r i c field intensity a r e normally assumed to be a r e a s of
unusually low resistivity. However, problems a r i s e in this straightforward
evaluation of dipole mapping surveys because in some c a s e s , an a r e a of
anomalously low e l e c t r i c field intensity may appear without there actually being
any subsurface region of low resistivity present. This occurs commonly a t a
fau€t-like boundary between a region with moderate resistivity values and
another region with high resistivity values. In o r d e r to avoid being misled by
such "false" anomalies, it i s n e c e s s a r y to provide multiple coverage of an a r e a
in which an anomaly has been found, using s e v e r a l differently situated bipole
sources. The rotating dipole method offers an approach to multiple coverage
which is superior to t h e u s e of multiple but randomly located s o u r c e s .
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
w i r e a s the source wire i s swung through a 360' rotation. As the source
r o t a t e s , the direction of c u r r e n t flow a t the receiver site will rotate through
all possible directions , and apparent resistivity values which a r e maximally
and minimally affected by boundaries in actual resistivity will be measured.
The field procedure consists of making only two s e t s of electric field m e a s u r e ments at a r e c e i v e r site, one for each of two orientations of the source wire.
Then, the two e l e c t r i c field vectors can be added in the proper proportions to
determine the apparent resistivity for any orientation of the source bipole.
-
Rotating dipole surveys w e r e c a r r i e d out in the F l y Ranch Gerlach area
using five s e t s of bipole sources. Each bipole source was one mile in length,
and powered with c u r r e n t steps with amplitudes ranging f r o m 4 0 to 120 a m p e r e s .
The c u r r e n t waveform was that of an asymmetrical square wave, with a r e p e tition r a t e of 3 p e r minute. Measurements of e l e c t r i c field strength w e r e made
at 320 r e c e i v e r ' s i t e s , a t distances f r o m the bipole source ranging up to
5 miles.
Many different resistivity values can be 'computed f r o m the data obtained
in a rotating dipole survey, but computations made for various theoretical
models suggest that the b e s t value i s an average of the2maximum and minimum
apparent resistivities obtained on rotation. A contour map of averaged maximum and minimum resistivities measured from one pair of bipole sources is
shown in F i g u r e 5. The sources a r e located in the Black Rock Desert, immediately south of the Hualapai Flat a r e a , and over the epicenters of the majority
77
of the earthquakes detected during the microseismicity survey. An area of
extremely low resistivity i s p r e s e n t at the contact between the bedrock saddle
and the Black Rock D e s e r t sediments, with values of less than 1 ohm-meter
being observed. Within the bedrock to the north, extending into Hualapai Flat,
apparent resistivities r e m a i n moderately low, being about 20 ohm-meters
along the trend of the hot spring activity.
VI.
SUMMARY
Ample evidence exists f o r a geothermal s y s t e m at F l y Ranch. Some of
the m o s t persuasive evidence is the surface expression of heat flow, r e p r e sented by hot springs at o r n e a r the boiling point, and widespread areas of
w a r m water seepage, seen on the infrared imagery. That this i s not s o m e
random surface manifestation of structurally controlled ground water flow
along a fault s y s t e m i s the evidence for an extensive r e s e r v o i r at depth p r o vided by the electrical resistivity surveys. The s e s i m i c activity along faults
bounding the area of low resistivity suggests that high t e m p e r a t u r e s at depth
have r a i s e d the p r e s s u r e of pore fluids above normal. On the other hand, the
geophysical and geological studies have not yet provided evidence for a local
heat s o u r c e at depth.
Additional detailed geophysical and geological studies m u s t be c a r r i e d out
to evaluate the potential of the geothermal s y s t e m properly. However, it will
be n e c e s s a r y to drill a t least one t e s t well to determine the effectiveness of the
exploration procedures.
REFERENCES
1.
Smith, J. G. , McKee, E. H. , Tatlock, D. B. , and Marvin, R. F. ,
"Mesozoic Granitic Rocks in Northwestern Nevada: A Link Between the
S i e r r a Nevada and Idaho Batholiths, 'I Geol. S O C . Am. Bull. , Vol. 82,
pp. 2933 - 2944, 1971.
2.
Bonham, H. F., Geology and Mineral Resources of Washoe and Storey
Counties, Nevada. Nevada Bur. Mines Bull. 70, 140 pp. , Reno, 1969.
3.
Hodder, D. T. , "Application of Remote Sensing to Geothermal P r o s p e c t ing, I' Geothermics, Vol. 2, Pt. 1, pp. 368 - 380, 1970.
4.
Hochstein, M. P. , and Dickinson, D. J. , !'Infra-Red Remote Sensing of
Thermal Ground in the Taupo Region, New Zealand, Geothermics,
Vol. 2, Pt. 1, pp. 420 423, 1970.
-
5.
Gomez Valle, R. , Friedman, J. D. , Gawarecki, S. J. , and Banwell, C. J.
"Photogeologic and T h e r m a l Infrared Reconnaissance Surveys of the
Los Negritos -Ixtlan De Los Hervores Geothermal Area, Michoacan,
Mexico, Geothermics, Vol. 2, P t . 1, pp. 381 - 398, 1970.
78
,
6.
7.
8.
Ward, P. L. , "Microearthquakes: Prospecting Tool and Possible Hazard
in the Development of Geothermal
." Resources, I'.Geothermics, Vol. 1,
Pt. 1, pp. 3 - 12, 1970.
Westphal, W. H. , and Lange, A. L. , ttLocal Seismic Monitoring Fairview P e a k A r e a , Nevada, Bull. Seis. SOC.A m . , Vol. 57,
pp. 1279 - 1298.
Stauder, W. , andeRyall, A. , tY3patial Distribution and Source Mechanisms
of Microearthquakes in Central Nevada, Bull. Seis. SOC.A m . , Vol. 57,
pp. 1317 1346, 1967.
-
9.
Oliver, J., Ryall, A . , Brune, J . N . , and Slemmons, D . B . , 1966,
"Micro-Earthquake Activity Recorded by Portable Seismographs of High
Sensitivity," Bull. Seis. SOC. Am., Vol. 56, pp. 899 - 924, 1966.
10.
Savage, W ;U. , "Microearthquake Clustering Near Fairview Peak, Nevada
and in the Nevada Seismic Zone, Geophys. R e s . , Vol. 77, No. 35,
pp. 7049 - 56, 1972.
11.
Keller, G. V . , "Induction Methods in Prospecting f o r Hot Water,
Geothermics, Vol. 2, Pt. 1, pp. 318 - 332, 1970.
. ..
1
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79
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I'
--
a
o
100"
~
F
200°F
Fig. 1. Contours of thermal spring t e m p e r a t u r e s in the
Basin and Range Province in Nevada and California
80
I I
i
Fig. 2. Reconnaissance geological map of the Black Rock Desert
area of northwestern Nevada
81
Fig. 3 . Infrared imagery of the vicinity around the Fly Ranch
hot springs, northern Nevada
82
,
Fig. 4. Strain release for a twenty-five day period in the
Black Rock Desert
83
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0
0
0
0
1
0
0
0
0
0
2
Kilometers
Contours in ohm-meters
0
Fig. 5. Example of a rotating dipole resistivity survey at the
western edge of the Black Rock D e s e r t
84
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