Could you tell us a little bit about yourself?
I got into geophysics by accident: I majored in astronomy at the time the first space probes were sent to the Moon, and I wished to study planets. But the counsellor of the University of Utrecht pointed out to me that the Netherlands did not have an active space program. “However,” said he, “since we just discovered all these hydrocarbons beneath the North Sea, you can now specialize in geophysics. Isn't the Earth also a planet?” Of course it is, but studying it did not seem quite as adventurous as studying Mars or Venus. Nevertheless I gave it a try, and I have been hooked ever since. My interests go deeper than the crust, though, and center on inverse problems, more in particular that of global seismic tomography. My career path has brought me from Utrecht to Princeton and, more recently, to the University of Nice in France. As a result I now have two emeritus desks on both sides of the Atlantic, which I use to study the problem that has kept me busy in the past ten years: how come the Earth has cooled off so little in four billion years, and how can my tomographic images be improved and help to find the answer to that riddle?
What are the most important recent developments in the field of research discussed in your chapter?
The first tomographic images were obtained assuming that seismic waves behave like waves in optics. However, their wavelengths – relative to the distance they travel – are orders of magnitude larger than those in the visual spectrum. With the late Tony Dahlen and a number of very talented students and postdocs, we have been able to step away from the optical approximation and now take diffraction effects into account in what is called "finite-frequency tomography".
A second very important development is the densification of seismic networks, and the open access of data from thousands of digital seismic stations via the web. This is a far cry from hand-digitizing seismograms, blown up from a microfiche in a dark room, as I had to do for my PhD. The next hurdle to take is the oceanic domain: 2/3 of the surface of the earth is practically without seismometers. Doing global tomography is like doing a CAT scan in the hospital with 2/3 of the sensors out of order. I believe that floats equipped with hydrophones may soon help us out.
How can other seismologists, planetary scientists and astrophysicists learn from this area of research?
Helioseismologists have already adopted finite-frequency tomography. Tomography on other planets is still far away though. We shall need to learn how to observe seismic waves from a satellite using remote sensing of the surface, or perhaps using electron density perturbations in the ionospheres of other planets caused by seismic waves.
Could you recommend one or two of your key research papers related to your book chapter? Could you tell us a bit more about the research?
My book "A Breviary of Seismic Tomography" (Cambridge University Press, 2008) is probably the most complete of the literature for non-specialists. An example of how tomography helps us attack fundamental questions about the Earth is: Nolet, G., S.-i. Karato and R. Montelli, Plume fluxes from seismic tomography, Earth Plan. Sci. Lett., 248, 685-699, 2006.
How important is it for you to be involved in international, collaborative, interdisciplinary research linked to seismology?
I get bored too easily when I work on too narrow a range of problems. Working with other people (and changing environment every 15 years or so, as in my case) is almost always inspirational.
Do you have any other messages to our readers?
We could cover the oceans with a network of seismometers for the same amount of money that we spend to send one seismometer to Mars. I think our next priority should be closer to home....