14 June 1999

Paul Simons, Director, Bath Spa Project

Prof. Clive McCann, University of Reading

Prof. David McCann, University of Reading

Dr Geofrey Kellaway, Geological Consultant

Andrew Mann, IMC Geophysics Ltd

After introducing the panel and providing a background to the search, Paul Simons invited Prof. Clive McCann to present the results of the investigations.

Geophysical Investigations of the Hot Springs of Bath

Professor C. McCann, Professor of Geophysics, Postgraduate Research Institute for Sedimentology, University of Reading

From Roman times the City of Bath in North-East Somerset has been famous for its hot springs, which have been used for pleasure and for the alleviation of rheumatism and other medical conditions. There are three springs, situated in the very centre of Bath within a meander of the River Avon, of which the most important is the King's Spring which flows 275,000 gallons (1,250,000 litres) of highly mineralised water each day at a temperature of about 470C. Geological, geochemical and hydrogeological evidence shows that the hot water emerges close to the ground surface from faults and fractures in the Carboniferous Limestone ( the rock which underlies the centre of Bath at a depth of about 50 metres), that it has been stored in the Carboniferous Limestones for some hundreds of years and that it must have reached a depth of at least 1500 m to attain the high temperature. One hypothesis for the origin of the springs is that cold water flows from the Mendip Hills, 12 km to the south of Bath, into a basin of the Carboniferous Limestone, reaching a depth of about 2.5 km. The water, now at high temperature, then moves rapidly up the northern limb of the basin to reach the ground surface at Bath. An alternative hypothesis requires only local flow of meteoric water from the hills around Bath; this hypothesis envisages this water percolating to a depth of about 2 km and then re-emerging along major fractures in the Carboniferous Limestone to the surface.
The springs are in the care of Bath and North-East Somerset Council, which recently made a successful application to the Millennium Commission for part of the funding to build a new Spa in the centre of Bath. The building of the new Spa and the need to maintain the water flow have made it urgent to establish which of the hypotheses correctly accounts for the origin of the thermal springs so that the effects on temperature and flow rate of both local and distant water extraction and of local and distant quarrying can be predicted with more confidence.

Understanding the origin of the thermal springs requires a detailed knowledge of both the shallow and deep (greater than 2 km) geology of the area around Bath and in particular the structure of the aquifer, the Carboniferous Limestone. Unfortunately the surface geology of Bath is very complex, due to faulting and slumping of the young clays and sands which cover the area, and there are few deep boreholes so that the deep geology has remained poorly understood.

The Council appointed Dr. G. A. Kellaway to supervise a programme of investigation of the geology of the Bath area, through a series of deep boreholes and geophysical surveys. Professor Clive McCann and Professor David McCann, both of the Postgraduate Research Institute for Sedimentology, University of Reading, were asked to advise on appropriate geophysical surveys to support the deep geology programme. Knowing that the Carboniferous Limestone is the aquifer and that this rock has both a very high seismic velocity (6000 to 6500 ms-1) and a very high density (2720 kg m-3), the technical team suggested that seismic and gravity geophysical data should be used to map its structure.

In early 1999 IMC Geophysics was awarded the contract to shoot, process and interpret 21 km of seismic lines in and around Bath and to re-process existing seismic data from areas to the south and east of the city. In addition digital gravity data were purchased from the British Geological Survey for a 30 km by 30 km area around Bath. The seismic data proved to be of excellent quality, giving images of the sub-surface to depths greater than 3 km. Seismic data normally require a deep borehole to calibrate the observed reflectors with the geological interfaces. However, in this study, the seismic images, the digital gravity data and shallow borehole data have been integrated together to provisionally identify the Carboniferous Limestone and map the depth to its top surface across an area of 100 km2 around the centre of Bath. The resulting interpretation has yet to be confirmed by one deep borehole, shortly to be drilled on the outskirts of Bath.

Within a circle of radius about 1 km around the King's Spring, the Carboniferous Limestone is at a depth of less than 50m below the ground surface. Provisional interpretation of the geophysical data shows that outside this circle the aquifer is always at a depth greater than about 100 m below the ground surface, either because it is dipping away from the ground surface, or has been strongly faulted downwards or because it is overlain by the high ground formed by the overlying younger rocks. Thus the new data have shown that it is only within this circle, in the very centre of Bath, that the aquifer is sufficiently close to the ground surface to allow the hot water to leak through the overlying younger rocks to form the hot springs. The new data have also demonstrated that the Carboniferous Limestone reaches depths exceeding 2 km south of Bath.

Further analysis of the present interpretation of the geophysical data, or of one which is modified by the results of the deep borehole, will enable the detailed structure of the Carboniferous Limestone aquifer to be established. This information will place tight constraints on theoretical models of the origin of the hot waters of Bath.

C. McCann