jbm wrote:
"Lawrence Jenkins" wrote in message
...
|
| "Joe Egginton" wrote in message
| ...
| I was watching a building program yesterday, and the builder said it was
| 12c constantly 2 metres underground. They was building a house in
| Maidstone, Kent.
|
| I know builders are known for telling porkies, but is it true that just
2
| metres underground the temperature is constant?
| --
| Joe Egginton
| Wolverhampton
| 175m asl
|
| Hmmm I wonder how that squares with heatpumps? May that's the temp they
| bring the mains up to.
|
|
Your builder was right. Sub-soil temperatures in the UK are standard at
about 50F (10C). This applies from 2 feet down to a couple of hundred feet,
but may vary depending on the depth of the water table. This is why
underground caves are always at pretty much the same temperature throughout
the year irrespective of surface weather conditions.

To use geothermal heat pumps, you would have to drill down a couple of miles
before finding any appreciable rise in temperature, and then the deeper you
go, the hotter it gets. Generally, for every kilometre depth, the
temperature rises by about 30C, so at 3km, the rock temperature will be 90C
above the surface temperature, enough to produce boiling water.

The above figures are not necessarily true in areas of volcanic activity,
and especially suspect in places like Iceland!

jim, Northampton



Groundwater in the UK at the watertable is a pretty constant 10C all
year round. If you geophysically log a borehole (as I have done many
times) the geothermal gradient is very easy to measure in even quite
short distances (100m) with instruments that can measure fractions of a
degree.

Inflows or outflows of colder or warmer water (usually seen as steps in
the gently increasing temperature profile) to the borehole give away the
location of fractures. If water is inflowing at one depth and outflowing
at another depth the temperature profile will show as a straight line
(constant temperature) between those depths.

Certain areas of the UK have a steeper geothermal gradient and are ideal
locations for geothermal power schemes. Southampton is the primary
example and geothermal energy is used there to heat a number of public
buildings. The water used is drawn from a depth of 1.8km and is 76C
(I've geophysically logged one of the original test bores there).