April 28, 2021

Team Profile:
Seismic Tomography: a cat scan of the Earth
contributor(s)

Stephen Bannister

photo credit:
Cameron Asher

What magma bodies are below geothermal resources? At what depths? What are their properties and habitat?

One of the ways we are imaging the Earth’s crust to look for supercritical geothermal resources is through the use of seismic tomography.

How does this work?

Before we explain seismic tomography, first imagine a CT (computerised tomography) scan of the human body. A CT scan uses X-rays. Thousands of measurements are made around a body and mathematical algorithms dissect those measurements and ‘image’ the bone-tissue-muscle density. If a tissue has a low attenuation (e.g. air), very little radiation is absorbed by the tissue, allowing most of the X-ray to pass through and hit the detector (appearing black in a CT image). But if a tissue/bone has high attenuation, it absorbs most of the radiation (then appearing white in a CT image).

A very similar principle works for seismic tomography – only instead of X-rays or radio-waves, seismic tomography images the Earth using seismic waves (earth vibrations).

Measuring seismic waves

Seismic waves can be caused by earthquakes or small explosions, and are recorded using seismometers on the surface.

The speed that seismic waves travel through the earth varies with many factors, such as composition, saturation, temperature, pressure, fracturing, porosity, grain-size, mineralogy, as well as the presence of melt or fluids.

For natural earthquakes, we have access to seismic data recorded across most of the central North Island, mostly on GeoNet seismometers, but also on temporary seismometers especially placed out for a few months, thanks to friendly farmers.

Location of permanent GeoNet (white) and our temporary (yellow) seismometers spread between Lake Taupo and Rotorua.

To create a seismic image, we need as many earthquakes in different positions and depths as possible, shooting into as many surface seismometers as possible – from different directions.  

We use mathematics to determine the seismic wave paths from the source to the receiver, as well as the properties of the rock/medium the seismic waves passed through (and the maths is more complicated than for a CT-scan!).

Cartoon showing earthquake wave paths. Seismic waves that pass through partial melt lose more energy (attenuated) than waves that travel outside of the melt bodies. We use the observations at the surface seismometers to find out where the partial melt bodies are located. Figure following Lees (2007).
Aerial photo of one of our temporary seismometer sites, south-west of Rotorua. Photo taken by Cameron Asher (GNS Science). The temporary fencing is to protect the recorder from cattle.

How can seismic tomography help image supercritical resources?

At the moment we don’t know in detail where shallower magma is, nor the pathways it takes as it moves closer to the surface, up shallower from the deep heat source.

In the GNG programme, we are trying to make this seismic attenuation method as good as it can possibly be, for the earthquakes and seismometers that we have in the central North Island. Our goal is to improve imaging capability and resolution especially for partial melt features in the 3-10 kilometre depth range, as they define the heat sources. We won’t be collecting more field data; instead, our research is all about the maths. We’ll be using the plethora of data already available to improve algorithms and data inversion techniques so that we get the most information possible out of the existing data.

Our first hurdle to tackle is dealing with the heterogeneity - large and rapid changes in the rock properties over small distances. These rapid changes mean that the earthquakes’ seismic waves travel in complex ways, bending as they pass through rocks with different properties. We need to unravel and understand those complex paths in order to correctly image the partial melt areas, to position them correctly in 3D. As we progress in the GNG programme, we hope that our new analytical approaches will enable us to find and image much smaller partial melt bodies, as well as correctly positioning those bodies.

The below figure shows some early results from our analysis, possibly imaging partial melt at around 5 to 15 kilometres depth below geothermal fields south of Rotorua.

An “early progress” image of seismic attenuation at shallower depths (<15 km depth), below some of the geothermal fields south of Rotorua. Earthquakes shown as black circles, blue represents low attenuation (basement rock), while white represents high attenuation.

References:

  • Lees, J.M. 2007. Seismic tomography of magmatic systems. Journal of Volcanology and Geothermal Research, v167.

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categories

Geophysics
Modelling
Science
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tags

geophysics
explore
resource delineation
seismology
seismic tomography
supercritical resources
Taupo Volcanic Zone (TVZ)

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