Grumbling the Earth "translated"

Seismic background noise provides valuable information

a: recording of the collected passive data, b: pattern after taking out surface waves and special filtering techniques. © Draganov et al.
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The earth is never quite still: movements in the underground, but also sound waves from the surface produce noises that propagate in the earth's crust. This "grumbling" of the earth, as Dutch researchers have now discovered, could provide valuable information about the nature and status of the subsoil, including the location of oil and gas fields.

When sound waves travel through a medium such as the Earth's crust, their properties are changed. If they are collected, they can therefore be used under certain conditions as an information carrier. With the help of ground microphones and a few mathematical calculations, a signal can be gained from the seismic background grunts.

In contrast to previous methods, in which only seismic waves have to be generated in order to study the subsurface - for example in the context of the search for oil deposits - the new method of so-called seismic interferometry alone uses the already existing background noise of the earth.

The idea for this method was initially developed by the geophysicists Kees Wapenaar and Evert Slob of the University of Delft together with their colleague Roel Snider of the Colorado School of Mines. They demonstrated theoretically that seismic "grumbling" could be used for a variety of purposes, including the search for new oil and gas fields.

Oil companies interested in procedures

Now the researchers, supplemented by the colleague Deyan Draganov, have also tested their theory in practice. In a desert region in the Middle East, they recorded the background noise using ground microphones and, thanks to special calculations, were able to extract data on the reflection behavior of the seismic waves. They were not only able to evaluate superficial sound waves, but also signals from greater depths. display

First oil companies, including Shell, which bore part of the cost of these trials, are already very interested in the new process. The results of the tests have now been published in the journal Geophysical Research Letters.

(University of Delft, 28.02.2007 - NPO)