BATH THERMAL RESOURCE PROJECT

24 July 2001

Rhodri Samuel, Bath Spa Project

Prof. David McCann, University of Reading

Prof. Clive McCann, University of Reading

Dr Geoffrey Kellaway, Geological Consultant

Andrew Mann, IMC Geophysics Ltd

A previous meeting was held on 14 June 1999 and is reported in BRLSI Proceedings, Volume 3, page 36. This gives the background to the project, which is to determine the source of the hot water emerging as springs in Bath so as to enable it to be protected.

This meeting reported on the work undertaken in the last two years. The original investigation covered an area of about 16 square miles close to Bath; this has been extended to 584 sq.miles, reaching to the Mendips in the south and for about 20 miles to the south-east and 10 miles to the north-east of Bath.

Three hypotheses are being examined:

The Mendip hypothesis, which postulates that the water comes from the rain falling on the Mendip Hills.

The Avon-Solent Fracture Zone hypothesis that the water has a local source within this zone along the Avon valley.

The Granite Pluton (or batholith) hypothesis that an intrusion of granite to the north-east of Bath supplied the water.

Wherever the water originates, it must descend to a depth of at least 2 km (1-1/4 miles) below sea level into the Carboniferous Limestone in order to attain a temperature above that at which it emerges in the hot springs, which is 47oC (117oF).

The seismic survey undertaken in 1999 and additional data, which was gathered in seismic surveys in 1979, 1982 and 1989 by the British Geological Survey and two oil exploration companies, has now been interpreted. The earlier results were excellent data but extra information could be extracted from those results with modern computers. The main objective was to determine the depth of the top of the Carboniferous Limestone, although subsidiary information on the rocks closer to the surface was also required and obtained.

In addition to the seismic survey, a gravity survey was used, particularly in connection with the third, Granite Pluton, hypothesis, since granite has a lower density than limestone and thus is revealed by this technique. The results of this gravity survey showed that this hypothesis could be dismissed; there was no granite pluton.

The long north-south seismic line (99-SPA-09) reached the Mendips and indicated that the Carboniferous Limestone surface, which was only 100 m below sea level at its northern end, north-west of Bath, dipped down to 1800 m at the Mendip Hills. However, the situation is not clear-cut; this is true for the section of the Mendips which lies east-west, but, where the hills turn to the north-west at the western end of the range, the survey is not yet complete. It is possible that in that area the Carboniferous Limestone surface does reach ground level (above sea level).

What is clear, and was unknown before this survey was carried out, is that the Carboniferous Limestone surface around Bath is at around 300 m below sea level, but at a distance of only 2 km south-west of the springs, it increases from 500 m to 1350 m over a distance of 1.75 km with an angle of dip of 45o. Within 4 km (2½ miles) it reaches a depth at which the temperature is sufficient to heat the Spa water to its 47oC. If it has a sufficiently open structure, this could provide a path for the rapid transport of hot water to the springs. The rock itself may not be sufficiently porous to allow this, but fractures through it could provide suitable paths.

There are many fractures close to Bath, some
of which get close to the surface of the ground. How deep they are is not yet known, but is being determined. It is possible that water enters these fractures from the ground level, flows down to sufficient depth to get heated, and then rises, because of its lower density when hot, through one or more faults to reach the surface as the springs. One such fault is known running from Gloucestershire to the Solent.

Work is continuing and a report on the next stage on the geology and hydrogeology of the area will be reported, probably next Spring.

The map on the next page shows a simplified layout of the position of the surface of the Carboniferous Limestone under Bath and its surrounding area. The general picture of the position of the Carboniferous Limestone bed around Bath is of a plateau on the eastern side of the City tilted slightly to the south down to 1000 m where it levels out, separated by a deep fault producing a `wall' 70 m high, separating it from the steeply inclined bed to the west of the City which tilts at 45o towards the south-west and again levels out.

Discussion

A number of questions were put to the speakers; the answers have in some cases been incorporated in the above report.

Is it assumed that the water trickles down slowly and flows quickly up faults?

The water has been chemically determined to be at least 1000 years old (perhaps much older) and to have reached 70oC. It must come up quickly and not mix with too much cooler water to emerge at 47oC.

Does wet and dry rock give different seismic or gravity signals?

The difference is very small except where rock is fractured.

How deep is the bed of Carboniferous Limestone?

Generally 600 1000 m for a level bed.

Are there many other similar hot springs in the world and is any research going on at any of them?

There are many. Not all of them involve Carboniferous Limestone beds. No research is known to be going on anywhere else, certainly not to the same extent as in Bath. The interesting construction of the Bath springs is the funnel-shape; this filled with clay and debris after it was formed, which restricts the flowrate of the water. If the debris was removed the flow would increase and the temperature of the water would fall.

Donald Lovell