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Initiator: ASTRON Netherlands Institute for Radio Astronomy

eu  SNN

This project was co-financed by the EU, the European Fund for Regional Development and the Northern Netherlands Provinces (SNN), and EZ/KOMPAS.

Seismic Interferometry works!

As part of the geophysics application of the LOFAR project a three-dimensional seismic array consisting of 126 sensors has been constructed over the Annerveen gas-field. This network of sensors will be used to sample the seismic wavefield on a kilometer scale, focussing on higher frequencies for seismic-exploration applications, which is up to 100 Hz. This geometry is most suitable for research on time-lapse seismic and the passive seismic interferometry.

Three weeks ago, the first correlation results for the seismic array near Annerveen were obtained: it proves it works as an interferometer! A major milestone of the LOFAR project for the seismic application.

In the picture on the left, a seismic recording of dynamite shot with the Annerveen array is given. From such recordings, images of the subsurface are obtained, typically after many noise-reduction and imaging steps. In such a dynamite recording the low-frequency events are mainly surface waves, while the high-frequency events are mainly reflections coming from the subsurface until some kilometers depth.

The two rightmost figures are obtained with correlated noise. The low-frequency components are typically interpreted as surface waves; they look somewhat different than the dynamite recording but there are sufficient arguments to explain those differences. The high-frequency components are also abundantly present (the rightmost figure), and this is good news. The further interpretation of this record needs further investigation but it seems there are many reflections of the subsurface present.
ASTRON initiated LOFAR as a new and innovative effort to force a breakthrough in sensitivity for astronomical observations at radio-frequencies below 250 MHz. 
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