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

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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.

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Marching orders (again!)

How time flies!

We have only just managed to get back into the swing of things after the Antarctica field mission, and they send us right back out again.

This time JY and I are going to the French Sub-Antarctic islands of Crozet, Kerguelen, St Paul and New Amsterdam. This will not be my first journey to this part of the world. I first went there in 2006, only a few weeks after starting my job here in Strasbourg. I have great memories of that trip!

These islands are all in the southern Indian Ocean, and are accessible only by ship. We shall be traveling on the Marion Dufresne, a multi purpose ship: part research vessel, part cruise ship, part cargo ship, part tanker, part helicopter carrier. As comfort and stability go, it is way better than the Astrolabe (the ship we took to Antarctica three months ago).

We set off on April 3rd from Reunion Island, then visit each of the islands in turn, starting with Crozet. We are expected back at Reunion Island on May 2nd. You can follow the ship's progress using either this web page or this kml file for Google Earth.

By popular request, I shall be field-blogging again, so stay tuned for more exciting stuff from the world of seismology...

I've been blogged...

It seems I have been found out...

I received an email today from one of the editors at Blogged.com, an outfit that rates and categorizes blogs. They had found my blog and rated it, giving me an 8 out of 10... for whatever that's worth.

Here is some blurb from their "About us" page:

Blogged.com is all about blog discovery. It's a place for readers to discover interesting blogs and for authors to discover who their readers are. [...]

Our blogs are reviewed, rated, and categorized by our editors, so you won't have to experience the frustration of filtering through blogs that are either spam, outdated, or irrelevant. You'll be able to find quality blogs that you would have unlikely found through a traditional blog search.

And here is the pretty widget they gave me to put on my blog:

Can better physics guarantee better tomographic models?

One of the key elements in discussing an inverse problem such as seismic tomography is the quality of the forward theory. The better the forward theory, the better synthetic data can be predicted from physical model parameters, and hence the better the solution to the inverse problem, right?

Unfortunately the issue is not so simple. Trampert & Spetzler (2006) come to the dual conclusions that better physics (in the form a finite-frequency formulation of the sensitivity kernels of seismic wave measurements) is a necessary but not sufficient condition for improvement of tomographic models, and that the null-space (due to uneven or insufficient data coverage) is currently too large to permit the improvements in resolution that better physics could provide.

Despite finite-frequency kernels being more accurate than the approximate sensitivity formulations of ray-theory, models constructed from either theory are statistically similar, i.e. one cannot construct a finite-frequency model (with a given data fit and horizontal resolution) which cannot also be obtained from ray theory by changing the regularization damping of the inversion accordingly. Regularization dominates the significant aspects of tomographic models, and affects both finite-frequency models and ray-theory models similarly. Data error propagation is worse for finite-frequency kernels, but given the large influence of regularization, this is a minor problem.

The authors maintain that in order to increase the resolution of tomographic inversions, we have to remove the ill posedness in the inverse problem (an ill posed inverse problem has more degrees of freedom than can be constrained by the available data) by increasing and/or homogenizing data coverage. I agree whole-heartedly with this statement! What can be done?

(1) The current distribution of seismic stations is in-homogeneous (see figure at bottom of post showing all FDSN seismic stations), and is limited by the accessibility of suitable installation sites. We should attempt to homogenize the distribution of seismic stations by installing more instruments in currently inaccessible locations such as the sea-floor (ocean-bottom seismometers) and my personal favorite, Antarctica. This solution requires lots of time, effort and a high level of funding that is becoming more and more difficult to obtain.

(2) So far we only use very little information from the complete seismogram (first arrival times of a few main waves, or the dispersion characteristics of surface waves). We should use more of the information available from the complete seismogram, given that modern adjoint methods have made it possible to associate a complete sensitivity kernel to each measurable wiggle in a seismogram. This solution is technically feasible given enough computing power and the development of new tools to automate the data selection and measurement processes.



References

Trampert, J., Spetzler, J. (2006). Surface wave tomography: finite-frequency effects lost in the null space. Geophysical Journal International, 164(2), 394-400. DOI: 10.1111/j.1365-246X.2006.02864.x

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