After 5 years, the Geothermal: the Next Generation programme is coming to a close.

Here, we summarise the Explore: Geophysics component of the research. This research is contributing to identification of prime location targets for exploration drilling to develop supercritical resources. You can also check out the summaries for Explore: Geology, and Explore: Modelling.

The aim of the geophysics research was to improve imaging of the crust.

Key knowledge advances include:

• New seismic, magnetic, heat flow and gravity models were produced with a specific focus on the southern part of the central Taupō Volcanic Zone / Te Ahi Tupua.
• From seismic tomography, we identified a deep large area of partial melt that is the heat source for the Wairakei – Tauhara and Rotokawa geothermal fields.
• We refined the buried geology underneath the area by proposing a new gravity model.

1.      New depth to Curie temperature for Zealandia – implication for heat mapping

Knowledge of the thermal structure of the crust is fundamental for understanding many geologic processes. We analysed a new compilation of magnetic data over central Zealandia continent, including onshore New Zealand, and calculated the depth to base of magnetic sources (DBMS) at regional scale. DBMS often reflects the 580°C isotherm, known as the Curie point temperature.

We found complex relationships between DBMS and markers such as the crust and elastic thickness and depth to which earthquakes occur, reflecting a range of processes additional to temperature influence DBMS (Figure 1). We used the depth to base of magnetic sources to generate heat flow models based on the assumption that magnetisation in rocks vanishes at the Curie point temperature. In the TVZ, we imaged the DMBS isotherm at around 10 km, consistent with the relative locations of the crustal thickness and brittle-ductile transition zone. DBMS is likely shallower than this in localized areas around geothermal fields however we cannot resolve small scale features in our regional scale model.

Accessible data & publications:

• Miller, C.A., Kirkby, A., Mortimer, N., Aden, F., Barretto, J., Black, J., Chambefort, I., Stagpoole, V., (2023). The Thermal Structure of Central Te Riu a Māui/Zealandia Continent as Determined From the Depth to Base of Magnetic Sources. J. Geophys. Res.: Solid Earth 128. Link
• Barretto, J., Caratori Tontini, F. (2022) Total magnetic intensity grid of the upper North Island, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2021/47. 29 p. Link

2.      New 3D magnetotelluric (MT) inversion code “FEMTIC”

The implementation of a new 3D magnetotelluric (MT) inversion code “FEMTIC” provides new insights into geothermal and magmatic systems in New Zealand. The new code includes topography and allows for fine meshing around areas of interest, allowing for more realistic models of crustal fluid distribution. This will benefit future work using MT to delineate geothermal systems and will allow for more detailed interpretation of results and refinement of targets for supercritical fluid.

The new code was first applied to test on a dataset at Mt Tongariro which has significant topographic relief and refined the model there which showed a trans-crustal magma plumbing system capped by the hydrothermal system.

Accessible data & publications:

• Heise, W., Bannister, S., Williams, C.A., McGavin, P., Caldwell, T.G., Bertrand, E.A., Usui, Y., Kilgour, G. (2024). Magmatic priming of a phreatic eruption sequence: the 2012 Te Maari eruptions at Mt Tongariro (New Zealand) imaged by magnetotellurics and seismicity. Geophys. J. Int. 236, 1848–1862. Link.

3.      New images of seismic properties in the mid-crust, below geothermal systems

In order to define background seismic properties (i.e. the ‘habitat’ surrounding potential supercritical fluid targets), we have investigated the seismic properties of the mid-crust, below the Wairakei-Rotokawa-Ngatamariki geothermal fields, using seismic wave data from more than 4000 well-recorded local earthquakes.

Our seismic tomographic analysis has focused on imaging properties in the 3-8 km depth range (Figure 2), although we can coarsely resolve properties down below 10-km depth. Analysis has highlighted considerable spatial heterogeneity of the properties beneath the region, including volumes of low P-wave velocity in the mid-crust, inferred to represent partial melt – the deep heat sources (Figure 3). ‍

Accessible data & publications:

• Journal paper in development, expected 2025.

4.      Refined geological models that honour geophysical observations

We have developed new capability to simulate geophysics, in particular gravity and magnetics, from existing geological models, allowing hypothesis testing where geological models are poorly constrained by drilling.

We applied this new method to the geological model of Wairakei-Tauhara-Rotokawa geothermal fields and were able to suggest improvements to the geological model where a poor match to the geophysical data was observed (Figure 3). A new model of the depth to greywacke basement was proposed in areas away from boreholes.

This workflow will allow geological model hypothesis testing and illustrates the importance of comprehensive geophysical datasets over geothermal fields. The new method has the potential to better target drilling to test geological hypothesis put forward to improve the match to geophysical observations.

Accessible data & publications:

• Barretto, J., Miller, C., Carson, L., Alcaraz, S.: Refining geological models through 3D gravity modelling at Wairakei, Tauhara and Rotokawa geothermal fields. Proc. 46th New Zealand Geothermal Workshop, Auckland, New Zealand. (2024). Link.
• Journal paper in development, expected 2025.