Jan Steinar Rønning

Resistivity measurements as a tool for outlining quick-clay extent and valley-fill stratigraphy: a feasibility study from Buvika, central Norway

Resistivity measurements as a tool for outlining quick-clay extents and valley-fill stratigraphy : feasability study from Buvika, Central Norway

Estimating groundwater flow velocity from changes in contact resistance during a saltwater tracer experiment

Abstract The contact resistance of a current electrode is the potential measured at the surface of the electrode divided by the current strength and is a function of the resistivities and geometry of the formations surrounding the current electrode. The equation describing the contact resistance of a half sphere electrode shows that the resistivity of the formation immediately surrounding the electrode dominates the size of the contact resistance. Measurements of the contact resistance of a current electrode over time can then be used to estimate changes in the formation resistivity close to the electrode over time. The formation resistivity is directly related to the pore fluid conductivity, which again, is related to the concentration of a conducting solute. Existing solute flow theory can be used to relate the change in concentration with time of a solute to the groundwater flow velocity, hence the measurements of contact resistance can be used to estimate a groundwater flow velocity. Hydrogeologists use the rate of dilution of a tracer injected into a borehole, monitored by water samples, to calculate the groundwater flow velocity. At Haslemoen, Norway, the exponential decay of the inverse contact resistance during a tracer experiment was used for estimating the velocity. A sodium-chloride solute slug of 0.5 m3 was injected into the groundwater at 4 m depth through a well in which there was installed a short iron bar electrode, reaching from the surface to just below the groundwater table. The well was cased with a plastic tube so that only the lowermost part of the iron bar electrode was in contact with the surrounding formation. The inverse contact resistance of the electrode was monitored every 2 h over approximately 600 h, showing an exponential decay by time. Fitting of an exponential function to the data gave the groundwater velocity parameter equal to approximately 0.26 m/day, a number which was in satisfactory agreement with velocity estimations using other methods.

Characterization of fracture zones in bedrock using 2D resistivity

2D resistivity profiling is used to foresee water leakage and construction problems in bedrock tunneling. Optical televiewer inspection and resistivity logging in boreholes indicate good correlation between open fracture frequency and moderate formation resistivity lows from resistivity profiling. More pronounced resistivity lows indicate clay filled and possible unstable fractured bedrock.

Detecting lateral resistivity inhomogeneities with the Schlumberger array

The Geological Survey of Norway has developed a method for recognizing lateral resistivity inhomogeneities with the Schlumberger cable (Ronning and Tonnesen 1990). The equipment used is constructed at the Geological Survey of Norway and consists of a multicore cable system. In this system the current and potential electrodes are connected to separate cables in order to avoid current leakage in the connections and cables.

Monitoring of a tracer experiment with electrical resistivity at Haslemoen, Hedmark County, Norway

Electrical resistivity measurements can be used in tracer studies to monitor the movement of an injected saltwater pulse (Bevc and Morrison 1991, Karous 1989, White 1988). A tracer experiment was conducted in the unconfined, sand-silt groundwater aquifer at Haslemoen in Hedmark County, Norway. In the study area, groundwater flows mainly towards the south, with a velocity of approximately 0.2 m/day. Depth to groundwater level is approximately 2.5 m.

Marine ERT modeling for the detection of fracture zones

Resistivity measurements in marine environments have already been tested in Norwegian landscapes in in the detection of subsea fracture zones (Lile et al. 1994; Dalsegg, 2012). Yet, most of the produced data have been processed without taking into account the special conditions created by the presence of seawater. Similar studies outside of Norway (Tsourlos et al., 2001; Satriani et al., 2011; Rucker & Noonan, 2013; Dahlin et al., 2014) have also utilized ERT in marine conditions however, most of these studies had deal with brackish water which is less conductive than pure seawater and therefore more favorable to the method (figure 1). This study summarizes our efforts to establish basic rules when considering whether or not pure sea water ERT can satisfactorily detect weak zones inside resistive bedrock. It is also in close connection to related ERT measuring (Ronning et al., 2009; Ganerod et al., 2006; Dalsegg, 2012) and modeling work (Reiser et al., 2007) carried out at the Geological Survey of Norway (NGU) and funded by the Norwegian Public Roads Administration. All results presented here are part of published NGU reports which were made to supplement the construction of an underwater tunnel in western Norway. Based on the modeling results, we were able to improve interpretations of ERT measurements made across seawater straits at Kvitsoy island but also conduct new marine ERT measurements in a more sophisticated manner. Nevertheless, we were also able to detect further limitations to the method.

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.

Detection and Characterization of Fracture Zones in Bedrock - Possibilities and Limitations

In Norway, resistivity measurements have already been tested in marine environments in order to detect subsea fracture zones. However, most of these data have been processed without taking into account the special conditions the presence of seawater creates. More recent studies worldwide have also applied ERT in marine conditions, but under more favorable conditions nevertheless since they dealt with brackish water of considerably higher resistivity than pure seawater. This study summarizes our efforts to establish basic rules when considering whether or not pure sea water ERT can satisfactorily detect weak zones inside resistive bedrock, a problem engineers in Norway usually come up against in tunnel construction sites. The scope for this study is related to the construction of a sub-sea tunnels and the potential application of ERT to detect fractured zones as part of the geotechnical study. Our results indicate that ERT surveys for fracture zone detection in Norwegian marine environments can be promising under certain conditions but at the same time ambiguous since they suffer from reduced resolution and major artificial effects. Based on the modeling results, we were able to improve interpretations of ERT measurements made across the straits at Kvitsoy and plan further investigations in southern Norway.