First Break - Volume 13, Issue 3, 1995

Radioactivity is part of our physical environment. The largest contribution to the radiation field is of naturaI origin: it is due to cosmic rays, the natural radioactivity of the ground and the radioactive decay products of radon in the air. Artificial radioactivity is emitted by nuclear power plants, industrial plants and research facilities. These emissions are very small in normal operation, although large amounts of radioactivity can be released to the environment through accidents.

There are two distinct approaches to the seismic modelling of crustal heterogeneity. In the first approach, which is of ten referred to as 'deterministic' modelling, individual terogeneities are modelled explicitly. That is, models are constructed in which the locations of these heterogeneities and parameters describing them (e.g. velocity, density etc.) are defined precisely by the modelIer.