72 M. Morris, J.S. Ronning, 0.8. Lile & N.O. Kitterod NGU . BULL 427. 1995

Monitoring of a tracer experiment with electrical resistivityat Haslemoen, Hedmark county, Norway

MARIAN MORRIS, JAN STEINAR RONNING, ots BERNT L1lE & Nll S OTTO KITTEROD

Marian Morris & Ole Bernt Lile, Institutt for petroleumsteknologi og anvendt geofysikk, Norges Tekniske Hoyskole, 7030 Trondheim,Norway.Jan Steinar Ronning, Norges geologiske undersokelse, P.O.8ox 3006-Lade, 7002 Trondheim, Norway.Nils Otto Kitterod, Norges vassdrags- og energiverk, P.D. Box 509 1,0301 Oslo, Norway.

IntroductionStudies have shown that surface resistivity mea­surements can be used to mon itor groundwaterflow . The data have been used in determiningvar ious parameters of importance to hydrogeolo­gists , such as natural groundwater flow direct i­ons and bulk groundwater velocit ies (White1988 , Karous et al. 1993), and the direction ofmaximum transm issivity (Bevc & Morr ison 1991),The method involves injection of a conductivetracer into an aqu ifer, and monitor ing the tracermovement over time with surface electrodes.

The opportunity unexpectedly arose to testlow-volume injection of a conductive tracer atHaslemoen in the county of Hedmark. The expe­riment was part of a tracer test conducted by theNorwegian Water Resources and EnergyAdministration (NVE). The purpose of the tracertest from NVE 's standpoint was to opt imise para­meters for future tracer stud ies and not to con ­duct a geoph ysical exper imen t. The refore , froma geophysical point of view, the test was not opt i­mum. However, as a test of the resistivity moni­tor ing method , the resu lts are useful.

Site descriptionIn the test area , the sediment thickness is about30 m. For the purposes of this exper iment, thesed iments of the upper 10 m, where the injectionoccurs, are of greatest importance. This sect ioncons ists predominantly of a med ium-grainedsand interpreted to have been deposited by abraided river system (Riis 1992). FromSchlumberger vert ical electrical sound ings , theresist ivity of this unit is estimated to be 8800 Qmabove the groundwater table and 2800 Qmbelow (Morris et al. 1993).

At the time and location of the exper iment, thedepth to the groundwater tab le was approximate­ly 2.5 m. The groundwater table inclines to thesouth in the ar ea w ith a gradient of approximate­ly 3 m/km. Permeability of the aquifer in the testarea ranges from around 1.1x10-2 crn/s to5.8x10-2 crn/s (Nordal 1986).

MethodElectrode ConfigurationA charged potential or mise-a-Ia-masse conf igu­ration was used to mon itor saltwater movement.The metal cas ing of the injection well served asthe near current electrode, while the far electro­de was placed 900 m to the north . The influenceof the far electrode in the measurement area wasnegl igible. Two non -polarising CU/CUS04 elec­trodes were used for the potentia l differencemeasurements. Potential differences were mea ­sured in a radial pattern. Positions for potentialelectrode placement were 1, 2, 4, 8, 12, 16, 20and 30 metres in eight direct ions away from theinjection well (Fig. 1).

Experimental ProcedureThe experiment took place from May 1 to May 5,1991. Twe lve kg of NaCI mixed with 700 I ofwater (0 = 4.6 S/m) were injected over a periodof one hour into the saturated zone through a fil-

Fig 1. Normalised apparent resistivity for the measurementsmade 17-18 hours after injection.

NGU - BULL 427, 1995 M. Morris. J.S , Ronning, OB , Life & N,O, Kitterad 73

ter extending from 4.3 to 5.2 m in a well.Because the formation has low permeability,pumping was commenced 3.5 hours prior toinject ion at a well located 3 m northwest of theinjection well. The filter in the pumping well waslocated between 4.1 to 5.0 m, Pumping was sus­tained throughout the experiment, starting with70 I/min and lowering to 55 l/min at the end of theexperimen t. Conductivity of the pumped waterand the pumping rate were recorded at frequentintervals , The output water from pumping wasreleased into an open ditch 18 m south of theinjection well.

Monopolar 200 mA square current pulseswere used for all measurements. Current 'on­time' was 2 seconds while 'off-time' was 6seconds. Prior to each time measurement, thepotential grid was leveled to 'infinity' to an elec­trode located 450 m southwest of the near cur­rent electrode. Possibl e self-potent ial field wascompensated before reading the effect of currentpulses, It took approximately one hour to measu­re the potential values for the entire array.

Results and discussionPotent ial differences were measured for the enti­re array eight times in a 70 hour period. Theelectric current was not measured correctly forthe initial (preinjection ) measurement. Therefore,these data could not be used in the interpretati­on. The conductivity of the pumped water wasmeasured frequently throughout the experiment.Appro ximately 40 hours after injection , the con­ductivity had returned to its initial backgroundlevel.

Establishing 'background' valuesIn the interpretation of the data, it is important tohave reliable estimates of the 'background' resis­tivity. Since the preinject ion resistivity measure ­ments had to be rejected , it was necessary toselect alternat ive 'background' values. Potent ialdifferences measured 67-68 hours after injecti­on, well after the conductivity of the pumpedwater had returned to its initial value, were cho­sen as reference or 'background' values, withwhich earlier measurements could be normali­sed. At this time , approximately 60% of the tracerhad been pumped out of the aquifer. Theassumption is then that , by this time , the remai­ning saltwater was sufficiently diluted. For geop­hysical purposes , the format ion water could beregarded as having returned to its original com­posit ion.

Determining groundwater flow directionsThe direction of tracer movement can best bedetermined by contouring the apparent resistiv ityvalues normalised with respect to backgroundvalues (Fig. 1). The asymmetric pattern with thesteep gradient to the northwest indicates thatpumping in that direction has the greatest effecton flow. Even though a large portion of the salt­water was removed , there is a steep northwestgradient as a result of the artificial barrier createdby pumping. It is also possible to determine thenatural direct ion of groundwater flow to the sout­heast through an examinat ion of normal isedapparent resistivity as a function of time.

Determ ining groundwater flow velocityThe dimensions, both in time and space , of this

7050 60

- - 0--- - -- - --0

40

___ NW 1-2m

- -<>- - SE 1-2 m-Pumped water

302010

0+-----+----+----1-----+-----1-----+------1

o

50

250

100

150

200

Time after injection [hrs.]

Fig, 2, Apparent conductivity calculated from the potential differences measured 1-2 m from the injection well compared to the con­ductivity of the outpumped water.

74 M. Morris, J.S. Ronning, 0.8. Lile & N.O. Kitterod

experiment were limited, and make it difficult todetermine a velocity. The distance between thewells was such that the second resistivity survey(conducted 3-4 hours after injection) was com­pleted approximately at the time of tracer break­through. This is illustrated by comparing valuesof calculated apparent conductivity (=1/Pa) withmeasured conductivity of the pumped water (Fig.2). The measured conductivity of the pumpedwater was greatest 7 hrs. and 40 min. after salt­water injection. The apparent conductivity mayhave reached a maximum some time before that.However, the frequency of potential measure­ments was not high enough to detect such amaximum.

ConclusionsIn a small-scale tracer experiment the artificialdirection of flow towards the pumping well wasrecognised. Through an examination of normali­sed apparent resistivity as a function of time, itwas also possible to determine the natural direc­tion of groundwater flow. With increased sam­pling in time, and greater distances of plumemovement, it should be possible to determinethe groundwater flow velocity.

In a similar experiment in the future, the injec­tion and pumping wells should be in a line paral­lel to natural flow, with much larger spacing bet­ween them. In order to obtain more frequentobservations, it is recommended to measurepotential differences with an automatic system.

AcknowledgementsNorges forskningsrAd (NFR) and the Geological Survey ofNorway (NGU) have supported this project. T. Lauritsen part ici­pated in the data acquisition. In addit ion, the Norweg ian Armyprovided assistance. E. Mauring and S.B. Nielsen gave valua­ble comments to the manuscript. Many thanks to all.

NGU • BULL 427. 1995

ReferencesBevc, D. & Morrison, H.F. 1991: Borehole-to-surtace electrical

resistivity mon itoring of a saltwater injection experiment.Geophysics 56 ,769·777 .

Karous , M., Kelly , W.E., Landa , I., Mares , S., Mazac, 0 .,Muller, K. & Mullerova, J. 1993: App lied geophysics inhydrogeological and engineer ing pract ice. In Kelly, W.E. &Mares, S. (eds.) Developments in Water Science 44,Elsevier, Amsterdam. 289 pp.

Morris, M., Ronning, J.S. & Lile, O.B. 1993: Detecting lateralresistivity inhomogeneities with the Schlumberger array .55th meeting and technical exhibition, Euro. Assoc. Expl.Geophys., Extended Abstracts , 0 043.

Nordal, O. 1986: Grunnvannsbalanse • en modellstudie treHaslemoen. M.Sc. thesis , Norweg ian AqriculturalUniversity, As, 105 pp.

Riis, V. 1992: Hydrogeo logi og avsetningsmodell avHaslemoen. M.Sc. thes is, Univers ity of Oslo, 168 pp.

White , P.A 1988: Measurement of ground·water parametersusing salt-water injection and surface resistivity . GroundWater 26, 179·186.