The implementation of a new 3D magnetotelluric (MT) inversion code “FEMTIC” allows new insights into geothermal and magmatic systems in New Zealand.

In GNG, the geophysics team have been improving the techniques used to model magnetotelluric data. Commonly available 3D MT inversion codes approximate the surface of the Earth as a flat plane. This approximation is acceptable for deep imaging in places where topographic variations are small to moderate. However, for shallow imaging and particularly in places with steep topographic changes (e.g. volcanoes), accurately representing the surface topography in the model is important.

The new code was first applied to test on a dataset at Mt Tongariro (New Zealand) which has significant topographic relief. This was published in a recent journal paper. A 3-D finite-element inversion model that includes topography allowed the hydrothermal system and its connection to the deeper magmatic system at Mount Tongariro to be better resolved than previous studies. The refined the model there showed a transcrustal magma plumbing system capped by the hydrothermal system.

A key factor in the increased resolution was a careful mesh design that included fine structure at the depths of interest, but coarsening outwards to also allowed fast computation time.

By including topography and allowing for fine meshing around areas of interest, the new code allows 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.

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