r
REPORT ON THE
RECONNAISSANCE DIPOLE-DIPOLE
RESISTIVITY SURVEY
IN THE
SODA LAKE AREA
CHURCHILL COUNTY, NEVADA
FOR
CHEVRON OIL COMPANY
r
'7y-'^sy^py-v.vi<;w^?.TgnT7?yy,!ffg^-y?p';yyy^
McPHAR GEOPHYSICS
(
NOTES ON GEOTHERMAL EXPLORATION
USING THE RESISTIVITY METHOD
Many geophysical m e t h o d s have been t r i e d in the exploration for
g e o t h e r m a l l y "hot" a r e a s in the upper r e g i o n s of the e a r t h ' s c r u s t .
The
only method that h a s been c o n s i s t e n t l y fovind to b e s u c c e s s f u l has b e e n the
r e s i s t i v i t y technique.
In this geophysical method, the specific r e s i s t i v i t y
(or its r e c i p r o c a l , the specific conductivity) of the e a r t h ' s s u b s u r f a c e i s
m e a s u r e d during t r a v e r s e s over the s u r f a c e .
The p r i n c i p l e of the technique is b a s e d on the fact that the r e s i s t i v i t y
f
of s o l u t i o n - s a t u r a t e d r o c k s will d e c r e a s e as the salinity of the solutions i s
i n c r e a s e d a n d / o r the t e m p e r a t u r e of the s y s t e m is i n c r e a s e d (see F i g u r e 1).
T h e r e f o r e , v o l u m e s of the e a r t h ' s c r u s t that contain a b n o r m a l l y hot and s a l i n e
solutions can often b e detected a s r e g i o n s of low r e s i s t i v i t y .
The r e s i s t i v i t y m e a s u r e m e n t s a r e u s u a l l y m a d e using grounded c u r r e n t
and potential e l e c t r o d e s , but s o m e usefvil data can s o m e t i m e s b e obtained using
electromagnetic techniques.
The field data shown on plan m a p s in F i g u r e 2 a r e
f r o m the B r o a d l a n d s A r e a in New Z e a l a n d ; in t h i s a r e a t h e r e a r e s u b s t a n t i a l
flows of hot w a t e r and s t e a m a t t h e s u r f a c e .
The resvilts show r e s i s t i v i t y lows m e a s u r e d with a Wenner Configuration
R e s i s t i v i t y S u r v e y and a loop-loop e l e c t r o m a g n e t i c s u r v e y .
c
The anomalous
p a t t e r n is m u c h the s a m e in both c a s e s and the r e g i o n s of low r e s i s t i v i t y c o r r e l a t e well with the a r e a s of i n c r e a s e d r o c k t e m p e r a t u r e .
SALINITY, parts per million
RESISTIVITY, ohm-meters
VARIATIONS OF SOLUTION RESISTIVITY
WITH TEMPERATURE AND SALINITY
FIG. I
GEOPHYSICAL SURVEY
BROAOUMOS AREA, W W Z E A U U C
A, TEMPERATURE AT S m DEPTH
a
APPARENT RESOTlvmr SURVEY USING
WENNER CONFIGURATION A ' l S O m ,
C APPARENT RESJSnviTY SURVEY USMG
LOOP TO UDOP ELGCTROMAGNETV: METHOD
FI0.2
**-
- 2 -
If the r o c k volume s a t u r a t e d with hot solutions does not extend to
the s u r f a c e it will be n e c e s s a r y to use l a r g e e l e c t r o d e i n t e r v a l s to d e t e c t
the r e s i s t i v i t y l o w s .
The r e s i s t i v i t y data shown in " p s e u d o - s e c t i o n " f o r m
in F i g u r e 3 is from. J a v a .
Along this line t h e r e a r e two deep r e g i o n s of low
r e s i s t i v i t y detected for the l a r g e r e l e c t r o d e i n t e r v a l s u s e d .
Zone A is
a s s o c i a t e d with s u r f a c e rnanifestations of g e o t h e r m a l activity.
The s o u r c e
of the r e s i s t i v i t y low at Zone B i s unknown.
If the a b n o r m a l l y hot region o c c u r s in a s e d i m e n t a r y b a s i n , the
g e n e r a l r e s i s t i v i t y level can b e quite low, due to the high p o r o s i t y in n o r m a l
sediments.
This is the c a s e in the I m p e r i a l Valley of California.
The r e s i s t i -
vities shown in F i g u r e 4 a r e f r o m an a r e a n e a r E l C e n t r e , California.
The
l a r g e s t e l e c t r o d e s e p a r a t i o n used was 12,000 feet.
The r e s u l t s show a t w o - l a y e r g e o m e t r y with t h e upper l a y e r having
a thickness of a p p r o x i m a t e l y one-half e l e c t r o d e i n t e r v a l (i. e. 1,000 feet).
The r e s i s t i v i t y in the upper l a y e r is 3. 0 o h m - m e t e r s ; the r e s i s t i v i t y of the
lower l a y e r is 1. 5 o h m - m e t e r s .
Due to the s m a l l r e s i s t i v i t y c o n t r a s t ,
additional m e a s u r e m e n t s would b e n e c e s s a r y to d e t e r m i n e the p o s s i b l e
g e o t h e r m a l i m p o r t a n c e of the lower r e s i s t i v i t y l a y e r at depth.
The r e s u l t s shown in F i g u r e 4 a r e from a dipole-dipole e l e c t r o d e configuration s u r v e y .
Our dipole-dipole data is plotted a s a " p s e u d o - s e c t i o n " for
s e v e r a l v a l u e s of n; the s e p a r a t i o n between the c u r r e n t e l e c t r o d e s and potential
e l e c t r o d e s , as well as the location of the e l e c t r o d e s along the s u r v e y l i n e ,
d e t e r m i n e the position of the plotting point.
The t w o - d i m e n s i o n a l a r r a y of
APPAPENT IC9STMTY S K ^ Y , CENO PLATEAU AREA, JAVA, tOOCaA
pMy* SwMw PMUniMteho* ttan DUU »*m Rw<
r^^>-~i
r4=n
r^^'—i
ZONE "A"
ZONE "B"
KLCCT.OM COiirMi«UtiOM . M nATTM. KWTt
,^1
. *n 1 *^ —. < ; ) —
I I '
\
1
'
; ,"
% A
nSTTMMmS
\v
nas
m
RESISTIVITY SURVEY, IMPERIAL VALLEY-CALIFORNIA.
LINE-"0", FREQUENCY-0I25HI,
2?W
ZpW
IfW
iy>
24W
I4W
(^)o-ohffl m t t r t t
t t
t t
It
l-«
t .
t t
i-r
n
l'«
It
1-4
i-«
It
••>
Oi^OtC-Di.QtE CLCCTKODC CONrittw.ATIOW
-»
M
NX -
r-^^
r^^
I.IOOOFtCT
,\—
^|.3fi-m
.1 I=H=R=f=
-i-...
.
loopri. _i ^_^
~l
!"
M l t U M B riILD
MT*
..1-Lu.
,!4
I ;
M t l t M t D FIELD M T .
C M M C T t D F M nOUCTIVI COUPUM i -
^,•190-"
^CLCULlTte MtltTIVITT MeFILI
•^'"TTI
-^^
-i"--r4-t^'ti
I
1
I
! i !
i
!
I
1" I !
J
J
i
1—ul
*
i
I I
t
/
^
xzi-xr-rx • •
I ! I HTii l
"iuil
I I
i
. 9
ris. 4
. 3 data i s then contoured (see below). The contour plots a r e not sections of the
DIPOLE-DIPOLE PLOTTING METHOD
nx
< — X — » •
^
< — X — * -
-(S>
7y7?'//////////y////////y////////y//y/y?7y//)//////////.
4,5-9,10
e l e c t r i c a l p r o p e r t i e s of the e a r t h ; they a r e convenient g r a p h i c a l r e p r e s e n t a t i o n s
of the m e a s u r e m e n t s m a d e .
H o w e v e r , with e x p e r i e n c e t h e contour p a t t e r n s c a n
be i n t e r p r e t e d to give s o m e information about the s o u r c e of the a n o m a l y .
If the contour p a t t e r n s indicate v e r y s i m p l e g e o m e t r i e s , m o r e quantitative
i n t e r p r e t a t i o n s can often b e m a d e .
F o r i n s t a n c e j if the contours a r e h o r i z o n t a l
for a l a t e r a l d i s t a n c e of four to s i x e l e c t r o d e i n t e r v a l s , a h o r i z o n t a l l y l a y e r e d
g e o m e t r y is indicated.
In this situation, t h e o r e t i c a l t y p e - c u r v e s for d i p o l e -
dipole m e a s u r e m e n t s in a l a y e r e d g e o m e t r y can b e used in " c u r v e fitting"
techniques to give the t r u e r e s i s t i v i t i e s and depths for t h e e a r t h .
McPHAR GEOPHYSICS
REPORT ON THE
RECONNAISSANCE D I P O L E - D I P O L E
RESISTIVITY SURVEY
IN T H E
SODA LAKE AREA
CHURCHILL COUNTY, NEVADA
FOR
CHEVRON OIL COMPANY
1.
INTRODUCTION
At the r e q u e s t of M r . William M e r o of Chevron Oil Conipany
M c P h a r Geophysics has cosnpleted a R e c o n n a i s s a n c e dipole-dipole
R e s i s t i v i t y Survey in the Soda Lake A r e a , Churchill CoTUty, Nevada.
Initially, the r e s i s t i v i t y s u r v e y of the Soda L a k e , Known
Geothermal Resource Area,
was to c o n s i s t of five n o r t h - s o u t h l i n e s
a c r o s s a n a r e a of high t e m p e r a t u r e g r a d i e n t .
It was a p p a r e n t , upon
completion of Line A through Line E , that a sone of low r e s i s t i v i t y
extended beyond the a r e a s u r v e y e d and additional lines w e r e proposed
to d e t e r m i n e the extent of this e o n e .
M e a s u r e m e n t s w e r e nnade with
2000 foot dipoles a t one-through-fotxr dipole s e p a r a t i o n s along eight
r e c o n n a i s s a n c e l i n e s s p a c e d ap|>roximately one m i l e a p a r t .
A frequency
of 0.125 Hz was u s e d in o r d e r to m i n i m i z e attenuation of the e l e c t r i c
- 2 -
field due to eddy c u r r e n t dissipation of e n e r g y and a t t h e s a m e t i m e
avoid t e l l u r i c n o i s e .
2.
PRESENTATION
OF
RESULTS
The r e s i s t i v i t y s u r v e y r e s u l t s a r e shown on the followijig data
plots in the m a n n e r d e s c r i b e d in the notes which a c c o m p a n y this r e p o r t .
Line
Electrode Intervals
Dwg.No.
A
2000 feet
R 6128-1
B
2000 feet
R 6128-1
C
2000 feet
R 6128-1
D
2000 feet
R 6128-2
E
2000 feet
R 6128-2
F
2000 feet
R 6128-2
G
2000 feet
R 6128-3
Hq
2000 feet
R 6128-3
J
2000 feet
R 6128-3
Also enclosed with this r e p o r t is Dwg. No. R P 4948, a plan m a p of
the Soda Lake A r e a g r i d a t a s c a l e of 1" « 2 0 0 0 ' .
T h e definite, probable
and p o s s i b l e R e s i s t i v i t y low a n o m a l i e s a r e indicated by b a r s , i n a znanner
shown in t h e legend, o n this plan m a p a s well a s on t h e data p l o t s .
These
b a r s r e p r e s e n t the s u r f a c e projection of the anomalous zones a s i n t e r p r e t e d
from the location of the t r a n s m i t t e r and r e c e i v e r e l e c t r o d e s when the
anomalous v a l u e s w e r e m e a s u r e d .
- 3 -
3.
DISCUSSION
OF
RESULTS
The r e c o n n a i s s a n c e r e s i s t i v i t y s u r v e y of the Soda Lake a r e a has
outlined a zone of low r e s i s t i v i t y extending for a p p r o x i m a t e l y 10 m i l e s in
a n o r t h e a s t e r l y direction and a v e r a g i n g two m i l e s in width, an a r e a of
a p p r o x i m a t e l y 13,000 a c r e s .
A d e s c r i p t i o n of the r e s i s t i v i t y r e s p o n s e
along each s u r v e y line follows.
Line A
Definite anomalous r e s p o n s e s (less than l . S ohm^ feet r e s i s t i v i t y )
o c c u r b e t w e e n SON to 130N and 150N t o 220N. T h i s definite r e s p o n s e i s
i n t e r r u p t e d between 130N and 150N b y a slight i n c r e a s e in r e s i s t i v i t y
which is i n t e r p r e t e d a s a probable an<»naly.
The r e s i s t i v i t y r e s p o n s e south of station 4pN a p p e a r s to indicate
a s i m p l e l a y e r e d e a r t h with a n e a x - s \ i r f a c e r e s i s t i v e l a y e r l e s s than
1000 foot thick overlying a conductive s u b s u r f a c e .
T h i s v a r i e s from the
r e s p o n s e on the n o r t h end of the line (north of station 280N).
The
r e s i s t i v i t y r e s u l t s of the n o r t h end of the l i n e s u g g e s t a uniforxu e a r t h ,
probably thick alluvial fill which does not contain any thercaal fluids
which produce
the conductive a n o m a l o u s r e s p o n s e .
Line E
A shallow, definite a n o m a l y occxirs between SOS and 190S and
extends a s a p o s s i b l e a n o m a l y from 1905 to 26pS. The definite r e s p o n s e
exhibits uniform r e s i s t i v i t y to depth.
T h e possible a n o m a l y located between 330S and 370S o c c u r s iat
- 4 -
depth b e n e a t h a r e s i s t i v e o v e r b u r d e n s i m i l a r to Line A .
Line C
A b r o a d , deep, definite ancanalous r e s p o n s e o c c u r s between
70S and 130N with the s t r o n g e s t r e s p o n s e between 40 and 60N.
This
r e s p o n s e extends as a p o s s i b l e anomaly iroza 130N to 210N.
The n e a r - h o r i z o n t a l contour p a t t e r n of the r e s i s t i v i t y data
b e t w e e n 80S and lOON s u g g e s t e d a s i m p l e l a y e r e d e a r t h beneath this
portion of the l i n e .
Theoretical curve matching indicates a two-layer
e a r t h with the upper l a y e r having an a p p r o x i m a t e r e s i s t i v i t y of 40 ohm
feet and a thickness of 1000 feet with the bottom l a y e r having an
approxiznate r e s i s t i v i t y of 1. 5 ohm feet.
The southern portion of this line was extended as Line I by
Chevron Oil, but since it was an extension of Line C the data have b e e n
i n c o r p o r a t e d with this l i n e .
Line D
The r e s i s t i v i t y r e s p o n s e on this line i s y e r y s i m i l a r to Line A,
one znile to the w e s t .
to 160N.
Definite anomialies occur a t 20S to 20N and SON
The i n t e r p r e t a t i o n of a p r o b a b l e anoznaly between 20N and SON
i s due to the i n c r e a s e d r e s i s t i v i t y at depth.
Line E •
A b r o a d , anomailous r e s p o n s e of v a r y i n g xnagnitude o c c u r s on
this l i n e .
T h e definite anomteily l o c a t e d a t 20N to 30S i s open on the south
s i d e due to Soda L a k e . A p r o b a b l e a n o m a l y extends fronx 20N to TON a n d
- 5 a possible a n o m a l y o c c u r s between TON to 120N.
Line F
T h i s line is located oh the south side of Soda L a k e .
The deftnite,
open-ended a n o m a l y l o c a t e d on Line E does not extend far enough south
to b e o b s e r v e d on this l i n e , but znay b e iziferred b y the possible anomaly
between 1 OS and ION.
The definite a n o m a l y l o c a t e d b e t w e e n 120S and 150S is open-ended
and extends a s a possible anomaly from 120S to 9PS.
Line G
A m o d e r a t e to deep definite anoznaly o c c u r s between 60N and SOS.
T h i s line did not extend far enough south to close off the anomaly which
a p p e a r s to b e shallowest on the south end of the l i n e .
T h e o r e t i c a l c u r v e m a t c h i n g of the n e a r - p a r a l l e l r e s i s t i v i t y p a t t e r n
b e t w e e n 0 and lOON indicates a t w o - l a y e r e d e a r t h .
The upper l a y e r h a s
a t h i c k n e s s a p p r o x i m a t e l y 1000 feet and a r e s i s t i v i t y of 20 ohm feet with
the lower l a y e r having a r e s i s t i v i t y of 1.4 ohm f e e t .
Line H
The r e s i s t i v i t y m e a s u r e m e n t s along this line a r e slightly higher
than any of the preceding l i n e s except for t h e definite anoznaly l o c a t e d at
240S to 260S.
This lizie is the m o s t e a s t e r l y line of the sxirvey.
The data suggest
a d e c r e a s e in t h e r m a l activity and naay r e p r e s e n t the e a s t e r n edge of the
geothermal reservoir.
. —
6
-
•
. _•
/'^
Line J
A definite a n o m a l y o c c u r s b e t w e e n 40N and 80S. The r e s i s t i v i t y
p a t t e r n for this line a l s o exhibits h o r i z o n t a l contours which i s indicative
of h o r i z o n t a l l a y e r i n g .
T h e o r e t i c a l c u r v e matching i n d i c a t e s a t w o - l a y e r
e a r t h v e r y s i m i l a r to Line G.
The p o s s i b l e anoznaly l o c a t e d b e t w e e n 190S and 21 OS i s open-ended
to the south.
4.
The extension of this line shoxild b e cozisidered.
CONCLUSIONS AND
RECOMMENDATIONS
The Reconziaissance R e s i s t i v i t y Survey of the Soda Lake a r e a has
outlined a zone of low r e s i s t i v i t y approxixnately two m i l e s wide and
extending for 10 zniles in a n o r t h e a s t e r l y d i r e c t i o n .
This zone a p p e a r s
to b e ending on the e a s t ; Line H, the m o s t e a s t e r l y line s u r v e y e d ,
exhibits a weak ancnsoalous r e s p o n s e which m a y r e p r e s e n t l a t e r a l effects
and s u g g e s t s that the potential g e o t h e r m a l r e s e r v o i r does not extend p a s t
this l i n e .
T h e zone i s not c l o s e d on the Southwest. Additiozial work should
b e c o n s i d e r e d to deterznine the a c t u a l lezigUi of the aiMsmalous zozxe.
T e z n p e r a t u r e g r a d i e n t m e a s u r e m e n t s have b e e n obtained over
p a r t of the l o w - r e s i s t i v i t y z o n e . Additional t e z n p e r a t u r e g r a d i e n t m e a s u r e «
znents znay b e r e q u i r e d for cosnplete c o r r e l a t i o n with t h e r e s i s t i v i t y data
which hJ9ive l o c a t e d the s t r o n g e s t a n o m a l o u s r e s p o n s e s on Line C between
40N and 60N and on L i n e E b e n e a t h 20S.
The definitei open*ended anomaly b e t w e e n 120S and 140S on
Line F w a r r a n t s f u r t h e r Investigaitlon.
T h i s azjiomaly znay r e p r e s e n t
- T -
another therznal zone which p a r a l l e l s the znain z o n e .
The p o s s i b l e anoznaly
on Line B b e t w e e n 330S and 3T0S znay r e p r e s e n t the north edge of the
second zone and the definite a n o m a l y on Line H b e t w e e n 240S and 260S
m a y a l s o r e p r e s e n t this z o n e .
Several temperature gradient caeasure-
znents in this a r e a , or the extension of Line A or Line E , woxild deterznizie
if this zone e x i s t s .
A c o m p l e t e c o r r e l a t i o n of a l l geological, geocheznical and
geophysical data is r e q u i r e d to d e t e r m i n e t h e location of a well to t e s t
the l a r g e anomalous z o n e .
McPHAR GEOPHYSICS INCORPORATED
E r u c e S, B e l l ,
Geologist
D a t e d i J a n u a r y 1 1 , 19T4