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My. J. E:.
J U R ~ 23,
Gilbert
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
E x p l o r a t i o n f o r Geothermal Power
i n the
Black Rock D e s e r t , Nevada
A Report Submitted
to
Dr.' George W. Berry, Corder0 Mining Company
Edward R.' Decker
Assistant Professor
Depte O f Geology
U n i v e r s i t y of Wyoming
Table of Contents
Introduction.......
.......................................
.....................................
Land ..........................
Page
1
N o t at i o n and Units....
1
Heat Flow Det erm inations on
1
R e s u l t s of Recent C o n t i n e n t a l Heat Flow S t u d i e s .
6
Heat Flow Research Near Corder0 Mining Company's
Hot S p r i n g s i n Northwestern Nevada
7
..........
.......................
................................. 7
.......
f o r Future
........................
16
................................................' 1 9
Previous R e s u l t s
Recommendation f o r F u t u r e Research.......
Cost
References
ReseaYeh
10
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, n i n e 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'
T a b l e 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 meter s ( m ) o r f e e t ( f t ) below t h e
s u r f a c e of t h e ground;
1 meterZ3,28 f e e t o
Temperature
-
-
OC
and OF
OC/km,
The l e z s t - s q u a r e s g r a d i e n t r e f e r s t o t h e s l o p e
Grad i en t
of a l e a s t - s q u a r e s s t r a i g h t line f i t t e d t o observed
t e m p e ratures and depths, The numbers i n p a r e n t h e s e s denot e
t h e depth i n t e r v a l s used.
-
Thermal C onduct ivity, K
millical/cm sec OC = loq3 cal/cm sec
The numbers i n p a r e n t h e s e s i n d i c a t e t h e number of samples
d e t ermining average c o n d u c t i v i t y o
OC,
-
For i s o t r o p i c media R = 1 / K ,
T h e r m 1 R e s i s t i v i t y , €3
c m sec O C / c a l ,
T h e numbers i n p a r e n t h e s e s i n d i c a t e t h e number of samples
d e t ernknlng average r e s i s t i v i t y ,
-
0
T h e rn al D i f f u s i v k t y , k
cm*/seco' k = K/Cp.g where K is c o n d u c t i v i t y ,
C p is s p e c i f i c h e a t , andykn density. ';'his pasametler determines
t h e changes of temperatur e 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 c a l c u l a t i o n s ,
-
Heat F ow or Heat F l x
1,O HFU (Heat Flow U n i t ) = l o o rnicrocal/
cm sec = 1,ox1o-z cal/cmZsec,,d
- In geology, t h e terms h e a t
f l o w and h e a t f l u x a r e used i n t e r c h a n g i b l y ,
In t h e theor y of
h e a t conCuction, h e a t f l o w r e f e r s t o t h e t r a n s f e r of h e a t p e r
unFt t i m e , whereas heat f l u x r e f e r s t o t h e 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 time,
3
-
P r e c i s i o n Indices
S t andar d e r r o r s a r e used as measures of t h e
i n t e r n a l consistency of t h e data, The e r r o r s a r e f o r
95% con2idence l i m i t s i ncltldf ng a "Students ttl m u l t i p l i e r
for (n-1) degrees of freedom,
I
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
~ . C 0 l o C , and thermal conductivities of individual samples can be
reproduced to within *2$.
The observed temperatures provide
gradient.
8
measure of the uncorrected
Uncorrected values of heat f l o w 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,
I
I
@
p. 481), or as the mean value of several heat flows determined over
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-
depth profiles for these holes are shown in Figures 1 through 4.
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 ) ,
!id
..-.
The h i g h precision (standard errors < +5%) of the calculations
I
:
I
I
Table 2. Xethods f o r heat f l o w calculationso
GK Y ~ t ~ h o d
Qz = G r a d i e n t
O
-
Q
K
Cerrillos, N. Flex,
DDH C H # 3
White P i m , Mich,
f?rJpf:Lj3
m
'
0
cm sec
C
Cd.
(90-280m)
24, 422°C/lim
Gradimt:
7 065
BOOB
80 58
9.08
Conductivit!::
5, O L Z ( 4.2) ~ l O - - ' c z l
cm s e c %
,,
w
H e s t Flow:
Heat Flow = 1,040 2 .003 HFU
( a f t e r R.F.
i+
Roy, unpublished)
1
0
Table 2 continued
I Method
QZ
S a n t a R i t a , N o Mexo
DDH #1
Depth
m
Gra dient
OC
I
_
km
Averzge
Conductivity
10-3cal
cm sec%
DDH #2
Heat
Flow
Depth
m
HFU
Gr adient
OC
km
Average
Conductivity
10'3cal
CIR
set%
Heat
Flow
HFU
26
22
20
24
22
20
28
22
22
22
22
Heat Flow = l.$:l
Cor r ected Heat Flow = 2 , 0 0 ~ , 0 2 HFU
HFU
C o rre c t ed Heat Flow =
l . 8 k e l HFU
TEMPERATURE, OC
e
Figure 1, Temperature vso depth in.drill hole CH-3, Cerrillos,
N e w Mexico (Decker, unpublished)
..-
,
. .
20
40
.
60
EO
.
.
120
v)
56- uo
.
.
,
w
z
2a
2a
Figure 2**Temperature v s o depth in drill h o l e #1, Santa Rita,
New Mexico (Decker, unpublished)
13
b
'11
I
IS
16
17
18
19
I
I
b
I
I
20
40
60
80
cn
G
eiJ
I-
w
z
100
120
r
-r
I=
a 140
w
c)
160
180
200
220
240
Figure 30hTemperature VS. depth in drill h o l e #2,
Santa Rita, New Mexico (Decker, unpublished).
1
W
I
1
I
I
.
.
I
'. I
I
Figure 4,, Temperature, gradient, conductivity, and
heat flow v s o depth in drill hole N-55, White Pine,
Michigan ( RoF Roy, unpublished)
3
Grs
demonstrates that the temperature and conductivity data are internally
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).
significantly lower ( 22,7f.2°C/km)
The gradient is
in DDH#l about two miles distant,
but the "topographically correctedttflux 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
If the regional flux were not known to be uniform
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
sources (hot waters, etc.) closer to the surface near this site.
Although modern techniques allow very precise (*lo$) determinations 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 ( ~ 2 3 0 0feet)
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
t h e r m a l equilibrium established when the surface temperature was
. The
different (higher and lower) throughout the areas
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.
,
'
'
'F
I
I
WELL TEMPERATURES
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 t M P t K A L U K t ('C)
I
I
9
10
I
12
11I
14
1I3
I
15
I
I
!
;
5c
.
I
. .
1OC
.
..
n
E
U
150
I-
I
Q
w
Q
200
. CAMBR/.DE,
MASS
o OBSERVED TEMPERATUUE
4
50 YEARS, TWO LAYERS,
'
250
300
GRAVEL
k = 0.009
K = 5.0
AT = 5°C
1
BEDROCK
0.01 cm7sec
7.0 m c o k m secOC
I
I
1
I
(a)
.
TEMPERATURE ('C)
F i g u r e 7. Effect of c u l t u r e and
c l i m a t i c change on s u b s u r f a c e
)Q
g r a d i e n t s o ( a ) E f f e c t of nearby
b u i l d i n g s a t s i t e i n Cambridge,
Masso (from Royo Blackwell and
Decker, 1971, i n p r e s s ) , . Open
c i r c l e s are observed t e m p e r a t u r e s z 1 ~ 0
S o l i d squares zapqesent temper-. - . $szE
atures obtained a f t e r CO%re6tiOn
shown dfffuslvities an&
_.
c o n d u c t l v i t i e s d (b) E f f e c t of
c l i m a t i c change a t t h r e e s i t e s
0
i n t h e Alaskan A r c t i c (after
Lachenbruch and Marshal l, 1969)
I
.
-
~
.*
900.
i
(b)
I
/
I
I
1-
-
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, This
technique has been used with good success in drill holes in the
New York-New England area (see, for example, Figure 8), Grouting
@
also could have been used to alleviate the water disturbance shown
in Figure 6. Although grouting does not remove climatically or
culturally induced disturbances, it does reduce those related to
circulating water and should be done at all sites drilled specifically
for heat flow research,
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
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
I
i
. . -
8
7
TEMPERATURE, OC
9
10
11
12
50
100
P
t
Q
150
200
!
i
250
..
.,
.
. . ~_.
Figure 8, Temperatures. v s o depth before and after grouting in
drill h o l e at Londonderry, Vermont (from Roy, Blackwell and
Decker, 1971, in press)o
.
13:
and Jaegers 1959; Jaeger, 1965) has developed a method for
calculating the first order effects of two-and three-dimensional
topography, H i s results, together with those of others (Roy, Decker,
Blackwell, and Birch, 1968, Table 5 ; Decker, 1969; Blackwell, 1969).
clearly demonstrate that uncorrected heat flows in deep (100-1000
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 $
The terrain
at localities where the depth of
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).
These lines are of the
form
.
Qs = Qo + ASH
where Qs is the flux where radioactive heat generation is A,,
Q0
is the heat flow where A s = 0, and H, with the dimension of thickness,
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
is given in Table 3,
- heat production l i f i e s
The similar slopes (total range 5 to 10 km)
of the lines imply that crustal radioactivity has undergone extreme
I
.
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
of the provinces,
Moreover, the intercepts
uniform throughout each province.
(ao) appear
to be.
Thus the average high surface
flux ( 1 0 9 - 2 0 1 HFU) in the Basin and Range province and Southern
Rocky Mountains is probably due to heat sources and temperature
anomalies at depth (,H) beneath each region, The very high values
of flux ( > 3 0 0 HFU) 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 h e a t f low pr ovinces
.
_-
.
.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 J a e g e r (1970)
Pro v i nce
Slope
km
Intercept
lO”6cal
c m2 s e c
Average
Flux
Range of
Flux
10’6cal
10-6ca1
cm2 see
cm2sec
-
--
Range of
Radioactivity
10-l3Ca1
cm3 sec
C o n t i n e n t a l U.S.
Basin and Range Province
9.4
1,4
E a s t e r n U.S.
7.5
08
102
Sierra Nevada
1001
04
0
So u t h ern Rocky Mtns.
10
109-2.1
94
1.2
1.9-2.1
.6
1.6
106-3.7
0
3
81-2
06-1.3
3.0-10,o
0
4-21 2
0
1e8-9.6
30~-160~
1.5-3.7
Australia
Western
4.5
7-2
o
3
1.2-21.9
Table 4;1Summaryof flow rates and mean temperatures of
not s p r i n g s in northwestern Nevada (from Badsell, Grose
and Berry, 1967)0
Temperature
Flow Rate
Locality
gal/min
OF
-, .
Soldier Meadows
.
-
2 8k2
1472
Fly Ranch
Gerlach
253
Pinto Mountains
142
-.
.
- __.
__.
..
.. .
.
.. .
.
,
. .. .
..
...
.- ...
. -..~
.
.. ..
.
. .
.-.-"
__
.
~-
~.
.
~.
~..
-
_ .
TEMPERATURE
RO
-
IN
._
O F
130
c3'
8C
120
I60
9
24
I
280
i
320
360.
-4;
400
I
F i g u r e 9 * Geothermal data from r o t r d r i l l hole i n the Pinto Mountains'
(from H a d s e l l , Grose and Berry, 19a67ye
;*
~
,
.
'
.
,r .
_.. .
~-
( f F o n H a d s e l l , Grose and Berry, 1967)o*
--- ._ _-I-_.
*
-
TEMPERATURE
60
EO
100
IN
OF
'
120
140
4c
I
--,
80
120
I
I
L
@-THERMOCOUPLE
BE M P ER AT UR 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
60
80
IO0
IN
OF
120
I40
..
ins
IN
TEMPERATURE
4
*
60
80
OF
120
IO0
,140
40
80
WEIGHTED AVERAGE
TEM PE RAT UR E CURW E
@-Y HERMOCOUPLE
TEMP E R AT U RE
G -AVERAGE GEOTHERMAL
I20
HF-HEAT FLOW IN
IO+ c a ~ / c r nsec
~
ASSU RA ED CON DUCTIV ITiES
IN 10-3 cQi/Cm s e c UC AND
STABILIZED EOTTOM HOLE
TEMPERATURE IN O F .
PINTO
M'OUNTAIMS
PROSPECT
-.-
-
___-.
.___
-
-.
. .,
HOLE
-.-.
__
,
.
_^__I_.__^...__.
,.
. ...
_,___-
IN
3
40
-r
'120
140
f"
80
W E I G HT E D AVER AG E
TEMPERATURE CURVE,
I2Ok
o-THERMOCOUPLE
TEMPERATURE
G-AVERAGE GEOTHERMAL
GRADIENT I N "F/IOO F T
b'[;!iiy;,
it"]!J&J+
1 . d
LL
I ,
IN
caI/crn s e c OC AND
STABILIZED BOTTOM HOLE
TEMPERATURE I N O F .
W.T,-ASSUMED WATER TABLE
F i g u r e 15, Geothermal data f r o m z-otary drill !?.ole in t h e Pinto Mountains
(from Hadsell, Grose and 5 m r y p 19670)
3
e
T b l e 5.Base
temp r a t u r e h e a t f l u x a t h o t springs
i n northwestern Nevadao Method a f t e r White (1968)
-
Heat Flux = Tf/A = ( F / A ) ,xodoCp
(Bt
St) where Tf = h e a t flow ( c a l / s e c ) , A is
area, x = 63.1 c m h n , F = flow r a t e (gal/min), d = d e n s i t y ( l o o ~ f t !) , c p = s p e c i f i c h e a t
gal s e c
em 3
(1.0 cal ) , V t = base temperature (OC), and S t = s u r f a c e temperature (OC)
- - g c'
_ _
._
Locality
Flow
Rate
gal/min
-
U
Bt
-
OC
OC
St
Surface** T o t a l
Area
Heat Flow
km2
105ca1
s ec
-
P i n t o Mountains
142
48.9
142
71.1
87a2
142
12.2
12.2
12.2
1.08
1008
1.08
R a t i o of Calculated Flux
t o Normal C o n t i n e n t a l
and Basin and Range Flux.+
Heat Flux
10-5ca1
cmzsec
5.3
6.7
26:.;2
4304
46.4
'3.3
J
F l y Ranch
Gerlach
S o l d i e r Meadows
14-72
253
2 842
58.9
1202
094
7g04
1202
a
43.3
12,2
02
10.7
535
4307
55.8
103
* A f t e r Had s ell, Grose and Berry (1967).
**
E s t i m a t ed from maps by Berry and Downs 1966a,b,c,
+ Normal
C o n t i n e n t a l Flux = 105x10- 6cal/~m
hsec:
Average Basin and Range F l ux = 2,OxlO" cal/cm2seco
1969)
.
Normal
20.3
3z06
41.0
Basin & Range
10
known and the surface temperatures can not be reliably
continued downward,
Recommendation for Future Research, Future evaluation of Cordero8s
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, A few recommendations for
futuro research are outlined below,
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.
The actual amount of heat
transfer will depend upon the undisturbed regional gradient,
the flow rate of the fluids, the thermal diffusivfties of
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 a r e
1, Seeo f o r example, Figure
6, Water enters the hole between
2300 and 2400 feet and flows to the surfaceo The flow rate
(1 gal/min) is low enough such that the water l o s e s 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 temperature at the surface is 20* lower f o r
the low flow rateo
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 m i g h t be
produced.
2 ) The thermal affects of local geology can be significant,
but many studies of hot s p r i n g s indicate that the flows
at
the surface consist l a r g e l y of meteoric water ( T o u l m i n
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 w i t h lower temperature ground water,
Thus
future exploration should: a) obtain reliable knowledge of
.,
each regional -groundwater 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',
and EM) in each area
DC resistivity
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
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
A
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, The other
I
should be 3OOO-to 4000-feet west of the West Spring,
Secause the granodiorite#appears.tobe compositionally
and texturally uniform, excellent background heat flow
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'-to400 feet of relief
in the area, terrain corrections would be needed at
/
\
I
14
a l l sites.
The T e r t i a r y v o l c a n i c s and sediments n e a r t h e
.. .
s u r f a c e 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 c a l c u l a t e d .
Although only t w o s p r i n g
s y s t e m s are e v i d e n t a t t h e P i n t o iYountnin p r o s p e c t , 'the
r e l a t i v e l y simple geology and exposures, t h e h i g h
temperat ures of t h e s p r i n g s ( T a b l e 4). and t h e e a r l i e r
shallow d r i l l i n g ( F i g u r e s 9-15) make i t an e x c e l l e n t
t e s t area.
B.' F l y Ranch Hot Springs,
Two h o l e s should be d r i l l e d
-
i n t h e h i g h 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 electr omagnetic
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 i t a p p e a r s t o
have low p o r o s i t y and reduced water c o n t e n t ; t n u s t h e r e
should be fewer t r a n s i e n t d i s t u r b a n c e s 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 b o r d e r i n g t h e e a s t s i d e of
t h e s p r i n g system, b u t a h e a t flow s i t e i n t h i s area
should n o t be s e l e c t e d without b e t t e r e s t i m a t e s for
t h e depth t o basement,.
C o s Gerlach
Hot S p r i n g s , One d r i l l h o l e i n t h e g r a n o d i o r i t e
c r o p i n g o u t t o t h e n o r t h and west of Mud and Gr eat B o i l i n g
S p r i n g s , r e s p e c t i v e l y , should pr ovide a n e x c e l l e n t v a l u e
. -
.
f o r t h e undist urbe d flux,
_-
be needed a t t h i s s i t e ,
A t e r r a i n c o r r e c t i o n would
A s i t e 2000-to 3000-feet e a s t
of Great B o i l i n g S p r i n g s should pr ovide data on t h e
underl ying, deep temperature anomalyo D r i l l i n g i n
t h e a l l u v i a l v a l l e y should be delayed u n t i l geophysical
t echniques a r e employed t o determine t h e subsur f ace geology.
D o Soldier Mendows.
Because of t h e l a r g e areal e x t e n t ,
t h e l a r g e t o t a l flow r a t e ( T a b l e
and t h e low temperatures
Lk),
of t h e s p r i n g s ( T a b l e 4) , thorough r e s e a r c h should be
conducted t o determine t h e 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
h o l e s a t t h e f o l l o w i n g 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
p o r t i o n 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 p o r t i o n s of S17 o r S6
(Sl&
G u l l i o n C r e e k ) , TMN, R25E.
A less thorough survey
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,
TltON, R24E bei ng d e l e t e d , and t h e l o c a t i o n on Sl9, T4ON,
.
I
R2JE b e i n g s u b s t i t u t e d f o r by a site r o u g h l y halfvaay
between S p r i n g s 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 h e e a s t e r n and wester n boundaries of t h e
Meadows should provide h e a t flow data f o r t h e a r e a s away
from t h e s p r i n g s d
Those i n t h e Meadows should l e a d t o
\
a c c u r a t e e s t i m a t e s for t h e magnitudes of t h e u n d e r l y i n g
thermai-anomaly.* A s t u d y of t h e h a l f - w i d t h s ,
etc,, of
t h e h e a t flow anomaly a c r o s s t h e meadows would pr ovide
data on t h e depths
temperatures:
t o anomalously
high subsurface
.
.
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
report by Keller and Pritchard (1966, p a 12-13)
With the
I
exception of t h e 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
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 r o m heat f l o w 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
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 $ ' j 0 o 0 to $30450per sample,
Gus
I
Approximately 150 to 200 samples would be needed at a total
I
cost of $525 to $7500: A cost for laboratory measurements
is not included, because our laboratory could handle 20-to
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 s o ,
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 nulldetectors).
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 interpretations in-each areao The rr,aximum cost for interpretation
at each prospect is estimated to be $800, including computer
time,
18
crs
The above discussion demonstrates that heat flow measurements
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, a s is strongly suggested by the deep (lOOO-5000 ft.),
/
non-producing holes near the anomalous springs at Beowafve and
Bradie Hot S p r i n g s , Nevadao*
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Geologic map of P i n t o Mountaim Hot Spr ing2 Area,
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Nevada, August 25, 1 9 6 6 ~ .
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20
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1
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e
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.--..
-. .
,
.