improved field techniques and integrated case histories ...directory.umm.ac.id/data...

Ž .Journal of Applied Geophysics 44 2000 181–196www.elsevier.nlrlocaterjappgeo

Improved field techniques and integrated case histories

Geoelectric exploration and monitoring in rock salt for the safetyassessment of underground waste disposal sites

Ugur Yaramanci )

Technical UniÕersity of Berlin, Department of Applied Geophysics, Ackerstr. 71–76, D-13355 Berlin, Germany

Received 24 March 1998; accepted 10 March 1999

Abstract

The safety of an underground waste disposal site depends to a large extent on the presence and migration of the water inthe rock. Geoelectrics is the most suitable method particularly in underground mining conditions to explore and monitor themoist zones in many different rocks. Recent developments in hardware, inversion schemes and petrophysical interpretationof the resistivity enable reliable and useful measurements to be conducted with direct current geoelectrics for exploration andmonitoring also in salt rock environment with rather high resistivities and resistivity changes. Examples are presented forresistivity exploration and monitoring in rock salt. The measurements are carried out in the research mine Asse in NorthGermany which is used for investigations into handling, storage, disposal and geological interaction of nuclear waste. Afully automated geoelectric system suitable for salt rock environment and long-term monitoring was used with a largenumber of electrodes installed permanently. Logistical conditions allow measurements only in profiles so that two-dimen-sional inversion schemes had to be used and their suitability and limitations are shown. At one site where the moisture isvisible in a limited area at the wall the extension of the moist zone in the rock is explored and determined. This helps toestimate the possible changes that might occur in future and which measures have to be taken. At another site in a large areain the rock salt the resistivity has been monitored over several months. The resistivity distribution in the area shows localvariations indicating changes in the state of rock salt which is otherwise usually homogeneous. The changes are related todisturbed rock zone at near surface around the voids due to the stress induced by the mining, to the neighbouring cavitiesand also disturbed zones in the deeper rock due to the stress redistribution in the last 30 years since the excavations tookplace. Also significant changes of resistivity with time are detected for which an estimate of water content can be given.These are attributed to fluctuations of the water content in the disturbed rock areas. q 2000 Elsevier Science B.V. All rightsreserved.

Keywords: Geoelectrics; Rock salt; Waste disposal; Mining; Resistivity

1. Introduction

An increasing importance is attached to rocksalt as it is generally considered to be very

) Fax: q49-30-314-72597; E-mail:[email protected]

suitable for the storage and disposal of high andmedium level nuclear waste as well as of toxic

Žchemical and industrial waste Matula, 1981;.Herrmann and Knipping, 1993 . Presently many

rock salt mines in Germany and around theworld are already used for waste storage pur-poses and also for storage of oil, gas and pres-surised air.

0926-9851r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0926-9851 99 00013-0

( )U. YaramancirJournal of Applied Geophysics 44 2000 181–196182

Geoscientific investigations of salt rocks,which are basically rock salt, anhydrite, carnalli-tite and salt clay, are expected to yield informa-tion about the tightness and stability of therocks and, hence, on the suitability of the rockto host hazardous material for a very long time.Apart from large scale geophysical surveyingfor geological site characterisation, there is anincreased need for small scale, but high resolu-tion investigations to assess the properties andthe state of the rock in the surroundings ofdrifts, shafts and caverns in the mine. The dis-turbed rock zone around any void in the mine isa potential migration path for dangerous mate-rial to interact with the biosphere even after thevoid is sealed.

Many geophysical methods are suitable forthe exploration of the structures in salt rocks.Main features like the boundaries of rock salt toanhydrite or salt clay may be found using seis-mics. Also disturbed rock zones can be charac-terised by their seismic velocities and attenua-

Ž .tion. Ground Penetrating Radar GPR has beenfound very useful in the past to explore thegeological structures. A high degree of resolu-tion can be achieved using tomographic modus.It is even possible to distinguish between reflec-tions from geological boundaries or from voidsby using reflection characteristics. However, themain draw back with GPR is that in moist areasthe electromagnetic waves are attenuatedstrongly, i.e., the depth of the investigation israther limited. Further limitation is due to strongreflections from geological features such as evenvery thin moist clay layers, and transmissionbeyond these layers is not possible. Because themoist areas are not penetrated by GPR it is alsonot possible to estimate any water content withGPR in salt rocks. In order to overcome someof these problems in exploring rock salt geo-electrics is utilised and adopted.

Geoelectrics has not been used very often insalt rocks as these are generally highly resistive,so large layouts are only possible with an ade-quate power source. Usually there are problemsconcerning the electrode coupling. But the most

important limitation is that a large number ofmeasurements is needed for a reasonable degreeof resolution and elaborate means are requiredfor processing and interpretation of the data.These problems are gradually being overcomein the past ten years or so. Automatic multielec-trode systems and also inversion algorithms formass data at a high degree of resolution and

Žaccuracy are now available Barker, 1992; Loke,.1995 . As a result, geoelectrics is being increas-

ingly used and it has turned out to be a valuabletool to assess various aspects of the salt rocksessential for the safety of a disposal site; forexample, to explore and monitor the presence ofnatural and man injected brine to derive quanti-tative estimates of parameters like porosity and

Ž .water i.e., brine content.Previous experiments for using direct current

geoelectrics in rock salt have helped to recog-nise and to overcome the difficulties particularly

Žconcerning the hardware used Kessels et al.,.1985; Skokan et al., 1989 . A special high

power automated multielectrode geoelectric sys-tem was successfully put to operation at the salt

Žmine Asse in North Germany Yaramanci and.Flach, 1989a,b . The system is used by a large

scale multimethodical investigation to assess thestability and the tightness of an underground

Žsealing in a drift Flach and Yaramanci, 1989;Walter and Yaramanci, 1993; Walter et al.,

.1993 . Using tomographic measurements aroundŽ .a sealing with a threedimensional 3D -elec-

trode array and 3D-resistivity modelling themoisture distribution in the disturbed rock zonearound a sealing has been mapped. This and alarge number of measurements in the driftsusing profiles yield very useful information ofthe geometry and state of different salt rocksrelevant to the safety assessment of an under-

Žground site Yaramanci, 1994a; Kurz, 1997;.Zimmer and Yaramanci, 1997 . Large data sets

collected for in situ resistivities of salt rock andcorresponding laboratory measurements havemade it possible to derive water contents from

Žthe resistivities Kessels et al., 1985; Kern et al.,1992; Kulenkampff and Yaramanci, 1993;

( )U. YaramancirJournal of Applied Geophysics 44 2000 181–196 183

.Yaramanci, 1994b; Yaramanci and Flach, 1992 .In this paper, two case histories are presentedwhich are representative of the exploration ofmoist zones and for monitoring large areas inrock salt. Also methodological and petrophysi-cal aspects of geoelectrical exploration in rocksalt are presented and the accuracy and resolu-tion which can be achieved shown.

2. Equipment and processing

In order to be able to conduct repeated mea-surements in large numbers, a geoelectrical

Žmeasurement system was designed Yaramanci.and Flach, 1989a . It consists of a commercially

available measurement instrument for geo-electrics, SYSCAL R2 of IRIS InstrumentsŽ .1993 , which is controlled fully by a computervia the serial link. This instrument was speciallymodified for use the in highly resistive saltrocks. The injection current range of the instru-ment is reduced from 1 A down to 200 mA aspossible currents in salt rocks are usually arounda few tens of milliampere. The accuracy of thecurrent measurement is also modified down to10 mA. The range of the measured voltage isincreased from 4 V up to 20 V as these are quitelarge in the resistive rock salt. The accuracy ofthe measured voltages is 50 mV which is stillsuitable for even very conductive moist areas.The instrument has an input impedance of 5MV which is suitable even for measurements inhigh resistive media. Measurements are con-ducted using many cycles of direct current withalternating sign and stacking. Effects from natu-ral self potential and electrode polarisation aretaken care for as this are measured and cor-rected for.

The voltage supply for injection current isfrom the mains via the usual ACrDC converterin steps of 50, 100, 200, 400 and 800 V. As theconverter can be set up manually for a certainvalue, measurements are organised in such away that they are grouped to suitable voltageranges. Meanwhile, experiments for automating

the input voltage also have been successfulusing a computer controlled voltage supply sothat by an over- or under-voltage the measure-ment will be repeated automatically with thenext appropriate voltage.

A multielectrode switch box was built withspecial relays which also is directly controlledby the software via serial link or, if necessary,manually. It can accommodate up to 480 elec-trodes and is free programmable; it is also ableto check electrode configurations for their cor-rectness so short circuits and zero connectionsare avoided.

A special software is designed to conduct themeasurements in arbitrary configurations andtime periods. The basic parts of the software

Ž .are: 1 the information about the measurementscontaining the numbers of the electrodes to beused, the duration of the measurement and num-

Ž .ber of cycles, 2 the information about theaddress of the electrodes at the multielectrode

Ž .switch box and 3 coordinates of the elec-trodes. After the measurements, apart from thefull information about the current and voltages,the apparent resistivity is also immediatelyavailable so that almost an online processingincluding the inversion is possible. The wholesystem can be used in remote control via mo-dem or internet so large numbers of measure-ments with any kind of configurations can bestarted at any time and all-around continuousmonitoring is possible.

The electrodes are made of common steelwith a length of 20 cm and a diameter of 2 cm.They are centred and cemented in holes of 3 cmdiameter. Because the rock is usually very dryand the contact resistances are very high theelectrodes had to be moistened occasionally.The electrodes are installed along profilesequidistantly and are usually 2 m apart.

For the measurements the usual four pointconfiguration is used with two electrodes for theinjection of the current and another two elec-trodes to measure the field. Several configura-tions were tested including pole–pole or

Ž .dipole–dipole arrays Kurz, 1997 . As the cur-


rents are very small, reasonable measurementsare only possible using linear arrays with thepotential electrodes between the current elec-trodes. The most convenient array is that ofWenner layout which is also used in most of thework.

A serious problem encountered in under-ground measurements is that the electrodes areon profiles on the walls so that for small layoutshalf-space conditions can be assumed safely butafter expanding to larger layouts full space con-ditions are valid. Usually, when profiles are indrifts, layouts larger than 5 times the diameterof the drift are to be considered in full space. Inthis work, the geometric factor is always calcu-lated for half space also because of the require-ments of the inversion scheme used. Conse-quently the apparent resistivities, as well as theinverted resistivities, for large layouts, are to bemultiplied by a factor of 2.

The inversion of the apparent resistivities arecalculated using the program RES2DINVŽ .Barker, 1992; Loke, 1995 which is based on aleast square iterative algorithm. It is comparableto other programs currently available and istherefore, representative. The merits of the in-version will be discussed in the last sectionalong with other possible sources of errors.

3. Examples of exploration and monitoring

In this section, two field examples from thesalt mine Asse in North Germany are given.

3.1. Delineation of a highly moist zone

Although rock salt is supposed to be veryŽ .dry, occasionally water i.e., brine might mi-

grate and seep in to the voids. This is notunusual and experienced often in salt mining. Infact, many mines are lost that way in northernGermany. Generally the mining activity itselfinduces stress release around the openings andthis favours the development of a disturbed rockzone in which brine may migrate easily. At one

Ž .location called A prominent moisture was ob-served on the walls and ceiling of the cavern K3Ž .Fig. 1 . In order to delineate the extension ofthe moist zone in the rock it was decided toconduct geoelectrical measurements. This wasthe only possible means of exploration, as, dueto safety reasons, boreholes in highly moistzones are generally not allowed by mining laws.

The area is entirely in rock salt bounded tothe South by almost vertical layers of anhydriteand sandstone which are not reached by thegeoelectrical measurements. A profile consist-ing of 50 electrodes 2.5 m apart was installed inthe caverns K2 and K3 over a length of approxi-

Ž .mately 140 m Fig. 1 . The caverns are 60 m inlength and 40 m in width and 12–15 m inheight. The electrodes along the profile are in-stalled in K2 on the wall at a height of 1.5 mfrom the floor and in a borehole in the pillarbetween K2 and K3. As the cavern K3 wasfilled partly with backfill the profile had to beinstalled towards the east with increasing height

Fig. 1. Map of the location A with 50 electrodes 2.5 mapart on the profile from 251 to 300. The electrodes areinstalled on the south face of the chambers, in a boreholebetween the chambers K2 and K3 and in the ceiling inchamber K3. The hatched area shows parties of rock saltcreeped into the chamber. The location is entirely in the

Ž .rock salt Na of different ages with minor differences inmineral content. At the south there is almost vertical

Ž . Ž .dipping anhydrite So1A and sandstone So .


at the wall and at y30 m even it had to becontinued at the ceiling.

The measurements are conducted with theconventional Wenner layout for every possible‘a’ and shifted always for ‘a’ so the maximumcoverage was possible. Starting with the small-est layout of as2.5 m this was systematicallyincreased to be 5 m, 7.5 m, 10 m, etc., up to 40m. The horizontal displacement of every layoutfor fixed ‘a’ has been D xs2.5 m so whenlayouts become larger also the overlapping areaof neighbouring measurements increased.

Ž .The results of the two-dimensional 2D -in-version are shown in Fig. 2. The depths herecorrespond to the horizontal distance from thewall of the cavern to the rock. There are basi-cally two low resistivity features observed whichare associated with the moist zones. The areaswith 105–106

Vm correspond to the undis-turbed rock salt which are found almost every-where in the mine and are considered to beunmoistened.

From the east end of the profile up to they60 m position there is a zone with extremelylow resistivity. The center of this zone is alongthe profile between y15 m to y35 m andabout 6 m depth. The resistivity in this centralzone is as low as 10 Vm and corresponds to avery large moisture content. The boundary ofthe moist zone to the west at y60 m is unusu-ally sharp. The depth extension of the zone isaround 10 m to 15 m at the east part but at thelocation around y60 m to y50 m even deeper.This area is therefore considered to be the mi-grating path of the brine from the levels above.The extension of the moist zone found by thegeoelectric measurements shows that it is muchlarger in the rock than it might had been as-sumed from the few moist zones visible on thewall and ceiling.

Another low resistivity anomaly is found atthe profile location from y80 m to y90 mhaving a center at a depth of about 4 m to 6 m.The resistivity is slightly lower than 104

Vm,

Ž .Fig. 2. Resistivity section at location A Fig. 1 with 0 m corresponding to electrode 251.


indicating only a small increase in water con-tent. It is a rather confined anomaly whichexplains why no moisture is observed on thewall at this location. This is an unexpectedmoist location which might be a recently openedpath for brine migration and in process of pro-gressing in to the cavern.

It should be noted that the investigated areais no longer accessible as the cavern K3 isclosed down by strongly creeping moist rocksalt from the ceiling and South wall. The cavernK2 was backfilled recently in order to increase

Žthe stability and avoid large breakings i.e.,.cracks . The electrode profile is still operating

and is used for further monitoring.

3.2. Monitoring of resistiÕity change at a largearea

Ž . ŽAt a location called B in the rock salt Fig..3 which is very close to the neighbouring

sandstone it was decided to investigate the stateof rock, i.e., the moisture content and stabilityover a long range of time. The area is typical ofthose encountered in salt mining where open-ings are very close to the non salt rock whichbehaves differently mechanically, i.e., as saltcreeps into the openings the rather stiff rocksbehind may crack and create potential migration

paths for water. This might cause serious prob-lems especially when the water paths are insome way connected to the aquifers. The mea-surements are also the very first large-scalegeoelectric monitoring in the rock salt so thatmethodical aspects for the suitability of themethod had to be considered.

The profile is located at the south wall of theopenings along the whole length available at

Ž .this location Fig. 3 . It consists of 249 elec-trodes with a separation of 2 m to cover a rangeof approximately 500 m. The openings alongthe profile are different so local conditions atthe profile are to be taken into account in theinterpretation. At this level only the caverns K4and K8 are of standard size with 60 m=40 mgroundfloor and 15 m height. Along the wholeprofile the electrodes are installed on the wall ata height of 1.5 m except in the cavern K3 at thewest end where the height is at 10 m becausethe cavern is backfilled up to that height. Thedrift between K3 and K4 was also backfilled sothat the electrodes in this area are installed in aborehole connecting the caverns and has a slightgradient to connect the different heights in K3and K4. Between K4 and K7 the profile is inthe drift of an approximately 4 m=3 m cross-section. K7 is a cavern with a height of only 4m but extends into the lower level for 18 m. K9

Fig. 3. Map of location B with 249 electrodes 2 m apart on a profile. The electrodes are installed in the chambers and driftsŽ .as well as in a borehole connecting K3 and K4. The location is entirely in rock salt Na . At the south there is almost vertical

Ž . Ž .dipping anhydrite So1A and sandstone So .


is similar to the standard size except that thewidth is 20 m instead of 40 m as in K8.

The south wall of the openings where theprofile was installed runs parallel to the strike ofthe geology. To the north there is solely rocksalt to a large extent. To the south there is

Ž .anhydrite So1A approximately 20 m thick withan almost vertical dip and parallel to the wall ina distance varying from 10 m to 30 m. Further

Ž .south there is sandstone So with rather largerthickness. There are further caverns just belowthis level in a similar geometry. Above thislevel, there is only pure rock salt.

An example of measured apparent resistivitysection is given in Fig. 4. The results of theinverted measurements are shown in Fig. 5.These are for 13 time points over 7.5 monthsselected from a lot more measurements todemonstrate the main features in the changes.The successive measurements are not equidis-tant in time, the time lapse is between 3 to 20days except for the last two measurements forwhich the time difference was 2.5 months.

The measurements consist of Wenner sec-tions for the ranges of ‘a’ starting with 2 m witha maximum of 48 m. For fixed ‘a’ the measure-ment location is displaced for ‘a’ along theprofile so that the coverage of neighbouringpoints is about 1r3. In every section there are878 measurement points in all which had also tobe kept to this size to fit the needs of theinversion schemes.

The main features of the resistivity distribu-tion do remain slightly similar in time and can

be easily followed through all the sections inFig. 5. In general, the resistivity is within arange 105–106

Vm. As this range is consideredto correspond to the normal state of the rocksalt, the deviations from these values are to beinvestigated closely. There is no resistivity be-low 5=104

Vm so that extreme moist zoneslike in location A in the previous example arenot present. At the higher end, the largest resis-tivities are slightly above 107

Vm.The resistivity change in the near surface

area is about a factor of 10 with some promi-nent higher resistivity zones around y435 m,y390 m, y340 m, y310 m. These are veryshallow but rather confined anomalies and areattributed to the near surface disturbed rockzone. It is well known that the creeping of thesalt into the openings is usually very high in theimmediate vicinity of the surface because of theadditional moisture adsorbed from the air circu-lating in the mine favouring the creep. The zonebehind has a moderate rate of creep so the rockmay crack in these areas. The size and extent ofthese zones depend on the local conditions, onactual creep rates, surrounding geometry ofopenings and, therefore, the actual stress distri-bution. To the east of the profile, the variationof the resistivity in the near surface area ismoderate with no significant extreme values sothat it can be concluded that the rock here is notdisturbed.

There are two further prominent high resistiv-Ž .ity areas which are too deep 6 to 8 m to be

considered near surface. The first area is located

Ž .Fig. 4. Example of the measured apparent resistivity section along the profile at the location B Fig. 3 with 0 mcorresponding to electrode 1.


at y220 m. It is present in all measurementsover the whole time and changes slightly, thesize extending into deeper regions. The otherarea is about from y150 m to 0 m with a ratherlarge change in size and resistivity in time.These areas, obviously also disturbed rockzones, are not directly the result of the differen-

tial creeping at the near surface area but due tosome other mechanism occurring in the deeperrange. The reason is not very clear yet. Mod-elling of stress and creep behaviour with thereal geometries of different geological layershere may give some indication on the mechani-cal behaviour.

Fig. 5. Inverted resistivity sections at location B for different times. Note the section is exaggerated for a factor ofapproximately 4 in the depth axis.


Ž .Fig. 5 continued .

Although the resistivity distribution seems toremain stationary with respect to time, signifi-cant changes can be revealed by examining thedifferences of measurements in time. For thisthe ratio relating the resistivity to that of theprevious measurement is very useful and shown

in Fig. 6. Basically the results for resistivitychanges are more accurate than the resistivitydistribution itself because systematic errors areeliminated to a great extent by taking the ratio.In general, there is a change of about a factor of2 to 3 which is not systematic locally and


temporally. The changes are larger for measure-ments taken over large time intervals.

One very prominent change occurs in thetime to 18.2.97 between y350 m to y300 m ina depth of about 26 m in a rather large area andprobably with an extension to deeper regions.These changes developed within a few days and

continued further at least till 28.2.97 as it couldbe followed in the measurements not shownhere. The range of the resistivity decrease isquite large at about a factor of 100. The area infact has previously a rather high resistivity indi-

Ž .cating a possible disturbed rock zone Fig. 5with higher porosity. The large decrease of re-

Ž .Fig. 6. Relation of resistivity to that of at previous measurement Ratios between successive measurements in Fig. 5 . Theratio is in logarithmic scale so that 0 means no change and an increase in resistivity is shown by positive values.


Ž .Fig. 6 continued .

sistivity suggests that this area might be filledwith migrating brine. This area is also promi-nent later on at 3.5.97 and 16.7.97, charac-terised by a higher resistivity. This would sup-port the idea of oscillating brine migrations in

the rock salt due to continuously changing stressand strain by relatively high creep rates. It is notclear yet if these kind of changes can be at-tributed to the usual behaviour before somebrine approach the openings.


4. Discussion

4.1. Accuracy of the obserÕations and inÕersion

The accuracy of geoelectrical measurementsdepends on many factors. A knowledge of thereliability of the estimates of the resistivity dis-tribution calculated by inversion is important byassigning structural elements to changing resis-tivities. Here, errors might be tolerated to acertain degree and the interpretation will bereduced only to significant changes over largeareas associated with geological boundaries ormoisture changes. Moreover, a precise determi-nation of resistivity is important if some petro-physical meaning is to be attached to thesevalues. Hence, some estimates are needed forthe degree of accuracies.

A large number of inversions has been calcu-lated for simulated structures and measurementsby the means of forward modelling using stan-dard geometries of dike like structures perpen-dicular and parallel to profiles at different depthsas well as for normal layering and local inho-mogeneities with rather large volumes. Gener-

Žally structures parallel to the profile i.e., in line.with the main current direction are mapped

Ž .better than otherwise Kurz, 1997 . It is alsowell known that finite difference type of inver-sions cannot cope with sharp and large changesin the resistivities and the distribution is some-what smeared or smoothed. Furthermore sys-tematic investigations show that by inversion ofapparent resistivities occasionally the modelmisfit can even increase when data misfit de-creases. For that the rms-data error by inver-sions might not be an appropriate indicator forthe fit; moreover, it is an integral measure of the

Ž .misfit Olayinka and Yaramanci, 1998 .Additional problems arise because the struc-

ture is actually 3D but the measurements areonly suitable for 2D inversion as they can onlybe conducted along profiles in the drift. Apartfrom that, some parts of the profiles are in largecaverns rather than in drifts and some otherparts are in boreholes so that the estimation of

the errors introduced by the geometry of theprofiles and surroundings is quite complicated.Numerical simulations using 3D finite differ-

Ž .ence resistivity modelling Fan, 1998 show thatthese effects may introduce errors as large as afactor of 2 or 3. However, by investigatingchanges, as is done by monitoring, it can safelybe assumed that some of the errors due to thedifferent factors discussed above are eliminated.

The error on the measurement, i.e., apparentresistivity is maximum a factor of 2. It is rea-sonable to assume that the error on the modelsection is of the same order of magnitude. Butsince the resistivity contrast between dry andwet rock salt is extremely high a factor of 2 isnot great importance in the geological interpre-tation of the inversion results.

In all, it is realistic and rather on the safe sideto assume that resistivities are mapped with anmaximum error of a factor of 5. But the resistiv-ity changes are certainly better in maximumerror which might be around a factor of 2.These are presently the limits of resistivity mea-surements concerning the possible layouts andlocations in the mining environment and canonly be improved when measurements are con-ducted and also inverted for 3D which, how-ever, is very time consuming and not affordableyet.

4.2. Petrophysical aspects of resistiÕity in rocksalt

In order to interpret the resistivity and itschanges in salt rocks, the physical cause ofresistivity and the influencing factors must bewell understood. A model used widely for avariety of rocks can be adopted for salt rockswhich in its simplest form is based on the

Ž .well-known equation of Archie 1942 . Thegeneral model can be considered in terms ofrock conductivity s as the sum of two conduc-tivities, s and s , in a parallel circuitv qŽSchopper, 1982; Gueguen and Paliciauskas,

.1994 .

sss qs . 1Ž .v q


s is the volume conductivity caused by thev

ohmic conductivity of the free electrolyte in thepores and s the capacitive interlayer conduc-q

tivity due to adsorbed water and cations at theinternal surface of the pores. s is, in contrastq

to s , strongly frequency dependent, being veryv

small for zero frequency and becoming largewith increasing frequency. For rocks with largeinternal surface for example containing a greatdeal of clay, s might be very high and it isq

therefore called the clay term. For salt rocks,particularly at low frequencies, s is muchv

larger than s and therefore sfs . Goingq v

back to the more familiar expression in terms ofresistivity, with rs1rs , the ohmic resistivityof rock is

rsr fymSyn sr FI 2Ž .w w

where r the resistivity of water, f the poros-wŽity, m the Archie exponent or cementation

.factor , S the degree of saturation, n the satura-tion exponent. Sometimes a constant called ‘a’

Ž .is introduced in Eq. 2 in order to get a betterfit of experimental data. The porosity is definedas fsV rV and the degree of saturation asp

SsV rV with V, V and V being the vol-w p p w

umes of the rock, pores and the water, respec-tively. The actual dependence of the resistivityon the pores is expressed with the formationfactor Fsfym and saturation index IsSyn.

Ž .For a rock with full saturation i.e., Ss1 theŽ .resistivity in Eq. 2 becomes

r sr fym 3Ž .o w

where the index o stands for fully saturatedrock. This is the well-known equation of ArchieŽ .1942 and is widely used particularly in inter-pretation of resistivity well logs. For rock salt

Ž .the formation water i.e., brine is chemicallyfully saturated by NaCl and has a resistivity ofr s0.035 Vm. For different type of salt rocksw

where the chemical constitution of the brine isdifferent including also partly MgCl and KCl,r might be lower by a factor of 2. The Archiew

exponent for salt rocks has been determined tobe ms1.9 based on an extensive program of

combined laboratory and in situ measurementsŽ .Yaramanci, 1994b .

Laboratory measurements have shown thatthe rock salt is usually not fully saturated, andthe degree of saturation might be between 10%

Ž .to 50% Kulenkampff and Yaramanci, 1993 .The effect of saturation on resistivity might beunderstood better by introducing the water con-tent directly into the resistivity equation. Therelative water content is given with

GsV rVsfS. 4Ž .w

Ž . Ž .By combining Eqs. 2 and 4 :

rsr GymSmyn . 5Ž .w

Ž .For full saturation i.e., Ss1 and thereforeŽ .Gsf the expression in Eq. 5 reduces to the

Ž . Ž . ymEq. 3 . In Eq. 5 the term G describesdirectly the effect of decreasing r as watercontent G becomes larger. The term Smyn de-scribes basically the effect of redistribution ofwater. However, even for large values of mynand low values of S the effect is not greaterthan a factor of 2. There are only few ownlaboratory measurements for n of rock salt yetbut they suggest that n is in the same range asm so that it can be assumed that mynf0 andconsequently Smyn f 1. However, measure-ments on individual samples especially fromlocations with recent deformations such as fromthe disturbed rock zone around the shifts showedsome significant departures from mfn so theeffect of Smyn cannot be ignored a priori.

The common value of resistivity of 5=105

Vm associated with the undisturbed rock saltcorresponds to a water content of approximately0.02%. This means for average porosities forrock salt of 0.1% to 0.5% a degree of saturationlower than 20% as has been verified many timeswith controlled laboratory measurements. Anincrease of resistivity to a value of 107

Vm canbe associated with a decrease of average watercontent from 0.02% down to 0.004%. A smalldecrease of the usual resistivity to 105

Vmmight mean that the water content is increasedby a factor 2. This certainly is just at the limitof significant detectability. To explain changes


of resistivity down to 10 Vm as encountered inthe location A, the water content must be about5% which in fact might be as large as that inhighly disturbed and flooded rock salt. In gen-eral, the estimation of water content is onlypossible within certain limits as the resistivityalso depends on the pore network. The geome-try of the pore structures, that open during stressbuild-up, and the pore network properties arenot satisfactorily known.

The resistivities of the major salt rocks canbe attributed directly to their water content. Inlarge scale surveys the mean resistivities fordifferent undisturbed salt rocks are found to be5=105

Vm for rock salt, 5=104Vm for

anhydrite, 104Vm for carnallitite and less than

2 Ž10 Vm for salt clay Yaramanci, 1994b; Zim-.mer and Yaramanci, 1997 . Laboratory mea-

surements show that these resistivities aremainly due to the amount of the pore elec-trolyte. Lower values of resistivity are ascribedto increased amount of water. Higher values ofresistivity are attributed in general either toclosed pores under high stress and water issqueezed out or to disturbed rock with increasedporosity or openings as some of the currentpaths get disrupted.

The knowledge about the explicit relationshipof resistivity to water content in rock salt, asdiscussed above, allows some more insight intothe nature of anomalies encountered. At loca-tion B the minimum resistivity of about 5=104

Vm corresponds to a water content of 0.06%and the maximum resistivity of about 107

Vmcorresponds to a water content of 0.004%. Thisa significant variation with a factor 15. Theminimum water content is somewhat higher thanthe usual porosity indicating that these areasmight have enlarged pore space because theyare subjected to a stress decrease.

The most prominent change at 18.2.97 be-tween y350 m to y300 m in a depth of 26 mwith a resistivity decrease of about 100 timeswithin few days corresponds to a water contentincrease of an factor 10. This somewhat signifi-cant as the area is in anhydrite and it cannot be

ruled out that in this area some migration pathshave opened. However, this happened temporar-ily as the area got resistive in following days.

In general the differences changes in resistiv-ities and water contents show that the disturbedrock zone is significantly not homogeneous andchanging continuously probably due to differentcreeping rate into openings depending on thesize of openings itself.

A major difficulty in interpreting the resistiv-ity in terms of water content is that no explicitinformation is available on the degree of satura-tion and therefore also on the porosity. The onlylimit to porosity is that it cannot be smaller thanwater content.

The capacitive interlayer conductivity s inqŽ .Eq. 1 is often neglected in rock salt. However

extensive laboratory measurements show thatwhen rock salt is drained the volume conductiv-ity s will decrease and the frequency depen-v

dent interlayer conductivity will dominateŽ .Kulenkampff and Yaramanci, 1993 . Thismeans that repeated frequency dependent mea-surements in situ may give an indication on thechanging degree of saturation. Measurements inthat respect carried out in the presented casestudies, however, are somewhat ambiguous anddid not give any significant clues probably dueto technical problems as the electromagneticcoupling between in cables could not com-pletely avoided.

5. Conclusions

Geoelectrics is a valuable and probably themost suitable tool to explore the geologicalstructure and particularly to monitor changesdue to water migration. With respect to explo-ration the salt rocks like rock salt, anhydrite,carnallite and salt clay have quite different resis-tivities and are, therefore, distinguishable. Withrespect to the monitoring particularly for waterpresence and migration, small changes in watercontent might cause large resistivity changes.Not only qualitative exploration but also some


quantification of the brine is possible. This isvery important because the amount of brine isessential to estimate the porosity and the degreeof saturation which influence relative and abso-lute permeability, storage capacity, creep prop-erties, nuclide transport properties, etc.

A combination of geoelectrics especially withradar is a promising tool as one method maycompensate the draw backs of the other. Manyinvestigations show that some boundaries in thesalt can be detected by radar even for thin layerswhich would not be resolved with geoelectrics.In contrast, the penetration of radar is limited bystrong reflectors and only with geoelectrics is itpossible to look behind the reflector.

It is necessary to increase efforts to evaluatethe capability of geoelectrics for salt rock envi-ronment and investigate the improvements pos-sible. The major draw back presently is that dueto logistical conditions only 2D measurementsare possible so the location of anomalies areoften not unique. 3D measurements should beused wherever possible, appropriate and afford-able.

Acknowledgements

The work presented here is carried out overmany years in corporation with ‘GSF—Re-search Center for Environment and Health’ inthe ‘Research Mine Asse’. I am grateful to themembers of the Research Mine Asse for theircontinuous support and particularly to M.W.Schmidt for his encouragements to the investi-gations and making the logistics available, to G.Gommlich and B. Hente for valuable discus-sions and to M. Pelz maintaining the hardware.Thanks are also due to G. Kurz who conductedand processed most of the measurements. Fur-thermore I would like to thank to M.A. Mejuand N.B. Christensen and two anonymous re-viewer whose comments are helped much toimprove the paper.

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