Background
In 2013, SN Power initiated an internal R&D project called “Geophysical investigation methods for deep underground structures”. The goal of this project was to assist with geotechnical surveys to better understand the rock formations in underground structures, especially for hydropower tunnel projects. The idea was to predict the geological conditions more accurately to decrease the overall risk of these projects in terms of cost and time management.
NGI acted as a geophysical consultant for SN Power, and their role was to supervise, control and ultimately interpret the airborne geoscanning survey. The experts at NGI later founded EMerald Geomodelling.
How it was done
Within the scope of the project, NGI carried out a demonstration survey to assess the use of airborne geoscanning in support of tunnel pre investigations in July 2013 in Zambia. The airborne geoscanning method was chosen because it can cover large areas within short time, without major conflict over land use. This was especially important as local topography and tree coverage presented a particular challenge regarding ground-based surveys. The purpose of the survey was to image deep weathering and to reveal potential weakness zones along the alignments. There are two tunnel alignments close to each other (Muchinga Waterway U/S and D/S) and a third tunnel alignment (Mulungushi) about 35 km away.
A total of 353.7 km flight lines was flown over two days in three separate blocks: Mulungushi, Muchinga U/S (tunnel option 1) and Muchinga D/S.
Deliveries to customer
Customer results and actions taken
The geophysical data revealed a rather complicated geology. Profiles and maps were created to visualize the resistivity changes and the resistivity maps made it possible to determine the optimal placement of future boreholes. The data also provided a good assessment of the weathered layer, and major areas with deeper weathering were identified. New drillings can provide even more detailed models.
At Mulungushi, two deep conductive zones were found to intersect the planned tunnel route. It was discovered that previously planned boreholes would have missed these weakness zones, and final drilling plans were adjusted based on the geoscanning models. Thanks to the bent design of Muchinga D/S, the planned tunnel was found to be likely to avoid any weakness zones. However, the deep outlet tunnel that is planned from the underground power houses at 500-meter altitude is anticipated to go through a major conductive zone, which is likely an extension of the mapped quartzite vein.
The assesment of the planned tunnel at Muchinga U/S, discovered that the tunnel would not pass through any major weakness zones. However, it was found that there were at least two minor weakness zones along the tunnel route. New and deeper drillings were recommended for the client to be able to determine this exactly.
In total, this illustrates how airborne geoscanning can provide valuable information and reduce drilling costs. Subsequently, borehole data could be used in the future for optimization. A more refined weathered layer model can be made and delivered after integration with further borehole data.
