1. Presentation for ISZA, Békéscsaba, Hungary 5-6. April 2013
2D and 2.5D multi-electrode resistivity survey on a potential tunnel construction site in Norway
Presentation for ISZA, Békéscsaba, Hungary
5-6. April 2013
Balázs Rigler, Geophysical-Engineer
Dorottya Bartucz, Geologist

2. Project objectives
Potential tunnel construction site was investigated by using surface resistivity
imaging technique near Drammensfjord in southern Norway.
The planned track of the tunnel comes through a tilted, flat area covered by
quick-clay and crystalline rock hills slashed by faults and dykes.
An area (L1-7) focused mainly on mapping of soil (potential quick clay) and rock
surface was surveyed with 2.5D resistivity array coverage (6 parallel ~300m long
survey lines 300m long each and one survey line, which was perpendicular to the
six parallel survey lines)
Two long ( m each) profiles (P2 and P3) along the track of tunnel focused
primarily on identification of zones of weakness and sediment distribution across
the mountains. Weakness zones can often be identified using multi-electrode
resistivity survey.

3. Oslo

5. Quick Clay Overview (1) Properties [5] [6]
Quick-clay material originates from highly porous clay deposited in a marine
environment during and/or following the last glacial period.
The former marine clays lie currently above sea-level and have been exposed to
fresh-water environment due to isostatic rebound as a result of lowering the
relative sea level.
Salt, which originally contributed to the bonding between the clay particles, may therefore
have been leached from these materials by groundwater and percolating surface water.
If sufficient leaching of salt occurred a highly sensitive or “quick” material may result.
A good knowledge of the groundwater flow system is a prerequisite for mapping of
potentially quick clay areas.
- The presence of permeable layers, such as sand and silt layers, which are connected to surface
water or other water conducting layers, greatly enhances the possibility of leaching.
The topography of the bedrock can also affect leaching: if there is a local high in the
surface of the bedrock underlying the soil, the outflow of groundwater may be
concentrated to this point.

6. Quick Clay Overview (2) Geohazard
Quick clay landslides are common hazard in formerly-glaciated margins like
Scandinavia and Canada
Specifically at construction works such as tunnel or bridge constructions
quick clay must be taken seriously as geohazard
Only small perturbations in stress conditions, such as human activity, erosion or
excess rainwater saturation, can trigger a failure [6].
[6]

7. Quick Clay Overview (3) The Prospecting Area
On upper side (east side) of the area are a number of source origins
- It is also reasonable to assume that a significant amount of water supplied to the
underlying sand layer between rock and clay by water from cracks in the granite.
- This means artesian conditions in many parts of project area, which contributes to
the leaching of electrolyte content of the supernatant clay formations. [1]
Water-bearing fractures from granite of eastern freshwater supplies
through several layers of sand, thus the whole camp volume
exposed to trapping of multiple and significant contact surfaces and
leaching of the electrolytes.

8. Classification of soil resistivity
[2]

9. [2]

10. Description of acquisition system and
principle
channel, 72 electrode Syscal Pro (IRIS Instruments) data acquisition system
- Stainless steel electrodes were used
Multi-electrode 2D and 2.5D resistivity surveying was carried out with continuous
profiling using 5m spacing with 42, 47, 49 and 60 electrodes on the different profiles.
- Wenner-Schlumberger configuration was used for each profile with 50m penetration
depth.
Electrodes were planted at 5m cable takeout locations.
Electrodes were watered thoroughly to avoid contact resistivity problems.
Dust spray was used before connecting cables.
- Resistivity check was run by Syscal Pro before data acquisition started. Contact errors were repaired.
Current injection time for WS array was 500 ms.
- Topography was surveyed by RTK GPS
Data processing was performed in Prosys (IRIS Instruments) software.
Geophysical inversion (2D only) was performed by using the commercially
available RES2DINV software (M.H. Loke, 2000) [8]
Inversion results are presented as inversion models following pseudosections
using a uniform color scale for quick clay site and for hard rock site
to make comparison of the results easier.

11. Measurement Configuration

19. Summary Results on area L1-7
Marine clay generally have very low resistivity (1-10 Ωm) NOT OBSERVED
Sensitive (quick) clay have medium low resistivity ( Ωm) OBSERVED AND DELIMITED
Rocks and sand with fresh pore water has very high resistivity (>100 Ωm) OBSERVED AND DELIMITED
Sandy layers were observed on top of underlying bedrock and also on the surface
with higher elevation
The dipping surface of bedrock could be easily identified and appears with very high >400 Ωm
resistivity values
In the prospecting area the formation consists of sensitive clay, with the possible exception of residues of dry
crust on the upper eastern portion (higher elevation). Clay layers thus seems to be air sensitive, but to varying
degrees. This is probably due to different degrees of leaching.
Results on Profile 2 and 3
Zones of weakness could be effectively identified by combining resistivity survey results and preliminary
geological information

20. References
[1] Final Report, Geoelektriske og Strukturgeologiske undersøkelser RV23 dagslet – Linnes.
R&P Geo Services AS, 2012.
[2] Solberg, Inger-L., Dalsegg, E. (2012): Resistivitetsmålinger for løsmassekartlegging i Kaldvelladalen
og ved Fallan i Melhus kommune, Sør-Trøndelag. Data og tolkninger. NGU Rapport,
[3] Solberg, Inger-L. et al. (2010): Veileder for bruk av resistivitetsmålinger i potensielle kvikkleireområder.
NGU Rapport,
[4] J.A. Mendoza, T. Dahlin (2008): Resistivity imaging in steep and weathered terrains.
Near Surface Geophysics, 2008, p
[5] G.Sauvin, I.Lecomte et al. (2012): Geophysical Investigations of Quick-clay Slide Prone Areas.
Near Surface Geoscience – 18th European Meeting of Environmental and Engineering Geophysics,
Paris, France, 3-5 September 2012
[6] G.Sauvin, I.Lecomte et al. (2013):Towards geophysical and geotechnical integration for quick-clay
mapping in Norway. Near Surface Geophysics, 2013, 11.
[7] A.Malehmir, M.Bastani et al. (2013): Geophysical assessment and geotechnical investigation of quick-clay
landslides – a Swedish case study. Near Surface Geophysics, 2013, 11.
[8] M.H.Loke (1998, 1999, 2000): Electrical imaging surveys for environmental and engineering studies.
A practical guide to 2-D and 3-D surveys.

21. Thank you for your attention!