Aerial Electromagnetic Surveying
(AEM)
AEM aquifer mapping is the core element of the AIA programme and is being undertaken in Northland, Gisborne, and Southland. AEM delivers a step change in the understanding of groundwater and its interaction with surface water. It is a leading edge technology that is used overseas to support decision making about the allocation and use of water. It will provide New Zealand with similar benefits to those provided by LIDAR in relation to land use.
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Groundwater is a foundational water resource that is not as well understood as surface water. Aquifers facilitate both storage and flows of groundwater. While less visible than surface water, groundwater also requires protection from overuse to ensure these vital stores are not depleted. As 80% of surface water comes from groundwater, aquifer mapping greatly enhances the understanding of water resources in a region. Better information on groundwater will prevent overuse that may arise from inadequate understanding of aquifer characteristics.
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​Figure 1: Building the picture of water for better decision making.
​AEM surveying involves flying over the land with a loop system suspended from beneath a helicopter (Figure 2). Transmitters on the loop send electromagnetic signals underground, and sensors measure the behaviour of the returning signals. Similar to radar, this method allows us to ‘see’ what’s under the ground by looking at and interpreting, the way the signals return.
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​Figure 2: Diagram showing the SkyTEM method of AEM (left), example data from AEM and the type of geology represented by low and high resistivity data.
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AEM provides a 3D continuous ‘picture’ of aquifers. This is a significant scientific advancement on the point-based evaluation and interpolation from bore log data alone, that is currently used to inform groundwater models. AEM provides councils with much greater resolution of data on which to model groundwater, test future scenarios and make allocation decisions. AEM markedly reduces the uncertainty in groundwater modelling (Figure 3) and thus improves the confidence that councils can have in setting groundwater allocation policies because groundwater systems are better understood (Figure 4). This leads to greater user certainty in water availability and investment security. It also improves surface water allocation policies, given the interface between the groundwater and surface water.
Figure 3: Higher data resolution to support greater understanding and management regimes
Figure 4: Groundwater elevation map based on 518 boreholes (left) compared to 1,400 TEM soundings.
The regions we are working with are interested in AEM because it can tell them about the interface with seawater and the impact of sea level rise, connectivity to surface water bodies, interactions between deep and shallow aquifers and the geometry of the basement. AEM can be used for other purposes too, such as identifying low temperature geothermal water for sustainable energy sources, identifying land faults to inform hazard assessment, mitigation, and infrastructure investments.
While AEM provides information to councils that is unrivalled in its explanatory power, undertaking AEM is a long-term investment that takes time to become fully incorporated into regional decision making. The AEM process involves a number of steps that take two to three years to complete, depending on the size of the area mapped. These include:
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working with a region to define the area to be mapped and to engage with the community about the mapping being undertaken,
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undertaking the AEM flights to collect the data,
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processing the data, such as cleansing the data to remove any noise (e.g., from infrastructure) and generating a resistivity model, and
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hydrological interpretation - generating 3D maps or profiles to show aquifer boundaries, inform recharge areas, and surface water connections.
Once this work has been completed, the region can incorporate it into regional models of the water flows that will occur in the aquifers and update regional freshwater allocation policies and limits. This process can also take time.
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FREQUENTLY ASKED QUESTIONS
Is AEM safe?
AEM is a safe and effective measurement tool that is used around the world. You may see the helicopter flying overhead but you will not notice any impact from the electromagnetic signals. Airborne SkyTEM is flown at high speed so there is limited exposure to the magnetic field generated from the equipment. The exposure is safer than watching a LCD or plasma TV or blow-drying your hair.
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What about my animals?
The technology is safe to use above animals. Experience in other farming areas is that stock generally aren’t disturbed much by the technology. In New Zealand the team has observed that horses moved to the other side of the paddock when the system came very near.
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What will I see?
You may see a low-flying helicopter towing a large loop hanging from a cable. It will fly over the area on the below map and ‘scan’ to about 300 metres deep underground, where we’ve never seen before. We won’t be flying over towns or built-up areas.
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How will the helicopter fly the survey?
The helicopter will fly at 80-120 km/hr in parallel lines generally 200 m apart and at a height of approximately 100 m. The measurement instruments are suspended under the helicopter and will be about 30 – 50 m above the ground. The noise from the helicopter has been described as equivalent to a truck going past on a motorway and lasts for around two to four minutes. The helicopter flies up and down in lines, so once it flies over, it will then return approximately 15 minutes later but be at least 200 m further away.
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Will the helicopter take aerial images too?
We will not be gathering data or information on anything above ground. While the helicopter will carry a camera, this is only to guide the crew managing the loop, and no photos or video are retained or shared.
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What will the information be used for?
The information gathered by the helicopter and loop will take some time to process and analyse. Once available, it will provide a much better picture of the underground water resource and reduce uncertainty to help guide decisions for environmental protection, development, resource consents, water management, and water availability for the local community. It will be available to councils, tangata whenua, and community groups, subject to agreed guidelines.
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We want to assure you that these flights are not in any way used for compliance monitoring. We only want to know about water.
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What if I have more questions or concerns?
Once the flight lines are planned for each region we publish them on this website and on information flyers. Please contact us if you have any questions or concerns.
