Variability in natural systems and means to characterise it

A common characteristic of natural geological media is that they display an immense spatial variability with respect to their structural, physical and chemical properties. It is, however, extremely difficult to describe this variability in space, given the amount of effort needed to explore directly natural media and the shortcomings of indirect geophysical methods as they are very often insufficiently sensitive to the variations of relevant parameters. A general review is presented of the existing approaches to describe variability and quantify its consequence with regards to migration processes through the geological medium. A number of key questions are discussed, and present knowledge and state of the art on these issues are overviewed. (R.P.)