Sub-basalt imaging: a new approach to model and understand the Faroe Islands basalt

Abstract

The main part of the work carried out in this thesis has been concentrated around the building of in-homogeneous velocity and density models, representing basalt. The models have a strong link to the real geology since real seismic data has been used as basis for the model building. 3D nite difference modelling has shown to produce synthetic data, which is surprisingly similar to the real seismic. To the author's knowledge there has not been published any similar results to those in this thesis.

A low velocity zone, most likely to represent inner flows, has been identifed to have great impact on the quality of the seismic image. Due to the decreasing velocity, more scattering is introduced to the seismic data. A link between the decreasing velocity and the presence of gas has been established, but not been confirmed. This gas theory has not been presented before, as far as the author know, and may be an important explanation for the poor sub-basalt images. The inner flows are thought to continue beneath the lava delta, and this may explain why the seismic image is poor in the whole sub-basalt zone at the escarpment. The results from this thesis indicate how more knowledge about inner ows could be of great importance related to the sub-basalt imaging problem. One of the major conclusions in this work is that in order to understand more of the sub-basalt imaging problem, it is vital to establish a good link between the geology and the geophysics.

Both a time migration and a depth migration have been performed on the synthetic data, and it is obvious how depth migration gives a much better image than the time migration.