3D geological model and field observations of the Eastern Mendips

This report was produced by the British Geological Survey (BGS) on behalf of the Environment Agency (EA). It provides background and methodological information on the development of a 3D geological model as well as field passive seismic survey observations of the Eastern Mendips.
The 3D geological model covers an area of approximately 225 km2 and comprises selected horizons from regional Palaeozoic and Mesozoic bedrock strata as well as selected faults and a major unconformity. It was constructed using historical data including borehole records, geological maps and structural data held by the BGS and seismic data provided by Heidelberg Materials.
The model was built in two parts, separated by the major unconformity. The stratigraphically upper part consists of horizons ranging from Triassic to Middle Jurassic, whereas the stratigraphically lower part consists of horizons ranging from Silurian to Carboniferous. Both parts, but especially the stratigraphically lower part of the model were constructed using relatively sparse data resulting in significant uncertainties, especially at greater depths. The modelled stratigraphical horizons are provided as ASCII grids and TSurfs.
The modelling was augmented by the collection of additional field data, principally in the large active working quarries (Torr Works, Whatley, Westdown, Colemans, and Halecombe quarries), and passive seismic transects across the eastern Mendips north of Frome. These data were used to assess the quality of the current geological mapping hat was used as input to the 3D geological model. The results confirmed that the existing geological data at surface, and thus much of the input data to the 3D model, is of good quality.
The 3D geological model provides an overarching architecture of the major geological units in the east Mendips area. As such, it cannot be used to predict flow pathways because these will be guided by sub-model scale heterogeneities within the modelled strata. However, it provides estimates of where important aquifer units may be present at depth and where there may (or may not) be likely groundwater connectivity between recharge zones and springs.