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Geothermal ReeOUCeS Council, TRAiJSACTIONS Vot. 4, September 1080
A COMPARISON OF DIPOLE-DIPOLE RESISTIVITY
AND ELECTROMAGNETIC INDUCTION SOUNDING OVER THE PANTHER CANYON
THERMAL ANOMALY, GRASS VALLEY, NEVADA
M. Wilc, J. H. Beyer'
and N. E. Goldstein*
*Lawrence Berkeley Laboratory, Berkeley California 94720
+Woodward-Clyde
Consultants, San Francisco, California
years,.but to datr no deep wells have been drilled.
ABSTRACT
.
y;,
,
.
A comparison is made between the dipole-dipole
resistivity method and electromagnetic sounding
method based on surveys over a geothermal anomaly
near Panther Canyon, Grass Valley, Nevada. Dipoledipole data were taken in conjunction with largescale geothermal studies in the area. Two orthogonal lines were measured over the heat flow
anomaly and two-dimensional modeling was performed
on the data. EM sounding data were taken with the
Lawrence Berkeley Laboratory EM-60 system which
is a large-moment, frequency-domain, horizontalloop system. Relative to single 50-meter-radius
transmitter coil, eight soundings were made with
detectors at distances of 0.5 to 1.6 km from the
loop. Interpreted results from the two surveys
indicate substantial agreement in the depth to
and thickness of a conductive zone that may be
associated with the thermal anomaly. The dipoledipole method is inherently better for resolving
resistive basement beneath the conductive anomaly,
and dc resistivity interpretation techniques are
presently better able to handle the complex twodimensional geology. However, the EM method is
far less labor intensive, requir'ing only onethird the field time for similar areal coverage.
* Field
e sile
m
v~\
q
Sonorno
Grass Valley
'I
!\\
x
EM - 6 0 Receiver stations
T-T' Geophysical survey lines
-7- h o t flow &WIS
(HFU)
I
I 0.5 0
I
I
1
I
1
?
?
? Kilometers
XBLB04- 7050
Figure 1. Survey location map Panther
Canyon area, Grass Valley,
Nevada.
INTRODUCTION
Electrical methods are commonly used for
geothermal exploration and have often proved
effective for locating areas of low resisitivity
associated with geothermal reservoirs. However,
controlled source electromagnetic (EM) induction
soundings have not been widely used. This paper
compares field data acquisition and interpretation for EM sounding with a more conventional
method, dipole-dipole resistivity. Data for both
methods were acquired at identical locations over
a thermal anomaly in the Panther Canyon region of
Grass Valley, Nevada (Figure 1).
The Panther Canyon area is located in the
southwestern portion of Grass Valley near the
intersection of the Tobin and Sonoma Ranges.
Exposed rocks in these ranges consist mainly of
the Paleozoic Havallah sequence of cherts,
argillites and sandstones. Figure 2 is an idealized
geologic cross section along one of the orthogonal
survey lines shown in Figure 1.
DIPOLE-DIPOLE RESISIVI'IY
GEOLOGY
Dipole-dipole resistivity data were acquired in
conjunction with large scale geothermal exploration and technique evaluation studies in Grass
Valley (Beyer, 1977). Using a 25 kw transmitter
and synchronous-detection receivers, we obtained
high quality data for dipoles 250 to 1000 m in
length and transmitter-receiver separations
,exceeding ten dipole lengths (>lo km).
Grass Valley is a northerly-trending Basin
and Range valley located in north-central Nevada
(Figure 1). The region is characterized by higher
than normal heat flow (Sass et al., 1977), active
hot springs (Olmsted et al., 1975) and recent
faulting (Noble, 1975). Surface geologic studies
consist of regional photogeology (Noble, 1975) and
detailed field mapping (Olmsted et al., 1975).
Grass Valley has been an area of fairly active
geothermal exploration for about the past eight
Reconnaissance geophysics and shallow heat
flow holes located a low resistivity, high heat
101
Wilt
Wail
EM INDUCTION SYSTEM
H
.
'
dH4
$2
013
ols T8 IOU5 TO
T9
MESOZOIC
p.LEOzOIC
TI1 I 0171
TI2 TI3
[ =,"."
M*IOIOK inttimamw
m
The LBL EM-60 frequency-domain induction
system shown schematically in Figure 4 , includes
two major components: (1) a transmitter section
consisiting of a power source, control and timing
electronics, and a transistorized switch capable
of handling large current; and (2) a receiver
section consisting of magnetic field detectors,
signal conditioning amplifiers and anti-alias
filters, and a multi-channel programmable receiver
(spectrum analyzer) (Morrison et al., 1978).
m a m h u Y i ~OCL.
2:y;
xtlL762-2519 A
Figure 2.
Idealized geologic cross section for
survey line H-H' (after Sass et al.,
1977).
The EF-60 transmitter is powered by a Hercules
gasoline engine linked to a 60 kW, 400 Hz, 38
alternator. The system is capable of transmitting
square-wave current pulses from 10-3 to 103 HZ at
up to 400 Amps into a coil of wire. Four turns
of #6 wire in a circular loop 50 m in radius
provide adequate signal for soundings where
transmitter-receiver separations are less than
about 5 km. This corresponds roughly to a
maximum depth of exploration of about 5 km.
flow anomaly near the mouth of Panther Canyon. To
better define the anomaly, orthogonal dipole-dipole
resisitivity lines were run across the feature.
For Line H-H', as shown in Figure 3 , the data were
interpreted in terms of a two-dimensional model.
The model data were fit by trial and error,
requiring about a dozen iterations using a finite
difference algorithm (Dey, 1976). The model
clearly indicates the low resisitivity zone
corresponding to the heat flow high and indicates
a fairly shallow basement.
Horizontal loop, M> IO6 mks at 100 Hz
Control unit
Transmitter
truck
GRASS VALLEY
Line H-H'
R-R'
A-A'
FT'
Telemetry
0 Clock
0 LOOP input current
0 Voice intercom
r3 Component
1"
A low frequency electromagnetic prospecting system
xaL 786-2575
Figure 4. Schematic diagram of the EM-60 horizontal loop electromagnetic prospecting
system as used in Nevada in 1979.
The magnetic field is detected at receiver
sitesaby means of a three-component SQUID magnetometer oriented to measure the vertical, radial
and tangential components with respect to loop.
Signals are amplified, anti-alias filtered and
input to a six-channel, programmable, multifrequency, phase-sensitive receiver (Fig. 1).
Data processing yields a raw amplitude estimate for
each component and a phase estimate with respect
to the phase of the current in the loop. Phase
referencing at the receiver is maintained with a
hard-wire link to a shunt resistor in the loop.
Raw amplitude estimates must be later corrected
for dipole moment (strength) and the distance
between loop and magnetometer.
Figure 3.
Dipole-dipole apparent resistivity pseudosection for 1 km dipoles along line H-H':
field data, model generated data, and twodimensional model.
102
Wilt
Sounding TT’ km 4
In practice, the hard-wire link was found to
be a source of noise, particularly above 50 Hz.
This has required the elimination of the absolute
phase reference at high frequencies in favor of
relative phase measurements between vertical and
radial components. With relative phase measurements, interpretation is based on the ellipticity
and tilt angle of the magnetic field rather than
on the amplitude and phase of the vertical and
radial components.
Basic interpretation is accomplished by
direct inversion of observed data to fit onedimensional models. The program fits amplitudephase and/or ellipse polarization parameters
jointly or separately to arbitrarily layered
models. This program allows the use of both:
(1) ellipse polarization parameters to fit high
frequency points where‘absolute phase data are
unreliable, and (2) absolute phase data at the
lower frequencies where the phase reference may
allow for better parameter resolution.
I
I
I
1
-
HR/
A Observed
Model
/’”
4
8.1 f 0,s i l m
f30 m
3.9 f 1.0n m 340
f 200m
150.0 (fixed 1
,
I
0,I
1.0
IO
Frequency ( Hz 1
IO0
-
XBL 802 6816
Figure 5. Normalized magnetic field amplitude
spectra normal and radial fields sounding TTS Panther Canyon.
EM INDUCTION RESULTS
Grass Valley, Line T-T’
The EM-60 field survey in Panther Canyon
consisted of eight soundings arranged in two orthogonal profiles about a central 4-turn, 50-m-radius
horizontal loop. Transmitter-receiver separations
varied from 400 m to 1.6 km, and data at each site
were recorded over at least two frequency decades
within the frequency band 0.033-500 Hz. Because
the depth of penetration for EM induction sounding
is proportional to both the transmitter-receiver
separation and the period of the transmitted wave,
we occupied receiver sites at varying distances
from the transmitter loop. At Panther Canyon,
four receiver locations were about 500 m from the
loop source for shallow information, and four
sites were at a distance of about 1.6 km for
better resolution of deeper horizons.
0-
250
-5
Dipole-Dipole Resistivity Model
N
-
500-
I
S
L
0
E
S
I
750-
loo0
-
1250
-
0
I
2
3
4
5
After Beyer ,1977
Distonce in kilometers, Resistivity in ohm-meters
E M - 6 0 Resistivity Profile
0
250
An example of an EM-60 amplitude spectra
sounding is given in Figure 5. The error bars
signify one standard deviation. The fit to a
three-layer model is fairly good, but the data
were interpreted only to 50 Hz because of high
noise resulting from the reference wire. Ellipticity data, however, could usually be interpreted
to 500 Hz.
5
-
500
O’
750
c
n
o
1000
1250
x
Loop tronsmitler locallon
Receiver locolion
0
I
2
3
4
5
2
3
4
5
Dirtonce in kilometers
DISCUSSION
Figures 6 and 7 are resistivity cross
sections along orthogonal lines over the Panther
Canyon thermal anomaly. Each figure gives a
comparison between dipole-dipole resisitivity and
EEI-60 electromagnetic interpretations. Along the
north-south line (Fig. 6) the EM and dipoledipole interpretations are similar. Both cross
sections indicate resistive surface material overlying an irreg.ular southward-dipping conductive
body.
Depth to resistive basement (ihvaliah
formation?) is shown to vaEy between 250 and 800 m
below the surface. The depth to and lateral extent
of the conductive body, which may be associated
with the theF.1 anomaly,. is. well resolved by both
methods. The two profiles disagree somewhat ‘on the
depth to resistive basement beneath the conductor.
Because the EM method is less sensitive to resistive formations and because the dipole-dipole
transmitter-receiver separations were five times
0
250
.5
500
150.0
c
n
750
1000
1250
0
I
XBL 802-6775
Figure 6 .
103
Resistivity cross section over line H-H’
in Panther Canyon (a) two-dimensional
dipole-dipole resistivity model (b)
profile of one-dimensional EM-60 electromagnetic soundings; (c) comparison of
parts a and b.
Wilt
g r e a t e r t h a n f o r t h e EM s u r v e y , t h e conventional
r e s i s t i v i t y s e c t i o n is probably more a c c u r a t e
i n determining t h i s parameter.
Although t h e i n t e r p r e t e d s e c t i o n s i n b o t h
cases are s i m i l a r , t h e EM r e s u l t s show a smoother
v a r i a t i o n , a consequence of one-dimensional i n t e r p r e t a t i o n . However, t h e r e are o t h e r d i f f e r e n c e s
between t h e EM and d c r e s i s t i v i t y surveys t h a t
are n o t a p p a r e n t i n t h e d a t a and i n t e r p r e t a t i o n s .
The d i p o l e - d i p o l e s e c t i o n s r e q u i r e d a crew of
f o u r working f o r more than 1 9 f i e l d days whereas
t h e same s i z e crew c o l l e c t e d t h e EM d a t a i n six
f i e l d days. The dc r e s i s t i v i t y d a t a cover an'
a r e a about 50 p e r c e n t l a r g e r , b u t f a r more l a b o r
was r e q u i r e d t o a c h i e v e coverage comparable t o
t h a t of t h e EM survey. I n t e r p r e t a t i o n t e c h n i q u e s
f o r dipole-dipole d a t a a r e p r e s e n t l y b e t t e r a b l e
t o h a n d l e complex geology, and t h e method i s
i n h e r e n t l y b e t t e r a b l e t o r e s o l v e r e s i s t i v e format i o n s . However, deep EM i n t e r p r e t a t i o n s r e q u i r e d
much s h o r t e r t r a n s m i t t e r - r e c e i v e r s e p a r a t i o n s ,
t h u s r e d u c i n g t h e e f f e c t s of lateral inhomogen e i t i e s on i n t e r p r e t a t i o n s . The two c r o s s s e c t i o n s
s u g g e s t t h a t , even i n r e g i o n s of two- and t h r e e dimensional geology, EM d a t a w i l l adequately r e s o l v e
major f e a t u r e s w i t h o u t s e v e r e d i s t o r t i o n .
F i g u r e 8 shows r e s u l t s f o r an east-west l i n e
over t h e c e n t r a l p o r t i o n of t h e thermal anomaly.
For t h i s c r o s s s e c t i o n t h e EM and dc r e s i s t i v i t y
i n t e r p r e t a t i o n s show moderate agreement. Both
c r o s s s e c t i o n s i n d i c a t e t h e p r e s e n c e of an
i r r e g u l a r l y shaped conductive body n e a r t h e c e n t r a l
p o r t i o n of t h e thermal anomaly. The EM d a t a p l a c e
t h e t h i c k e s t p o r t i o n s l i g h t l y w e s t of t h e thermal
maximum. The d c r e s i s t i v i t y d a t a i n d i c a t e
t h a t basement d i p s s t e e p l y westward from 250 m t o
Grass Valley, Line H-H'
W
7
0
250
-E
-
500
E
750
8
r
a
,
150
T h i s work was supported by t h e Department
of Energy under C o n t r a c t W-7405-ENG-48.
1000
0
3
2
I
5
4
1250
X
0
EM-60 Resistivity Profile
x
a
X
8.0
9,J
/
I'3.2
250
REFERENCES
Alter Beyer ,1977
Distance i n kilometers, Resistivity in ohm-meters
Beyer, J. H., 1977, T e l l u r i c a n d d c r e s i s t i v i t y
t e c h n i q u e s a p p l i e d t o t h e geophysical invest i g a t i o n of Basin and Range geothermal syst e m s , P a r t 111: The a n a l y s i s of d a t a from
Grass V a l l e y , Nevada: Lawrence Berkeley
Laboratory, Report LBL-6325, 3/3, 115 p
X
15.0
-0
2B
2.7
.
25.0
o
1000
X
0
1250
I
2
3
Dey, A., 1976, R e s i s t i v i t y modeling f o r a r b i t r a r i l y shaped two-dimensional s t r u c t u r e s ,
P a r t 11: Users guide t o t h e FORTRAN algor i t h m RESIS2D: Lawrence Berkeley Laboratory,
Report LBL-5283.
Loop transmitter location
Receiver location
4
5
Distance in kilometers
Morrison, 'H. F.,- G o l d s t e i n , N. E., Hoversten, M . ,
Oppliger, G., and Riveros, C . , 1978, Descript i o n , f i e l d test and d a t a a n a l y s i s of a
c o n t r o l l e d - s o u r c e EM System (EM-60): Lawrence
Berkeley Laboratory, Report LBL-7088.
0-
-
E
250
-
.. ....:. .
.,. ... . ........
........
.
5
Noble, D. C . , 1975, Geological h i s t o r y and
geothermal p o t e n t i a l of t h e Leach Hot
S p r i n g s area, P e r s h i n g County, Nevada:
P r e l i m i n a r y r e p o r t t o t h e Lawrence Berkeley
Laboratory.
500-
c
a
u
750
-
1000
-
1250
-
2 2.
. ...
.... ....,.:,.
<
>
Figure 1 .
25.0
I50
R e s i s t i v i t y c r o s s s e c t i o n o v e r l i n e H-H'
i n P a n t h e r Canyon (a) two-dimensional
d i p o l e - d i p o l e r e s i s t i v i t y model (b) prof i l e of one-dimensional EM-60 electromagn e t i c soundings; (c) comparison of p a r t s
a and b.
about 800 m a d j a c e n t Lo t h e edge of' t h e Tobln
Range; t h e EM i n d u c t i o n d a t a show a similar
behavior, b u t w i t h fewer p o i n t s and l a r g e r uncertainty.
104
Olmsted, F. H., Glancy, P. A., H a r r i l l , J. R.,
Rush, F. E.,, Van Denburgh, A. S., 1975,
P r e l i m i n a r y h y d r o l o g i c a p p r a i s a l of s e l e c t e d
hydrothermal systems i n n o r t h e r n and c e n t r a l
Nevada: U.S. G e o l o g i c a l Survey Open F i l e
Report 75-56.
Sass, J. H.,
Ziagos, J. P., Wollenberg, H. A.,
Munroe, R. J., DiSomma, D., and Lachenbruch,
A. H., 1977, A p p l i c a t i o n of h e a t flow
t e c h n i q u e s t o geothermal energy e x p l o r a t i o n ,
Leach Hot S p r i n g s , Grass Valley, Nevada:
Lawrence Berkeley Laboratory, Report LBL-6809
a l s o U.S. G e o l o g i c a l Survey Open F i l e Report
77-762.