February 3, 2021

Team Profile:
Using Magnetics to Map Geothermal Resources

Jenny Barretto

photo credit:
Vaughan Stagpoole

GNG researchers are using magnetics as one geophysics tool to help delineate New Zealand’s supercritical geothermal resources.

*Stay tuned to the GNG website for future posts explaining magnetotellurics, seismics and other geoscience methods we are using to search for supercritical resource…

What is Aeromagnetics?

Magnetics is a type of geophysical method. Geophysics is the study of physical processes and properties of the Earth. One of these properties is the magnetic field emanating from the Earth’s interior, which provides information about the Earth’s internal geological structure. Magnetic surveys can be ground-based, airborne and shipborne.

In an aeromagnetic survey, a magnetometer (which measures magnetic field) is mounted in a stinger assembly attached in front or rear of the aircraft. Flight allows data collection over large areas and over difficult to access terrain. Typically, the aircraft flies in a grid-like pattern, and the height and line spacing determines the data resolution.

A magnetometer mounted in a stinger assembly protruding in front of an aircraft. Source: New Zealand Petroleum & Minerals (2020).

What do we measure?

Magnetic anomalies are a local variation of the Earth’s magnetic field intensity, due to rock properties. Magnetic anomalies of geological origin are commonly less than 1% of the Earth’s magnetic field, so magnetometers are designed with high sensitivities to detect these very small differences.

Rocks differ in their magnetization – this is a measure of how much each rock type becomes magnetised by the Earth’s magnetic field, and depends on what type of minerals they contain. When rocks form, their minerals with strong magnetization (e.g. magnetite) become magnetised and align themselves in the same direction as the Earth’s magnetic field, just like a compass needle. Iron sands in Taranaki are highly magnetic sediments.

Volcanic rocks like basalt commonly have larger magnetization compared to sedimentary rocks like limestone. We can therefore expect that basalt will show a very intense magnetic anomaly, and limestone will have a smaller magnetic anomaly. However, the Earth’s magnetic poles sometimes flip directions such that magnetic minerals formed during those times will have reversed polarity and show negative anomalies when measured today. Heat can also reduce magnetisation intensity — above the Curie point, a magnetic mineral loses any permanent magnetisation.

Magnetic Anomaly Maps

A magnetic anomaly map (such as the one below) visualises the local variations in Earth’s magnetic field intensity caused by geologic materials and structures in the crust. This information can infer the shape, depth and properties of the rock bodies, and the presence of faults and folds.

A magnetic anomaly map will show the spatial distribution and relative abundance of magnetic minerals on and beneath the surface in the upper crust, up to Curie point depth, which is a theoretical surface with a temperature of about 580°C.

Magnetic anomalies (in nanotesla or nT units) over Northland. Numbered features are identified by authors as prominent and discussed in their paper. Source: Stagpoole et al. (2016).

What does this mean for New Zealand’s Supercritical Resources?

In GNG, we are not flying any new surveys. Instead we are focussing on the abundance of data already available for the Central North Island (collected between 1977 and 2014). However, since this data was collected using different platforms, observation heights and resolutions, we have to integrate the data and assemble this puzzle.

We are working with data from surveys collected at fine scale (e.g. less than 100m line spacing), and larger scale (collected at 200m to 2km line spacing). Our aim is to create for the Taupo Volcanic Zone 100m regional magnetic grids - one preserving original observation levels of individual surveys and another draped 150m above the topography.

We are using the magnetic information to investigate the deep crustal structure, identify magnetisation variations related to deep magmatic bodies, and to estimate the depth-to- the-bottom of a magnetic layer (i.e. the Curie point depth).

Curie point depth map of Taiwan derived from magnetic data. Source: Hsien et al. (2014).

References & Sources:

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