2D Electrical Resistivity Imaging for Quick Clay Mapping
Figure 1. 2D resistivity survey being undertaken after a quick clay slide at Byneset in January, 2012.
2D resistivity measurments are performed by laying out cables on the ground and attaching electrodes to them (Figure 1). The electrodes are inserted into the soil. An automatically controlled measurement process begins, where different electrode pairs are used as current electrodes while the voltage that is produced is measured between two other electrodes. The resistivity is mapped at different depths. The measurement values collected are apparently resistivity of the ground, and data must therefore be inverted to arrive at the "true" resistivity. When the resistivity in areas of potential clay is interpreted, the NGU classification shown in Figure 2 is used as a starting point. After supplementary examinations and comparison with geotechnical data, the limits for the specific site may be adjusted.
Figure 3 shows an example of 2D resistivity measurements from Central Norway. 3A shows a profile of a slope where geotechnical investigations were carried out in 2002, along with the interpretation of soil conditions. 3B shows a 2D resistivity profile from the same slope as in (A), measured in 2010. Here, the blue colour is stable, saline clay, the green-yellow colour is potential quick clay and the red-purple colour is coarse gravel or bedrock. The interpretation of figure (A) is added over: The transparent layer is quick clay distribution interpreted only from interpolation between the geotechnical data. The 2D resistivity profile shows that leached clay is not continuous throughout the slope, but intersected by a salty, stable layer of clay. This shows that 2D resistivity data give crucial information regarding interpolation between geotechnical boreholes.
2D resistivity profiles could provide information on the prevalence of various types of sediments, their distribution, characteristics and stratification (in a coarse scale), as well as indications of the depth to bedrock. Information about protruding basement and/or soil types other than clay, will be very useful for understanding the drainage conditions and extent of a possible quick clay landslide. Since the profiles often cover a relatively large area, the measurements will provide a more comprehensive understanding of the geological conditions, which may be of great importance in order to better predict whether landslides in clay can occur. It must be pointed out that 2D resistivity measurements are not a substitute for traditional geotechnical investigations, such as sampling and drilling, but the method can be a valuable supplement, particularly in the initial phase of a project.
There is a number of NGU reports with data from different 2D resistivity projects:
- Buvika (Sør-Trøndelag): 2006.006
- Finneidfjord (Nordland): 2008.032
- Rissa (Sør-Trøndelag) (3 stk): 2010.045, 2012.018, 2012.025
- Namsos (Nord-Trøndelag): 2010.046
- Melhus (Sør-Trøndelag): 2008.084, 2012.013, 2014.022
- Byneset (Sør-Trøndelag): 2012.025, 2015.002
It is also made a guide for using 2D resistivity measurements in potential quick clay areas:
- NGU rapport 2010.048
And also an atlas which shows resistivity modelling
- NGU rapport 2010.047