Fracture Mapping in Clays: Soil Gas Surveys at Down Ampney, Gloucestershire.

摘要

At Down Ampney airfield, Gloucestershire, a faulted sequence of Jurassic sediments, comprising the Cornbrash (limestone), Kellaways Beds (sands and clays) and Oxford Clay (clay) is covered by about 5m of Pleistocene gravel capped by 1 m of soil. Soil gas samples from a depth of 0.5m were taken at intervals of 12.5m along eight traverses and 25m along one traverse, run approximately north-south. The samples were analysed for /sup 4/He, Rn, CO/sub 2/ and O/sub 2/. Anomalous concentrations of these gases were found in a number of linear zones trending approximately east-west. These define the positions of fractures. It is not possible to positively identify which of the fractures exhibit vertical displacements, but the most significant feature occurs about 50m north of the east-west runway. This is bounded by less well-developed fractures in a pattern consistent with a fault accompanied by sub-parallel minor fracturing. Consequently, the zone 50m north of the east-west runway is considered to be the main target for future work. (Atomindex citation 20:025706)