92 Jan Steinar Ronning, Torteit Lauritsen & Eirik Mauring NGU . BULL 427, 1995

Evaluation of geophysical methods for the delineation ofbedrock fractures - a case history from Utengen, Hvaler,southeastern Norway

JAN STEI AR RD I G, TORlEIF LAURITSE & EIRIK MAURI G

Jan Steinar Ronning, Tor/errLauritsen and Eirik Mauring, Norges geo /ogiske undersoketse, P.O.Box 3006- Lade , N-7002 Trondhe im,

Norway

IntroductionResearchers have used a wide variety of qeop­hysical methods to address the problem ofdetecting bedrock fractures (e.g. Warrick &Winslow 1960, Zohdy & Jackson 1969, Palackyet al. 1981, Aubert et al. 1984). The location offractures is essential in the initial stages of theinvestigation of water resource potential.However, very little work has been focused onthe combined use of various methods for fracturelocation. At the Geological Survey of Norway, theapplicability of several geophysical methods hasbeen studied (Ronning 1985). These includerefraction seismics, electromagnetic methods(VLF & slingram), resistivity methods and mag­netic profiling. The introduction of the ground­penetrating radar has further contributed to thedelineation and mapping of bedrock fractures(e.g. Davis & Annan 1989).

A test-site at Utengen, Hvaler, southeasternNorway, has been subject to geophysical investi­gations using several methods. The main objecti­ve of the study was to evaluate the suitability ofthe various methods in the mapping of bedrock

fractures.Utengen test-siteUtengen is situated on the northwestern penin­sula of the island of Kjerkoy at Hvaler in southe­astern Norway (Fig. 1). The lithology is domina­ted by the Precambrian Iddefjord Granite.Several locations in the Hvaler area have beensubject to extensive hydrogeological investigati­ons (e.g. Banks & Rohr-Torp 1990, Banks et al.1992, 1993a, b). Utengen was one of these loca­tions. It was chosen for detailed geophysicalinvestigations because it lies at the intersectionof several fracture lineaments, and also becausethe central part of the site is covered by thinQuaternary deposits, allowing us to investigatethe performance of geophysical methodsthrough such a sediment cover (Banks et al.1993c). The geophysical results from Utengenare reported by Lauritsen & Ronning (1992) andBanks et al. (1993c).

Geophysical methodsThe following is a brief outline of various geophy­sical methods applied to mapping of fractures.

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NGU - BULL 427, 1995 Jan Steinar Ronning, Tor/eif Lauritsen & Eirik Mauring 93

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94 Jan Steinar Ronning, Torleif Lauritsen & Eirik Mauring

More comprehensive descriptions of the met­hods can be found in standard texts, e.g. Telfordet al. (1976) and Kearey & Brooks (1991).

Refraction seismicsAmong other paramete rs, the seismic refractionmethod yields information about acoustic wavevelocities in bedrock. Fractures are associatedwith velocity lows. This can be an accurate met­hod in determining fracture position and thedegree of fracturing. The method is, however,relatively expensive.

Very low frequency electromagneticprofiling (VLF)This method utilises electromagnetic (EM) fieldsfrom military transmitters. Due to elect rical con­ductors, a secondary field is superimposed onthe primary field creating an anomaly which canbe detected on a receiver. Fractures commonlyact as electrical conductors due to increasedporosity. A general disadvantage of the methodis that spurious anomalies can be generated bynumerous sources other than fractures, such aselectrically conductive overburden, mineralizati­ons, power-lines and severe topography.

Magnetic profilingThe method can only be employed in areas whe­re the bedrock has some content of magnetite.Fractures are normally associated with magneticlows due to oxidation and hydration of magnetite(Henkel & Guzrnan 1977). The method shouldbe used with great care in areas with overbur­den. Sediments are usually non-magnetic, andan increase in thickness will also give rise tomagnetic lows.

Resistivity profilingFractures generally have lower electrical resisti­v ity than fresh bedrock, and will appear as resis­t ivity lows on resistivity profiles. The methodoffers information on fracture location and thick­ness. The main disadvantage with this method isthe strong influence of conductive overburdenwhich, if present, masks the response from theunderlying bedrock.

Ground-p enetrating radar (GPR)GPR transmits electromagnetic pulses into theground and records reflections from interfacesseparating layers of different dielectric propert i­es. The dielectricity increases with increasingwater content. Fractures normally have a higherwater content than the surrounding non-fracturedbedrock, and may therefore be seen on GPRrecords. The method is not suitable in areas with

NGU - BULL 427. 1995

electrically conductive overburden (e.g. marineclay) because of energy absorption.

Borehole geophysicsSeveral logging methods have been developedto give a great amount of downho le information.The Geological Survey of Norway applies a sim­ple data acquisiti on system , measur ing formationresistivity with three different electrode configu­rations. The system also records fluid resistivity,temperature and self potential. Recorded datacan provide information on the location of fractu­res/fissures in bedrock and the sites of waterinflow.

ResultsThe test profile is shown in Fig. 1. Data fromeach applied method are shown in Fig. 2. A risein the VLF-Re curve (Fig. 2A) from position 20 iscaused by a power -line at position -20. A steepdip in the curve from positions 60 to 80 is inter­preted to be caused by a conduct ive zone in theground. Small variations in the VLF-Im curve(ellipticity) indicate moderate conduct ivity.

Magnetic data (Fig. 2B) show irregularitiesfrom positions 0 to 30 due to the power- line. Amagnetic low from positions 50 to 80 coinc ideswith the VLF anomaly. The magnet ic low is pro­bably caused by a combination of fracturedbedrock and increased soil thickness as indica­ted by a GPR record (not shown).

The resistivity profile (measured with gradientconfiguration, Fig. 2C) has a low from positions45 to 80. This coincides with both VLF and mag­netic anomalies , but also with increased soilthickness. Soil resistivity measured by a pole­dipole configuration is in the order of 30-50ohmm , while the bedrock has a resist ivity in theorder of 2000-4000 ohmm. Modell ing of VLF,magnetic and resistivity data (not presentedhere) shows that the anomalies can be explainedby overburden mater ial alone with a maximumdepth of 5 m. However, from GPR profiling thesoil thickness is interpreted to be less than 2 mwhich means that the anomalies are partly cau­sed by bedrock fracturing. In glaciated terrain,outcropp ing fracture zones are commonly ero­ded by ice making a depression in the bedrocksurface. Thus, the overburden is often thickerabove fracture zones.

The GPR record is shown in Fig. 20 , and it ispresented with the subtraction of neighbour ingtraces to enhance dipping reflectors. The recordreveals reflectors dipping in both directions . Thereflectors are probably representing bedrockfractures. The fractures appear most dense lydistributed between positions 60 and 90.

NGU • BULL 427, 1995 Jan Steinar Ronning, Toneit Lauritsen & Eirik Mauring 95

ConclusionsAt the Utengen test-site , several geophysicalmethods have been employed for the detectionof bedrock fractures. The results are in goodagreement with the location of fractures actuallypenetrated during drilling, and confirmed byborehole geophysics. VLF, magnetics and resis­tivity all gave anomalies in an area which couldbe explained by a combination of overburdenand bedrock fracturing. Ground-penetratingradar was particularly successfu l at detectinglow-angle fractures even with low overburdenresistivity. The combined use of geophysicalmethods yielded most information.

Reflectors are not as steep as they appear dueto differing length and depth scale. True dips arein the order of 20-30°. The good penetration israther surprising due to the low overburdenresistivity (30-50 ohmm).

Borehole geophysics has been performed in aborehole shown in Fig. 1. A section showing theborehole on the interpreted GPR record is pre­sented in Fig. 3. Resistivity lows (Fig. 2E) can beseen at depths of 7,9 and 25-27 m. The first twoof these correspond with reflections on the GPRrecord. The deepest reflector had to be migratedto match the resistivity log. The SP log (Fig. 2F)indicates porous structures at 5-7.5 and 25-33m, coinciding with the position of resistivity ano­malies. Fluid resistivity (Fig. 2G) and temperatu­re (Fig. 2H) logs are anomalous at 7 and 9 mindicating water inflow. The largest fluid resistivi­ty anomaly is at a depth of 9 m. There is no indi­cation of water inflow at 26 m. Results from bore­hole geophysics correspond with observationsmade during drilling, where fractures wereencountered at 7.5 and 9.3 metres.

Fig. 3. Interpretation of the GPR record.

References

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Banks, D., Rohr-Torp, E. & Skarphagen , H. 1993b:Groundwater chemistry in a Precambrian graniteisland aquifer, Hvaler, Southeastern Norway. InBanks, S.B. & Banks, D. (eds.). Hydrogeo logy ofHard Rocks \ Proc. XXIVth Congress Int. Assoc .Hydrogeo l., AS, 395-406.

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AcknowledgementsThe authors wish to thank Conall MacNiocaill and Ole B. Ulefor reviewing this paper. Norges almenvitenskapelige forsk­ningsn'ld (NAVF), Norsk Hydrologisk Komlte (NHK) and theGeological Survey of Norway have supported this project.

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