Learning more about Ruamāhanga Valley potential aquifers using SkyTEM

The insights we gain from this SkyTEM project is the first step to help us better understand potential groundwater resources, their patterns, and interconnections with surface water.  

The resistivity and hydrogeological models produced by this project will provide us with valuable information to update our conceptual understanding of potential groundwater systems in areas, and depths of the Ruamāhanga Valley that we do not hold bore data or other information for.  

However, it is important to understand that these models only identify and map potential aquifers i.e., areas where the geology suggests an aquifer might be. They can also suggest where groundwater and surface water may be connected. 

The models do not provide a map of groundwater and do not tell us how much water is flowing in or out of different aquifers over time. This sort of water yield understanding will be critical to ensure decisions on future use of water are good for the community and the environment. In the meantime, the provisions in the Natural Resources Plan still apply. 

The SkyTEM survey is a fundamental piece of work will enable other pieces of work to build off. There is still a lot of work to do to validate / ground truth what we see in these models to the real world and then further work needed to understand how the groundwater moves and behaves so we can make the best-informed decision of how to manage our natural resources efficiently and responsibly. This further work is out of scope of the current SkyTEM project. Greater Wellington will be using the model to work with Councils and communities to identify priority areas to start any further investigations 

The Wairarapa Water Resilience Strategy recognises the importance of the SkyTEM work to provide key information on potential groundwater availability. However, it is only one piece of the puzzle towards water resilience which will need many solutions such as water use efficiency, water capture, retention and detention, among other things. 

As your Regional Council, Greater Wellington is responsible for making decisions about the Region’s water; for example, how it’s used.    

Right now, we know a lot about the water above ground in rivers, streams, lakes, and wetlands. But we don’t yet know much about the water below ground (groundwater).   

The results of this aerial survey will give us information that will help to:  

• Fill in the gaps in our knowledge about substrates under the Ruamāhanga Valley that have the potential to hold water 
• Better understand the interconnections between surface water and groundwater.  

This information will help us make better decisions about water.  This is especially important when you consider how climate change could impact freshwater including aquifers and the different interests in water, including: 

• Security of municipal water supply  
• Managing groundwater recharge zones 
• Managed aquifer recharge 
• Irrigation and other commercial activities 
• Tourism activities 
• Understanding the effects of groundwater abstraction on lakes, rivers, wetlands and also saltwater.   

Only some of the information from the SkyTEM outputs such as the resistivity and hydrogeological models can be used immediately. For example, it will help Greater Wellington redraw the Category A, B, and C groundwater boundaries more accurately and better understand potential connections between known and potential aquifers. It could also help landowners make decisions on what they might like to think about for their land depending on what is below ground.  

Once Greater Wellington have the final model outputs and other further information, we will need to work out how to change the NRP to reflect the model outputs and in the meantime the existing NRP provisions (such as allocation limits and minimum flow restrictions) apply. Because of this, landowners thinking about implications for their access to water should contact us before they make any significant decision (such as investing in water storage options or deep drilling investigations).

The data collected by SkyTEM is not being used for compliance monitoring. We only want to know about the geology under the valley floor – not what’s happening on your property.  

Groundwater is the water found underground in the cracks and spaces in soil, sand, and rock. It is stored in and moves through geological formations of soil, sand and rocks called aquifers.  

If you were in the area during early 2023, you might have seen a helicopter carrying the SkyTEM equipment which looked like a large hoop hanging from cables - around 50-metres above the ground.    

This technology is safe for people and animals. It’s safer than everyday activities like watching an LCD or plasma TV or blow-drying your hair.  The Ruamāhanga Valley floor is roughly 1000 square kilometres. The helicopter scanned almost the entire valley floor flying in parallel lines to gain as complete a picture as possible of what is happening below the ground. The bulk of the survey lines were flown approximately 200-metres apart but in some areas where the geology is known to be more constant these flight lines were extended to 400 metres without affecting the resolution of the data. 

As metal and electricity interfere with the signal from the SkyTEM equipment, the pilot avoided towns and houses in rural areas, and crossed perpendicular to roads, rail lines and power lines where possible. 

This map shows the flight lines of the SkyTEM helicopter took during its survey scan over the valley floor (the inset map shows the whole Ruamāhanga catchment in grey).  

A total of 5684 km of data were collected during this aerial survey! 

That’s a lot of data, which is why it has taken two years for GNS to process check and stitch the data together and interpret what it all means. 

We owe thanks to SkyTEM for conducting this survey using their technology, which has been used successfully all over the world including Hawke’s Bay here in Aotearoa, and in sensitive environments like the Galapagos Islands and Antarctica.  

Watch to find out more 

In basic terms: The electromagnetic SkyTEM system creates a gentle electrical current that travels down to an average depth of 400-metres underground.   

This current behaves differently based on the materials it passes through underground. The equipment records how easily the current passes through different materials which helps us identify what materials underground layers are made up of. For example: sand, gravel, and clay.    

The current generated by this equipment is safe for people and animals. It’s no different from watching an LCD or plasma TV or blow-drying your hair.  

In scientific detail: The SkyTEM system is an airborne transient electromagnetic system. The system uses a magnetic field to generate an electric current in the subsurface (an eddy current) and then measures the behaviour of that induced electric current. Such measurements can be used to infer subsurface properties relevant to understanding potential groundwater resources.  

As rocks are composed mostly of silicate minerals that are essentially insulators, electrical conduction is dominated by electrolytic conduction, and depends mainly upon porosity, permeability, moisture content, concentration of dissolved electrolytes, temperature and phase of pore fluid, and amount and composition of clay content.  

Groundwater acts as an electrolyte, and so increases the conductivity of water-bearing lithologies. This conductivity increase depends on the volume of water as well as the chemical composition of the water. Fresh water is resistive, but contaminants such as clays and/or salt will increase its conductivity.  

SkyTEM airborne electromagnetic survey technology was developed by the Danish Government and Universities to make efficient and effective mapping of groundwater resources possible.  

The technology has been used commercially since 2004 to map large aquifer systems in countries including Denmark, Netherlands, India, USA, Canada, Australia, and New Zealand.   

SkyTEM technology can map the subtle changes between sand, clay, silt, and gravel that define the location of lithologies capable of carrying groundwater resources.   

Find out more about SkyTEM

The data from the Ruamāhanga Valley SkyTEM survey flight lines were first processed to correct for things like flight altitude, tilt, roll and pitch of the helicopter and equipment, and to remove any electromagnetic ‘noise’ from roads, powerlines, buildings, underground cables, railway tracks etc that may give false readings. In areas closer to the coast salinity was also corrected for. 

Creating a resistivity model

The data was then converted from electrical conductivity to electrical resistivity to tell us more about the geological layers in the data. 

Download the Electrical Resistivity of Geographical Materials graph (JPG 45 KB)

There is not a one-to-one relationship between electrical resistivity and the different types of layers we might find underground. Electromagnetic waves are sensitive to many properties like rock type, porosity, permeability, clay content, moisture content and other fluid properties such a temperature and dissolved electrolytes. However, we can make some general correlations based on what we do know about the ranges of resistivity values. For example, gravels are poor conductors of electricity which means they have high resistivity and indicate layers that are more likely to hold groundwater. Clays and silts (unlikely to hold groundwater) are good conductors and therefore have low resistivity.

A resistivity model for the Ruamāhanga has been built to help identify the layers of the different rocks including gravels, sands, silts and clays to depths we haven’t seen before. This model was built using established geophysical approaches. Scientists from GNS, Greater Wellington, and consultants worked together to refine the processing and modelling against what we already know using supporting evidence like geological maps, previous geophysical studies, and borehole logs. They also assigned areas where they had high confidence in the model and areas where they had lower confidence. Aarhus University in Denmark also reviewed the data processing and model development.  

An example of this process is shown in the cross section across the valley showing resistivity against the lithology profile of three boreholes. Here, areas of high resistivity are shown in blues which match the gravels (in orange) within the three bores, while areas of low resistivity shown in reds are consistent with the underlying clay (in grey) material in one of the bores. The more transparent colours of the resistivity model indicate depths where there is less confidence in the model. 

Download the Cross Section West to East graph (JPG 165 KB)

Creating a 3D hydrogeological model 

The next step is to develop a 3D representation of the hydrogeological units to identify which areas have the potential to hold water-bearing material.   

This model is built using the resistivity model and the supporting geological evidence to identify and group areas of rocks and sediment with similar resistivity characteristics into major hydrogeological units. For example, silts and clays indicating confining layers that do not allow much water to travel through, gravels and sands at shallow or deeper depths likely to indicate potential shallow or deep aquifer units, and non-aquifer basement rock.  

Creating a 3D potential aquifer mapping tool

The resistivity data and hydrogeological units will then be combined to create aquifer potential classes in a 3D potential aquifer map. High aquifer potential indicates an area has a greater potential for hosting water-bearing material (i.e., a productive aquifer) than a low aquifer potential area.  The model will allow the user to draw a transect line anywhere in the Ruamāhanga Valley to see the resistivity, hydrogeological unit and aquifer potential of the subsurface along a line, or at a point in place to view a virtual bore hole showing the same information.

Of the 3456 boreholes within the SkyTEM survey area we have depth and lithological information for 1953 which have varying degrees of detail and reliability. Most of these boreholes are quite shallow – the majority being less than 20 metres. Just 17 boreholes are over 50 metres deep and considered to hold reliable descriptions of their lithology (these are shown in red in the figure below); the deepest bore we have records for is 177 metres deep.

Download the borehole map (JPG 453 KB)

The information provided by the resistivity and hydrogeological models will provide a more complete picture of the Ruamāhanga Valley’s complex geology and potential aquifers, up to 300-metres underground.  However, over time, we need more information such as deep bore data to validate these models to improve our confidence in the potential aquifer areas and aquifer testing to improve our understanding of the movement of groundwater and more accurately assess any effects of our decisions around allocation.

The information from the SkyTEM project will also provide valuable input for any next generation of 3D groundwater flow models in the Ruamāhanga Valley. These flow models will help us understand how ground and surface water interact so we can more accurately assess the impacts of groundwater pumping, land use changes, and effects of our decisions around allocation.

The table below shows where and when the SkyTEM helicopter flew during the survey period (29 January – 2 March 2023)

When	Where	Map
3 March 2023	And that’s a wrap! Thanks to a few good days of weather, we were able to finish our final flights and wave goodbye to the helicopter and loop. We collected a huge amount of data about Wairarapa aquifers during these flights! So, it’ll take two years to analyse this data and develop a 3D map of the Ruamāhanga Valley’s aquifers.	Click for full picture
23 Feb - 2 Mar 2023	We’ve passed the halfway point in this survey!   Over the last few days, we’ve been surveying the area west of Greytown, Carterton, and Masterton.  From 23 February we’ll fly around the outside of these towns and to the east of Masterton.  Important note: We don’t fly over towns, just rural areas. That’s because metal interferes with the equipment signal and there is a lot of metal in buildings in urban areas.   Do you know how high we fly?  Our helicopter files around 80-metres above the ground. That means the survey equipment suspended below it will be around 50-metres above the ground. People have told us the level of noise is like a big truck driving past on the highway.	Click for full picture
21 – 22 Feb 2023	Next, we’ll focus on the northern part of the Wairarapa Valley, which includes working around the edges of Greytown, Carterton and Masterton.  But, we don’t fly over towns, just rural areas. So you won’t see us in the air over these towns.	Click for full picture
18 – 20 Feb 2023	If the weather is fine enough – we’ll carry on surveying the area we were covering before Cyclone Gabrielle. This area was the central and northern part of Lake Wairarapa - extending across to Martinborough and the hills to the east. Work in this location had been taking a bit longer than we originally planned due to poor weather conditions.  A reminder that we don’t fly over towns, just rural areas. So you won’t see us in the air over Martinborough or Featherston.	Click for full picture
14 Feb 2023	We’ve had to pause flights due to severe weather and strong winds caused by Cyclone Gabrielle. We’ll update you again when flights resume.
11 – 13 Feb 2023	Flights continue over the central and northern part of Lake Wairarapa and extend across to Martinborough and the hills to the east.  If the weather is fine and settled, we’ll start approaching the southern edge of Featherston.  But we don’t fly over towns, just rural areas. So you won’t see us in the air over Martinborough or Featherston.	Click for full picture
8 - 10 Feb 2023	We’re planning to fly north of Lake Onoke - potentially as far north as central Lake Wairarapa.	Click for full image
3 - 7 Feb 2023	We’ll continue flying in long lines across the valley to calibrate our equipment.   Then we’ll start surveying from the coast in the south to the southern end of Lake Wairarapa. But at the time of posting rain is predicted, so this plan could change.  Note: Strong winds impacted this flight plan.	Click for full image
29 Jan – 2 Feb 2023	We’re conducting initial flights across the Ruamāhanga Valley to help us prepare to survey specific areas.	Click for full image

If you have any questions about SkyTEM or the data collected, you can email us at info@gw.govt.nz.