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May 2003

This attenuation apparatus measures the seismic wave speed through samples of earth materials by exerting a torsional force on the sample. This test rig is the only one of its kind in the world (and this technique was devised by the ANU Rock Physics group).

Seismic wave speed tester
Placing rocks under extreme heat and pressure
is helping earth scientists understand
what's going on deep in Earth's mantle.

For centuries scientists have been attempting to model the processes that drive and form our planet. That the surface crust of Earth consists of a series of tectonic plates that are constantly moving and being recycled was a major conceptual breakthrough. However, an understanding of how movement in the upper mantle is driving this continental drift has proved frustratingly elusive. This is not an easy place to do science.

Some rock samples from depths as great as 100 km reach the surface in volcanic magmas, and we also have a comprehensive description of how seismic waves (like sound waves) travel through this region of the Earth. But to truly understand the behaviour of geological materials from the mantle and successfully interpret the seismic data we need a detailed model of the mechanical properties of these materials. And to develop that model it's necessary to test them in conditions equivalent to those found deep inside the planet.

Who runs it: The seismic wave speed test rigs form part of the Rock Physics research group at the Research School of Earth Sciences. The Rock Physics group is investigating a range of properties of geological materials in conditions that simulate the Earth's mantle. Using two purpose-built test rigs, the researchers can subject samples of earth materials to pressures of 3000 atmospheres and temperatures of around 1300° C, approximating conditions experienced tens of kilometres deep in the Earth's mantle.

How does it work: The first test rig uses a gas (argon) charged pressure vessel with an internal electrical heater for the ultrasonic measurement of elastic wave speeds at high temperature. The samples are small cylinders of rock (both natural and synthetic) ranging in size between 7 and 3 mm in diameter.

The use of ultrasonic methods to measure elastic wave speed in rocks is a well known and widely practiced procedure. It involves firing radio-frequency (10-100 MHz) sound pulses through one end of the sample and measuring the interference pattern created as part of the signal bounces back from each end of the specimen. It has been difficult to know how accurately this method simulates seismic waves (at frequencies below 1 Hz) because it's not easy to directly measure seismic waves in rock samples under these high pressure/high temperature conditions. However, direct testing of rock properties at seismic frequencies has been achieved by the Rock Physics group with their second test rig known as the 'attenuation apparatus'.

This equipment allows laboratory investigation of the viscoelastic behaviour of earth materials at high temperatures through torsional forced oscillation and microcreep tests. As with the ultrasonic method, small cylindrical samples are heated up electrically in gas-charged pressure vessels. The sample being studied is mounted within a thin-walled iron sleeve between torsion rods of alumina and steel and connected to a standard of known mechanical properties. The combination of sample + standard is then twisted at seismic frequencies. The resulting angular distortions of sample and standard provide data from which the seismic shear wave speed and attenuation can be calculated.

Results from the attenuation apparatus have allowed researchers to check the validity of the speeds estimated from the ultrasonic studies. There is broad agreement in the results between the two techniques for temperatures below 1200 K, but considerable divergence above this temperature. This suggests that care needs to be exercised when applying the results of ultrasonic estimates at high temperatures. These results have important implications for the interpretation of models of seismic wave speed variability for the upper mantle.

This is just one part of the work being carried out by the Rock Physics group. Other inter-related projects include the creation of a range of synthetic earth materials (such as synthetic polycrystalline specimens of the major upper mantle mineral olivine) to test the systematic variation of material properties with key parameters such as grain size and basaltic melt fraction; and an array of light microscope, TEM and SEM studies of the microstructures of samples. By integrating a knowledge of the macroscopic physical properties of earth materials (such as strength, seismic wave speeds and attenuation) with an understanding of their microstructure the group is providing deep insights on how the Earth's mantle behaves.

More information: http://rses.anu.edu.au/petrophysics/PetroHome.html