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My. J. E:. G i l b e r t J U R ~ 2 3 , 1971 Page 2
has c o n s i d e r a b l e spade dome. c m a I t h i n k , work very em V! e
Heat Flow Research
and
Explorat ion f o r Geothermal Power
i n t h e
Black Rock Deser t , Nevada
A Report Submitted
t o
Dr.' George W. Berry, Corder0 Mining Company
Edward R.' Decker Assistant Professor
Univers i ty of Wyoming Depte O f Geology
Table of Contents
Page
Introduction....... ....................................... 1
Notation and Units.... ..................................... 1
Heat Flow Determinations on Land .......................... 1
6
7
Resul t s of Recent Cont inental Heat Flow Studies . .......... Hot Springs i n Northwestern Nevada ....................... Heat Flow Research Near Corder0 Mining Company's
Previous Resul t s ................................. 7
Recommendation f o r Future Research....... ....... 10
Cost f o r Future ReseaYeh ........................ 1 6
References ................................................ ' 1 9
Introduction
Measurements of heat flow near the earth's surface provide
the most reliable boundary conditions to be employed in the
calculation of models of subsurface temperatures and the spatial
distribution of crustal sources of heat. The subsurface temperature
and heat source distributions, in turn, have many broad implications
in the explanation of several geologically-observed phenomena and
must be considered in exploration for geothermal power. In this
report, several aspects of modern heat flow research are briefly
summarized and reviewed. These comments are then used as a basis
for a heat flow research project that would provide more definite
data on the geothermal power potential of the Black Rock Desert
area in northwestern Nevada.
Notation and Units @*
Table 1 summarizes the notation and units that are used in ,
this report. More complete discussions of the thermal parameters
and conventions may be found in Roy, Decker, Blackwell and Birch
(1968) and Decker (1969)
Heat Flow Determinations on Land
Determinations of heat flow require the measurement of
temperatures at known positions in underground openings (boreholes,
tunnels, nine shafts, etc.) and the measurement, usually in the
laboratory, of the thermal conductivity or representative samples
of rock from the same openings. Temperatures are frequently measured
with thermistor probes in combination with cables and three-and four- -@bt
\4'
Table 1, Sunmary of n o t a t i o n and u n i t s
Depth - v e r t i c a l depth i n meters ( m ) o r f e e t ( f t ) below the su r face of t h e ground; 1 meterZ3,28 f e e t o
Temperature - OC and OF
Gradient - OC/km, The lezs t - squares grad ien t r e f e r s t o t h e s lope of a leas t - squares s t r a i g h t line f i t t e d t o observed temperatures and depths, The numbers i n parentheses denote t h e depth i n t e r v a l s used.
Thermal Conductivity, K - millical/cm sec OC = loq3 cal/cm sec OC, The numbers i n parentheses i n d i c a t e t h e number of samples determining average conduct iv i tyo
Therm1 R e s i s t i v i t y , €3 - c m sec O C / c a l , The numbers i n parentheses i n d i c a t e t h e number of samples deternknlng average r e s i s t i v i t y ,
C p is s p e c i f i c hea t , andykn density. ';'his pasametler determines t h e changes of temperature w k t h time and dhstacce i n a body and is used i n time-dependent ( t r a n s i e n t ) h e a t conduction ca l cu la t ions ,
For i s o t r o p i c media R = 1 / K ,
Thernal Diffusivkty, k - cm*/seco' k = K/Cp.g where K is conduct iv i ty ,
0 Heat F ow or Heat F l x - 1,O HFU (Heat Flow Unit) = loo rnicrocal/
f low and h e a t f l u x are used interchangibly, In t he theory of h e a t conCuction, hea t f l o w refers t o t h e t r a n s f e r of h e a t per unFt t i m e , whereas heat f l u x r e f e r s t o t he t r a n s f e r of heat per u n i t a r e a 2nd in u n i t t ime,
- In geology, t h e terms heat cm 3 sec = 1,ox1o-z cal/cmZsec,,d
I
Prec i s ion Indices - Standard e r r o r s are used as measures of t h e i n t e r n a l consistency of t h e data, The e r rors are f o r 95% con2idence l i m i t s inclt ldfng a "Students ttl m u l t i p l i e r for (n-1) degrees of freedom,
2
0: lead compensated DC bridges and null-detectors (see Roy, Decker,
Blackwell and Birch, 1.968) . Thermal conductivities are normally measured using divided-bar systems (Birch, 1950). With these
systems and techniques, temperatures may be measured to within
~.C0loC, and thermal conductivities of individual samples can be
reproduced to within *2$.
The observed temperatures provide 8 measure of the uncorrected
gradient. Uncorrected values of heat flow combine the observed
temperatures and conductivities and are usually calculated by one
of three nethods, If the gradient profile is linear, the heat
flow may be calculated as the product of mean conductivity multiplied
by the least-squares gradient (GK); otherwise, the flux should be
calculated from the resistance integral (RI) (after Bullard, 1939,
p. 481) , or as the mean value of several heat flows determined over
I
I
@ several separate intervals of depth (I) ,, The results obtained after
these methods were applied to geothermal data collected in four
different drill holes are summarized in Table 2. The temperature-
I
: I depth profiles for these holes are shown in Figures 1 through 4. I
The gradient is very uniform i n DDH#CH3 at C e r r i l l o s , N e w Mexico
(Fig, 1) ; therefore, heat flow was calculated as the simple product
of least-squares gradient times mean conductivity. At DDH#l and
DDH#2 near Santa Rita, New Mexico and DDH#N-55 near White Pine,
Michigan, however, the gradients significantly varied with depth
and heat flows were calculated using t h e interval and the resistance
integralmethsds, respectively (see Figures 2, 3 and 4; also Table 2) , _.. '
The high precision (standard errors < +5%) of the calculations !id ..-.
Table 2. Xethods f o r heat f low calculationso
GK Y ~ t ~ h o d -
Qz = Gradient O K Q Cerrillos, N. Flex,
White P i m , Mich, DDH C H # 3 f? rJpf :Lj3 '
0 cm sec C C d .
m
,,
G r a d i m t : (90-280m) 24, 422°C/lim
Conductivit!:: 5, OLZ ( 4.2) ~ lO- - ' cz l
cm sec %
H e s t Flow: w
7 0 65 B O O B 80 58 9.08
Heat Flow = 1,040 2 .003 HFU
i+ ( a f t e r R.F. Roy, unpublished)
1
0 Table 2 continued
I Method
Z Q
Santa R i t a , N o Mexo
DDH #1 DDH #2
Depth Gradient Averzge Heat Depth Gradient Average Heat Conduc- Flow Conduc- Flow
m I_ OC t i v i t y m - OC t i v i t y km 10-3cal HFU km 10'3cal HFU
cm sec% CIR set%
26 22 20 2 4 22 20 28 22 22 22 22
Heat Flow = l.$:l HFU Corrected Heat Flow = 2 , 0 0 ~ , 0 2 HFU
Corrected Heat Flow = l . 8 k e l HFU
TEMPERATURE, OC
Figure 1, Temperature vso depth in. drill hole CH-3, Cerrillos, N e w Mexico (Decker, unpublished) e
, ..- . .
20
40
60
EO
120
v)
6- w 5 uo z
2a
2a
.
.
.
.
. ,
Figure 2** Temperature vso depth in drill hole #1, Santa Rita, New Mexico (Decker, unpublished)
cn G eiJ I- w z
r -r
I= a w c)
20
40
60
80
100
120
140
160
180
200
220
240
13 '11 IS 16 17 18 19 b I I I b I I 1
Figure 30h Temperature VS. depth in drill hole #2, Santa Rita, New Mexico (Decker, unpublished).
1
W I
I
I
I
. . I
'. I
Figure 4,, Temperature, gradient, conductivity, and heat flow vso depth in drill hole N-55, White Pine, Michigan ( RoF Roy, unpublished)
3
demonstrates that the temperature and conductivity data are internally Grs consistent at all four sites and clearly illustrates the need for
different calculation methods in the analyses of geothermal
measurements at different localities.
The geothermal data from Santa Rita, New Mexico also illustrate
the value of heat flow in the determination of deep temperature
and heat source distributions, In particular, the gradient in
DDH#2 systematically increases ( 34-58'C/km) with depth below 180
meters, but the conductivity varies inversely such that the vertical
component of flux is uniform (2.002.02 HFU) throughout the 180-to
240-meter interval (see Figure 3: also, Table 2). The gradient is
significantly lower ( 22,7f.2°C/km) in DDH#l about two miles distant,
but the "topographically correctedtt flux at this site is l . 8 o k l
@: HFU, a value close to that obtained in DDH#2. . The good agreement
between the heat flows obtained in the two different drill holes
shows that high and uniform regional flux is characteristic of the
Santa Rita areao
in the Santa Rita area, and if the inverse correlation between
temperatures and conductivity had not been observed in DDH#2,
it would have been reasonable to speculate that the increasing
and higher gradients in DDH#2 provided evidence for anomalous heat
If the regional flux were not known to be uniform
sources (hot waters, etc.) closer to the surface near this site.
Although modern techniques allow very precise (*lo$) determin-
ations of flux over short intervals of depth (20-50 meters) in
drill holes, the experience of much logging has shown that various
disturbances (climatic changes, culture, ground water circulation,
4
u' etc.) are likely to affect temperatures in the first few tens of
meters beneath the earth's surface. As a result, values of thermal
gradient representative of the heat flow from below are usually
not obtainable at depths of less than 100 meters. For example,
the highly irregular gradients in the upper portions of DDH#2 at
Santa Rita, New Mexico (Figure 3) and the drill holes near Hesperus
and Colorado Springs, Colorado (Figures 5 and 6) represent the disturbing effects of circulating waters that are entering the holes
at various locations. The heat flows calculated in these zones
are significantly different (A10 to &loof%, Decker, 1966 and 1971, unpublished) from those obtained in the lower, undisturbed intervals.
Tho effect of circulating water is more regular at the site near
Colorado Springs, Colorado (Figure 6), but the flux also is variable and inaccurate for a distance of 800 feet above the point (~2300 feet)
where water enters the hole,,
Figure 7 illustrates the effect of transient changes of culture and climate on near-surface temperatures. The linear portions
of the deep (below 100 meters, or so) temperature curves represent
thermal equilibrium established when the surface temperature was
different (higher and lower) throughout the areas . The curvature i n the shallower profiles, however, represents the adjustment of
near-surface temperatures to recent changes of mean surface
temperatures associated with climatic variation and changes of
vegetation or construction (see, for examples, Lachenbruch and
Marshall, 1969; Roy, Blackwell, and Decker, 1971, in press). If
the magnitudes and durations of the surface temperature fluctuations
I
0.
,
'
' I WELL TEMPERATURES ' F I REO CREEK ANTICLINE I
Figure 60 Temperature vso depth i n a d r i l l h o l e near Colorado Springs, Colorado (after Birch, 1947)
. , '
. .
I I t M P t K A L U K t ('C) I
I
!
; 5c
1OC
n
E U
150 I- Q w Q
200
' 250
300
9 10 1 1 - 12 13 14 15 I I I I I
I
. I
. .
. . .
. CAMBR/.DE, MASS o OBSERVED TEMPERATUUE 4
50 YEARS, TWO LAYERS,
GRAVEL BEDROCK k = 0.009 0.01 cm7sec K = 5.0 7.0 m c o k m secOC AT = 5°C
1 I I I 1 . (a )
TEMPERATURE ('C)
Figure 7. Effect of c u l t u r e and c l i m a t i c change on subsurface g rad ien t so (a ) Ef fec t of nearby )Q
bui ld ings a t s i t e i n Cambridge, Masso (from Royo Blackwell and Decker, 1971, i n press) , . Open - c i r c l e s are observed t empera tu resz 1 ~ 0 .
S o l i d squares zapqesent temper-. - .$E atures obtained a f t e r CO%re6tiOn
conduct lv i t iesd (b) Effec t of ~
shown dfffuslvities an& _.
c l ima t i c change a t t h r e e s i t e s 0
i n t he Alaskan Arc t i c (after Lachenbruch and Marshall , 1969) .*
I
sz
-
900.
i
I ( b )
I 1 - -
/
I I
5
--. are known in a quantitative sense, corrections (after Birch, 1948;
Roy, Blackwell, and Decker, 1971: also, Figure 7a) may be applied to the near-surface temperatures to obtain corrected values for
the geothermal gradient; otherwise, as is usually the case, any
attempts to use uncorrected nonlinear near-surface temperature
profiles would obviously lead to unreliable estimates for the
heat flow from below, or the true regional flux.
The disturbing affect of circulating ground water can be of
the transient type and, at drill sites where it is economically
feasible, may be greatly reduced or alleviated by grouting (AM-9,
or cement) casing in place at the termination of drilling,
technique has been used with good success in drill holes in the
New York-New England area (see, for example, Figure 8) , also could have been used to alleviate the water disturbance shown
in Figure 6.
culturally induced disturbances, it does reduce those related to
circulating water and should be done at all sites drilled specifically
for heat flow research,
This
Grouting
@ Although grouting does not remove climatically or
Like measurements of gravity at the earth's surface, observations
of underground temperature obtain their full significance only
after certain kinds of topographic reduction, In general, the
I
temperatures follow the topography such that the isothermal surfaces
are more widely separated beneath peaks than under valleys. As a
result, more heat is conducted out through the valley floors than
through the sides of adjacent h i l l s or mountains,
Birch (1950) (see also Bullard, 1938: Jeffreys, 1938: Carslaw @
50
100 P t
150 Q
200
!
i 250
7 TEMPERATURE, OC
8 9 10
.. . , . . . ~ _.
i . . -
11 12 13:
Figure 8, Temperatures. vso depth before and after grouting in drill hole at Londonderry, Vermont (from Roy, Blackwell and Decker, 1971, in press)o
.
and Jaegers 1959; Jaeger, 1965) has developed a method for
calculating the first order effects of two-and three-dimensional
topography,
Blackwell, and Birch, 1968, Table 5 ; Decker, 1969; Blackwell, 1969).
clearly demonstrate that uncorrected heat flows in deep (100-1000
H i s results, together with those of others (Roy, Decker,
meters) drill holes or tunnels may be 10-40$ different from those
obtained after correction for steady-state topography.
correction may be 2 5 O - 2 O O $ at localities where the depth of
The terrain
measurement (shallow drill holes) is small relative to the distance
to moderate relief (Lachenbruch, 1969)~ Thus the distorting effects of local topography must be considered at each temperature station,'
if regional heat flow surveys are to provide reliable quantitative
information on the subsurface temperature and heat source regimes.
Results of Recent Continental Heat Flow Studies @
and Birch, 1968; Lachenbruch, 1968; Roy, Blackwell and Decker,
1971, in press) suggest that the earth's thermal field may be
subdivided into "heat f l o w provincos,Il within which there are
local basement radioactivity (U, Th, K).
form
where Qs is the flux where radioactive heat generation is A,, Q0
is the heat flow where As = 0, and H, with the dimension of thickness,
These lines are of the
. Qs = Qo + ASH
is the slope of. the line, The intercept Q is considered to provide
a meaure of the heat from the lower crust and/or upper mantle,
and it can be analytically shown that the steady-state heat from
a layer with vertically uniform heat production As and thickness
H would be the excess flux (Qs- Qo = As H) at a given site.
A list of well-determined "heat flow provinces" and the
parameters of their respective heat flow - heat production l i f ies
is given in Table 3,
of the lines imply that crustal radioactivity has undergone extreme
upward differentiation and readily accounts for 1.0 to 1.5
YFU variations of heat flow at the surface, The significantly
different Qo values (.4 to 1,4 HFU), on the other harid, provide
the first reliable demonstration that heat flows from the lower
crust and upper mantle are the characteristic thermal parameters
The similar slopes (total range 5 to 10 km)
I .
_.
of the provinces,
uniform throughout
flux (109 -201 HFU)
Rocky Mountains is
anomalies at depth
of flux ( > 3 0 0 HFU)
Moreover, the intercepts (ao) appear to be. each province. Thus the average high surface
in the Basin and Range province and Southern
probably due to heat sources and temperature
(,H) beneath each region, The very high values
observed in these areas suggest that anonalous
heat sources and temperatures are locally closer to the surfaceo
Heat Flow Research Near Corder0
Mlnir,g Company8 s Hot Springs In Northwestern Nevada .- . . . _
-
Previous Results, Hadsell, Grose and Berry (1967) summarized flow
rates and mean water temperatures for the hot springs at Soldier
Meadows, Fly Rancho Gerlach and the Pinto Mountalns (Table 4) , and calculated heat flows in seven shallow (65-385 ft, deep) drill
holes at the Pinto Plountain Prospect (Figures 9-15). Their data
c
Table 3.’ Summary of hea t f low provinces . _ -
.Data a f t e r Roy, Blackwell and B i r c h (19681, Roy, Slackwell and Decker (1971, i n p r e s s ) , and Jaeger (1970) .
Average Range of Range of In t e rcep t - Flux -- Flux Radioact ivi ty Slope Province
km lO”6cal 10’6cal 10-6ca1 10-l3Ca1 3 cm2sec cm sec 2 cm see 2 c m sec
Continental U.S.
Basin and Range Province 9.4 1,4 109-2.1 106-3.7 3.0-10,o
Eas te rn U.S. 7.5 08 102 0 81-2 3 0 4-21 0 2
Sierra Nevada 1001 04 0 94 06-1.3 1e8-9.6
Southern Rocky Mtns. 10 1.2 1.9-2.1 1.5-3.7 3 0 ~ - 1 6 0 ~
A u s t r a l i a
Western 4.5 .6 1.6 7-2 o 3 1.2-21.9
Table 4;1Summary of flow rates and mean temperatures of
not spr ings in northwestern Nevada (from Badsell, Grose
and Berry, 1967)0
Locality
Soldier Meadows
Fly Ranch
Gerlach
Pinto Mountains
Flow Rate gal/min
2 8k2 -, .
. -
1472
253 142
-.
Temperature O F
. - __. __. .. .. . . .. . . , . .. . . . .. . .- ... . -..~ . . . .. . .. . .-.-" __ . ~. . ~- ~. ~..
- ._ - _.
TEMPERATURE I N O F
RO 130
c3'
8C
120
I60
9
24
I 280
i
320
360. -4;
400 I ~
Figure 9* Geothermal data from r o t r d r i l l hole i n the Pinto Mountains' (from Hadsel l , Grose and Berry, 19a67ye ;* ,
. '
r . , ..
_. . .
* ~-
( fFon Hadsell , Grose and Berry, 1967)o* -- -- ._ _-I-_.
-
TEMPERATURE I N OF
4c
I - - ,
80
120
I
I L
60 EO 100 ' 120 140
@-THERMOCOUPLE BE M P E R AT U R E
G-AVERAGE GEOTHERMAL
ASSUMED COMDUCTlVlTIES IN 10-3 cal/cm s e c OC AND STABILIZED BOTTOM HOLE TEMPERATURE IN O F . W.T-ASSUMED WATER TABLE
PROSPEC? MOLE
TEMPERATURE IN OF
6 0 80 IO0 120 I40
. .
ins
TEMPERATURE IN OF 4
* 40
80
I20
60 80 IO0 120 ,140
WEIGHTED AVERAGE TEM PE RAT U R E CURW E
TEMP E R AT U RE @-Y HERMOCOUPLE
G -AVERAGE GEOTHERMAL
HF-HEAT FLOW IN IO+ c a ~ / c r n ~ sec
ASSU RA ED CON DUCTIV ITiES
STABILIZED EOTTOM HOLE TEMPERATURE IN O F .
IN 10-3 cQi/Cm sec UC AND
PINTO M'OUNTAIMS
PROSPECT HOLE
- . . ., -.-. __ , . _^__I_.__^...__. ,. . ... _ ,___- -.- - .___ ___-. -
40
f "
80
W E I G HT E D AVER AG E TEMPERATURE CURVE,
o-THERMOCOUPLE I 2 O k TEMPERATURE
it"]!J&J+ G-AVERAGE GEOTHERMAL b'[;!iiy;,
1 . d LL I , GRADIENT I N "F/IOO FT
I N -r 3 '120 140
IN caI/crn sec OC AND STABILIZED BOTTOM HOLE TEMPERATURE I N O F .
W.T,-ASSUMED WATER TABLE
Figure 15, Geothermal data from z-otary drill !?.ole in t h e Pinto Mountains (from Hadsell, Grose and 5 m r y p 19670)
3
T
e ble 5.Base temp r a t u r e h e a t f l u x a t ho t springs
Heat Flux = Tf/A
i n northwestern Nevadao Method a f t e r White (1968)
= ( F / A ) ,xodoCp ( B t - St) where Tf = heat f low ( c a l / s e c ) , A is area, x = 63.1 c m h n , F = flow rate (gal/min), d = densi ty ( l o o ~ f t ! ) , c p = s p e c i f i c heat
(1.0 cal ) , V t = base temperature (OC), and S t = sur face temperature (OC) . gal sec em 3
- - g c ' _ _
. _ U
St Local i ty - Rate - Bt - gal/min OC OC
Flow
-
Pin to Mountains 142 48.9 12.2 142 71.1 12.2 142 87a2 12.2
J
F l y Ranch 14-72 58.9 1202
Gerlach 253 7g04 1202
S o l d i e r Meadows 2 842 43.3 12,2
Surface** T o t a l Area Heat Flow km2 105ca1
s ec
1.08 '3.3 1008 5.3 1.08 6.7
094 4304
a 02 10.7
4307 55.8
* A f t e r Hadsell , Grose and Berry (1967). ** Estimated from maps by Berry and Downs 1966a,b,c, 1969) + Normal Continental Flux = 105x10-6cal/~m h sec:
Average Basin and Range Flux = 2,OxlO" cal/cm2seco
Heat Flux
cmzsec 10-5ca1
2:; 6.2
46.4
535 103
Rat io of Calculated Flux t o Normal Cont inental and Basin and Range Flux.+
Normal Basin & Range
20.3 3z06 41.0
10
known and the surface temperatures can not be reliably
continued downward,
Recommendation for Future Research, Future evaluation of Cordero8 s
geothermal prospects should be directed toward the acquisition of
reliable heat flow data and a better knowledge of the deep subsurface
temperature regimes at each area,
futuro research are outlined below,
A few recommendations for
1) Since most of the springs crop out near or along obvious
or postulated faults or fault systemss the tempemtures
measured near and in them undoubtedly represent the end
result of a complex history of heat transfer between host
rock and circulating fluids migrating upward and along
fault planes or fractures.
transfer will depend upon the undisturbed regional gradient,
the flow rate of the fluids, the thermal diffusivfties of
The actual amount of heat
t h e host rocks, and the orientation of the fault planes o r
systems, but the general effect i s that hot waters from depth
lose heat to the lower temperature surroundings and the
temperatures measured in springs at the surface do provide
direct measures of the true magnitude of the underlying thermal
anomaly,’ Moreover, If the above mentioned parameters are
1, Seeo f o r example, Figure 6, 2300 and 2400 feet and flows to the surfaceo The flow rate (1 gal/min) is low enough such that the water l o ses heat to the rock during upward migration. If a fast flow occurred, the water temperature could be 83OP all of the way to the surfacea the low flow rateo
Water enters the hole between
The temperature at the surface is 20* lower f o r
11
known, the reduction of water temperature can be analytically
determined by approximating the springs and fault planes
as line, cylindrical or plane sources of heat (see Birch,
1947; Carslaw and Jaeger, 1959, Chandrasekhar, 1961; Levich, 1962; Jakob, 1957) Therefore additional exploration near
the hot spring areas should include more extensive gravity
and magnetic surveys to determine the subsurface geology and
the extent and orientation of folding and faulting at depth.
Considered with direct observations of regional flux, thermal
conductivity, and presently available water-flow rates (Table 4;
and Berry, personal communication, 1970 and 1971), detailed
analyses for the thermal effects of the local geology could
lead to better estimates for the highest subsurface temperatures
and the depths at which large quantities of dry steam might be
produced.
2 ) The thermal affects of local geology can be significant,
but many studies of hot springs indicate that the flows
a t the surface consist l a rge ly of meteoric water (Toulmin
and Clark, 1967; White, 1.967)~ As a result, the temperatures
measured in hot springs at the surface also may be abnormally
low due to mixing with lower temperature ground water, Thus
future exploration should: a) obtain reliable knowledge of
each regional -ground water regime; b) arrive at estimates
for the age and volumes, etc, of meteoric and primary waters;
and c) use background heat flow data to obtain estimates
for the temperatures of meteoric waters before mixing with
hotter waters from belowo
.,
The thermal consequences of a
12
mixing of meteoric and primary waters are complex, but may
be treated analytically if local hydrology, water ages, and
regional heat flow are known in a quantitative senseo Studies
of this type could be especially pertinent in Soldier
Meadows where the low temperatures of the springs (Table 4)
suggest quenching by colder near-surface waters.
This research would require chemical analyses and
additional geologic mapping, It would be desirable also
to conduct deep electrical surveys ( > lOOO', DC resistivity
and EM) in each area since changes in resistivity can
reflect the degree of saturation in subsurface rock (Keller
and Frischkneckt, 1966; Grant and West, 1965), Moreover,
electrical surveys provide information on subsurface structure
and subsurface temperature anomalies (Keller and ?rktchard, 1966).
Thus the electrical surveys could yield data on regional
hydrology, geologic structure, and subsurface temperature
models that could be compared with those based on heat flow
studfes to arrive at additional estimates for the depths at
which high temperatures might occuro
3) Deep rotary and core drilling to deternine reliable values
of heat flow near the hot springso The holes should be at
least 200 meters deep, penetrate competent rock units
(not just aiiuvi&) in. the bottoms of the holes, and be
continuously coredI(NX or BX size) for the bottom 30 to 50
meters, The holes should be cased and left accessible for
subsequent temperature loggings. The casing should be grouted
A
in place with cement or chemical grout (Am-9, American
Cyanamide CO:) to alleviate disturbances associated with
circulating water, Because the primary objective is to use
the heat flow measurements to determine the deep temperature
regimes and estimate the depths appropriate for the production
of dry steam, the holes should - not be drilled near individual
hot springs; temperatures close to the springs obviously
would be anomalous and'provide little information on. the ,
flux at depth,'
Although the final selection of hole locations would
require further consultations with Cordero's Geologists,
existing maps (Berry and Downs, 1966a, b, c, 1969) and a brief visit to each property in July, 1970 suggest the following numbers and distribution of sites:
A,. Pinto Mountain Hot Springs, Two drill holes would be
needede2 One should be drilled in the granodiorite
about half way between West and East Spring,
should be 3OOO-to 4000-feet west of the West Spring,
The other I
Secause the granodiorite#appears.to be compositionally
and texturally uniform, excellent background heat flow I
should be obtained at this site, The other site would
provide data away from the springs and detect deep
temperature anomalies associated with circulating waters , etc, along the north-south trending fault very near
West Spring, Since there is 3OO'-to 400 feet of relief
in the area, terrain corrections would be needed at / \
14
a l l s i t e s . The T e r t i a r y volcanics and sediments near the
su r face a l s o should be sampled f o r s t u d l e s of their
tnermnl p r o p e r t i e s , and t o determfne i f r e f r a c t i o n
.. .
- - .
models should be ca lcu la ted . Although only t w o sp r ing
systems are evident a t t h e P i n t o iYountnin prospec t , 'the
r e l a t i v e l y simple geology and exposures, t h e h igh
temperatures of t h e spr ings (Table 4). and t h e e a r l i e r
shallow d r i l l i n g (F igures 9-15) make it an e x c e l l e n t
t e s t a rea .
B.' F l y Ranch Hot Springs, Two holes should be d r i l l e d -
i n t h e high r e s i s t i v i t y l a y e r ( 3 5 ohm-meters) t h a t
K e l l e r and P r i t c h a r d (1966) mapped w i t h electromagnetic
and DC r e s i s t i v i t y surveys. T h i s u n i t should provide
t h e b e s t deep geothermal data because it appears t o
have low poros i ty and reduced water conten t ; tnus t h e r e
should be fewer t r a n s i e n t d i s turbances due t o c i r c u l a t i n g
water, I t would be d e s i r a b l e t o have data on t h e
downthrown s i d e of t h e f a u l t bordering t h e e a s t s i d e of
t h e sp r ing system, bu t a hea t flow s i t e i n t h i s area
should no t be s e l e c t e d without b e t t e r es t imates for
t h e depth t o basement,.
C o s Gerlach Hot Springs, One d r i l l ho le i n the g ranod io r i t e
c roping o u t t o t h e north and west of Mud and Great Bo i l ing
Springs, r e spec t ive ly , should provide an e x c e l l e n t va lue
f o r t h e undisturbed flux, A t e r r a i n c o r r e c t i o n would . - .
_ - be needed a t t h i s s i t e , A s i t e 2000-to 3000-feet e a s t
of Great Boi l ing Spr ings should provide data on the
underlying, deep temperature anomalyo D r i l l i n g i n
t h e a l l u v i a l va l l ey should be delayed u n t i l geophysical
techniques a r e employed t o determine the subsurface geology.
Do Soldier Mendows. Because of t he large areal ex ten t ,
t h e l a r g e t o t a l flow r a t e (Table Lk), and t h e low temperatures
of t h e sp r ings (Table 4) , thorough research should be
conducted t o determine the deep f l u x i n t h i s a r e a o A very
comprehensive survey would r e q u i r e t h e d r i l l i n g of f i v e
ho le s a t t h e fol lowing rough l o c a t i o n s ( s e e Berry and
Downso 1969): t h e western boundary of S27, T4ON, R24E;
t h e northwest quarter of S25, T40N, R24E; t h e c e n t r a l
po r t ion of Sl3, TLCON, R24E: t h e northwest q u a r t e r of
Sl9, TbON, R25E; and t h e e a s t e r n po r t ions of S17 o r S6
(Sl& Gul l ion Creek) , TMN, R25E.
would r e q u i r e f o u r d r i l l holes, w i t h t h e s i t e i n Sl3,
A less thorough survey
TltON, R24E being de le t ed , and t h e l o c a t i o n on Sl9, T4ON,
R2JE being s u b s t i t u t e d f o r by a site roughly halfvaay
between Springs E and F and B and C. I n e i t h e r case,
t h e s i t e s off t he e a s t e r n and western boundaries of t h e
Meadows should provide hea t flow data f o r t he a r e a s away
from t h e spr ingsd Those i n t h e Meadows should l e a d t o
accu ra t e es t imates for t h e magnitudes of t h e underlying
thermai-anomaly.* A study of t h e ha l f -wid ths , etc,, of
. . . I
\
t h e h e a t flow anomaly a c r o s s t h e meadows would provide
data on t h e depths t o anomalously high subsurface
temperatures:
16
@ Cost f o r F u t u r e Resezrc.,. The approximate expense for c:ravity and
deep electrical surveys i n each area are summarized in the excellent I
report by Keller and Pritchard (1966, pa 12-13) With the
exception of t he proposed shallow drilling, it is my opinion that
their proposals have great merit and could be of special value
at the Gerlach and Soldier Meadows prospects, where more subsurface
I
I
control (geologic, hydrologic, and temperature) is needed, I also
believe that the final evaluation of a l l of the hot spring areas
should include a comparison of subsurface temperature models
arrived at f rom heat f low and resistivity surveys.
Heat flow measursmnts would involve expenses f o r contract
drilling, casing and grouting, preparation of thermal conductivity
samples, field support f o r temperature logging, ar,d costs for data
@ reduction and interpretation,\ The average expenses that would be
Gus
incurred if the work were done using equipmen% and personnel at the
University of Wyoming as summarized below:
Drilling, Casing and Grouting. Rotary and core drilling is
estimted at $7 to $8 per linear foot of drill hole. whereas experience shows that casing (with l i l t black iron pipe) ar,d
grouting costs about $.'75 per foot of hole, including materials
and rig-time,. Therefore a 200 meter hole would cost $4900
to $5600, A lower cost might be incurred at Soldier Meadows,
if four to five holes were drilled,*
Thermal Conductivity.' Commercial grinding and edging of
conductivity samples averages about $' j0o0 to $30450 per sample,
Approximately 150 to 200 samples would be needed at a total
cost of $525 to $7500:
is not included, because our laboratory could handle 20-to
A cost for laboratory measurements
30-additional samples per day without interrupting the usual
routine;
F i e l d Support for Temperature Measurements. A 200 meter hole
may be readily logged in two hours using portable equipment;
Therefore one day, including travel from the nearest base,
would be required to measure temperatures at any prospect.
Two loggings on differer.t days, separated by a month, or so,
would be needed. Excluding costs for air travel, etc, to
and from the nearest base, temperature logging would cost
about $100/day plus a $15-$16 per d l e n per man (one or two),
If temperatures were logged with equipment already in existence, ..
no cost for equipment would be incurred; new equipment would
cost $2000 to $2500 for cable, (teflon insulated), thermistors,
construction ar,d calibration of probes, and DC circuitry for
resistance measurements at the surface (bridges and null-
detectors).
Intermetation, This would involve data reduction and
interpretation, and the preparation of reports. Since most
of the necessary computer programs are in existence, only
6 to 7 man-days would be needed for rather complete inter-
pretations in- each areao The rr,aximum cost for interpretation
at each prospect is estimated to be $800, including computer
time,
18
The above discussion demonstrates that heat flow measurements crs at Cordero*s Geothermal Power prospects would be costly, For
example,the proposed, reszarch at all four prospects
would cost $60,000 to $70,000; However, without reliable h e a t
flow datr; and hence more reliable subsurface tenperature modelsp
drilling for geothermal power in these areas would be very
speculative, as is strongly suggested by the deep (lOOO-5000 ft.),
non-producing holes near the anomalous springs at Beowafve and /
Bradie Hot Spr ings , Nevadao*
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Blackwell , D.D., Heat flow determinations i n t h e northwestern United S t a t e s , J. Geophys, Res,, 74, 992-1007, 1969.
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20
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