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SELF POTENTIAL SURVEY, SAN EMIDIO, NEVADA
for
STANDARD OIL OF CALIFORNIA
SENTURION SCIENCES, INC.
TULSA, U.S.A.
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SENTURION SCIENCES, INC.
8841 EAST 11TH STREET. TULSA. OKLAHOMA
P.o. lOX 1544"1. TULSA. OKLAHOMA 74115
PHONE 1118t 838-6748
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IMAGINEERING
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EXPLORATION, ENGINEERING and ENVIRONMENT
SELF POTENTIAL SURVEY, SAN EMIDIO, NEVADA
STANDARD OIL OF CALI FORN IJ',
INTRODUCTION
A spontaneous potential survey ~as , conducted for Standard Oil of
California at a geothennal prospect in Nevada. The site was in the
San Emidio Valley (T29N, R23E), ~Jashoe County. ·A total of 126 measurements were made from Hay 13-20, 1974.
INSTRUMENTATION AND FIELD PROCEDURE
The S-P measuremen~ were made with a Kiethley 155 Microvo1tmeter
with 10 megaohms input impedence. The potential difference between
points 100Qft. ~part was measured continuously for each ' traverse.
Electroae preparatfbn was difficult in some part~ of the valley, particularly in the dry sand and gravel alluvial fans. The first readings
were made on the isolated southern traverse shown on the map. This
, traverse was discontinued because of the poor repeatability of the
readings in the dry alluvials ,. On the valley floor the soil was damp
at depths from 2-3 inches to 1-2 feet and electrodes could be planted
which gave consistent and highly repeatable readings. At each of the
stations shown on the map, 2 seperate readings were made using different
electrode plants as a continuous check on the validity of the measurements.
DATA PRESENTATION
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The station loc~tions are shown on the map.
eac station is
the readin a
cumu a lV 0 a e a ong eac traverse relative to an arbitrar oint
a benchmark a
e n r o e cen er traverse. ac 1n lV1 ua1
S-P reading is actually a measurement of the gradient of the electric
field in the direction of the line of traverse. times the line length.
The sum 9f these measurements, relative to the arbitrary origin is contoured at levels of equal potential. The sign of the readings was kept
consistent by connecting the north end of the dipole to the negative. terminal of the instrument and the south end to the positive terminal. The
cumulative· curves of the S-P data along each of the north south traverses
are· shown as profiles. It should be noted that the eastern and western
lines are cumulative relative to their starting p01nts and not relative
to the benchmark to the north which is the arbitrary origin of the contoured data map. As can be seen from the profile curves a typical s-p reading in the valley varied from + 0.1 to + 15 mV. The cumulative effect
of these measurements was a weTl defined high on each of the profiles.
Around the closed traverses the c~mulative voltage was very small, being
amV for the western loop and 3.4mV for the eastern loop. These small
errors in, the cumulative voltage are on indirect indication of the general reliability of the measurements, for they come close to satisfying
the basic physical law that the sum of the voltages around any closed
path in an electric field be zero.
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DISCUSSION
The well defined high on the north south traverses and the~losure on
:he contour i:iap are in the generJl vicinity of the springs tl-:(lt red : h
the surface in the valley. It is possible that the hydrologic regime
responsible for the springs gives rise to the S-P anomaly. Differential
oxidation or cation exchange by trace minerals such as zeolites might
give rise to electro-chemical gradients large enough to be measured at
the surface. The rna nitud of the anomaly is not
those measured over su p ide bo 1 es norma y 00'" 1OOOm
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dera (Anderson,
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otential Survey of Long Valley a era,
U.S.G.S. open file report, Menlo Park, 1974). The well defined and
consistent gradients and closure and their proximity to the springs are
important in terms of the application of S-P measurements to geothermal
exploration. If there is any correlation ben~een the S-P anomaly and
the results of any other geophysics in the area the interpretation of
the S-P data could be put on firmer ground. It is possible that the
S-P data is an indication of the hydrology or cnemistry of geothermal
systems but the value of S-P
. if i
evaluate because
virtually none have een run over kno~~ geothermal areas. Comparison
should be made of this s-p data with the results of heat flow, resistivity, and passive seisrnics. Without such a comparison the value of
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the S-P data alone cannot be determined.
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