The Norwegian Geotechnical Institute (NGI) has said that ground investigation could be undertaken using helicopter-based surveys to reduce time and costs for ground investigations on complicated, large-scale infrastructure developments.
NGI is developing the technology with Kjeller Innovation with the support of the Research Council Norway FORNY 2020 programme.
NGI started on adapting an airborne geophysical method for geotechnical site investigation 10 years ago.
According to NGI, helicopter geo-scanning surveys integrate complicated geophysical models with sparse geotechnical drillings to produce an integrated ground model with statistical uncertainty.
“Machine learning algorithms are key to the success of this technology because they integrate enormous amounts and varying types of data,” said the organisation.
The technology is based on Airborne Electromagnetic (AEM) that were first developed in the 1950s and have evolved into highly accurate, quantitative mapping devices that facilitate new geo-modelling applications. NGI has said that AEM provides 3D data in a cost-efficient way, applicable even in very rough terrain with the use of helicopter-based platforms.
NGI has said that the technology can be used in a variety of applications including bedrock surface modelling by integration of AEM and geotechnical data, investigation of possible quick-clay areas, mapping of weakness zones in hard rock and landslide studies through identification of weakness zones and sliding planes.
How does AEM work?
A typical AEM system carries a transmitter loop that induces electric eddy currents in the subsurface. The ground response, picked up by one or more receiver coils (on the airborne platform), is dependent on the conductivity distribution in the Earth. Depending on the system parameters (signal strength, antenna size, flight height, noise characteristics) and the data processing, the penetration depth ranges from several tens to several hundreds of meters. A survey grid with depth sections (with a few metre spacing) along the flight lines finally provides a 3D model of the earth resistivity over the survey area.
As a rule, higher porosity and concentration of saline elements in the pore water will result in rock or sediments with higher conductivity. There is no general correlation of the lithology with resistivity, but a broad classification is possible. Moraine sediments (gravel, sand, tills) are resistive to poorly conductive (50-10000Ωm) while clays are highly conductive (5-100Ωm). In sedimentary areas, conductivity depends on clay content, porosity, dissolved mineral content, and water saturation.