Seismic rocker aims to cut the noise

A Canadian seismologist is arguing that our understanding of the structure of the Earth’s interior is based on the equivalent of fuzzy ultrasound images that leave room for improvement.

Once seismic images are fine-tuned to remove background noise, they may tell a very different story of the world below, says Dr. Felix Herrmann, a seismologist at the University of British Columbia. And oil companies are already lining up to cash in on his clearer view of the Earth’s underbelly.

Dr. Herrmann says that there has been a push to gather more data from beneath the ground through initiatives such as US ARRAY, but that these data cannot be properly used without a better understanding of how seismic images are produced.

Until now, he says, scientists have interpreted transitions in the mantle in terms of changes in a complex cocktail of substances below the earth’s surface. However, he says, his research has led to a theory that, instead of this accepted complex mineralogical composition, the seismic data could be showing a complex interplay of only two different types of substances.

“Think of what happens when you mix a French and a Swiss cheese,” says Dr. Herrmann. “You have Swiss cheese and you start to mix in more and more French cheese. At some point, you would call it a French cheese. When this happens, basically there is a critical point. That’s a point where all of a sudden the material starts to become like the French cheese – and a sharp transition occurs, whose fine structure we hope to observe from noisy seismic data.”

He will present the results of his latest seismic imaging research at the 2005 American Association for the Advancement of Science meeting in Washington D.C. on February 19.

With seismic imaging, scientists bounce sound waves into the Earth, or record natural tremors, and use the reflected waves to construct an image of the Earth’s interior structure in much the same way that ultrasound technology enables us to peer into the womb and see a fetus.

But Dr. Herrmann says that previous seismic images of the Earth’s interior were distorted by noise. Noise is created by things like tremors, wind or even a truck driving on the surface, as well as by flaws in measuring devices and mathematical models.

As a result, while we were expecting to be painting in geological blue, we should really be thinking in an earthy pink. “What I’m proposing is certainly a different way of thinking,” says Dr. Herrmann.

While the scientific world has yet to fully adopt Dr. Herrmann’s view, oil companies are eagerly taking a closer look at his “de-noising” and mixture-model techniques. His seismic imaging research has led to a recent partnership with four large oil companies to use these clearer imaging techniques to find the world’s most elusive oil patches.

“Oil prices are not high for nothing,” notes Dr. Herrmann. “It’s more and more difficult to find petroleum resources. The big oil fields have been found, and now people want to find smaller fields. That means we need to look with a finer resolution, and also to look deeper.”

We also need to better understand what we see in these images, he adds.

“That’s where both the ’cheese model’ and noise come in, because noise limits how fine a structure you can actually resolve to learn about these rapid changes in the Earth’s properties.”

Dr. Herrmann’s new theory of the earth’s transitions, co-written with MIT’s Dr. Yves Bernabé, was published in the December 2004 issue of Geophysical Journal International.