Airborne mapping of sensitive clay—stretching the limits of AEM resolution and accuracy
Due to postglacial uplift, lowlands in Canada, Norway, Sweden and Russia are prone to formation of highly unstable, sensitive, and leached marine clay (quick clay). Quick-clay failures are dramatic due to its high water content, resulting in liquefaction. It thus poses a major hazard for society and construction projects in particular, and knowledge of its extent is of vital importance. Quick-clay assessment is usually undertaken by geotechnical boreholes having the disadvantage of giving only information at the borehole location. To overcome this limitation, geophysical ground-based methods like electrical resistivity tomography have been used successfully. However, when a larger area has to be investigated, electrical resistivity tomography surveys become costly and time consuming. We show results from an airborne electromagnetic survey aiming at detection of different clay units for a road project in southeastern Norway. Airborne electromagnetic data clearly show structures within the sediment layer that correspond well with results from geotechnical boreholes. While a clear distinction between clay and quick clay cannot be derived from airborne electromagnetic alone, our study shows that this method has high-enough resolution and accuracy to map differences in clay units, which can subsequently be probed at specified locations. Thus, by using airborne electromagnetics to target borehole locations, the costs for the geotechnical drilling program can be reduced significantly.