Lena Persson

Geophysical assessment and geotechnical investigation of quick-clay landslides – a Swedish case study

We present a preliminary assessment of the potential utility of various geophysical measurements carried out over a quick-clay landslide site in south-west Sweden. The multidisciplinary approach includes active P- and S-wave seismic investigations, including 2D and 3D reflection and refraction surveys, passive single and 3C seismic surveys, electrical resistivity tomography and electromagnetic surveys including controlled-source and radio-magnetotellurics, ground-penetrating radar and potential field studies. The P-wave and particularly S-wave reflection seismic data show a highresolution image of bedrock topography and the stratigraphy of a 100 m thick sequence of sediments that lies on top, which include lightly consolidated quick-clays. Of particular interest is the identification of a layer of relatively coarse-grained material between 10–20 m below the ground surface. Geotechnical investigations indicate that most but not all quick-clays at the site are located above this layer. Further studies are required to determine the importance of their relationship and whether the coarse-grained layer may have had a role in triggering quick-clay landslides in the region. Geoelectrical and electromagnetic methods provide high-resolution images of the unconsolidated subsurface and particularly the normal and leached clays. Radio-magnetotelluric methods proved valuable near the river where traditional geoelectrical methods failed to provide sufficient depth coverage. The study shows that geophysical data are able to image major subsurface structures associated with quick-clay landslides.

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.

Boat-towed radio-magnetotellurics — A new technique and case study from the city of Stockholm

We have developed a new data acquisition system and technique to measure the radio magnetotelluric (RMT) signals from distant radio transmitters with the objective of mapping and modeling electric ...

Radiomagnetotelluric mapping of marlstone and limestone in the Silurian bedrock of Gotland

The Silurian of Gotland is composed of a series of stacked carbonate platforms. Airborne electromagnetic measurements show areas with high electrical resistivity, dominated by dense limestone, and areas with lower resistivity, dominated by marlstone. On Gotland, three main regional, up to 80 m thick, limestone-dominated bedrock packages overlie marlstone-dominated strata. The thickness of the limestone and the subsurface relationship between limestone and marlstone was investigated in the area of Ale–Ardre on central Gotland by use of the radiomagnetotelluric (RMT) method. A significant difference in resistivity between the limestone (1000–10 000 Ωm) and the marlstone ( < 100 Ωm) renders favorable RMT conditions. Four profiles, up to 4.3 km long, were measured across the upper part of the Klinteberg Formation and lower part of the Hemse Group (late Wenlock–early Ludlow). The results show a distinct change in resistivity at 50–80 m depth, correlating with the transition between biostromal–biohermal limestone and shelf marlstone–wackestone lithofacies, as observed in boreholes. The measurements were used to create resistivity models that make it possible to map the subsurface lateral relationship between limestone- and marlstone-dominated sequences. The results also provide important information on distribution and bedding, which give supporting information regarding the sequence stratigraphic model for the Klinteberg Fm–Hemse Group in the Ala–Ardre–Sigsarve area. The RMT method utilized in this study proved to be a very useful method for mapping subsurface bedrock in carbonate-dominated geological settings such as those on Gotland.

The Use of Geophysics for Sensitive Clay Investigations

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. Recently, considerable efforts by a number of researchers have been made to investigate areas of sensitive clay using a range of geophysical techniques. Although the majority of this work has focussed on measurements of electrical resistivity, other electromagnetic and seismic geophysical techniques have also received attention in the literature. The purpose of this paper is to review recent research concerning the effectiveness of a number of geophysical techniques for investigating sensitive clays. In addition to discussing a number of case studies, this review will also consider recent work showing the correlation of geophysical measurements, and in particular electrical resistivity, with a range of relevant engineering properties.

Development of a Methodology for Quick Clay Mapping

Quick clays involve considerable risks because small initial slips may evolve into large landslides involving the entire quick clay formation. Most large clay slides in Sweden, Norway and Canada have been in quick clay areas. It is therefore necessary to develop cost-effective methods for mapping the extent of quick clay formations and areas with probable quick clay. The most important areas are those with existing infrastructure and buildings as well as areas planned for exploitation. The methodology should be well-designed in proper steps with different levels of accuracy and include overview studies of topographical and geological methods, detailed geophysical and geotechnical ground investigations, as well as airborne geophysical methods with greater coverage but less accuracy. Three areas in southwest Sweden and one area in the northeast have been selected for this study. In these areas, locations with and without quick-clay have been identified by geotechnical sounding and sampling. Airborne electromagnetic (ATEM) measurements have been carried out in all four areas. The preliminary results from the first two areas, which are presented in this paper, show a reasonable correlation between the different methods used.

Analysis of Ground Geophysical, Airborne TEM, and Geotechnical Data for Mapping Quick Clays in Sweden

Airborne transient electromagnetic (ATEM) data for mapping clay areas are acquired in four areas in Sweden. The resistivity models from the inversions of ATEM data are compared to the existing geotechnical, geological and ground geophysical data in one of the areas at Slumpan located in the Gota River valley. The ATEM models reveal information about layering and thickness of the sediments, the river depth and bedrock undulations. The estimated resistivities at the known locations of quick clays are within the range of 8–40 Ωm. The variation is dependent on the type of the surrounding sediments and the leaching process. The resistivity models have a limited resolution and must always be integrated with geotechnical and geological information for a confident and precise interpretation that leads to a realistic model. The method can be utilized as an effective tool prior to planning of any detailed and costly ground geotechnical investigations.

Aggregate Quality Mapping of Sustainable Substitutes for River Sand for Concrete Production in Eastern Botswana

Botswana is one of the most developed countries in Southern Africa. It has progressed steadily from being one of the world’s poorest countries in the early 70s to the middle income country it is today. In Botswana the classical, raw material for buildings, e.g. concrete and bricks, is the local river sand. Due to poor annual inflow of river sand and intensive construction in the area of the capital Gaborone, the few traditional resources that exist today are decreasing very rapidly. A major change from the use of river sand to crushed rock is identified as the most promising solution. The change is urgently needed to prevent a halt in development due to material deficit. A second alternative is to import most aggregates. However, such a solution is not considered advantageous for a sustainable and independent material supply. The results so far indicates that the good hard rock resources of Botswana manufactured with suitable crushing technique will remove one of the obstacles for the development of a modern society, e.g. the lack of river sand, and enable an infrastructural hub for trading.

New developments in the processing and interpretation of airborne VLF-EM data

Summary In a research project carried out by the Geological Survey of Sweden (SGU) and Uppsala University, several new methods in the processing and interpretation of airborne VLF data have been developed. In the new tensor VLF concept, a response can be calculated that is independent of the transmitter direction. A new method that provides a direct transformation from VLF data into apparent resistivities and phases has been developed. A remarkable correlation between the calculated apparent resistivity and the lithological units on the bedrock map was achieved. Furthermore, selected flight lines were interpreted using the 2D-inversion code Rebocc. The 2D-code was found to be a fast and convenient way for quantitative interpretation of airborne VLF data and rather detailed information about the resistivity distribution in the ground was obtained. One field example from an area near Stockholm will be shown.