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(200J
UNITED STATES DEPARTMENT OF THE INTERIOR
GEOLOGICAL SURVEY
Evaluation of Audio-magnetotelluricTechniques as a Reconnaissance
Exploration Tool in Long Valley, Mono and Inyo Comities,
California
By
D. B. Hoover, F. C. Frischknecht, and C. L. Tippens
Open-file report
1973
This report is preliminary and has not been edited or reviewed for conformity with U.S. Geological Survey standards and nomenclature.
Contents
PageIntroduction 1
Basis of the technique 1
Equipment 4
Field operations 4n A a** 1 4» a - - _ ____ -_- - . - __ _ _ ___ _ ____ __ _ _-_____.__ _ _ _ _ ^1IxSSUiuS^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ D
Summary ' 10
References 11
Illustrations
[Plates are in pocket]
Plates 1-7. Maps of Long Valley, California:
1. AMI station locations and generalized geology.
2. 26-Hz apparent resistivity (east-west electric
dipole).
3. 26-Hz apparent resistivity (north-south
electric dipole).
A. 8-Hz apparent resistivity.
5. 86-Hz apparent resistivity.
6. 270-Hz apparent resistivity.
7. 7,000-Hz apparent resistivity.
8-9. AMT pseudosections along section A-£f :
8. East-west electric dipole
9. North-south electric dipole.
10-11. Skin-depth pseudosections along section A^A 1 :
10. East-west electric dipole.
11. North-south electric dipole.
Page
Figure 1. Diagram of audio-magnetotelluric system 5
2-27. AMT soundings, Long Valley (Nos. 1-6 and 13-32) 13-38
Evaluation of audio-magnetotelluric techniques as a
reconnaissance exploration tool in Long Valley,
Mono and Inyo Counties, California
By D. B. Hoover, F. C. Frischknecht, and C. L. Tippens
Introduction
The audio-magnetotelluric technique (AMI) has been suggested as a
useful tool for reconnaissance exploration of conductive anomalies in
geothermal areas. To access the potential of the technique, AMT studies
were made in the Long Valley caldera where extensive geological and
geophysical studies provided adequate background information (Bailey,
Lanphere, and Dalrymple, 1973; Hill, McHugh, and Pakiser, 1973; Kane and
Mabey, 1973; Lackenbruch, Lewis, and Sass, 1973; Willey, Rapp, and Baraes,
1973; Stanley, Jackson, and Zohdy, 1973; Anderson and Johnson, 1973; lyer and
Hitchcock, 1973; Steeples and Pitt, 1973). In particular the geological
work of Bailey, Lanphere, and Dalrymple (1973), and electrical studies of
Stanley, Jackson, and Zohdy (1973) provided principal control for evaluation
of the technique.
As no commercial equipment is available as a package, the results
reported here were obtained from equipment and field techniques developed
by the Geological Survey. These methods and techniques should be
considered only preliminary as modifications to both equipment and
operations are continuing as experience is gained.
Basis of the technique
The magnetotelluric method is one of three electromagnetic
exploration techniques in which naturally occurring electromagnetic fields
are used. The telluric and AFMAG (audio-frequency magnetics) methods are
the other techniques, and all 3 suffer from being dependent upon vagarities
in the natural fields as anyone who has worked with the techniques knows.
In this work the frequency range used was from 8 Hz to 18.6 KHz and the
technique is accordingly called audio-magnetotelluric exploration. This
range of frequencies provides exploration depths of interest in minerals
exploration and spans the frequency range in which AFMAG measurements
are made.
In using the AMI method it is assumed that the sources of energy
are plane electromagnetic waves propagating essentially vertically
downward into the earth. For the plane wave assumption to be valid the
source must be at a distance greater than about four skin depths. The
downward propagating plane wave consists of mutually perpendicular
horizontal magnetic and electric fields. If the earth consists of
homogeneous horizontal layers the electric field in the earth is radial
to the source and the magnetic field is tangential to the source. It
should be noted that above the earth ELF (extra low frequency) and VLF
(very low frequency) energy propagates for long distances around the
earth in the cavity or wave guide formed by the earth and the ionosphere.
The fields above the earth are approximately plane waves at grazing
incidence. For a homogeneous or horizontally stratified earth the
apparent resistivity of the earth is a function of these fields and is
given by (Cagniard, 1953):
f «* frequency in Hertz (1)
E » electric field in microvolts/meter
H «* magnetic field din gammas
p « apparent resistivity in ohm-meters3>
As the apparent resistivity is a function of frequency, a means is
provided for determining the variation of resistivity with depth. As the
electromagnetic energy propagates into the earth it is attenuated. The
depth at which the current density falls to of its surface value is
called the skin depth and provides an approximate measure of the depth
of exploration. Thus if the apparent resistivity is measured as a function
of frequency a sounding is made much as in direct-current geometric
sounding (Keller and Frischknecht, 1966).
For rocks which are not strongly magnetic' the skin depth, 5, is
given by
5 503 / p/f meters, where p is in ohm-meters (2)
For example, with our equipment, if measurements were made over a
100 ohm-meter earth we would be measuring the bulk properties from the
surface to a depth of approximately 37 meters at 18.6 KHz and to 1800
meters at 8 Hz.
The source for the observed electromagnetic energy is from world
wide lightning storms. Storms in tropical regions account for the
preponderance of the energy. Bleil (1964) and Ward (1967) discuss the
temporal and spacial variations of these storm signals. The main feature
of interest here are the weakness of signals during the winter months,
and the nonuniformity of thunderstorm centers from day to day. The
decrease in energy makes operations difficult or impractical during
winter months. The nonuniformity in source position gives some data scatter
or nonrepeatability of data depending on source location and lateral
resistivity variations (Strangway and Vozoff, 1970). This is more apparent
at the higher frequencies. The energy from the storm centers propagates
in the earth-ionosphere wave guide which has its first resonant mode at
about 8 Hz. For about a decade below this frequency the energy is weak
enough to be impractical to use in reconnaissance exploration. The
frequency dependence of the wave guide produces a region of low signal
strength around 2,000 Hz, and limits data acquisition in this band.
In our operations we seldom obtained usable data at 2,000 Hz, and in late
October energy was often too weak to use at 700 Hz.
Within the AMT frequency band, manmade signals are also present.
Host troublesome are powerline radiations, their harmonics, and in the
higher ranges, VLF radio stations. These latter signals may be used, and,
in our equipment, VLF stations at 10.2 KHz and 18.6 KHz are employed.
During the rare periods when the transmitters are off, natural energy is
sufficient for operations. In the field, however, it would be difficult
to insure for powerline radiations that the source distance was at least
four skin depths without severely restricting operations to remote regions.
Thus the large amount of radiation from powerlines generally constitutes only
a difficult noise problem. With our present equipment we prefer to
operate no closer than 1 km from powerlines, although equipment
modifications are underway to permit closer operations.
Equipment
Figure 1 shows a diagram of our instrumentation. To measure
the electric field two steel stakes are used as electrodes, generally
separated by 100 meters. The electric field signal is amplified and
prefiltered using R-C bandpass filters so as to prevent limiting of strong
transients in the early stages. Narrow band active notch filters are
used to remove 60 and 180 Hz powerline signals. The signals then enter
a universal active filter connected in a high-Q bandpass configuration.
Approximately constant Q is maintained at all filter settings. The 6 db
bandwidth at 8 Hz is 0.3 Hz; 9 selectable frequencies are used to define
a sounding curve spaced logarithmically throughout the band, but selected
so as to avoid midband harmonics of 60 Hz. At present our operating
frequencies are 8, 26, 86, 270, 700, 2,000, 7,000, 10,200 and 18,600 Hz.
The output of this narrow band filter is rectified, integrated, and
displayed on a strip chart recorder to show the envelope of the received
energy. Maximum system gain will provide about 30.0 mm chart deflection
for a 1 microvolt signal*
An induction pickup is used for the magnetic field sensor, consisting
of a ferrite core upon which are wound many thousands of turns of wire.
In order to span the broad range of frequencies we have found it necessary
to use two separate coils. One covers the range of 8 to 700 Hz and the
other 2 KHz to 18.6 KHz. An integral part of each coil is a low noise
preamplifier which feeds the magnetic field signal to a second channel,
essentially similar to that described for recording the electric field.
Coil sensitivity is about 0.1 microvolt per milligamma at 8 Hz.
Phase information is preserved by means of a phase-locked loop and
synchronous detectors as shown in figure 1. The usefulness of the phase
information is still being evaluated so no further discussion of it will
be made in this report.
Field operations
The strip-chart recorder and high gain selective filters are operated
from the back end of a carryall truck with power supplied by an inverter
connected to the truck battery. The coil and common electrode of the
electric line are located 100 ft from the truck to avoid electrical noise
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from the vicinity of the truck. Signals are brought to the truck over
coaxial cable.
The electric line is laid out in either an east-west or north-south
direction and the coil placed at a right angle. System gains are adjusted
so as to give 20 to 40 mm chart deflection of peak energy bursts on each
channel. The amplitude of corresponding electric and magnetic signals are
measured and their ratio computed for a sufficient number of signals so
that a reliable average ratio is obtained. The Cagniard resistivity is
then computed from a knowledge of system gain and equation 1.
Data is computed and plotted in the field while recording is
underway giving a sounding curve by switching through the various
frequencies. The electric dipole and coil are then rotated 90 degrees,
and a second sounding made and plotted. This permits the operators to
correct any obvious errors and to check any data points that appear
aberrant. The second sounding also provides information on lateral
variations in conductivity or anisotropy of the earth.
Operations are made by two persons, one acting as observer and the
other acting as computer. Typical production is 8 soundings or 4 stations
per day. Most of the time is spent waiting for a sufficient number of
strong signals so as to provide good statistics on the ratio of £ to H.
Our experience has shown that the 8 Hz signals are often insufficient
to provide good statistics; 26, 86, and 270 Hz signals are always good;
700 Hz signals tend to be variable; 2 KHz signals virtually nonexistent;
and 7 KHz and greater the signals are very good.
Results
Figures 2-27 are the AMT sounding curves obtained in the Long Valley
caldera. Station locations are shown on plate 1. The soundings are
plotted on a logarithmic base with the frequency increasing to the left,
contrary to conventional use. They are similar in appearance to a
Schlumberger sounding curve and hence permit easier reference to the work
of Stanley, Jackson, and Zohdy (1973) in Long Valley. Examination of the
AMT sounding curves shows the data scatter that is inherent in the method
when soundings at one site are compared for both east-west and north-south
orientation of the electric dipole. This data scatter is dependent on
station location and in part reflects lateral variations in resistivity.
Some sounding locations such as stations AMT 14 and 16 show
consistent variations in apparent resistivity between east-west and
north-south sensor orientations. These differences are due to major
lateral variations which may be produced by fault zones oriented
subparallel to one of the sensors. The absence of these consistent
variations, however, doesn't necessarily imply the absence of lateral
inhomogeneities.
As Strangway and Vozoff (1970) have pointed out, the AMT method,
as it is an inductive technique, is excellent for locating conductors
because it tends to "look through" high resistivity material. This is the
reason for its application in geothermal exploration. However, as
Strangway and Vozoff also point out, its depth resolution suffers
accordingly. An intermediate'layer of higher resistivity sandwiched
between two lower ones can easily be undetected as a low velocity layer
can be undetected in seismic refraction work.
Strangway and Vozoff also note that in practice many more lateral
variations appear in AMT work than are perhaps expected. This also is
evident in the Long Valley soundings. Even some soundings which, at first
glance, appear consistent in both east-west and north-south directions do
not give horizontal layer interpretation in terms of reasonable layer
resistivities. This is evident from the slope of the sounding curves.
These problems suggest that the AMT method might best be used as a
reconnaissance technique to look for the location of major conductive
anomalies. However workers should not put too much reliance on it for
definitive depth information. Also, as is found for the AFMAG method, the
AMT method would be expected to detect fault zones along which conductive
zones tend to concentrate the telluric currents. Because of these
problems, data presentation in Long Valley has been limited to the
preparation of a series of anomaly maps at several frequencies and some
pseudosections.
In principle, one could assume that the earth is not horizontally
stratified and attempt to interpret the AMI soundings in terms of
7
three-dimensional models. In practice, techniques for solving the
forward problem of three-dimensional modelling are just now being
developed and it will be some time before we learn to quantitatively
interpret AMI data in terms of complex models.
Plate 1 shows the location of 26 sounding stations occupied in the
caldera and the hot springs and fault patterns taken from Bailey, Lanphere,
and Dalrymple (1973). One of the major features seen on this map is the
branching of the Hilton Creek fault to the northwest as it leaves the
Sierra block near Whitmore Hot Springs, with many of the inter-caldera
branches trending to the north and northwest across the resurgent dome.
Plates 2 and 3 are maps of the apparent resistivity variations at
26 Hz for orientation of the electric field dipole east-west and north-south
respectively. The differences reflect the effect of lateral inhomogeneities
within the caldera. In a gross sense, conductors oriented subparallel to
the electric line will tend to give lower values than conductors at right
angles to the electric line. This is complicated by current concentration
within the conductor and one finds resistivity highs associated with the
ends of conductors. The anomaly pattern reflects in a gross way, but not
in detail, the position and shape of the conductors.
In view of the low station density, plate 2 shows an anomaly pattern
remarkably similar to the total field resistivity anomaly of Stanley,
Jackson, and Zohdy (1973). A small closed low is seen at Casa Diablo Hot
Springs and an elongate low which trends northwest near the Cashbaugh
Ranch. It should be noted on these maps that resistivity contouring is
based on a logarithmic scale. The Cashbaugh Ranch low however is much
more restricted in area to the northeast than is indicated on the total
field map. The reason for this difference is not entirely clear but is
related at least in part to the structural complexities in this region as
discussed by Stanley, Jackson, and Zohdy (1973).
Plate 3 shows the 26 Hz data for a N-S orientation of the electric
dipole. Again the general pattern is the same as plate 2. It shows a
V-shaped low with one arm east-west and the other northwest and the center
about where the Hilton Creek fault enters the caldera. The low is now
centered on a hot spring just north of the Whitmore Hot Springs. The
8
differences in plates 2 and 3 are due to lateral resistivity variations,
and some inferences can be made regarding the conductor orientation.
Stations 16 and 14 primarily define the east-west low in plate 3; however,
the magnitude of the apparent resistivity is much lower at these stations
where the electric dipole is oriented north-south rather than east-west.
These data imply that the conductor has a generally north-south trend.
By referring to plate 1 it can be seen that these stations are very close
to north-trending faults.
Examination of all the sounding curves shows that at the lower
frequencies most conductors tend to have a more north-south orientation
and only in a few cases, as soundings 21 and 22, is an east-west
orientation indicated.
The southern edge of the surveyed area near the caldera boundary
clearly shows by the abrupt increase in resistivity the presence of the
basement rocks associated with the Sierra front.
Plates 4 through 7 show apparent resistivity maps of 8, 86, 270, and
7,000 Hz. Where substantial differences were measured between the two
orientations of the electric dipole an average value of apparent
resistivity was plotted. These figures show very similar trends to the
two 26 Hz maps (pis. 2 and 3). The 8 Hz map (pi. 4), which is looking deeper
than the others, has a greater similarity to the total field map of Stanley
and others, particularly if the sounding data is examined. At the higher
frequencies the flows associated with the resurgent dome become more
apparent by the high resistivity values on the northwest end of the mapped
area.
Plate 7 represents the near surface resistivity variations, but the
sampling density is very poor as at this high frequency a relatively
small volume is being measured. This map shows a strong east-west
orientation of the low resistivity area running from Casa Diablo to Lake
Crowley. The low resistivity probably indicates the region where hot-spring
activity has caused most of the near surface alteration.
In magnetotelluric work, electrical cross sections are often used
as aids to interpret variations of electrical properties with depth.
These are usually called pseudosections and plotted with frequency
decreasing downward on a logarithmic scale. Plates 8 and 9 are
pseudosections along section A-A 1 , oriented approximately north-south
through Whitmore Hot Springs. The two sections are plotted for east-west
and north-south orientation of the electric dipole. The sections
clearly show a vertical low resistivity feature at station 14. This low
is interpreted to be a fault zone along which thermal waters are rising
and along which extensive alteration has taken place.
An obvious disadvantage of these pseudosections is that they give a
very distorted idea of the depth of exploration at each data point. For
instance, at station 27 the depth from which information is obtained is
about 5 times deeper than at station 14. An alternative presentation is
shown in plates 10 and 11 in which the skin depth is plotted on the vertical
axis corresponding to a measured apparent resistivity and frequency. These
are still pseudosections in that they do not represent an anlytical solution
in terms of a resistivity-depth model. Their justification is in their
reduced distortion of the resistivity section.
These skin-depth pseudosections illustrate the ability of AMT
surveying to look through large thicknesses of high resistivity rock as on
the south ends of the profiles (pis. 10 and 11) where depths of more than
2 km are sampled. They also clearly show the exploration limits where
low resistivities are encountered. This difference in exploration depth
should be kept clearly in mind when examining the anomaly maps (pis. 2-7).
The low resistivity feature at station 14 still remains but its inferred
depth range is clearly limited a fact not directly apparent from
plates 8 and 9.
Summary
The described AMT technique was developed for use as a reconnaissance
geothermal exploration tool to search for conductive anomalies associated
with hot saline waters and related altered rock. The exploration philosophy
is that a relatively inexpensive technique would be followed by more
definitive electrical techniques in appropriate areas. Long Valley was
used as a test area for equipment and field-technique development
described here.
The fieldwork was performed in one week by two persons, and therefore
represents no large field effort. Cost of the survey is minimal. The
10
correspondence with the other detailed electrical work in Long Valley
by Stanley and others is considered very good. In fact, if the AMT survey
had been used to pinpoint an intensive exploration program, this area
would not have significantly differed from that identified by the more
detailed electrical methods. This study, at least for Long Valley,
demonstrates the effectiveness of AMT as a reconnaissance technique and we
believe clearly demonstrates that AMT is a cost-effective tool for
reconnaissance exploration of geothermal areas. This was the major purpose
for the work described here.
In terms of the geothermal potential of Long Valley, the hot water
and associated altered zones appear from the AMT data to be restricted to
a V-shaped area that extends from Casa Diablo east to Whitmore Hot Springs
and then northwest to the head of Little Hot Creek. Within this area the
hot springs and their alteration zones are concentrated along north- to
northwest-trending fault zones. These fault zones appear to be channels
along which hot waters leak from a more poorly defined deeper geothermal'.
reservoir in the southwest part of the caldera. The AMT data implies that
shallow exploration in Long Valley should be restricted to faults within
the described V-shaped zone. The method offers little regarding deep
exploration.
References
Anderson, L. A., and Johnson, G. R., 197A, A self-potential survey
of Long Valley caldera, Mono County, Calif.: U.S. Geol. Survey
open-file rept.
Bleil, D. F., 1964, Natural electromagnetic phenomena below 30 Kc/s:
Plenium Press, 470 p.
Bailey, R. A., Lanphere, M. A., and Dalrymple, G. B., 1973, Vulcanism
and geochronology of Long Valley caldera, Mono County, Calif.
£abSt7: EOS (Am. Geophys. Union Trans.), v. p. (in press).
Cagniard, Louis, 1953, Basic theory of the magneto-telluric method of
geophysical prospecting: Geophysics, v. 18, no. 3, p. 605-635.
Hill, D. P., McHugh, S., and Pakiser, L. C., 1973, Structure of the Long
Valley caldera from detailed seismic refraction measurements
IjabsJ: EOS (Am. Geophys. Union Trans.), v. p. (in press).
11
lyer, H. M., and Hitchcock, T., 1973, A seismic noise survey in Long
Valley, Calif, [absji EOS (Am. Geophys. Union Trans.), v. ,
p. (in press).
Kane, M. F., and Mabey, D. R., 1973, Gravity and magnetic anomalies in
Long Valley, Calif. £absj: EOS (Am. Geophys. Union Trans.), v. ,
p. (in press).
Keller, G. V., and Frischknecht, F. C., 1966, Electrical methods in
geophysical prospecting: Pergamon Press, 519 p.
Lackenbruch, A. H., Lewis, R. E., and Sass, I. H., 1973, Prospecting for
heat in Long Valley JlabsJ: EOS (Am. Geophys. Union Trans.), v. ,
p. (in press).
Stanley, W. D., Jackson, D. B., and Zohdy, A. A. R., 1973, Preliminary
results of deep electrical studies in the Long Valley caldera,
Mono-lnyo County, Calif.: EOS (Am. Geophys. Union Trans.), v. ,
p. (in press).
Steeples, D. W., and Pitt, A. M., 1973, Microearthquakes in and near
Long Valley, .Calif. Zabs._7: EOS ( Am. Geophys. Union Trans.), v. ,
p. (in press).
Strangway, D. W., and Vozoff, K., 1970, Mining exploration with natural
electromagnetic fields, in Marly, L. W., ed., Mining and groundwater
geophysics/1967: Ottawa, Queens Printer, p. 109-122.
Ward, S. H., 1967, The electromagnetic method, in Mining Geophysics v. 2,
Theory: Tulsa, Okla., Soc. Explor. Geophysicists, p. 224-372.
Willey, L. M., Rapp, J. B., and Barnes, Ivan, 1973, Geochemistry of
thermal waters in Long alley, Calif. £absj: EOS (Am. Geophys. Union
Trans.), v. , p. (in press).
12
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Fre
quen
cy in
Her
tz
1987
6 543
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02
19
87
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4
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10
Fig
ure
IQ
. A
MI
sou
ndin
g,
Lon
g -V
alle
y. 1
i5.,._
_x,
_ e0
.ect
ric_
line,
east
,^^_.e
le_c tr
ie li
ne .
sou
tlu
.
Fi I 3 00
roT
:§
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^ * 0,0^0*0- i
I
4 B-3N
01
fO
O _
09
. >..-.
mi'W
>;. {--.
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Frequency
in H
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Fig
ure
AM
I so
un
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uth
.
Apparent resistivity in ohm-meters
r
300
?
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J~.y
s?1^rrN
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»J
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w 4^ 01roO*
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I(Di IX-u3
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ort
f
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§
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Fre
qu
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in
Her
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Fig
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17
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MI
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ing
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62
, Apparent resistivity in ohm-meters
0QQ
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ocresistivity in ohm-meters
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<x>00
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Fre
quen
cy
In H
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19876
543
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21987 6
543
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10
Fig
ure
20
.._ A
MT
soun
ding
, .L
ong.
Val
ley.
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h..
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00
s-
r& o rt
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Fre
qu
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in
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1987 6
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Figure 22
... AMT
sounding,
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g.Va
lley
. //
27^
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lectric..line. wes
t, CL.electric
line
south._
%:= :i'
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i 'i
ife
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a
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o ft
rtH
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ro CM 4^ 01 o>-jo>to--r01 ^ 010 ro w *,
5
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i
w
r
ro oi
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CO
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N) ON
u»
r I-ot
£
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I
0101
rt N
o _
1
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