Airborne Geophysical Solutions for Geotechnical Challenges
Assessing geological risk is a significant part of infrastructure planning and managing project cost overruns and delays are persistent challenges for engineers and project managers. Risks can be difficult to control given the high cost of detailed ground investigation programs using traditional approaches (i.e. geotechnical drillings). Airborne electromagnetic surveys (AEM) are increasingly being used to mitigate geological uncertainty and save time and money.
The SkyTEM method of airborne time-domain electromagnetics is flown with helicopters (HTEM) and is an innovative and advanced technology capable of mapping the top 500 metres of the Earth in fine detail and in 3 dimensions. SkyTEM technology and data help geological organizations and government agencies on all seven continents unearth a wealth of geological information. HTEM data can be of substantial value to pre-engineering and construction projects and in this White Paper we give examples of the applicability of HTEM surveys to geotechnical engineering. The case studies selected provide examples of how and why engineers and others use SkyTEM data as a critical input for decision making. The high-resolution images available from SkyTEM are used to quickly develop a geological model and are used with available ground truth data to adjust, update and improve the geological interpretation and selection of appropriate sites for development. Coverage of large and/or remote areas requires only a few days. Airborne electromagnetic surveys (AEM) have been used routinely for mineral and oil and gas exploration since the 1950s. See “Airborne Electromagnetic Systems - 50 years of development.” See https://www.tandfonline.com/doi/abs/10.1071/EG998001.
As the human populations grow so does the infrastructure we build to accommodate people. Civil engineering is called upon to build bigger and bigger structures while geotechnical engineers must constantly develop new techniques to ensure the underlying ground will support those structures. The conventional approach to infrastructure pre-studies involves the use of geotechnical drillings and other ground-based techniques. The advantage is generally highly accurate point information however drillings and the application of ground-based techniques are time-consuming and costly. They can also often be challenging or impossible to carry out at locations with limited access. Consequentially, information may be scattered with significant gaps in data coverage which requires interpolating between data points and the introduction of risk to decision making. On the grounds that conventional approach’s have limitations, helicopter-borne time-domain electromagnetics (“HTEM”) is increasingly employed as a tool to complement existing methods in large scope prestudies.
HTEM is a highly cost-efficient technique for rapid mapping of large land stretches and coast lines without being constrained by access to the ground. The method maps changes in ground electrical conductivity (or it’s inverse, resistivity) from the surface down to several hundred metres, virtually continuously along each line of flight. Variances in the conductivity of earth materials can be correlated with different characteristics of rock and soil and HTEM data can be used to interpret geological settings to update geological and hydrogeological models. HTEM data can be employed to interpolate lithological information between boreholes and soundings where information may be deficient. Interpretation is reinforced with available borehole information. From a practical point, it is advantageous to perform HTEM as one of the first of the earth science disciplines since results can guide further subsurface investigations and ground follow up, including strategic targeting of new boreholes. Some studies and experience suggest that up to 50% of geotechnical drillings can be reduced if HTEM is carried out prior to commencing drilling programs. One limitation of HTEM must be mentioned, that is, HTEM data are significantly distorted when collected within 100-200 m of electrical installations such as powerlines. It is therefore not feasible to employ HTEM in urban areas or close to these installations.