8/18/2019 Wilt & Goldstein Cerro prieto resistivity.pdf
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THIRD SYMPOSIUM
ON THE CERRO PRIETO GEOTHERMAL FIELD,
BAJA
CALIFORNIA, MEXICO
Sponsored by
United States Department of Energy, Office of Renewable Technology
Geothermal and Hydropower Technology Division
in Cooperation with
Comisidn Federal de Electricidad de Me xico
\
Q
DO
NOT M l R O f l L
PROCEEDINGSIACTAS
March 24 26 1981
San Francisco California
Earth Sciences Division
Lawrence Berkeley Laboratory
University of California
Berkeley, California 94720
R
Coordinadora Ejecutiva
de Cerro Prieto
Mexicali, Baja California,
Mgxico
Prepared for the
U S .
Department
o
Energy under Contract DE-AC03-76SFOCO98
k J T U
IJ
tM
bbtU Wtif
W U U l T E 8
CONF-810399-27
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RESULTS FROM
TW
YEARS
OF
RESISTIVITY MONITORING
AT
CERRO PRIETO
M.
J.
Wilt and N. E. Goldstein
Lawrence
Berkel ey Laborat ory
Uni versi ty
o
Cal i fornia
Berkel ey, Cal i fornia, U.S.A.
ABSTRACT
Di pol e-di pol e r esi st i vi t y measurement s
f or t he combi ned pur poses of r eser voi r del i neati on
and r esi sti vi t y moni t ori ng were f i r st made at
Cer r o Pr i et o i n 1978 and have cont i nued on an
annual basi s si nce then.
di pol e l i nes w t h per manent l y empl aced el ect r odes
at one ki l omet er spaci ngs wer e est abl i shed over
t he f i el d ar ea; one of t hese l i nes i s r emeasur ed
annual l y. Resi st i vi t y measurement s are t aken
usi ng a 25 kW gener ator capabl e of up t o 8OA
out put and a m cr opr ocessor cont r ol l ed si gnal
aver agi ng recei ver ; t hi s hi gh power- l ow noi se
syst em i s capabl e of hi ghl y accurat e measur ement s
even at l ar ge t r ansm t t er - r ecei ver separ at i ons.
St andar d er r or cal cul at i ons f or col l ected data
i ndi cat e er rors l ess than 5 percent f or al l
poi nt s, but 95 per cent conf i dence i nt er val s
show err or l i m t s about 2- 4 ti mes hi gher .
Anal ysi s of col l ected dat a i ndi cat e l i t t l e
change i n the apparent r esi st i vi t y of t he upper
300
m over t he f i el d producti on zone and t hat
i n thi s secti on measur ement s are r el at i ve-
l y i nsensi ti ve to the annual rai nf al l cycl e.
Apparent r esi st i vi t y i ncr eases were observed
over t he ol der pr oduci ng zone at Cerr o Pri eto
at depths of 1 km and gr eat er. Lar ge zones of
decr easi ng appar ent r esi st i vi t y were observed
f l anki ng t he zone of i ncr eases on bot h si des.
The i ncrease i n appar ent r esi st i vi t y i n the
product i on r egi on may be due to an i ncreasi ng
f r acti on of steam i n t he reservoi r resul t i ng
f r oma pr oducti on r el at ed decl i ne i n reser voi r
pr essur e.
t he resul t of f resh wat er i nf l ux fr om the
Col or ado ri ver. The zone of decl i ni ng resi sti v-
i t y f l anki ng t he area of i ncr ease may be due to
t he movement of sal i ne wat er s i nt o the r eser voi r
r egi on as a resul t of t he pr essur e decl i ne.
Quant i t at i ve model i ng of observed changes i s
i mpr act i cal ow ng to t he hi gh uncer t ai nt y i n
est i mati ng appar ent r esi st i vi t y changes and t he
nonuni queness of model s.
Two 20 km l ong di pol e-
Al t ernat i vel y t he i ncreases may be
I NTRODUCTI ON
Begi nni ng i n 1978, Lawr ence Berkel ey
Laborat ory (LBL), i n cooper ati on w t h t he Comsi bn
Federal de El ectr i ci dad i n Mexi co (CFE), began a
pr oj ect of moni t or i ng changes i n subsur f ace
r esi st i vi t y w t h surf ace resi st i vi t y measurements
over an area of i nt ense st eam and water producti on
at t he Cer r o Pri eto geot her mal f i el d i n Baj a
Cal i f orni a, Mexi co (Fi gure 1.
i ncl ude: (a) t he del i neati on of subsur f ace
r esi st i vi t y str ucture at Cerr o Pri et o and reservoi r
boundari es and
(b)
the f easi bi l i t y of det ecti ng
The pr oj ect goal s
changes i n t he subsur f ace r esi st i vi t y (e. g. , due
t o cont i nui ng f l ui d producti on) f r om sur f ace
measurement s. The proj ect pl an was to est abl i sh
a permanent arr ay of st ati ons and to dupl i cate
t he measurement on a year l y basi s
f or t he pur pose
of observi ng changes i n subsur f ace condi t i ons.
The r esi st i vi t y str ucture deri ved f rom
sur f ace measurement s has been descri bed i n Wl t
and Gol dst ei n (1979).
t he resul t s of two years of r esi st i vi t y moni t ori ng
at Cer r o Pri et o. We descr i be the f i el d system
used f or measurement s and t he met hods f or obt ai ni ng
hi gh qual i t y r epeat abl e dat a.
observed changes over t he t wo year span w l l be
examned i n t erms of t he exi st i ng t wo- di mensi onal
r esi st i vi t y model . Fi nal l y, an at t empt i s made
t o expl ai n geol ogi cal and hydr ol ogi cal processes
r el ated to groundwater w t hdr awal and t ect oni sm
I n thi s paper , we pr esent
I n addi t i on,
EXPERI MENT DESI GN
The di pol e-di pol e resi st i vi t y method was
chosen f or t he resi st i vi t y moni t or i ng arr ay at
Cerr o Pri eto. Thi s met hod, whi ch i s commonl y
used i n m ni ng expl orat i on, was chosen f or
sever al r easons: (a) t he ease i n establ i shi ng
t he r el ati vel y f ew per manent el ectr odes needed
f or moni t or i ng; (b) t he i nherent sensi t i vi t y of
t he met hod to l at eral l y di scont i nuous resi st i vi t y
st r uct ur e; and (c) t he r el ati vel y short l engt hs
of w r e needed f or f i el d operat i on.
A schemati c di agr amof t he f i el d syst em
i s shown i n Fi gur e 2.
capabl e of provi di ng square wave curr ent s of up
t o 80 amps peak t o peak i nto the ground at up to
1200 vol t s f or square wave per i ods f r om1 t o 1000
seconds. Thi s power source pr oved i deal f or t he
Cer r o Pri eto sur vey si nce i t i s por t abl e yet
powerf ul enough to pr ovi de adequat e si gnal s f or
di st ant st at i ons. Because of t he hi ghl y conduct i ve
gr ound at Cer r o Pr i et o, 40 second peri od squar e
waves wer e used t o m ni m ze i nduct i ve coupl i ng
eff ects. Tri al s of 10 second peri od square waves
showed severe i nduct i ve at t enuat i on f or di st ant
si t es whi ch resul t ed i n er r oneousl y l ow apparent
res i st i vi t y est i mates.
The 25 kW generator i s
W t h t he LBL syst em si gnal s are recei ved
at f our di pol es si mul t aneousl y at i nt eger mul t i pl es
of
1
t o 10 t i mes t he 1 tan t ransmt t er di pol e
l engt h.
copper - copper sul f ate el ectr odes and el ect r oni cal - -
l y f i l t ered and ampl i f i ed.
t o r emove
6 0
Hz and t el l ur i c noi se. Af t er anal og
pr ocessi ng, t he si gnal s ar e di gi t i zed, decomposed
i nto Four i er components and st acked usi ng a
The si gnal s are detect ed w t h porous pot
Fi l t er i ng i s n e c e s s a b
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DISCLAIMER
This report was prepared as an account of work sponsored by anagency of the United States Government. Neither the United StatesGovernment nor any agency Thereof, nor any of their employees,makes any warranty, express or implied, or assumes any legalliability or responsibility for the accuracy, completeness, orusefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privatelyowned rights. Reference herein to any specific commercial product,process, or service by trade name, trademark, manufacturer, orotherwise does not necessarily constitute or imply its endorsement,recommendation, or favoring by the United States Government or anyagency thereof. The views and opinions of authors expressed hereindo not necessarily state or reflect those of the United StatesGovernment or any agency thereof.
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multichannel spectrum analyzer (Morrison et el.
,
1978).
for obtaining high quality data since it was
possible to eliminate much of the noise prior to
--tacking and efficiently reduce the remaining
This system was found to be very effective
ise by stacking the signals from four dipoles
tdmult aneously.
Two
2
km long dipole-dipole lines
oriented east-west were established in the field
area (Figure 1). Line D-D' is outside of the
producing field area and lies adjacent to the
Cerro Prieto volcano; this line was primarily
used for background information. Line E-E'
crosses directly over the production zone (Figure
3) and is remeasured on an annual basis for
monitoring purposes. Measurements are take n-at
130 points to a maximum n-spacing of
8,
which
corresponds to a transmitter-receiver separation
of 9 km and a maximum depth of penetration of
about 3 km. A minimum of 30 square wave cycles
were averaged at each site; and, for the distant
sites, where signals are weakest, more than 2
cycles were averaged. Measurements were often
taken during the evening hours and during
weekends, when telluric and cultural noise levels
were lowest. Almost half of the points were
measured twice or more during each annual field
session. This was done to estimate repeatibility
of measurements over a short time interval and to
compare short time repeatability with errors
estimated from individual data sets.
\
RESULTS
Estimate of Error.
For all measurements,
means and standard deviations (g) of apparent
resistivities were computed, and from these
quantities and
N,
the number
of
cycles averaged,
the percent standard error (SEI was computed,
U
percent SE
*
7100
' FT
average apparent resistivity.
This number estimates the error in the mean
of
a
given data set. With only random noise in the
signal this is a reasonable estimate of the
measurement error. Unfortunately, signals are
often contaminated by non-random sources (i.e.,
power lines, fence lines, vehicular traffic), and
such sources could provide a bias to the data
measurements. To estimate how severe this bias
can be, we made repeat measurements over particular
points two and three times during each annual
survey.
ranged from 12 hours to 8 days.
these data, 9 5 percent confidence 1
calculated, as displayed in Table 1.
In all cases, we find that the calc
The time interval between measurements
For some of
exceed the standard errors by a
In addition, the differences
s of remeasured Val
fall within the confidence interval
two
entries in the table show the results when
transmitter and receiver position were
w
erchanged. The calculated means for this case
are very close to each other; however, the errors
are much larger when the transmitter is located
at stations 12 and 13. The error difference is
probably due to the higher noise level at stations
7
and 8, which are close to the power plant.
Although it is likely that the confidence
intervals provide a more accurate representation
of actual error, their calculation for three sets
of data is a formidable task. Because of the
limited number of observations taken for each
point, the accuracy of these more rigorous
confidence interval calculations is still suspect.
We have therefore limited error calculations to
the standard error.
approach is that it gives a fairly accurate
representation of relative error for. values in
the pseudosection. A pseudosection plot of
standard errors the fall 1980 data set is
given in Figure
increasing with separation, and relatively
greater errors are observed in the western end of
the line where it is comparatively more difficult
to impress large currents in the ground. Errors
are relatively low in the central and eastern
portions of the line, which overlie the reservoir
region.
The advantage of this
This figure shows errors
Observed Apparent Resistivity Changes.
Figure 5 is a line plot of observed apparent
resistivity differences for h-spacings of 1 and
4
over line E-E'.
calculations for two sets of data relative to
baseline measurements taken in the spring of
1979. For the n=1 line plot (Figure 5a), the
differences are large for both sets of data at
the western end of the line but relatively small
elsewhere.
The plot shows percent difference
To assess the effect of the annual
rainfall cycle on subsurface conditions, a set of
measurements taken in the fall of 1980, at the
end of the dry season, is compared with measure-
ments taken in the spring of the same year.
Figure 5a shows that the apparent resistivity
differences for measurements taken over the
producing field (stations 6-13) at the n=l
spacing are very small for both data sets. This
indicates that the clays and muds in this region
are relatively insensitive to the annual rainfall
cycle. In contrast, the apparent resistivity
differences of the near surface in the alluvial
fan material adjacent to the Cucapa mountains
(stations 1-5) is much greater; this suggests
that the resistivity of the near surface in this
region is very sensitive to the annual rainfall
In Figure 5b, the apparent resistivity
ences for an n=4 spacing ar
both spring and fall of 1980 data
to the 1979 baseline. The
n==4
p
to a maximum depth of penetration of about 1300
m.
The figure indicates a sign
in.apparent resistivity over th
zone at Cerro Prieto (stations 7-11) and signifi-
cant decreases in an apparent resistivity for the
areas flanking the high.
A
similar pattern is
observed for n spacings.of 3 through
7.
Because
these changes are observed only at the larger
separation, this suggests that significant
resistivity changes are occurring at depth in the
producing reservoir at Cerro Prieto as well as in
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the region surrounding the reservoir.
Interpretation of results. Figures 6a
and 6b are pseudosection plots of apparent
resistivity 'differences from the spring and fall
of 1980 data respectively, relative to the 1979
baseline measurements. The differences, plotted
in percent, show apparent resistivity increases
as greater than 25 percent for points adjacent to
the Cucapk mountains, but also show decreases as
much as 25 percent in regions immediately eastward
and westward from the present steam production
zone.
For n spacings greater than 2, a significant
increase in apparent resistivity is observed over
the present steam production zone flanked on
either side by large regions of decreasing
apparent resistivity. Differences for both
figures are contoured at 2 and 5 percent, which
is close to or within the confidence limits for
these data. Because the changes occur for
clusters or groups of points in the psuedosection
that increase or decrease together, the observed
pattern of differences is probably significant,
although the actual shape of the patterns may be
sensitive to measurement error.
such changes does, however, suggest significant
subsurface variations caused by extensive fluid
withdrawal and subsequent groundwater recharge
into the system.
Both sets of data show a similar pattern.
The magnitude of
Figure 7 shows the present two-dimensional
dipole-dipole resistivity model over the region
encompassing the producing zone. For the purpose
of
analyzing apparent resistivity changes, we
briefly discuss the working two-dimensional model
presented in
1979). The most striking feature of the model is
the relatively resistive 4.0 ohm-m) body associ-
ated with the zone of present steam production.
The body is also associated with a zone of
increased consolidation (de la Peiia et al.,
1979) and metamorphic minerals (Elders et el. ,
1979).
sands and shales increase in resistivity in this
region (although for the shales the increase is
more dramatic)
and that the bulk density is
greater and bulk porosity lower than for corre-
sponding rocks outside this zone (Lyons and van
de Kamp, 1979; Elders et al., 1981).
an earlier paper (Wilt and Goldstein,
Well log analysis has indicated that both
Immediately east of this 4.0 ohm-m
resistivity zone lies a thin, steeply inclined
conductive body. This region correlates well
with a plane of microearthquake hypocenters on
the Hidalgo fault (Majer and McEvilly, 1981) and
the inferred source plane location for the
observed self-potential anomaly (Corwin et al.,
1979). Well log analysis indicates that this
region is characterized by warm water and low
resistivity sands and shales (Diaz et al.,
1981).
sponds to a plume of ascending hot waters connec-
ting the zone of deep production east of the
power plant to the shallower production zone
adjacent to and west of the plant (Elders et
al., 1981).
represents a mixing area for upward-moving hot
waters and colder waters moving in from the sides
or downward from above. East of this conductor,
the rocks gradually increase in resisitivity
One explanation is that this area corre-
It is also possible that this zone
indicating fresher pore waters as we approach the
Colorado River.
zone the resistivity at depth is low, less than
1.5 ohm-m.
interpreted this section as a sequence of marine
beds saturated with partially evaporated sea
water.
West
of
the steam production
Lyons and van de Kamp (1979) have
With this conceptual model,
it is
possible to explain in a general way the observed
apparent resistivity changes at Cerro Prieto. A n
increase in apparent resistivity in the older,
shallower-producing zone can be explained by an
increase in steam fraction in the formation due
to production or by a replacement of produced
waters by less saline Colorado River water, or a
combination of these two. Evidence for the
former comes from the observation that enthalpy
has increased and pressure has decreased in many
of the older wells over the past several years
(Goyal et al., 1981). The chemistry of produced
waters has also changed markedly over the past
several years to more closely resemble Colorado
River water (Grant et al., 1981). This, along
with isotopic evidence (Williams and Elders,
19811, suggests significant fresh water recharge
for the geothermal system. For
a
10 percent
increase in resistivity in a year, a 15 percent
replacement of reservoir waters with waters
one-tenth as saline would be required.
ing the annual fluid production at Cerro Prieto
(Goyal et al., 19811, this is not unreasonable.
Consider-
A possible explanation for the regions
of resistivity decrease on either side of the
high is that more saline waters are moving
towards the reservoir in response to the pressure
drop caused by production.
plant, the apparent resistivity decreased by as
much as 25 percent for a large region extending
from the surface to great depths. If faulting is
important in this region, as suggested by self-
potential measurements and microearthquake
surveys, then flow channels may be created by
fault induced fracturing. The pore fluids may
then be moving fairly rapidly in response to
the production pressure drop.
East of the power
In order to quantify these observations,
an attempt was made to match the observed differ-
ences by perturbing the working two-dimensional
model. It was quickly discovered, however, that
the range
of
acceptable models that match the
data is very large. A second drawback is that
the pattern of observed differences is not
well-defined due to the margin of error in the
field data.
CONCLUSIONS
The two year resistivity monitoring
experiment has yielded some significant results:
(1) It is possible and feasible to monitor a
geothermal reservoir with surface resistivity
measurements but even in the most ideal situation
the measurement error may be large relative to
the expected change.
changes in apparent resistivity were observed
for the production region and surrounding area.
The observed 10 percent increase in apparent
(2) At Cerro Prieto, large
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re si s i t i v i ty i n the o lder product ion region may
be due to local boi l ing or to f resh water
invasi on. The lar ge decrease in apparent
re si s t iv i t y on the f lanks of the producing zone
may be due t o th e movement of more sa l i n e
he pressure drop. ( 3 ) Because of the
oundwater in to the res erv oir region in response
nonunique-ness of models and the pres ent le ve l of
measurement error, i t
i s
no t f eas ib l e t o quan t it a -
t i ve ly in t er pre t data by per turbing the working
two-dimensional model i n order t o match th e
observed changes.
ACKNOWLEDGMENT
The aut hor s wish t o acknowledge t he h elp
We als o wish t o thank Alfred Truesde ll ,
of Deborah Hopkins who was invo lved i n e rr o r calcu -
l a t i o n s .
Ern est Majer, Keshav Goyal, Serg io Diaz, and HCctor
Fonseca for valuable d iscussions .
This work was supported by t he As si sta nt
Secretary €or Conservation and Renewable Energy,
Of fi ce of Renewable Technology, Divisi on of Geother-
mal and Hydropower Technologies of the
U.S.
Depart-
ment
of
Energy under Contract DE-AC03-76SF00098.
REFERENCES
Corwin, R. F., H. F. Morrison, S . Dfaz C . , and
B. Rodriguez
J . ,
1978. Self po ten tia l
s tud ies a t t he Cerro Pr i e t o geothermal
f i e l d , in Procee dings, F i r s t Symposium
on
the Cerro Pri et o Geothermal Field ,
Baja
Ca li fo rn ia , Mexico, September 1978. Lawrence
Berk eley Lab ora tory Rep ort LBL-7098, p.
204-
210.
de l a Peaa L., A. , I . Puente C . , and E. Dfaz C.,
1979. Modelo geo ldgic o d e l
campo geothrmi-
co de Cerro Pr ie to , &Proc eeding s, Second
Symposium on th e C erro P ri e t o Geothermal
Fie ld, Baja Cal iforn ia, Mexico, i n MexiCali,
Mexico, October 1979, Comisidn Federal de
Electricidad, pp. 29-52.
Dfaz
C., S . , I.
Puente
C. ,
A. de l a Pe3a
L . ,
Proposed geologic model based on981.
geophysical w e l l
logs,
Proceedings , Third
Symposium on the Cerro Prieto Geothermal
Fie ld, Baja Cal iforn ia, Mexico, ( th i s volume).
Elde r s ,
W.
A.,
J.
R. Hoagland, and A.
E Williams,
1979. Dis tr ib uti on of hydrothermal mineral
zones i n the Cerro Pr ie to geothermal f ie ld
o f
Baja Ca lif orn ia, Mexico, Proceedin gs,
Second Symposium on th e C erro P r i e t o Geother-
mal Fie ld , B aja Ca lif orn ia, Mexico, i n Mexi-
Cali, Mexico, October 1979, Comisidn
Federal de El ect ric ida d, pp. 57-65.
Elde r s , W. A.,
A. E . Williams,
and
J . R.
Hoagland, 1981. An int eg rat ed model fo r
the natural f low regime in the Cerro
Pr ie to geothermal f ie ld based upon petro-
logic al and isotope geochemical c r i te r i a
-
n Proceed ings, Third Symposium on th e Ce rro
Pr i e t o Geothermal F ie ld , Baja C al i fornia ,
Mexico, ( t h i s volume).
Goyal,
K.
P. ,
C.
W.
Miller ,
M.
J.
Lippmann, and
S. P.
Vonder Haar, 1981. Ana lysi s of Cerro
Pri eto production dat a Proceedings, Third
Symposium
on
the Cerro Prieto Geothermal
Fiel d, Baja Cal iforn ia, Mexico, ( t h is volume).
Grant, M. A., A. H. Truesdell and A. Mafidn M.
1981. Production induced bo il in g and col d
water e nt ry i n the Cerro Pr ie t o geothermal
res erv oir in dica ted by chemical and physical
measurements Pro cee ding s, Thi rd Symposium
on the Cerro Pr ie to Geothermal Field , Baja
Ca lif orn ia, Mexico, ( th i s volume).
Lyons, D.
J.,
P. C. van de Kamp,
S.
Vonder
B a r ,
J. Noble, and
J. H .
Howard, 1980. Subsurface
geolog ical and geophysical study of th e Cerro
Prieto
geothermal f ie ld ,
i p
Proceedings,
Second Symposium on the Cerro P ri et o Geother-
mal
Fie ld , Baja Ca l i fo rn i a ,
Mexico, in
Mexi-
C a l i , Mexico, October 1979, Comisibn
Fed era l de El ec tri cid ad, pp. 173-186.
Majer, E. L. and
T.
V. McEvilly, 1981. A de ta i l ed
microearthquake study a t th e Cerro Pri et o geo-
thermal fi e ld Proceedin gs, Third Symposium
on
t he Cerro Pr ie to Geothermal Fie ld, Baja
California, Mexico ,
(
h i s volume).
Morrison,
H. F., N . E.
Coldste in ,
W.
Hoversten,
G.
Oppliger, and C. Riveros, 1978. Descr ip t ion,
f i e l d test and data analysis of a con t ro l l ed
source EM system
(EM-60),
Lawrence Berkeley
Laboratory Report LBL-7088.
W i l l i a m s
A
E. and W.A. Eldere, 1981. Oxygen
i so tope
exchange in
rocks and minerals from
t he
Cerro
Pr i e t o geothermal system; i nd ica tor s
o f
temperature
d i s t r i b u t i o n and f l u i d f l o w ,
-
n
Proc eed ing s, Thi rd Symposium
on
the Cerro
Prieto Geothermal Field,
Baja California,
Mexico, ( t h i s volume).
W i l t
M.
J
and
H-
E. Goldstein, 1979. R e s i s t i v i t y
monitoring a t Cerro P r i e t o , g P roc ee ding s,
Second Symposium
on
t he
Cerro
Prieto Geother-
m a l Fie ld ,
Baja
California, Mexico,
in Wexi-
Cali, Mexico, October
1979,
Comisibn Federal
de El ec tr ic id ad , pp. 419-428.
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Figure
2.
dipole-dipole re si s t iv i t y system.
Schematic design diagram of t he
LBL
x eL 811-2532C
Figure 3.
d i p ol e r e s i s t i v i t y l i n e
E-E .
Figura 3.
de l a l i n ea de r e s i s t i v id ad d ipo lo-d ipo lo E-E
.
Sta t io n locat ion map for d ipole-
Mapa de ubicaci6n de
l a s
es t ac iones
Figura
2.
r e s i s t i v idad d ipo lo -d ipo lo de l LBL.
Diagrama e s q u d t i c o d e l
sistema
de
kilometers
0 I 2 3
4
5 6 7 8 9
IO
I I 12 13 i 4
15
16 17 18 19
:
=
7
8
9 -
10
-
.
XEL812-2717
Figure
4.
d a t a se t .
Figura 4.
otoiio de 1980.
Pseudosection pl ot of s tandard er r or s (SE) f o r f a l l 1 980
Seudosecci6n de errores estgndar SE) para
10s
d a t o s d e l
376
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30 - -
FALL 1980
-*- PR. 1980
20
c
.
’j IO
0
&
-10
.-
>
l
c
al
“ 1
20
n = l
SPACING
Figure 5a.
t o 1979 ba se lin e measurements, spac ing n = 1.
Figura 5a.
t i v a s
a
l a s mediciones de l in ea de base de 1979, separaci6n n =
1.
Line p lo t of pe r cen t apparen t r e s i s t i v i ty d i f f e r ences r e l a t i ve
Porcentaje de l a s d i f e r enc ias de r e s i s i t i v idad apa ren te
rela-
‘O r
t
20
L
n=4SPACING
PA
DIFFERENCES%
XBL 813-2715A
Figure 5b.
t o 1979 bas elin e measurements, sp
Line plot
of
percent
r e l a t i v e
377
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kilometers
0 1
2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 ~ 1 5 1 6 1 7 1 8 1 9
h
I
I 1
1 1 ' 1
14.
SPRING 1980
Figure 6a.
spring 1980 data set re l a t i ve t o sp r ing 1979 da ta
se t .
Pigura 6a.
para 10s datos de l o toso de 1980 re la t i vo a
10s
datos de l a primavera
de 1979.
Pseudosect ion p lo t o f appa ren t r e s i s t iv i t y d i f fe rences fo r
Seudosecc i6n de l a s d i fe renc ia s de r e s i s t iv i dad apa ren te
kilometers
I
2
3 4 5 6
7
8
9
IO
II
12
13 I I I I : ~4 15
16
17 18 19
FALL 1980
PA
DIFFERENCES
%
XBL 813-27188
Figure 6b.
f a l l 1980 da ta set relative to spring 1979 measurements.
Figura 6b.
para
10s
datos d e l o to iio de 1980 re la t iv o a
10s
datos de l a pr imavera
de 1979.
Pseudosect ion p l o t of apparent re s i s t iv i t y d if ferences fo r
Seudosecci6n de
l a s
d i fe renc ia s de re s i s t iv i dad aparen te
318
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I - 8.5
-
2.0
Fi gur e
7.
l i ne E-E .
Fi gur a 7.
a l o l ar go de l a l i nea E-E .
muest r a l a ubi caci bn de 10s pozos.
Expanded ver si on of t wo- di mensi onal r esi st i vi t y model over
Wel l l ocat i ons ar e shown at t he top of t he f i gur e.
Versi bn ampl i ada del model o de resi st i vi dad bi - di mensi onal
En l a par t e super i or de l a f i gur a se
-
Tabl e 1 Conf i dence l i m t s of appar ent r esi sti vi t y
esti mat es f or var i ous t r ansm t t er- r ecei ver di pol e
separ at i ons.
Tabl a 1
r ent e par a vari as separ aci ones ent r e el di pol a
Lf m t es de conf i anza de resi st i vi dad apa-
transmsor
y
r ecept or .
DATA POI NT
T R P A S E ( ) 95 CI(+)
C I / S E
8-9 5-6 1.686 .8 2.0 2.5
8-9 5-6 2.4 2.7
6-7 11-12 6.0 3.3
6-7 11-12 2.083 1.6 4.6 2.9
7-8 13-14 2.807 1.8 5.0 2.8
7-8 13-14 2.692 1.9 4.9 2.6
12-13 7-8
1.813 3.6
10.0
2.9
7-8 12-13
1.808 3.0 6.5 3.2
T
=
Transmt t i ng di pol e st at i ons
R
-
Recei vi ng di pol e st at i ons
PA Appar ent r esi st i vi t y
SE = Per cent St andard Er r or
CI - 95 Conf i dence I nt er val
CI f SE = 95 CI / SE
379
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RESULTADOS DE DOS AGOS D E MONITOREO
DE LA RESISTIVIDAD EN CERRO PRIETO
RESUMEN
Mediciones de res i t i vi dad dipolo-dipolo
c on e l dob le prop6s ito de de l ine a r e l yacimiento y
de monitorear l a res i s t i vid ad de l campo se han l l e -
vado
a
cab0 anualmente en Cerro Pri et o desde
1978.
En e l
&rea
d e l campo
se
e s ta b le c ie ron dos
l h e a s
dipolo-dipolo de 20
km
de longitud con e lec tr odos
permanentes emplazados
a
1
km
de d i s ta nc ia e n t re
si;
una de
esas
l i n e a s
se
mide anualmente.
Las
medi-
c i on e s d e r e s i s t i v i d a d s e r e a l i z a n u t i l i z a n d o u n
generador de 25
kW
con una corr ien te d e s a l i d a d e
ha s ta 8 0 A y un receptor promediador de se&les con-
trolado por un microprocesador. Este
sistema
de
a l t a potenc ia y ba jo ruido
es
capaz d e mediciones
altamente precisas, abn con grandes separaciones
e n t r e e l t ransmisor
y
e l recepto r. Los c6lc ulo s
e s t b d a r d e e r r o r p a r a d a t o s o bt en i do s i n d i ca n
errores menores de l
5
para todos
10s
puntos.
l h i t e s de confianza de 95%muestran Grgenes
de er ro r 2-4 veces
mas
a l t o s .
datos obtenidos muestran un cambio pequezo en
l a
r e s i s t iv ida d a pa re n te e n
10s 3 m
supe r io re s en l a
zona de producci6n de l campo; en dicha re gi 6n las
mediciones son re la t ivamente insens ibles
a1
c i c l o
pluvia l anua l .
gua de Cerro Prieto se observaron incrementos de
l a
r e s i s t i v i d a d a p a r e nt e a profundidades de
1
km y
mayores. Grandes zonas de disminuci6n de l a res is -
t iv idad aparente se observaron
a
ambos lados de l a
zona d e incremento. E l aumento de l a r e s i s t i v i d a d
aparente en l a reg& de producci6n puede debers e
a1 incremento de l a f r a c c i6n de va por e n e l ya c i -
miento como resu lta do d el abatimiento de l a pres i6n
d e l mismo rel acio nado con l a produccibn. Alterna-
tivamente, 10s aumentos de resistividad pueden ser
e l
r e su l ta do de l a entr ada de agua dulce d el Rio
Colorado.
L a
zona de res i s t i vi dad de c l inan te que
f l an q ue a e l
&ea
de aumento puede deberse a l a en-
trada de aguas sa lobres en l a regi 'on de l yacimiento
como consecuencia del abatimiento de
l a
presidn.
E l
modelado cuantita tivo de
10s
cambios observados
es impriictico debido a l a gran incer t idumbre en l a
estimaci6n de
10s
cambios de res is t ividad aparente
y l a f a l t a de unicidad de 10s modelos.
L o s
E l a d i s i s d e
10s
En l a
zona de produccidn
m6s
a n t i -
INTRODUCCION
A
comienzos de 1978, e l Lawrence Ber keley
Laboratory (LBL) en cooperaci'on con l a Comisi6n
Federal de Electricidad de M6xico
CFE)
comenzd en
e l k e a de inten sa produccidn de vapor y agua de l
campo geot6rmico de Cerro Pr ie to , Baja Ca l i fo rn ia ,
Mgxico (Fig. 1)
un
proyecto de monitoreo de cambios
en
l a
resistividad del subsuelo mediante mediciones
de l a r e s i s t iv ida d de sde l a sup erf ici e . Los obje-
t i vos de l proyecto inc luyen: a ) l a de l ineac i6n
de
l a
e s t ru c tu r a de r e s i s t i v ida d en e l subsuelo de
Cerro Pr i e to y de 10s l im ite s de l yac imiento, y
b) l a fac t ibi l idad de de tec ta r cambios en l a re-
s is t iv ida d de l subsuelo (e.8. debido a
l a
produc-
c i b continua de f luidos) , a par t i r de medic iones
hechas en l a supe r f ic ie .
lecer
un
conjunto permanente de estaciones
y
repe-
E l programa
era
de estab-
w
t i r anualmente l a s mediciones con e l f i n de obser-
va r cambios en l a s condiciones subterrgneas.
i l t
y Coldstein
(1979)
d e s c ri b i e r o n l a
e s t r u c t u r a d e l a r e s i s t iv ida d de r iva da de
l a s
me-
d ic ione s
d e
s u p e r f i c i e .
mos
10s
resul tados de dos
6 0 s
de monitoreo de l a
res i s t i vid ad en Cerro Pr ie t o . Descr ibimos e l equi-
PO
u t i l i z a d o e n
l as
mediciones y
10s
mgtodos para
ob te ner da tos r e p e t ib le s de a l t a c al idad .
A d d s ,
se examinarzn 10s cambios observados en
e l
lapso
de dos azos en tgrminos d e l modelo bi-dimensi onal
de res i s t i vid ad exis t ente . F inalmente, se hace un
i n t e n t o d e e x p l i c a r loa procesos geol6gicos e hi-
drol6gicos relacionados con l a extracci6n de aguas
subte rrgneas y e l tec tonismo.
En e s te t r a b a jo p re se nta -
DISEfJO DEL EXPERIMENT0
E l
mgtodo de resistividad dipolo-dipolo fu6
seleccionado para
e l
monitoreo de
l a
r e s i s t i v i d a d
en Cerro Pr ie to . Este mgtodo, ut il iz ad o comunmente
en exploraciones d e minerTa, fu6 seleccionado por
va r i a s r az ones : a ) l a f a c i l ida d de e s ta b le c e r loa
re la t vamente pocos e lec tro dos
pe
manentes necesa-
r i o s p a r a
e l
monitoreo; b)
rente de es te mgtodo a e s t r u c t u r a s d e r e s i s t i v i d a d
la te ra lme nte d i s c on t inua s y c )
cable re la t ivamente c or t as que
son
necesar ias para
l a s
operaciones de campo.
l a s e n s i b i l i d a d inhe-
l a s longi tudes de
En l a F ig u ra
2
s e muestra un diagrama esque-
E l
generador de
25
kWtic 0 de l equ ipo u t i l i z a do .
es capaz d e proveer co rri en tes de onda cuadrada de
has t a 8 0 amps de c re s ta
a
c r e s t a ,
y
de hasta 1200
volts para periodos de onda cuadrada
d e 1
a 1000
segundos.
para e l est udi o en Cerro Pr ie to dado que, aunque
po rt a t i l , ea suf ic ientemente poderosa
para
proveer
seza les adecuadas a l as e s tac io nes
a
d i s t a n t e s .
Se ut il iz ar on ondas cuadradas de un period0 de 40
segundos para minimizar 10s efectos de acoplamiento
induc tivo re su l ta n te s de l a
a l t a
conductividad del
ter ren o en Cerro Pri eto . Pruebas de ondas cuadra-
das de un perio d0 de 1 0 segundos mostraron atenua-
c i6n induc t iva severa en lug ares dis t ant es , dando
como resultado estimaciones d e r e s i s t iv ida d a pa re n-
t e erroneamente bajas.
Esta fue n te de e ne rg ia r e su l t6 ide a l
Con e l sistema del
LBL,
las seGales se r e c i -
ben en cuat ro dipolos simultaneamente a d i s t a n c i a s
que son mfi l t ip les en te ros , ent re
1
y 10 veces, de
l a longitud de 1 km del dipolo t ransmisor . Las
se na1es son detectadas con electrodos
porosos
de
cobre-sulfa to de cobre, y f i l t ra da s y amplif icadas
electronicamente. Es necesar io
f i l t r a r
l a s
s ek l e s
para
r emover e l r u ido t e l i k i c o y
e l
de 60
Hz.
Des-
pu'es d e l procesado analSg ico, l a s
se&les
s e
d i g i t a l i z a n ,
se descomponen en
SUB
componentes de
Fourie r , y s e adic ionan ( s tacking ) ut i l izando un -
ana l i zador de es pec tro mult icana l (Morrison
et a
1978).
t i vo pa ra ob te ner da tos de a l t a calidad dado que es
pos ib le e l im ina r buena pa r te d e l ru ido a n te s de l
L d
e encontr6 que este sistema es muy efec-
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adicionado ( stacking ) y redu cir eficientemente e l
ruido restante adicionando simultaneamente
las
se-
:ales de c uatro d ipolos .
Doe lin ea e dipolo-dipolo de 20 km de longi-
o r ien tadas e s te -oes te se e s tab lec ie ron en e l
wmpo (Figura 1). La l i n e a D-D est& f u e r a d e l
6r ea de producci'on d e l campo y es adyacente a1
volc'an Cerro Pr iet o.
cipalmente para obtener informacign de fondo.
l b e a
E-E ,
que cruza directamente sobre l a zona
de producci'on (Figura
3 ) .
se usa para medir anual-
mente l a res ist ivi dad con fi ne s de monitoreo. La8
mediciones se efectGan en 130 puntos a una separa-
ci'on
(n)
m6xima de 8, que corresponde
a
una separa-
ci'on transmiaor -receptor de 9
km
y a una profundi-
dad m'axima de penetraci'on de 3
km.
se
promediaron
un
m'inimo de 30 cicloe de onda cuad-
rada. Para es tac iones d is tan tes , donde l a s s e k l e s
son
m6s &bi le s ,
se
promediaron m'as de 200 ciclos.
Las
mediciones s e efect uaron a menudo en horas de
l a
noche y en f i n e s de semana, cuando 10s n ive le s
d e r ui do t e l b i c o y c u l t u r a l
son
m'as bajos.
t e cada camp&a an ua l, c a si l a mitad de loa puntos
se
midieron doe veces
o
&s.
estimar l a repe t ib i l idad de las mediciones en in-
tervalos de tiempo cortos, y para comparar l a repe-
t i b i l i da d de cor t o p lazo con error es es timadoa de
conjuntos individuales de datos.
RESULTADOS
Dicha l h e a
se
uti l iz 'o pr in-
La
En
cada lugar
Duran-
Esto
se
hizo para
Estimaci'on de error.
Para todas las medi-
c iones se computaron promedios y desviaciones es-
t 'andares (a) de re s i s t iv idad apa ren te , y de
esas
cantidades y d el n b e r o de ci cl os promediado (N),
s e comput'o e l er ro r es tandar porcentual (SE),
U
per cen taj e SE = x 100
P * d
donde
V
es l a res ia t iv i d ia d aparente promedio.
E l
e r r o r e k n d a r (SEI
estima
e l
e r r o r e n
e l
promedio
de un conjunto dado de datos.
razonable de l e r ro r de l a medici'on s i
e l
ruido en
l a se na1 fue ra solamente aleat ori o. Desafortunada- .
mente,
l a s s e b l e s
est'& a menudo contaminadas
por
fuen te s no al ea to r i as ( i .e . l in eas de tensi 'on , 15-
neas de alambrados, t r6 nsi to vehicular) y d ichas
fuentes podrian provocar errores sistem'aticoa en
l a s
mediciones de datos. Para estimar cu& sever08
pueden se r es to s er rore s, realizamoa mediciones re
petidas 2 y
3
veces en determinados puntos durante
cada levantamiento anual. E l interval0 entre me-
dic iones var i6 entre 12 horas y 8 dias.
muestra en
l a
Tabla
1, se
calcularon l fm ites de
confianza de 95% para algunos de es to s datos.
todos loa casos, encontramos que
10s
l imi te s de con
fianza calculados excedieron 10s errores es t i indares
por un fa c t or de 2 a
4.
Adem&, l a s d i fe renc ia s
en t re 10s promedios de va lor es remedidos c ayemn,
usualmente, d entro d el l h i t e de confianza.
Las
Gltimas
d o s
en t radas de la Tabla 1muestran 10s re
sultados cuando
se
intercambiaron
la8
posiciones
de l r ecep to r y e l tran smis or. Loa promedios cal -
culados para
este
cas0 est& muy pr6ximos entre sf;
n
embargo, 10s e r r o r e s son mucho mayores cuando
u ransmisor se ubica en l as estac iones 12 y
13.
La d i fe renc ia de e r ro r se debe, probablemente, a 1
nivel de ru ido &s elevado en l as es tac iones 7 y 8,
Es una estimacibn
Como se
En
381
que se e ncuentran pr6ximas
a la
planta generadora.
Aunque es probable que 10s l h i t e s de
confianza den una represent aci6n e prec i sa de l
e r r o r r ea l , su c6 lculo pa ra
tres
conjuntos de datos
e s
una
tar ea enorme. Dado e l n b e r o limitado de
observaciones tomadas en cada punto,
l a
prec i s i6n
de es tos l -b i t es de confianza m
rigurosos
es
sospechosa. Por l o tanto, hemos lim itad o nues tros
cglculos de error a 1 de loa e rro res es t6ndar . La
venta ja de esta apr oxi mac ib ea que da
una
repre-
sentac i 'on bas tante prec isa de l error r e l a t i v o p a r a
valores de
l a
seudosecci6n.
muestra una r epresentaci' on en seudosecci6n de
err ores es tgndares para e l conjunto de da tos de l
otoso de
1980.
aumentan con l a separaci6n entre transmisor y re-
cept or, y s e observan er ro re s relativamente mayores
en e l extremo occidental de l a linea donde e8 com-
parativamente
6 s
i i c i l i n t ro d uc i r c o rr i e nt es
mayores en e l suelo.
ba jos en l ea pa r te s ce n t ra l y o r ien ta l de
l a
l i n e a
que cruza l a regi'0n del yacimiento.
En
l a Figura 4 se
Esta f i gura muestra err ore s que
Los e r r o r e s son relativamente
CABIBIOS OBSERVADOS
EN
LA RESISTIVIDAD APARENTE
En l a
Figura
5
se
mueatran
l a s
d i fe renc ia s
observadas en l a re s i s t iv idad apa ren te de l a l k e a
E-E , para
separaciones
(n)
igua le s
a 1
y 4.
gr if ic o muestra c6 lculos d e l porcenta je de d ifere;
c i a para dos conjuntos de da tos re la t ivos a medi-
ciones de linea de base hechos en l a primavera de
1979.
(Figura 5a) para ambos conjuntos de datos
las
d i -
fe renc ia s
son
grandes en e l extremo oeste de l a
l inea, pero relativamente pequeEas en todo e l
re s to .
E l
Para e l grgf ico correspondiente
a n =
1
Para
estimar
10s e f e ct o s d e l c i c l o p l u v ia l
anual sobre l as condiciones en e l subsuelo, se e-
fectu aron una a er i e de mediciones en e l otoso de
1980, a1 f i n a l d e l a esta ci6n seca, con e l f i n de
compararlas con mediciones efectuadas
en
l a
prima-
ve ra d e l mismo aiio. La Figura 5a muestra que las
d i fe renc ia s de resis t ividad aparente para medicio-
nes efectuadas sobre
e l
campo de producci6n (esta-
ciones 6-13), con separaciones
n
=
1,
son muy pe-
queiias en ambos con jun tos de dato s. Es to demues-
t r a qu e l a s a r c i l l a s y lodos de la regi'on
son
rela
t ivamente insensib les
a1
c i c l o
pluvi a l anual .
Por
con t ra s te ,
las
d i fe renc ia s de re s i s t iv idad
aparen-
t e son mucho mayores cerca de la superf ic ie en e l
mater ia l de abanico a luv ia l adyacente a l a s i e r r a
Cucap; (estaciones
1-5).
Esto sugiere que l a re-
s i s t iv idad de l a parte somera d e esta regi6n es
muy se ns ib le
e l
cic lo p luvia l anual .
En l a Figura
5b
se gra f ican las diferen-
c i as de res is t iv idad aparente para separac iones
n
-
1
para
l a s
se ri es de datos de primavera y
o t o -
50 de
1980,
r e l a t i v a s
a
l a l h e a de base de
1979.
Las separaciones n =
4
corresponden a una profundi
dad &xima de penetraci 6n de alr sde dor de 1300 m.
La
f i g u r a i n d i c a
un
incremento s ign if i ca t ivo en
l a
re s i s t iv idad apa ren te en l a zona de producci6n
116s
antigua
de
Cerro
Pr ie to (e s tac iones
7
11) y
una
dieminuci6n significativa en
las
&reas
que
f lu
quean l a zona antedicha.
Un
pa t& similar se ob-
serv a para separaciones con n e n t r e
3
y 7. Dado
que estos cambios se observan 8610 con las separa-
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ciones m grandes, es to su gier e que en Cerro
P r i e t o
est&
ocurr iendo cambios s igni f icat ivos de
r e s i s t i v i d a d a profundidad ta nto en e l yacimiento
bajo producci6n como en l a regi'on que rodea a1 ya-
cimient 0 .
In t e rp r e t ac idn de
10s
Resultados: Las
Figuras
6a
y 6b son seudosecciones de di ferencias
de r e s i s t i v idad apa ren te co r respond ientes r e sp ec t i
vamente
a
10s da tos de
la
primavera y d e l otoEo de
1980,
r e l a t i v o s
a las
mediciones
d e
linea
de base
de 1979.
b s
diferen cias , dadas en porcentaje ,
muest ran incrementos de res is t iv i dad aparen te de
has ta 25% en puntos adyacentes
a
l a s i e r r a de Cuca
p6. Tambi'en muestran disminuciones de hasta 25%
en regiones inmediatamente hacia
e l es te
y
e l
o e s t e
de l a actual zona de producci6n.
dat os muestran un pat &
similar.
Para separacio-
nes mayores de 2
se
observa
un
incremento s igni f i -
ca t ivo de r e s i s t i v idad apa ren te en l a actual zona
de producci6n, flanqueada a ambos lados por grandes
zonas donde l a r e s i s t i v i d a d a p a r e nt e
est&
disminu-
yendo. En ambas f i g u r a s l a s d i f e r e n c i a s se contor-
nean para 2 y 52, l o que est6 cerca o dent ro de
10s l ' imites de conf ianza para e sto s datos.
que en
l a
seudosecci6n
10s
cambios ocurren en
gru-
pos de puntos que aumentan
o
dismirmyen conjunta-
mente,
e l
pat& observado de di fe renc ias es pro-
bablemente signif icativo, aunque su
forma
real puz
de s e r s e n s i b l e a errores de medici6n.
Sin
embar-
g o , l a magnitud de
t a les
cambios sugiere variacio-
nes s ign i f i ca t ivas en
e l
subsuelo causadas por una
extensa ext raccidn de f lu i d0 y subsecuente recarga
subter rgnea de l s i s tema.
Ambas
series
de
Dado
La Figura
7
muestra
e l
actual modelo de
re-
si st iv id ad bi-dimensional dipolo-dipolo para la re
gi6n que abarca
l a
zona de prod ucc ibn. Con e l
pro_
p 6 s i t o d e a n a l i z a r
10s
cambios de resis t iv idad apa
rente, discutimos brevemente
a
continuaci6n
e l
mo-
del0 bi-dimensional d e l campo presentado en un t
bajo anterior (Wilt y Goldstein, 1979).
E l
rasgo
m6s
sobresaliente del modelo
es
e l c ue rp o r e l a t i v a
mente resis t ivo (4.0 ohm-m) a so ci ad o con la zona
ac tu al de producci6n. Este cuerpo ta mb ib
est6 a s 2
ciad o con una zona de mayor consol idac i6n (de l a
Pe& et al . , 1979), 9 de m inera les metamo'rficos
(Elde r s
e t
al. ,
1979).
An6l i s i s d e regist ros geo-
f i e i co s de pozos indicaron que tant o
l a
r e s i s t i v i -
dad de l as arenas como l a de las lu t i tas aumentan
en ea ta zona (aunque para
l a s
l u t i t a s
e l
incremen-
t o es s d r h t i c o ) , y que l a densidad es mayor y
l a
porosidad &s
baja
que l a de
l a s
rocas cor res-
pondientes loca l iza das fuera de esta zona (Lyons y
van de Kamp, 1979; E ld er s
e t al . ,
1981).
Inmediatamente
a1 es t e
de
es ta
zona de
4.0
ohm-m de re si st iv id ad , yace
un
cuerpo conduc-
t i v o delgado, sumamente inclina do. Esta regi'on co
rrelaciona bien con un plano de hipocentros micro-
sismicoa local izado sobre
l a
f a l l a
Hidalgo (Majer
y McEvilly, 1981) y con l a ub icac i6n in f e r ida de l
plano de
l a
fuente de l a anomalia de autopotencial
observada en e l campo (Corwin e t al . , 1979). An6li
s i s de regi st r os de pozos in dican que es ta zona se
ca rac t e r i za po r
aguas
c a l i e n t e s , y a r e n a s l u t i -
tas de baja resi t iv idad (Diaz e t al . , 198l3. Una
expl icacidn e s que
esta
zona corresponde a un pe-
nacho de aguas calientes ascendentes que conectan
l a
zona de producci6n profunda situada a1
es t e
de
la planta generadora con la regi'on de producci6n
de menor profundidad situada
a1
oeste de y adsacen
t e a l a planta (Elders e t al . , 1981).
posibl e que
6sta sea un
;rea de mezcla e nt re
aguas
ca l i en t es a scenden tes y aguas m6s f r i as descenden-
tes
o que fluyen horizontalmente.
A 1
es te de es t e
cuerpo conductivo,
l a s
rocas aumentan gradualmente-'
en resi s t iv idad indicando
aguas
de poro
s
d u l c e
a
medida que
nos
aproximamos
a1 Rio
Colorado.
oes t e de
l a zona
de producci6n de vapor,
l a
resis-
t i v idad a profundidad es ba ja, menos de 1.5 ohm-m.
Lyons y van de Kamp (1979) in te rp re ta ro n
esta
sez
ci6n como una secu enci a de es tr a t os marinos s atu-
rados con
agua
de mar parcialmente evaporada.
Tambi'en
e8
A1
Con este modelo conceptual
e8
pos ib l e e5
p l i c a r , d e
un
modo general, 10s cambios de
resistL
vidad aparente observados en Cerro Prie to. Un au-
mento en l a r e s i s t i v idad apa ren te en l a zona de pro
ducci6n m6s an ti gu a y poco profunda puede explicaf
se por un incremento en
l a
fracci6n de vapor en l a
formaci6n debido a la produccio'n, o por un reempla
eo de l a s
aguas
profundas por
aguas
menos salobres
d e l Rio Colorado,
o por una
combinaci6n de ambas
causas. hridencia de
l a
primera viene de la obsef
vaci6n de que en muchos de 10s pozos a nt ig uo s du-
r a n t e
10s
Gltimos 6 0 s
l a
e n t a l p b se ha elevado y
ha descendido l a presi'on (Goyal
e t
al., 1981).
bi'en han variado marcadamente en 10s Gltimos
6 0 s
l a s
ca rac t e r i s t i cas qu5u icas de las
aguas
produci-
da s hacia un mayor pareci do con las d e l Rio Colo-
rado (Grant e t
al.,
1981). Esto, junt o con l a e v i
dencia isot6pica (Will iams y Elders, 198l), sugie:
r e que ex i s t e
una
r eca rga s ign i f i ca t iva
de
agua
f r es c a en e l sistema geot6rmico. Para provocar en
un 6
n aumento de l 10%e n l a r e s i st i v id a d
S e r b
nece sari o un reemplazo d e l 15% de las aguas d e l y&
cimiento con
aguas
d e s a l i n i d a d
10
veces menor.
Esto
no es
i r razona ble dada
l a
producci6n
armal
de
f lu ido s en Cerro Pr i e to (Goyal e t
al . ,
1981).
T B ~
Una
posible expl icaci6n para
las
zonas
donde l a r e s i t i v idad dec rece a ambos lados del -
ximo es que otr as
aguas
sal obr es s e mueven hacia e l
yacimiento en respuesta a
l a
caida de l a i r e s i 6 n
causada por l a producci6n.
A 1
este de l a p lan ta
generadora, en una
gran
&ea
que s e extiende desde
l a
super f i c i e has t a gran profundidad, l a r e s i s t i v i
dad aparente disminuy6 hasta un 25%. S i
e l
a fa l l a
miento es importa nte en e s ta zona, como l o sugi eren
l a s mediciones
d e
autopo tencia l y
l a s
i n v e s t i g a c i o
nes microsismicas, pueden
crearse
cana les de f lu jo
mediante e l fracturamiento inducido por
fallas.
Los
f lu id os de poro podrian entonces moverse bas-
tante
rapidamente en respuesta a
l a
c d d a de p re -
si6n debida a l a produccio'n.
Con e l f i n de cuan t i f i ca r estas observa-
ciones,
se
real izb un in tento de reproduci r loa
ca=
bios observados perturbando e l modelo bi-dimensio
n a l elaborado. Si n embargo, pro nto descubrimos que
e l
n h e r o de modelos acep tab les que pueden corres-
ponder a 10s da tos
es
muy grande.
Una
segunda des
ventaja
es
que
e l
pa t& de cambios observado no
est6 bien definido debido
a1 margen
de e r r or de
loa datos de campo.
onclusiones
E l monitoreo de resis t iv ida d durante d
o s ha rendido a lgunos resul tados s igni f icat ivos:
1 )
geot6rmico midiendo l a r e s i s t i v idad desde l a s u p e r
E 8
poaible y v iabl e moni torear
un
yacimiento
382
8/18/2019 Wilt & Goldstein Cerro prieto resistivity.pdf
15/15
f i c i e ; s i n embargo, aGn ba jo las mejores condicio- Agradecimientos
nes,
e l
e rr o r de medici6n puede ser grande en rela
ci'on a1 cambio esperado.
2
En Germ P r i e t o s e
02
servan grandes cambios de reaist ividad aparente en
l a zona de produccidn y
Qreas
circundantes. E l i
temento de
10%
en re sis t iv i dad aparente observado
l a zona de produccidn s antigua puede deberse
a
e b u l l i c i d n l o c a l o a invasidn de agua dulce. La
gran disminuci6n de
l a
resis t iv idad puede deberse
a l a entrada de agua subter rgnea 6 s a l o b r e a la
regi6n del
presi6n. 37 Debido a l a f a l t a de unicidad de
10s
modelos y
a1
niv el a ctu al de er ro r de medicidn,
no
e s f a c t i b l e i n t e r p r e t a r cu a nt i ta t iv a me n te l o a cam-
bio s de re si st iv id ad perturbando e l modelo bi-dimen
s ion a l e l aborado pa ra i gua l a r
10s
cambios observa-
Loa auto res desean agradecer l a ayuda de
Deborah Hopkins que colabor6 en
1 8
d l c u l o s d e
e r ro r .
' d e l l , E r n e s t
Majer,
Keshav Goyal, Sergio D h z y
H6ctor Fonseca por v ali osa s discusionea.
Tambih quie ren agradecer a Alf red Trues-
Este t rab ajo cont6 con e l apoyo del
Ass ist ant S ecr eta ry f o r Conservation and Renewable
Energy, Office o f Renewable Technology, D ivi sio n
of Geothermal and Hydropower Technology de l Depar-
tamento de Energia de Estados Unidos bajo contrato
acimiento en respuesta
a
l a cafda de l a
DE-AC03-76SF00098.
dose
1