Andreas Aspmo Pfaffhuber

Multi-method geophysical mapping of quick clay

Marine clay deposits in coastal, post-submarine areas of Scandinavia and North America may be subjected to quick clay landslides and hence significant efforts are being taken to map their occurrence and extent. The purpose of this paper is to assess the use of a number of geophysical techniques for identifying quick clay. The investigated area, Smorgrav, located in southern Norway has a history of quick clay sliding, the most recent event occurring in 1984. Geophysical techniques that are used include electromagnetic conductivity mapping, electrical resistivity tomography, seismic refraction and multichannel analysis of surface waves. These results are compared to geotechnical data from bore samples, rotary pressure soundings and cone penetration testing. A number of these approaches have proved promising for identifying quick clay, in particular electrical resistivity tomography and electromagnetics, which delineated a zone of quick clay that had previously been confirmed by rotary pressure soundings and sampling. Seismic refraction was useful for determining the sediment distribution as well as for indicating the presence of shallow bedrock whereas the multichannel analysis of surface-waves approach suggested differences between the intact stiffness of quick and unleached clay. It is observed that quick clay investigations using discrete rotary pressure soundings can be significantly enhanced by using, in particular, electrical resistivity tomography profiles to link together the information between test locations, perhaps significantly reducing the need for large numbers of soundings.

Airborne EM mapping of rockslides and tunneling hazards

We have investigated potential rockslides in Western Norway using a time- and cost-efficient airborne electromagnetic (AEM) survey approach. The study area comprises phyllite, a low-grade metamorphic rock type that tends to be reworked to clay in disturbed zones. Mapping these electrically conductive clay zones was the aim of the AEM survey. Based on indications that precipitation drives the reported rockslide movements, the local municipality and regional hydroelectricity company are evaluating the option of draining the unstable area to a nearby hydropower reservoir using a drainage tunnel of more than 10 km. We conducted the AEM mapping survey to locate the sliding planes and to investigate the tunnel corridor for areas with potential tunneling hazards. Spatially constrained inversion of the data set (250 km) reveals extended conductive zones interpreted as sliding planes and/or gneiss/phyllite interface. Detailed follow-up of these initial results is planned with targeted percussion drilling and groun...

Airborne electromagnetic hydrocarbon mapping in Mozambique

The Inhaminga hydrocarbon exploration licence in central Mozambique sets the location for a multi-method airborne geophysical survey. The size of the Inhaminga block, spanning some 16 500 km2 from Beira to the Zambezi, limited available data and a tight exploration schedule made an airborne survey attractive for the exploration portfolio. The aim of the survey was to map hydrocarbon seepage zones based on the evidence that seepage may create resistivity, radiometric and sometimes magnetic anomalies. The survey involved a helicopter-borne time domain electromagnetic induction system (AEM) and a fixed wing magnetic gradiometer and radiometer. Our data analysis highlights an anomaly extending some tens of kilometres through the survey area along the eastern margin of the Urema Graben. The area is imaged by AEM as a shallow resistive unit below a strong surface conductor and shows high Uranium and low Potassium concentrations (normalised to mean Thorium ratios). A seismic dimming zone on a 2D seismic line crossing the area coincides with the resistivity and radiometric anomaly. The geological exploration model expects seepage to be linked to the graben fault systems and an active seep has been sampled close to the anomaly. We thus interpret this anomaly to be associated with a gas seepage zone. Further geological ground work and seismic investigations are planned to assess this lead. Airborne data has further improved the general understanding of the regional geology allowing spatial mapping of faults and other features from 2D seismic lines crossing the survey area.

Geophysical Mapping of Quick Clay - A Case Study from Smørgrav, Norway

Marine clay deposits in coastal, post-submarine areas of Scandinavia and North America may be subjected to quick clay landslides. Quick clay may be described as highly sensitive marine clay, deposited in a marine environment during the last glaciation. In Norway some of the most densely inhabited areas, such as the areas around Oslo and Trondheim are located in potential quick clay areas and hence significant efforts are being taken to map its occurrence and extent. In this paper Electromagnetic (EM-31), Electrical Resistivity Tomography (ERT) and Multichannel Analysis of Surface Waves (MASW) methods were tested on a site known to contain quick clay. The site under investigation, Smorgrav, has a history of quick clay sliding, the most recent event occurring in 1984. A number of these approaches have proved promising, in particular ERT, which delineated a zone of quick clay that had previously been confirmed by rotary pressure soundings and borings.

An Integrated Approach to Quick-Clay Mapping Based on Resistivity Measurements and Geotechnical Investigations

Quick clay is highly sensitive, marine clay with an unstable mineral structure due to post-glacial heaving and subsequent leaching of saline pore fluids by surface- and groundwater. Quick-clay layers pose a serious geo-hazard in Scandinavia and North America and need to be delineated in detail. Geophysical methods, especially resistivity methods, have been tested for quick-clay mapping at several sites across Norway. By scrutinizing results from Electric Resistivity Tomography (ERT) and integrating them with geotechnical borehole data including Resistivity Cone Penetrometer Testing (RCPT), we confirm the value of an integrated study for quick-clay hazard zonation. ERT is an ideal tool to interpolate limited borehole results and thus to provide a more cost efficient and detailed result than with borehole data alone. Our resistivity data from ERT, RCPT and lab measurements are generally consistent and appear isotropic. Geochemical analysis confirms that changes in resistivity are directly related to changes in clay salt content and secondarily related to sensitivity. It’s a challenge to resolve small contrast in resistivity (to distinguish unleached from leached clay), in close vicinity to drastic changes in earth resistivity, for example the transition from clay to bedrock. To cope with this we improve results by means of constrained ERT inversion approaches based on drilldata and/or seismic refraction bedrock data. Though ERT is no silver bullet solution to detailed quick-clay mapping, it can provide a significant contribution to improve the risk assessment at comparably lower costs than extensive drilling campaigns. Remaining methodological ambiguities need to be handled by integration with further data.

Towards Joint Inversion/Interpretation for Landslide-prone Areas in Norway - Integrating Geophysics and Geotechnique

Quick clay may be described as highly sensitive marine clay that changes from a relatively stiff condition to a liquid mass when disturbed. Extended quick clay layers account for a lot of geo-hazards in Scandinavia and North-America and hence their occurrence and extent need to be mapped. Geophysical methods have been tested for small scale quick-clay mapping at a research site (Valen) close to Oslo, Norway. By scrutinizing results from Electric Resistivity Tomography (ERT) and Multi-channel Analysis of Surface Waves (MASW) and integrating them with geotechnical borehole data with the help of a resistivity logging tool (RCPTu), we confirm the value for such integrated studies in for quick-clay hazard zonation. Geophysical investigations allow indeed interpolation in between limited borehole results and thus provide a more cost-efficient and extended result than with boreholes alone.

Multi-method high resolution geophysical and geotechnical quick clay mapping

Quick clay is highly sensitive, marine clay with an unstable mineral structure due to post glacial heaving and consequent leaching of saline pore fluids by surface- and groundwater. Extended quick clay layers pose a serious geo-hazard in Scandinavia and North America and need to be delineated in detail. Geophysical methods, especially resistivity methods, have been tested for small scale quick clay mapping at a research site close to Oslo, Norway. By scrutinizing results from Electric Resistivity Tomography (ERT) and Controlled Source Radiomagnetotellurics (CSRMT) and integrating them to geotechnical borehole data with the help of a resistivity logging tool (RCPT) we confirm the value of this integrated study for quick clay hazard zonation. ERT is an ideal tool to interpolate limited borehole results and thus to provide a more cost efficient and detailed result than with boreholes alone. Our resistivity data from ERT, RCPT and lab measurements are consistent and appear isotropic.

Towards Using AEM for Sensitive Clay Mapping - A Case Study from Norway

An AEM survey has been carried out supporting a highway construction project in Norway in order to obtain information about possible sensitive clay (leached marine clay, also called quick clay). In addition, an ERT profile was acquired along a flight line in an area where geotechnical boreholes indicate a layer of sensitive clay. AEM and ERT data agree very well, indicating only small changes in electrical resistivity throughout the sediment layer. The AEM data show more structures than ERT, demonstrating that resolution and accuracy of AEM are high enough to resolve subtle changes in resistivity charecterizing different clay units. Based on our results, AEM can be used to target zones of possible sensitive clay which can subsequently be investigated in more detail through geotechnical analyses like boreholes and sampling. Thus, using AEM to design the geotechnical program has the potential to provide significant cost reductions for similar projects.

Comparison between 2D and 3D ERT inversion for engineering site investigations – a case study from Oslo Harbour

Excavation and piling works related to seafront development in Oslo’s historic harbour area need to mitigate the risk of damaging buried archaeological objects. In the Bjorvika harbour in Oslo, Norway, electrical resistivity tomography was performed to detect structures with potential archaeological value. A 2.5 dataset consisting of four equally spaced parallel lines was collected, trimmed, and systematically processed with both 2D and 3D inversion routines. The results were in good agreement with known underground features, and for the present dataset, an iteratively reweighted least squares 2D inversion was clearly preferable over a 3D inversion. This conclusion is based on differences in model resolution, data processing costs, and the value of the final product for engineering decision-making.

Helicopter Electromagnetic Scanning as a First Step in Regional Quick Clay Mapping

Identification of sediment types and in particular delineation of leached, possibly sensitive marine clays is of crucial importance for geotechnical design of infrastructure projects in Norway. Since leached clays normally have a lower salt content than intact marine clays, the electrical resistivity is consequently higher, and thus clay characterization may be based on data from high-resolution airborne electromagnetics (AEM) collected from helicopter. However, the resistivity difference between leached and unleached clays is small compared to the transition to bedrock and may furthermore vary locally. Therefore, indication of leached clays based on resistivity data has so far been done by manual interpretation. Here, we present a new procedure to calculate the likelihood of possible sensitive clays directly from AEM data. Geotechnical ground investigations are used to locally determine the expected resistivity of sensitive clay. The computation results are compared with well-known quick clay zones. The procedure is not intended as a simple solution to delineate quick clay, but to evaluate an area’s likelihood of sensitive clays that can be used as a cost-saving tool to efficiently place geotechnical investigations.