Inger-Lise Solberg

Developments in Mapping and Web Presentation of Fjord-Marine Deposit Distributions for Quick-Clay Related Work in Norway

For several decades mapping of Quaternary geology has been the basis for quick-clay mapping in Norway. In this context it has been, and still is, of particular importance to define the distribution of fjord-marine deposits that commonly contain clays and silts where layers or pockets of quick clay may have developed. Quick clay may collapse under certain conditions and give rise to disastrous landsliding with severe consequences. It requires careful communication of Quaternary map information to get its full use for quick-clay mapping, landslide hazard assessment and other purposes. For this reason, there has been an increased focus in recent years at the Geological Survey of Norway (NGU) to improve existing web-based map services. This includes for example a National overview of the marine limit (ML) in Norway as the upper natural boundary for the occurrence of marine clays. In addition, a filtered version of Quaternary map information below ML has been added called clay-deposit susceptibility. This map service gives an overview over areas where clay deposits with some probability may be present even under other deposit types. The next step in the development of web services is to include information where the occurrence of fjord-marine clay deposits and quick clay are registered, for example, from drill-hole information. This is now made possible especially with the help of the newly established National Database of Ground investigations (NADAG) hosted by NGU.

Applications of 2D Resistivity Measurements for Quick-Clay Mapping in Mid Norway

During the last 10 years, several Norwegian projects have explored the possibilities of using 2D resistivity measurements for quick-clay mapping in combination with traditional geotechnical methods. Experience has shown that an effective first-order interpretation of 2D resistivity profiles can be performed by using the following classification: unleached clay deposits (1–10 Ωm); leached clay deposits, possibly quick (10–100 Ωm), and dry crust clay deposits and coarse sediments (>100 Ωm). However, resistivity values are influenced by local conditions and there is an overlap between the classes. The 2D resistivity method can prioritize areas for further investigation using other geophysical methods or drilling to refine the interpretation of the subsurface. Through several case-studies, the 2D resistivity method has proven useful for detecting potential layers of quick clay, for outlining the extent of these layers and their positions in slopes (also near the shoreline), and for engineering applications such as construction planning. To this end, the method has shown to be applicable for adjusting the extent of hazard zones to improve stability evaluations. Another important application is landslide investigations, to identify barriers that may deter further landslide propagation.

The Norwegian National Database for Ground Investigations (NADAG): A Tool to Assist in Landslide Hazard Zonation and Other Quick-Clay Related Issues

Exploitation of the subsurface is becoming more frequent and the demand for knowledge about ground conditions is increasing. A vast amount of data from ground investigations such as geotechnical drilling, bedrock drilling and ground water wells exists in Norway. However, many of these are not easy to access as they are spread between multiple data owners and users. Following the development of The National Database for Ground investigations (NADAG) during the last years, the registration of geotechnical data has started. With increased accessibility of data, re-use will lead to considerable savings for the society. Importantly, the information will allow for better landslide hazard zonation. In addition, the effectiveness of emergency planning and response will improve with access to relevant, accurate and timely information about the local ground conditions. This may be crucial after landslide events with regard to the assessment of potential landslide expansion and the safe evacuation of people. NADAG aims to collect and make publically available data from ground investigation important for the society. The database contains various amounts of data, depending on availability – ranging from metadata (location, drill type, drill depth, company, date, report no., etc.) to full reports and raw data. NADAG will initially be populated by data from geotechnical investigations. A primary objective for NADAG is to distribute data from all types of ground investigations in Norway and to present the data coverage through a map-enabled web application.

Investigation of a Sensitive Clay Landslide Area Using Frequency-Domain Helicopter-Borne EM and Ground Geophysical Methods

Mapping of the distribution and properties of marine clay is important in Norway due to numerous landslides in sensitive clay. The degree of leaching of salt in marine clay may be reflected by its electrical resistivity. However, the degree of sensitivity of the clay needs to be confirmed by geotechnical studies. Electrical resistivity and various electromagnetic (EM) methods are common geophysical methods to investigate the resistivity of an area. Helicopter EM surveys are helpful to investigate a large area in rather shorter time compared to ground EM or resistivity surveys. Frequency domain helicopter-borne EM (FHEM), electrical resistivity tomography (ERT) and seismic refraction were performed in 2014 at Byneset outside of Trondheim, Norway where a landslide occurred in 2012. Geotechnical surveys were performed in the region before, but mainly after the landslide. There was a good correlation between the results from the different surveys. FHEM interpretation revealed that unleached marine clay was covered by varying thickness of leached clay in the survey area. At some places, bedrock was very shallow and even exposed at the surface. FHEM is proven to be a very good tool to get an overview of the leached and unleached clay zones and to map 3D resistivity of the region.

DELINEATION OF MARINE SEDIMENTS IN A LANDSLIDE AREA IN NORWAY USING FREQUENCY DOMAIN HELICOPTER-BORNE EM AND GROUND GEOPHYSICAL SURVEYS

There was a quick-clay landslide in Byneset, Mid Norway on 1st January 2012. The landslide area is surrounded mostly by agricultural lands. The geology in the area consists of old ocean floor with outcropped bedrock at several places. Prehistorically, the sea-level was ~160 m higher than the present sea level. Resistivity survey is a powerful tool to investigate marine sediments but it is relatively time consuming. Airborne EM survey is a faster way to investigate large areas. Therefore, Airborne and ground geophysical surveys were performed in the study area in Autumn 2013. Main aim for these surveys was to see usefulness of Frequency-domain Helicopter-borne ElectroMagnetic (FHEM) data in mapping of the marine sediments and cross-check it with 2D resistivity, refraction seismic and other available data like geotechnical investigations including RCPTu. Interpretation of FHEM data showed some correlation with 2D resistivity and refraction seismic data. Comparison of FHEM results with 2D resistivity, refraction seismic results, known bedrock depth from drilling and exposed bedrock locations suggests that FHEM data can be used for clay layer mapping and to indicate a rough bedrock depth. It can differentiate between layers of unleached marine clay (< 10 Ωm) and leached marine clay or possible quick clay (10-100 Ωm). However, similar resistivity values of possible quick clay (10-100 Ωm) can also suggest non-quick, leached clay and silty sediments. FHEM data is poor to resolve geological boundaries in case of thicker marine clay deposits, due to low skin depth in a conductive environment. Data acquisition and Processing A frequency domain helicopter-borne electromagnetic (FHEM) survey was performed in July 2013, to investigate subsurface resistivity in the landslide area. Five frequencies (880, 6606, 34133 Hz in coplanar and 980, 7001 Hz in co-axial settings) EM data were acquired at ~ 100 m line spacing and at average ~ 59 m height above the ground level (black lines in Figure 1). FHEM data drift was corrected automatically and manually with the help of high altitude flights. Laterally and specially constrained inversion (LCI and SCI) of FHEM data using workbench software of Aarhus Hydrogeophysics Group (Viezzoli et al., 2009) was performed. Airborne EM survey was further followed by four 2D resistivity profiles (red lines in Figure 1) in October 2013 along parts of the helicopter flight lines to compare the