report on the reconnaissance resist1v ity and...
TRANSCRIPT
REPORT ON THE RECONNAISSANCE RESIST1V ITY AND VLF-EM SURVEYS
, .
OF THE
SAFFORD VALLEY AREA, GRAHMI CO., ARIZONA
by Phoenix Geophysics Incorporated
Phoenix, Arizona
Bureau of Geology and Mineral Technology Geological Survey Branch
Geothermal Group 845 N. Park Avenue Tucson, AZ 85719
OPEN-FILE REPORT 81-23
November 1979
Interpretations and conclusions in this report are those of the consultant and do not r.~:::css::-dy coincide with those of the SLt~: of the D reau of Geology and Mh.-.:ral TechnoloflV.
PIIOEN LA GEOPHYSICS INC.
NOTES ON ( ;SOTHERMAL EXPLORATION
USING THE RESISTIVITY METHOD
Many geophysical methods have been ,tried in the exploration for
geothermally "hot" areas in the upper regions of the earth's crust. The
only method that has been cbnsistently found to be succes sful has been the
resistivity technique. In this geophysical method, the specific resistivity
(or its reciprocal, the specific conductivity) of the earth's subsurface is
measured during traverses over the surface.
The principle of the technique is bas ed on the fact that the resistivity
of solution-saturated rocks will decrease as the salinity of the solutions is
increased and! or the temperature of the system is increased (see _Figure 1).
I Therefore, volumes of the earth's crust that contain abnormally hot and saline
solutions can often be detected as regions of low resistivity.
The resistivity measurements are usually made using grounded current
and potential electrodes, but some useful data can sometimes be obtained using
electr'omagnetic techniques. The field data shown on plan maps in Figure Z are
from the Broadlands Area in New Zealand; in this area there are substantial
flows of hot water and steam at the surface.
The results show resistivity lows measured with a Wenner Configuration
Resistivity Survey and a loop-loop electromagnetic survey. The anomalous
pattern is much the same in both cases and the regions of low resistivity cor-
relate well with the areas of increased rock temperature.
- 2 -
If the rock volume saturated with hot solutions does not extend to
the surface it will be neces sary to use large electrode intervals to detect
the resistivity lows. The resistivity data shown in "pseudo-section" form
in Figure 3 is from Java. Along this line there are two deep regions of low
resistivity detected for the larger electrode intervals used. Zone A is
associated with surface Ir?-anifestations of geothermal activity. The source
of the resistivity low at Zone B is unknown.
If the abnonnally hot region occur s in a sedimentary basin,:i-the
general resistivity level can be quite low, due to the high porosity in normal
sediments. This is the cas e in the Imperial Valley of California. The resisti
vities shown in Figure 4 are from an area near El Centro, California. The
largest electrode separation used was 12, 000 feet.
The results show a two-layer geometry with the upper layer having
a thickness of approximately one-half electrode interval (i. e. 1,000 feet).
The..resistivity in the upper layer is 3.0 ohm-meters; the resistivity of the
lower layer is 1.5 ohm-meters. Due to the small resistivity contrast,
additional measurements would be necessary to detennine the possible
geothermal importance of the lower resistivity layer at depth.
The results shown in Figure 4 are from a dipole-dipole electrode con
figuration survey. Our dipole-dipole data is plotted as a "pseudo-section" for
several values of n; the separation between the current electrodes and potential
electrodes, as well as the location of the electrodes along the survey line,
determine the position of the plotting point. The two-dimensional array of
- 3 -
data is then contoured (s ee below). The contour plots are not sections of the
DIPOLE-DIPOLE PLOTTING METHOD
I~X~I~ nx >1+-X4
/%/ff/Y/Offff//7//ffh?O/-/,/ffh7/?
2 3 4 5 6 7
N-I
N-2------ / N-3
4,5-9,10
electrical properties of the earth; they are convenient graphical repres entations
of the measurements made. However, with experience the contour patterns can
be interpreted to give some information about the source of the anomaly.
If the contour patterns indicate very siInple geometries, more quantitative •
interpretations can often be made. For instance, if the contours are horizontal
for a lateral distance of four to six electrode intervals, a horizontally layered
geometry is indicated. In this situation, theoretical type-curves for dipole-
dipole measurements in a layered geometry can be used in "curve fitting"
techniques to give the true' resis tivities and depths for the earth.
...
25
LIIJ a: 75 => ..-«100 0::: W 125 0-~ 150 W IT5 '-200
SALINITY, ports". mil/ion
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v V V
/ / V / / /
J II
V V V If J /
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/ j / J J I I
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RESISTIVITY, ohm - meters
VARIATIONS OF SOLUTION RESISTIVITY
WITH TEMPERATURE AND SALINITY
FIG. I
GEOPHYSICAL SURVEY
A. TEW£AATUM AT 111m O£I'TH a ~ PlDISTMTY SlJIM;Y UI!IING
W£NNEIt COHf'lQUItA T10H A· 11011\
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PHOENIX GEOPHYSICS INCORPORATED
REPORT ON THE
RECONNAISSANCE RESISTIVITY AND VLF-EM SURVEYS
OF THE
SAFFORD VALLEY AREA
GRAHAM COUNTY, ARIZONA
FOR
STATE OF ARIZONA
BUREAU OF GEOLOGY & MINERAL TECHNOLOGY
1. INTRODUCTION
At the request of Mr. W. Richard Hahman Sr., Research Geologist for
the State of Arizona, Bureau of Geology and Mineral Technology, Phoenix
Geophysics has completed a reconnaissance dipole-dipole resistivity and
VLF-EM survey of the Safford Valley Area, Graham County, Arizona.
A geological-geochemical compilation of the Safford Valley Area
(see DWG NO. R-U-5057M) indicates several regions of high temperature
gradient and high chemical geothermometry. T\oJo distinct lineament trends
transect the area and several hot srrinqs have been located near the
vicinity of major lineament intersections. The Arizona State, ~eothermal
Group has determined that this area may be a potemial geotherma1 resource.
The purpose of the Reconnaissance resistivity survey was to locate
and delineate 10vl resistivity zones that might indicate areas of concentrated
thermal activity. Measurements were made with 2000 foot dipoles at one
- 2 -
through five dipole separations along parallel survey lines. A fre-
quency of .12S HZ was used in order to minimize attenuation of the
electric field due to eddy current dissipation of energy and at the
same time avoid telluric noise.
The VLF-EM survey was simultaneously conducted to locate major
conducti ve structures whi ch may represent therma 1 condu~: ts. VLF -EM
measurements were make at sao foot intervals along each survey line
using orthogonal transmitting stations at Seattle Washington and
Bangor Maine.
2. PRESENTATION OF RESULTS
The resistivity survey results are shown on the following data
plots in the manner described in the notes which accomDany thi~ report.
LINE ELECTRODE INTERVALS OW€;. NO.
1 2000 feet R-U-SOS7-l 2 2000 feet R-U-SOS7-1 3 2000 feet R-U-50S7-1 4 2000 feet R-U-SOS7-2 S 2000 feet R-U-SOS7-2 6 2000 feet R-U-SOS7-2 7 2000 feet R-U-SOS7-3 8 2000 feet R-U-SOS7-3 9 2000 feet R-U-SOS7-3 10 2000 feet R-U-SOS7-4 11 2000 feet R-U-SOS7-4
An interpreted true resistivity section along each survey line
;s shown on the following plots:
ELECTRODE INTERVALS DWG. NO. -LINE
1 2000 feet IR-U-SOS7-l 2 2000 feet IR-U-SOS7-1 3 2000 feet IR-U-SOS7-1 4 2000 feet IR-U-SOS7-2 S 2000 feet IR-U-SOS7-2
- 3 -
LINE ELECTRODE INTERVALS OWG. NO. ~
r
6 2000 feet IR-U-SOS7-2 7 2000 feet IR-U-SOS7-3
.8 2000 feet IR-U-SOS7-3 9 2000 feet IR-U-S057-3 10 2000 feet IR-U-5057-4 11 2000 feet IR-U-SOS7-4
The interpreted true resistivity sections along each survey line
have been compiled with the aid of two-dimensional theoretical cur~es,
three dimensional model studies and a computer program for the direct
inversion of apparent resistivity data for layered media.
The VLF-EM data is plotted in profile form along each survey line
as follows:
LINE OWG. NO.
1 VLF-SOS7-l .. J
2 VLF-SOS7-l 3 VLF-50S7-1 4 VLF-SOS7-2 5 VLF-S057-2 6 VLF-S057-2 7 VLF-SOS7-3 8 VLF-50S7-3 9 VLF-S057-3 10 VLF-5057-4 11 VLF-SOS7-4
Also enclosed with this report is DWG. NO. R-U-5057, pl an map of
the survey area at a scale of 1" = one mile showing the location of the
survey lines. The definite, probable and possible Resistivity low anomalies
are indicated by bars, in a manner shown in the legend, on the plan map
as well as on the data plots. These bars represent the surface projection
of the anomalous respon~es as interpreted from the location of the trans-
mitter and receiver electrodes when the anomalous values were measured.
- 4 -
Since the Resistivity measurements is essentially an averaging ~
process, as are all potential methods, it is frequently difficult to
exactly ,pinpoint the source of an anomaly. Certainly, no anomaly can
be located with more accuracy than the electrode interval length. In
order to locate sources at some depth, larger electrode intervals must
be used, with a corresponding increase in the uncertainties of location.
Therefore, while the center of the indicated anomaly probably corresponds
fairly well with source, the length of the indicated anomaly along the
line should not be taken to represent the exact edges of the anomalous
material.
The anomalies shown on the plan map are designated apparent depths
of shallow, moderate, or deep. At larger dipole separations a greater
volume of rock is averaged, in lateral extent as well as depth. Thus,
the source of a deep-appearing anomaly detected along a single line may
be at shallow depth to one side of the line. The data plots, therefore,
cannot represent true depth. Depths can be calculated from the apparent
resistivity data in the case of ideal horizontal layers, but even this
calculation depends on an assumed resistivity contrast between the zone
at depth and the overlying rock. Although ambiguous, the following simple
d~pth designations are useful for correlating or comparing anomalous zones
obtained on adjacent survey lines.
Shallow
Moderate
Deep
Apparent Depth (dipole separations)
- 2
2 - 3
3 - 5
Drill Hole Depth (in dipole lengths)
1/2 - 1
1 - 1-1/2
1-1/2 - 2+
- 5 -
Thus, a shallow zone is one detected at a one-to-two dipole separation ..
and should be tested by a drill hole from a half-to-one dipole length
deep.
3. DISCUSSION OF RESULTS
The reconnaissance resistivity survey of the Safford Valley Area
\vas planned to cover as much area as possible \vithin a limited bud~t.
Approximately 105 line miles were surveyed in three possibly potenti~l
lJeothermal zones Itlithin the 600 square mile valley. These zones of
interest were selected in part from the qeological compilation enclosed
in this report and information provided by the 0eothermal staff of
Arizona State.
The first three surveyed lines were conducted along the east~rn
boundary of the area across the Gila River. Comparatively the apparent
resistivity is high along these lines especially on the north end. Low
resistivities occur on the south end of Line 3 from 12N to beyond 6S
with a probable anomaly interpreted at shallow depth between 10N and O.
This low resistivity does not appear on Line 2, two miles to the east,
but there is an indication that the resistivities are decreasing on the
extreme south end of this line and may be anomalous beyond this data.
The apparent resisti~ity over the remaining portion of these lines
are not indicative of a geothermal source. There is a decrease in re-
sistivity on Line 3 at depth b~~ween 38N and 42N but the measurements
are n~t low enough to be considered as thermal activity. The numerous
old pits and surface mineralization in this area suggest the source may
be a base ~etal prospects.
- 6 -
Several resistivity contacts have been interpreted on the data plots . and plan mar that indicate rock type change that may be structurally con-
troll ed . .
The second area of interes~ smith of the town of Safford was surveyed
along six near parallel east-west lines approximately two miles apart.
This data has outlined an extremely conductive zone having a true re-o
sistivity less than one ohm meter. Generally this anomalous zone co-
incides vlith or extends across an area of hinh temperature gradient and
appears to be restricted on the west by the down-dropped fault shown on
the geology map. The east side of the zone has not been defined and
possibly the previously discussed anomaly on Line 3 could represent the
eastern boundary but the true resistivity of this response is not as low
as the anomalous zone. The south end of Line 3 from a to 6S also exhibits
an increase in resistivity so it must be assumed that the northern part
of the zone does not extend to Line 3.
The apparent resistivity within parts of the anomalous area appears
to represent a simple layered media and computer interpretation has been
attempted at several locations on Line 5 and 6. It can be seen from the
data plot that the conductive zone on Line 5 is overlain by a resistive
layer. The computer interpretation sU9gests that the surface layer is
approximately 1000 feet thick and the conductive zone is approximately
500 feet with a true resisitivity near .2 ohm meter. i
On Line 6 the anomalous zone occurs at shallow depth. The computer
inversion for a two layer earth indicates a thickness in excess of 1000
feet with a true resistivity of approximately .4 ohm meters. A Schlumberger
- 7 -
-electrical resistivity sounding was also completed on Line 6 centered
at l6E. These results suggest a thi-n, (220 feet) higher resistivity
layer (6,5 ohm meters) overlies the conductive zone which is approximately
900 feet thick and .5 ohm meters true resistivity.
Generally the apparent resistivity within the anomalous zone in-
dicates an increase on N=4 or N=5 measurements; suggesting that the. low·
resistivity layer is underlain by a more resi~ive rock unit. There are,
however, several locations where the resistivity contour pattern extends
to d~rth and these may represent thermal conduits extending to depth.
These decreased resistivities occur at:
Line 6 Line 7
Line 9
between l6E and l8E between l2E and l4E between 8E and 10E.
The VLF data for these three lines indicates a very strong conductor
on Line 9 at approximately 9E, possibly two weaker conductors on Line 7 ~
between 12E ~nd l5E and mixed response on Line 6. The conductor in Line 9
is one of the strongest observed during the survey which is not associated
with a power line. Most definitely the VLF data has been affected by
cultural noise. Attempts were made to note power lines and operating
pumps that might distort measurements but quite obviously we have located
the buried pipeline on several lines and there are probably other cultural
anomalies located. However on several lines, Lines 1 and 2 for example, ~ .
several mapped lineaments are coincident with VLF anomalies, thus it
must be assumed that the VLF-EM system has located some of the more pro
minent lineaments which can assist the geol09ical evaluation of this area.
-
- 8 -
The remaining two lines that comprise this survey were conducted ~
in the north part of the Valley. The anomalous response interpreted on
the south end of Line 10 from 4N to beyond 8S appears similar in mag
nitude to the anomalous zone south of Safford. The weaker response on
Line 11 is similar to that on Line 3 and may represent the northwest
boundary of the area of interest.
Line 11 was surveyed past one of the few hot springs located in the
area and it is situated on this line near station 24N. There is .no
apparent resistivity change in this vicinity, nor is the resistivity
anomalous. It must be assumed that the source must be extremely narrow
or possibly fluids are moving along the major lineament through the hot
springs from the west.
4. CONCLUSIONS & RECOMMENDATIONS
The reconnaissance resistivity survey of the Safford Valley area has
located a large zone of extremely low resistivity that may represent a
geothermal reservior. This zone extends across six surveyed lines south
of the town of Safford and is, in part, coincident with an area of high
temperature gradients. The west side of the zone appears to be defined
by the down-dropped fault shown on the geological compilation; the east
side has not been indentified on any of the east-west lines. It is poss-
ible to assume from the data on the south side of Line 3 that the zone
does not extend beyond Line 3 and it may be postulated to be confined
within the suspected graben structure in this area.
The dipole-dipole data obviously illustrates that the zone is shal-
lowest in the north and computer interpretation of selected locations
- 9 -
along two lines indicated it is also thicker on the northern lihes. The i
Schlumberger vertical electrical sounding on Line 6 shows that the depth
t.o the a~omalous source is approximately 250 feet.
The only other defi ni.te anollla 1 ous resronse located duri ng the survey
occurs on the south end of Line 10. The magnitude of this anomaly is
similar to the anomalous zone and possibly represents the north-west ex-
tenion of the low resistivity area.
It is essential to consider additional exploration in this . area to
determine that the extremely low resistivity of this zone is du~ to thermal
activity and not just conductive sediments. Part of the zone of interest
is coincident with an area of high temperature gradients but additional
temperature gradient information is required across the entire zone and
especially where the suggested thermal conduits have been located on
Line 6, 7 and 9. Micro-seismic data may also determine which of the map-
ped lineaments are actual conduits.
It is strongly recommended that the source of this extremely low
resistivity ZOlle be determined prior to determining the actually size of
the conductive area.
Dated: July 24, 1979
dC:~!L-Bruce S. Bell Geologist
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I ~. ",. t" L ·:'1" to oj :1 P,: h:::.q F:ho 1 F' ,-, 0:0:;': Thl 1 (I 1 E- Ol (1O ~~' 25 4 e (1 14':::~. 1 .' .
1 I: - (11 0 0,,: 1 13 4 c-'-' . O 1 4 ?~'. 6.
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Pet. ::: t. oj It E' , '1 'i : i ,) n ::: 1 9. 0::" O. (1 -, -, -, .. ' .:....::.. ,
C 0:0 t" r e' 1 :. t . i c, n 11:. t 1- 1 .:~ 1.00013 o. OO~:10 LJ. 0000
.9796 0.0000 1.0000
I 11= P =t e O b~. C .:.1 Fe t. oj
1 t-.. , ~ -. 2 C"
.' " . '.' :::: 2 -, , 7 '. .;..
4 2 -. , ,. .'. .;..
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;;. ... ':-l .;. '3 -.~
";-' 4 1 2 1 .'. .:. .:;. ','
::: 4 1 "- 1 , '. oO. ;.
.:, 0::" 1 .:' . ,' .. ' 1 4 -4 1 0 c · .• ' 1
" 1 4 :.: 0
::: H F F I) F: It ' .... H L l. F " I" f:. 1J f 1 1[' H1 'i L L I 11 E 6 2 ~ E
E 1 ",. C ~. r ' 0:0 oj ';:' I ,', t ':; r ' .. ' :l. 1 = :;: (I 0 (1
I ~ E"'-' L.:'fI"Iboj "'- P,: h:::: q Rhol 1':1'102 0 1 E-Ol CH~175;;' · 4 0= .' . [1
1 E-Ol 00204 4 S. 0 .'. 1 E-n ':: CHJ202 .:: 4 c- O · ,_I •
Pet :::td It",·, .... ; .:it. 101'1:::: O. 0 O. 0
Correlation M:.t r ix 0.13000 0.0000 0.0000
i f c-. ' 5 .;: '. .,: . 0
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Thl . 1000. 1204 . 1217.
4.
1 l '
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0 .0000 0.0000 1.0000
11=.p .:.,: Ob s. 1 e 1 "
2 1 6 . :. .' . ';I ',' .::
4 .-. ::: .:: 0=- ~. 1 I) . ,' .; . 6 ::;: 1 1 ~ 4 1 r:-, . ., , ::: 4 1 6 .~ c-.,' 1 6
J 0 c-~, 1 '::-
C."l Pc to -6 '~
· '3 -1 .'. .:: -1 .-:.
.:...
1 4 1 4 1 6 1 ,;
t. oj
1 -~ 4 .'. .::
1 0 1 .:: -5
5 1 1 .-. -,:;.
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7 6 6
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1
PHOENIX GEOPHYSICS
APPENDIX
INTERPRETATION OF DIPOLE-DIPOLE
RESISTIVITY DATA
IN
GEOTHERMAL EXPLORATION
Electrical resistivity surveys have proven to be th~ most used, and useful, technique of all the conventional geophysical tools for geothermal exploration because the parameter it measures is related not only to heat but also to the fluid chemistry and porosity of formations. In most of the field cases reported in the literature, the resistivity survey has been conducted using grounded current and potential electrodes and one of the conventional, large interval, electrode configurations usually Wenner or Schlumberger. These configurations easily provide the variations in resistivity with depth, providing the area surveyed has a comparatively simple layered geometry. Unfortunately, the areas of most geothermal potential occur in the vicinity of recent volcanic activity which generally exhibits lateral variations and the interpretation of these two configurations becomes arduous.
Our experience has shown that the Dipole-Dipole electrode configuration has definite advantages in doing general exploration for zones of low resistivity. Hhen the data is plotted in a "pseudosection" manner, (see notes), it is possible to separate effects of vertical resistivity variations from lateral changes. Continuous profiles of four or five values of (N) give complete information concerning the resistivity variations along the line surveyed and provide better geologically interpreted results than the other methods.
If the geometry of the earth is simple enough to be approximated by horizontal layering, quantitative interpretation of the resistivities and thicknesses of layers can be compiled, with the aid of two-dimensional theoretical curves, three-dimensional model studies and a computer program for the direct inversion of apparent resistivity data for layered media. This is shown in Figure 1 and Figure 2. The interpreted parameters are shown on the data plots which is typical of those surveys in which the country
- 3 -
The anomalous source in Figure 5 bottom, could represent an area of thermal activity overlain by recent volcanics. The highly resistive surface layer distorts the anomalous pattern and would produce, on a plan map for N=4, resistivity lows from 8 to 9 and 12 to 13 and centered on 8 and 13 for N=5. Drill holes located by this interpretation would completely miss the anomalous source.
~
The pseudosection presentation of dipole-dipole data is a convenient graphical representation of the measurements made and with experience, the contour patterns can be interpreted to give information about the source of the anomaly.
Most dipole-dipole resistivity surveys for geothermal exploration initially require large electrode spacing to provide data to a depth approaching 2 kilometers. The large electrode intervals permits a fairly rapid detection of large areas of low resistivity but this type of survey must be considered as reconnaissance since it is difficult to locate drill targets within 600 meters to 750 meters or identify faults carrying thermal fluids. Quite often, anomalous sources located during the reconnaissance survey require detailed work employing shorter electrode int~vals that better define the response and permits the geophysicist ~o make a better evaluation of it's importance.
The 600 meter dipole data shown in Figure 6 top, identifies a resistivity low at surface between 3E and 9E. This response was interpreted as anomalous and detailed with 200 meter dipoles shown in Figure 6 bottom. The highly conductive area beneath 7E to 9E has been located on additional parallel lines and proven to be a : thermally active fault which is part of a geothermal production zone sustaining a 150 Megawatt plant. The completion of this type of detail is very important in order to get the maximum usefulness from a reconnaissance dipole-dipole resistivity survey.
[
- 2 -
rocks were thick sections of volcanic flows and fragmental rocks.
The results shown in Figure 1 have outlined a relatively narrow zone of lower resistivities, at considerable depth. One mile to the west, (Figure 2), the low resistivity zone has a much greater width. At about 100S, the apparent resistivities were relatively uniform, with a magnitude about equal to those at the southern end of Figure 1. \
The broad resistivity lows associated with geothermal reservoirs are generally believed to be the result of thermal fluids circulating away from a fault or faults which transports the fluids up from a heat source. Certainly, these faults would be the ultimate drilling target, but since the resistivity measurement is essential as an averaging process, it is frequently difficult to locate a comparatively narrow source when employing large dipole separations. The scale model studies on Figure 3 and Figure 4 show the response of a narrow conductive source.
The top half of Figure 3 shows a vertical conductor that has a width approximately 1/5 of a dipole length (i.e. 150 meters for 750 meter dipole surveys), and a resistivity contrast with bafkground of I to 4. The resulting resistivity pattern is difficult to - interpret as anomalous. However, in the bottom diagram of Figure 3, when the resistivity contrast is 1 to 40, a definite source is easily recongnized between stations 10 and 11.
The conductor in the top half of Fmgure 4 is now 1/3 the dipole length at a ratio of 1:4 and still the resistivity pattern cannot be interpreted as anomalous. The bottom diagram of Figure 4 represents a conductive shear zone or fault between two rock units having different resistivities. No anomalous pattern can be recognized but certainly the resistivity contact is observed between 10 and 11.
These diagrams illustrate that narrow conductive zones are difficult to detect but can be recognized when the resistivity contrast between the source and background is sufficiently high.
Quite frequently, persons unfamiliar with the dipole-dipole data presentation assume that the pseudosections are exact electrical property profiles of the earth and attempt to interpret this data from contoured plan maps of similar N values. This procedure is incorrect and can result in misinterpretation. The case model studies shown in Figure 5 attempt to correct this misconception.
Figure 5 top, could again represent a conductive fault that , now has a moderately steep dip. The anomalous pattern suggests the resistivity low dips to the left and plan maps produced from similar data on parallel lines would show a shallow eN=I) resistivity low centered between 9 and 10 and occurring at depth between 7 and 8 (N=5).
"\ , RESISTIVITY SURVEY IN WESTERN U.S.A.
LINE - ONE MILE WEST
DIPOLE -DIPOLE ELECTRODE CONFIGURATION
x = 2 000 FT., - N = I, 2 ,3 , 4,- F R E QUE N C Y - O' 05 Hz.
Po/2TT' IN OHM FEET (0·52 X Po-OHM METRES)
Theoretical fit for
One-layer earth.
""'~""" '-:\ ""'''''''''''''''' d PI (GEOTHERMAL)
ZONE I C d = infinity P, = 100 -110 ohm metres .
I "~--"I o 20N 40N 60N eON
>.»t»»»)».>))>>>) Pz
Z ON E I B d = 2000feet = 600 metres P, = 105- 115 ohm metre~ P z = 10-11 ohm metres
I---J" I
ZONE I A d = infinity P, = 105- 115 ohm metres
" ,~--J _____ ..... , lOON '20N 140N 160N ISON 200N 220N 240N 260N 2eON
I I ~ ,U '
~2 49 M '.~ " ,. 51 ~z ~o " ~ " --N-I
61 ~3 ~2 52 40 33~ 31 3;-~~ '7 'II 84 '4 -N-2
'I ,. 40 27 18 17 18 19- IJ/~ ,. '8 ''I ---N-3
'7 " '4 38\ 21\ (II II ' 10 ' II"'") 30 113 n I~ 63 84 -N-4
)
;\1
RESISTIVITY SURVEY IN WESTERN U.S.A.
LINE - TWO MILES WEST
DIPOLE - DIPOLE ELECTRODE CONFIGURATION
x = 2 000 FT., - N = I, 2 t:3, 4,- F R E QUE N C Y - O' 05 Hz.
Pa/2rr IN OH M FEET (0'52 X Pa-OHM METRES)
Theoretical fir for
One-layer earth.
(GEOTHERMAL) ~""'~-I\ ,,"""""''''''''''''''' ,,~""'" d PI
ZONE II C d :; 1000 feet:; 300 metres
PI:; 100-110 ohrn metres P 2 :; 5-0-5-5 ohm metres
r--_JA\...._---.
»)~»~~)j»»»~ P2
ZONE IT B d:; 2000 feet:; 600 m'3tres
PI:; 100 -110 ohm metres
P 2 :; 5 · 0- 5·5 ohm metre~ r--_-IA~ _ _,
ZONE II A d = infinity PI:; 100-110 ohm metres
,.....---Jfl.L-----,
20N 40N 60N eON lOON 120N 140t~ 160N IBON 200N 220N 240N 260N 2eON 300N , "
18 19 19 20
5-2 '-5 5 -I "3
3 • 5 3 - I
2.8 . 2-9
., .... ..~
]a
3'
30
33 42 ---N-I
'4 -N-2
80 ---N-l
88 - N-4
C'lr ')
PHOENIX GEOPHYSICS INC. Theoreti ca I Resistivity Studies
Calculated Cases
0 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 I I I I I I I I I I J I I I
N-I-- 10 10 10 10 10 10 8 10 10 10 10
N-2 10 10 10 10 10 9 9 10 10 10
N-3 10 10 10 10 10 9 9 9 10 10 10
N-4 10 10 10 10 9 9 9 9 9 9 10 10 10
N-5 10 10 10 9 9 9 9 9 9 9 1(. 10 10
N-6 10 10 9 9 9 9 9 9 9 9 10 10 10
P,=IO 1'3 = '0
0 2 3 4 5 6 8 9 10 II 12 13 14 15 16 17 18 I I I I
~. .. ~
PI P2= P3
co
• , 0 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18
I J I I I I I I I I I I I I - I I I
N-I -- 100 100 100 100 101 \!V 101
N-2 100 100 101 101 104 101
N-3 101 101 103 106 42 106 103
N-4 102 104 107 118 107 104
N-5 105 109 120 51 120 109 105
N-6 //0 121 54 54 54 90 121 110 105
PI =100 P3= 100
0 . 2 3 4 5 6 7 8 9 10 " 12 13 14 15 16 17 18
P2=2~
co
• FIG. :3
PHOENIX GEOPHYSICS INC. Theoretical Resistivity Studies
Calculated Cases
0 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 I I I I I I I I I I I I I I I
N-I-- 10 10 10 10 10 10 10/ 9 8 9 10 10 11)
N-2 10 10 10 10 10 10 9 8 8 9 10 10 10
N-3 10 10 10 10 II 9 9 9 9 9 " 10 10
N-4 · 10 10 10 " 9 9 9 9 9 9 " 10 10
N-~ 10 10 II 8 9 9 9 9 9 8 II 10 10
N-6 10 II 8 9 9 9 9 9 9 8 " 10 10
PI= 10 P3 =10
0 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18
P2=2] ~
PI 1'3
co
+ , 0 2 3 4 5 6 7 8 9 10 " 12 13 14 15 16 I T 18
I I I I I I I I I I I I I F I I I
N-I -- 10 10 10 10 10 10 15 48 54 5 I 50
N-2 10 10 9 15 15 47 58 53 51
N-3 10 9 16 16 16 46 61 54 52
N-4 10 9 16 16 16 16 46 63 56 53
N-5 9 9 8 9 16 16 16 16 16 46 65 58 54
N-6 8 7 9 16 16 16 16 16 16 46 67 59 55
PI= 10 f'3=50
00
+
FIG.4
0
N-I
N-2
N-3
N-4
N-5
o I
0
.PI:: 50
N-I
N-2
N-3
2 I
2
2 I
3 I
3 I
3 I
4 I
4 I
4 I
PHOE IX GEOPHYSICS INC.
51
124
Theoretical Resistivity Studies
5 I
51
5 I
5 I
51
56
337
202
123
6
50
55
6 I
6 I
211
50
55
57
346
7
7 I
7 I
Scale Model Cases
52
' :f'77
8 I
8 I
8 I
373
9 I
37
9
9 I
350
10 II
I
40 44
10 II
10 II I I
366 382
12 I
51
59
12
12 I
216 20~00 124 1~6 95 97
13 I
50 49
50
51 50
50
60 50
13
13 I
369 358
221
14 I
49
50
50
50
51
14
14 I
341
231
15 I
50
50
15
I~ I
230
83 9~
N-4 95 93 ~?~~ 65 66 ~~~ ~ N-5 83 86 81 (15 34 36 62 62 36 37 61
OVERBURDEN - ;> 3 = 350 o I' 2 3 4 ' 5 6 7 8 9 10 " 12 13 14
!~~' ~~~L~~'~%:~' %% 0.5 UNtTS ~;ZZC'~~>ZYffi2/a3Zm
.P I:: 62.5
P2 = 2.5} DEPTHTOTO I UNIT
N
~~~~~~~~~~
I--- 3.5 UNITS ---I
1,I
50
50
51
142
83
16 I
" .
50
50
16
16 ,.
97
16 I
50
50
86
17 I
50
17
17 I
17 I
50
18 !
18
18 I
18 I
FIG. 5
"T\
o m
PHOENIX GEOPHYSICS INC. RESISTIVITY SURVEY
GEOTHERMAL AREA '
45W 39W 33W 27W 21W 15W 9W 3W 3E 9E 15E 21E 27E 33E 39E I I I I I I I I I I I I I I
N - 1 ~~ N -2 3 .6 4.1
N - 3 5 .6
N -4 6.4
44 ~~3S 39~~~3.4
_____ 4 ._4---. e~1 1 j2.~) 3 .9...--...3 .4 4.4.....-~ 5.1 ~""""""·4.4 3.2
/~ )6.~ ~ 3.6
3.2
5 .1 3 .1 6 . 2 5 . 1 3.3
1.0 6.1 ~ 6 . 2 0=600 meters
IIW 9W 7W 5W 3W IW IE 3£ 5E 7E 9£ lIE 13E 15£ 17E 19£ 21£ I I I I I I I I
N -I
N-2 195
N-3
N·-4 I.
N-5 7.0' . ..
N-6 7.0 a = 200 meters