In their use of geophysical methods to supplement sparse borehole data for locating a buried tunnel-valley in the Chalk of East Anglia, the authors include an interesting application of detailed gravity surveys. The thick boulder clay in-fill of the narrow valley gives rise to small Bouguer anomaly highs and a density contrast is derived of +0.26 t/m3 between this material and the Chalk. The sand and gravel in-fill is concluded to have a negligible density contrast. The authors then interpret gravity profiles over parts of the tunnel-valley with both types of in-fill, by adopting an ‘effective density contrast’ weighted according to the proportion of boulder clay present in adjacent boreholes.
This approach to the gravity modelling of the overall form of an anomalous multi-component body is, however, only valid if the dimensions of the individual components of the body are significantly smaller, perhaps by an order of magnitude, than that of the body itself. On the evidence of Fig. 2 in the example described, the ‘effective density contrast’ appears not to be valid for the whole tunnel-valley, as there is only one horizon of boulder clay and, at depth in the tunnel-valley, one horizon of sand and gravel in both the Cavendish and Glemsford boreholes. If the sand and gravel has no density contrast with the Chalk then it is surely not possible to derive a model from the gravity data for the bottom part of the valley, below the boulder clay. The adoption of an incorrect density contrast additionally
- © The Geological Society 1986