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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.

Passive seismic imaging

People have always been interested in discovering the hidden and unknown. And one such “hidden and unknown” is the Earth’s interior. The only method to see how the Earth looks like from inside is to cut it open. But this is quite unpractical as we are living on it. So, to overcome this slight problem geophysicists proposed to build an image of the Earth’s interior by investigating it with non-destructive methods: seismic, gravitational, electro-magnetic, etc.

When using the seismic method, a seismic source generates at a certain place at the surface seismic waves.These waves propagate through the Earth. When the waves encounter on their path rocks with different seismic properties, part of the waves’ energy is reflected back. At the Earth’s surface these reflected waves are detected using sensors for seismic trilling (seismic receivers or geophones). Knowing the time of initiation and the time of the detection of the reflected waves, one can in principle say how deep lays the reflector (boundary).

With the seismic daylight imaging method the geophysicists are trying to see the Earth without the help of a specially dedicated seismic source the surface. A use is made of the fact that there always are sources of seismic noise in the subsurface. When the waves resulting from these sources encounter on their propagation path rocks with different seismic properties, part of their energy is transmitted up towards the surface. And again at the surface the transmitted waves are recoded by geophones.

The idea of the method is to simulate a conventional survey as described above (reflection survey) as if in the presence of an impulsive source at a chosen position at the surface. Once the reflection picture is simulated, the well-established processing and imaging techniques for the normal reflection investigations can be used.

To reconstruct the reflection response of the subsurface measured at point A in the presence of an impulsive source at point B, the recorded at the points A and B transmission responses have to be cross-correlated. From the cross-correlation result the positive part of the signal is only needed.
See illustration for the case of the acoustic model.



At the surface the transmission response on figure 1 of the subsurface to all the white noise sources is recorded.


Then the transmission recordings are cross-correlated and the positive part is taken resulting in figure 2.


The directly modelled reflection response of the this model to an impulsive source at the surface is shown in figure 3.






ASTRON initiated LOFAR as a new and innovative effort to force a breakthrough in sensitivity for astronomical observations at radio-frequencies below 250 MHz. 
Development: Dripl | Design: Kuenst   © copyright 2014 Lofar