wilt & goldstein cerro prieto resistivity.pdf

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

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IJ

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W U U l T E 8

CONF-810399-27


2/15

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

372


3/15

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

373


6/15

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

374


7/15

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

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geothermal f ie ld ,

i p

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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).

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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.

375


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XBL 7881632

Figure

1.

pro jec t

.

Figura

1.

d e r e s i s t i v i d a d d e l LBL.

Pro jec t loc ati on map

LBL

r e s i s t i v i t y

Mapa de ubicaci6n d e l levantamiento

Permanent

FWOUS

pot

Current Electrodes Electrodes

XBL 8163203

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


9/15

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


10/15

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


11/15

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


12/15

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-


13/15

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-


14/15

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


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

wilt & goldstein cerro prieto resistivity.pdf

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