Contact: Andi A. Pfaffhuber, +47 414 93 753 [email protected] Key personnel: Helgard Anschütz Sara Bazin Kristine Ekseth Asgeir Kydland Lysdahl Jürgen Scheibz Helge Smebye ERT Application NGI (Norwegian Geotechnical Institute) is a leading international centre for research and consulting in the geosciences. NGI develops optimum solutions for society, and offers expertise on the behaviour of soil, rock and snow and their interaction with the natural and built environment. NGI works within the oil, gas and energy, building and construction, transportation, natural hazards and environment sectors. NGI is a private foundation with office and laboratory in Oslo, branch office in Trondheim and daughter company in Houston, Texas, USA. NGI was awarded Centre of Excellence status as the leader of the research consortium International Centre for Geohazards (ICG) from 2003-2012. Case histories using the Electrical Main office: PO Box 3930 Ullevaal Stadion Resistivity Tomography (ERT) method NO-0806 Oslo, Norway Experience and capabilities supporting Street address: Sognsveien 72, NO-0855 Oslo geotechnical and engineering projects T: (+47) 22 02 30 00, F: (+47) 22 23 04 48 [email protected] Bedrock investigations Quick clay mapping Rock Quality www.ngi.no Natural Hazards Contamination Archaeology 4 Background Table of contents 5 Summary Table of contents What is ERT? 6 This catalogue summarizes selected Case Histories 8 Electrical Resistivity Tomography (ERT) Skøyen, Enebakk 8 case histories performed by NGI Rove/Ekeberg, Holmestrand 10 with an ABEM Terrameter LS system. Holmestrand, along the railway 12 Since 2010 we have gained extensive Vålen/Smørgrav, Øvre Eiker 14 knowledge and experience about the Korsgården, Nedre Eiker 16 feasibility, applicability and reliability Tømmerås, Grong 18 of the ERT/IP survey system. Aurland 20 Numerous applications and different Trofors 22 types of targets have been prospect- Gåseid/Veddemarka, Ålesund 24 ed and evaluated. Here, we briefly Industrial Site 26 highlight and discuss possible applica- Bjørvika, Oslo 28 tions, advantages and also limitations. Appendix 32 • The ERT Method • Instrumentation • Data processing ERT Applications 6 What is ERT? What is ERT? 7 What is ERT? Electric resistivity tomography (ERT) is a near-surface- • Bedrock depth/sediment mapping: With a calibrating geophysical tool for ground investigations that has geotechnical survey (CPT, total sounding, RPS), a continu- increasingly been used by NGI to reveal ground features ous and detailed bedrock profile can be delivered for most and map the soil structure in both research projects and bedrock types. commercial surveys. An electrical current is injected into • Quick clay: The salt content and thereby the conductivity the ground through short steel electrodes distributed of quick clay is normally lower than for non-sensitive clay. along a straight line, and the resistance values between ERT thus distinguishes between them. the injection points are measured. By careful and correct • Geo-hazards/rock quality: ERT can be used to map weak- processing, a two-dimensional resistivity profile of the ness zones in bedrock, which often are characterized by ground is derived. higher water content and/or sediment inclusions. From resistivity to soil structure • Archaeological artefacts: Timber structures buried in clay The resistivity (inverse of conductivity) is often highly vary- appear as confined resistivity anomalies on the profiles. ing between ground materials of different types. While • Contamination mapping: Contamination sources and clay is one of the most conducting substances to be plumes containing minerals or other chargeable materials found, quick clay, other sediments, soft rock and hard that can be discovered and mapped by processing and rock have all higher resistivity and can be distinguished. interpretation of both ERT and IP data. Experienced geophysicists and geologists at NGI inter- General advantages Generelle fordeler pret the resistivity profiles, relate them to the local geol- ogy and are thereby able to describe the subsurface What to be aware of structure several tens of meters into the ground. In order to benefit from the advantages ERT has over other near-surface geophysical and geotechnical tech- niques, one should be familiar to the methodology and General advantages the way the system works. Firstly, the cable length, elec- Silent and non-invasive: No digging, large-scale drilling or trode spacing, resistance threshold and instrument settings 1. Silent and non-invasive u t 1. Ingen støy eller ødeleggelser • blasting is required. Even indoor surveys can be performed. should be wisely chosen and tailored to the problem at hand. Secondly, the data processing and interpretation • Very versatile: All walkable terrain can be assessed with ERT, should be done by skilled geophysicists and geologists, in and measurements can be performed on almost all types order to detect inversion artefacts or recognize certain of ground cover. geological structures. Further important points are: • Economic and efficient: One line is usually acquired within 3-6 hours, depending on the local conditions. • Mapping is based solely on resistivity and chargeability 2. Versatile and u t 2. Mange anvendelser og • Fast and easy processing: Preliminary results can be as- measurements, meaning that materials of similar resistivity widely applicable stor tilgjengelighet sessed in the field. will be poorly resolved (e.g. clay over shale). • Detailed: Contrary to borehole surveys, ERT provides con- • The data processing algorithms tend to smear out sharp tinuous and detailed lateral information. geological interfaces. A geotechnical calibration is there- • IP as a bonus: Without extra cost, IP (induced polarization) fore recommended in order to find a boundary value for data can be acquired. This is often used to map contami- the relevant interface (e.g. the bedrock topography). Ad- nation plumes or mineralization. vanced, joint inversion algorithms can significantly increase 3. Detailed results u t 3. Detaljerte resultater resolution and accuracy. with low cost til en lav kostnad Although ERT cannot completely substitute geotechnical • In case the geology is very heterogeneous (far from a surveys, it is valuable as a complementing tool and the layered structure), 2D ERT surveying might falsely display a number of required boreholes can be drastically reduced. side-lying artefact as a geologic layer. In such cases, 3D ERT As opposed to reflection seismology, ERT can distinguish or a grid of 2D surveys should be considered. between materials of similar mechanical properties like • Even though it’s possible to measure even on snow and ice clay and quick clay. ERT is also often superior to ground- it is generally not recommended to carry out ERT in winter. penetrating radars, because clay and / or water saturat- Field work becomes significantly more elaborate and thus 4. Fast and easy processing u t 4. Rask og enkel dataprosessering ed ground is an advantage rather than an obstacle. expensive and further, results are usually not as good as under warmer conditions due to the high resistance of Geotechnical applications frozen ground. Mapping of resistivity lets us distinguish between numer- ous kinds of materials. Chargeability, inferred from IP measurements, is another physical parameter which can be used to give confidence to resistivity measurements and find chargeable zones. ERT Applications 8 Case Histories Case Histories 9 Case Histories Bedrock mapping The resistivity zones in the model can be attributed to rock or sedimentary layers by considering the geology of the area and performing calibration borehole surveys. In this area, the bedrock is composed of muscovite-biotite gneiss which is a metamorphic rock. Unweathered, metamorphic rocks have resistivities above 1000 Ωm, whereas resistivities in weathered metamorphic rocks span from 2 to 2000 Ωm. Solid bedrock can therefore be attributed to the high-resistivity zones (> 1000 Ωm) highlighted with a solid line (yellow-green boundary) in Figure 2. In addition, the gradient between the low- and high-resistivity zones is very high, indicating that the bed- rock indeed is reached. Thus, the qualitative interpreta- Corrective bedrock mapping tion of the model is relatively clear: A thick marine clay + Detailed and precise bedrock mapping unit lies above a metamorphic bedrock. + Adjustment of bedrock topography from As to the quantitative interpretation, we have to con- borehole surveys sider the following: The applied data processing routine + Discovery of sand / silt in clay layer utilizes a so-called “smoothing” constraint, such that un- necessary model complexity (resistivity variations not re- quired by the data) is avoided. Therefore, one should be aware of the fact that large gradients are prohibited. In Skøyen, Enebakk this case, the abrupt resistivity drop from the basement (several thousand Ωm) to the clay (less than 10 Ωm) will Figure 1: The ERT profile (yellow line) recorded in the farm lands Motivation be imaged as a smooth transition and the postulated This project demonstrates how ERT effectively can be used This ERT survey was conducted by NGI in order to assess and forests of Skøyen. 5 of the 8 existing boreholes are indicated in bedrock profile given by the solid contourρ = 1000 Ωm to map the bedrock topography as a supplement to tradi- red. The boreholes were marked with wooden sticks on site. the bedrock profile prior to a road construction project. might be uncertain. tional geotechnical