1. A method of analysing controlled source electromagnetic (CSEM) survey data to determine probability density functions (PDFs) for values of an electromagnetic parameter at locations in a subterranean region of interest, the method comprising the steps of: (a) providing structural information for the subterranean region; (b) identifying structural features in the subterranean region based on the structural information; (c) assigning an initial probability density function (PDF) for values of the electromagnetic parameter in each of the structural features; (d) generating on a computer apparatus a plurality of subterranean models specifying a value for the electromagnetic parameter in each of the structural features by statistical sampling of the respective PDFs; (e) identifying a subset of the subterranean models which are determined to meet an acceptance criterion; (f) generating replacement PDFs for each structural feature based on a distribution of the values for the electromagnetic parameter in the subset of subterranean models identified in step (e); and (g) iteratively performing steps (d), (e) and (f) a number of times so as to obtain a final PDF for values of the electromagnetic parameter for each of the structural features identified in the subterranean region of interest, whereby the PDFs used in step (d) of one iteration are based on the replacement PDFs generated in step (f) of a previous iteration.