K. Smith, R. W. Gatliff & N. J. P. Smith write: The sonic velocity method of uplift analysis compares the observed velocity/depth values in a particular well with a reference curve which defines the characteristic velocity/depth relationship in a normally compacted sequence. Increased sonic velocities relative to those that might be expected at present depth of burial are thought to be related directly to the amount of the original sequence which has been removed by erosion. Hillis (1993) studied sonic velocity variation in the Middle and Upper Chalk, the Kimmeridge Clay, the Lias and the Bunter Sandstone from the Southern North Sea and East Midlands Shelf. His results apparently confirm the scale of Tertiary erosion in this area which had been previously inferred from the interpretation of apatite fission track (AFT) and vitrinite reflectance (VR) data (Green 1989; Bray et al. 1992 and references therein). A similar study by Hillis (1991) of Chalk Group velocities from the SW Approaches area of the UK continental shelf also revealed substantial amounts of Tertiary uplift. Sonic velocity analysis ought to provide an independent measurement of apparent uplift which avoids the potential pitfalls associated with temperature-dependent assessments such as AFT and VR (Holliday 1993). This discussion concentrates on the Chalk Group which was analysed in detail in both of Hillis's studies (1991 (1993). We suggest that there are four important factors in Chalk velocity analysis which might contribute to an over-estimation of burial depths and amounts of subsequent erosion.
(1) The porosity/depth relationship derived