Using seismic waves to image Earth's internal structure

Life on board the Marion Dufresne continues uneventfully. I am taking advantage of the relatively clement sea conditions to work on a manuscript due for submission soon, and to read some scientific literature.

Earth science is a frustrating subject at times. It seems the more we investigate the Earth, the less we understand how it works. Controversy is rife, generating much confusion in the minds of students and researchers alike. Questions like 'How do subduction zones work?' and 'Where does hotspot volcanism originate?' are still hotly debated at international conferences and in print. One of the ways we are trying to address these open questions is by striving for clearer and higher resolution images of the Earth's interior.

Romanowicz (2008) gives a good three-page summary of past progress and outstanding issues in the use of seismic waves for this type of imaging.

To address these controversies, seismology has been brought to bear to image Earth's deep interior. From the construction of accurate models of Earth's one-dimensional radial structure to the current models of its three-dimensional structure, progress in seismic imaging has gone hand in hand with improvements in the design of seismic sensors, the capacity to record digitally increasingly massive quantities of data, theoretical progress in handling seismic-wave propagation through complex three-dimensional media and the development of powerful computers for simulating seismic waves and for the inversion of large matrices.

We are now at a point where there is a certain consensus regarding the long wavelength heterogeneities within the Earth. The next steps according to Romanowicz should be

characterizing the sharpness or fuzziness of the boundaries of the heterogeneous structures deep inside the planet, and detecting and mapping small-scale heterogeneity [...] This will mean extracting more information from seismograms than has traditionally been done.

Romanowicz goes on to describe some of the recent advances that are already leading to improvements in tomographic imaging techniques, including first order scattering theory, spectral element wavefield simulation methods and the extraction of structural information from the cross-correlation of noise records.

The problem of data coverage still remains:

A significant challenge is the limited distribution of seismic-wave sources and receivers. Ideally, one would want to sample the volume of Earth uniformly. But unlike other disciplines that use imaging, such as medical tomography or petroleum exploration, earthquake seismologists cannot optimize their experimental geometry.

Here we return the main theme of the last few research blogging posts: the need to obtain more raw data and to exploit them more fully to improve the imaging of Earth's interior.

Quoted text reprinted by permission from Macmillan Publishers Ltd: Nature, copyright (2008).

References

Romanowicz, B. (2008). Using seismic waves to image Earth's internal structure. Nature, 451(7176), 266-268. DOI: 10.1038/nature06583

Life on board the Marion Dufresne

No two field trips are alike, not even when they take you to exactly the same sites. A lot of the atmosphere of such a trip is created by the participants, the rest is determined by the weather, and neither element can be influenced.

I first went on this trip to the French Austral Islands two years ago, and for me it was a voyage of adventure. I had only recently started a new job, was traveling alone, and had only a vague idea of what was going to happen. This second time round, the trip has a cozy and familiar feel to it. I know the ship (the Marion Dufresne is a luxury cruise liner compared to the Astrolabe), I know how life on the scientific bases of the sub-Antarctic works, there are many familiar faces amongst the passengers and the crew, and I shall be meeting more people I know on the bases themselves.

There are few scientists on this leg of the voyage (more will come aboard as we pick up the last of the summer campaigners on each island). Most of the passenger list is made up of logistics people, with the addition of a few tourists. The Marion Dufresne regularly takes tourists along on these trips. They get to visit the bases and talk to the scientists, and they also visit a number of protected sites elsewhere on the islands with a specialized tour guide. The trip is not cheap, and apparently there is a year long waiting list for the few available slots. The people who come are usually highly motivated and curious about all aspects of the sites and of the science that is being done there. They learn more about the islands and what is being studied there than we do, given that we spend most of our time working on our respective projects.

We got treated to a fantastic spectacle last night, courtesy of a late summer storm. The heat and humidity that had been mounting throughout the day finally gave way to thunder and lightening. We stood outside on the covered deck, with the rain drumming down on all sides and sloshing off the top deck, gasping at the sight of the mauve sky and mauve ocean suddenly revealed then immediately hidden again. Truly a breathtaking sight.