Seismic research without artificial source

Researchers at TU Delft have made progress in the theoretical foundation of a special subsoil imaging technique. This technique could be used to chart underground mineral resources, it is called “acoustic daylight imaging”. The method uses natural acoustic signals, already present in the earth, to create an image of the subsurface layers. This week, Professor Kees Wapenaar will publish an article in the renowned scientific magazine “Physical Review Letters”.

Usually, the composition of the subsurface is researched using generated acoustic signals that are sent into he ground. The sonic reflections are then analysed (the basic principle of seismics). This is no longer necessary with acoustic daylight imaging. Theoretically, taking surface measurements and subjecting the results to a series of mathematical calculations would be enough to create an image of the subsurface.

The theoretical possibility of seismic imaging using only naturally occurring sources of sound has previously been shown. This phenomenon is, however, no longer of purely theoretical importance. A current example of the possible application and development of acoustic daylight imaging is the Lofar-project in Exloo (in the Dutch province of Drente). This large scale scientific project not only encompasses the construction of the world’s largest radio-telescope, but also the realisation of the largest sensor network in Europe. This network would provide a development platform for, for example, geophysical applications.

In October of 2004, geophysicists from TU Delft, TNO and the KNMI therefore started placing geophones (a sort of microphones used to measure underground sound waves) in the testing field in Exloo. During ten years they will use these geophones to ‘listen’ to naturally occurring underground sounds. The expectation is that this will provide more understanding about the structure of the subsurface as well as providing a more accurate scientific model of how the subsurface is moving. The latter is of importance in the mining of resources such as gas, oil and coal.