A bit of jargon for you: the rate at which the temperature increases with depth into the Earth's interior is called the "geothermal gradient". To get a number for this in nice round figures (without resorting to lots of zeroes) I normally express the geothermal gradient in degrees C per hundred metres (deg. C/100m). In most of the UK, the background geothermal gradient is on the order of 2.2 to 2.5 deg. C/100m.

In the work we did a few years ago in Weardale, where we drilled right into the buried granite up there, which naturally produces heat to add to that conducted upwards from the semi-molten mantle of the Earth (more than a dozen tens of kilometres below), we found a geothermal gradient of about 3.8 deg. C/100m.

Now, our theory about Tyneside is that the main hope of geothermal resources right here relates to the same granite, although that is not thought to lie closer to the City Centre than the Winlaton area. Thus any heat coming from the granite is likely to be moving towards Newcastle along a large system of faults and deeply-buried beds of sandstone. Well, we definitely drilled into the right sandstones, from 1500 to 1800m below the City. But given the distance to the granite, I still thought we'd be doing well if the geothermal gradient exceeded 3 deg. C/100m. It was hard to tell exactly what geothermal gradient we were getting during drilling, when the drilling fluid is sloshing up and down the hole, cooling in the air at surface and so on. The indications were positive, but quote how good would it be once it had had a chance to settle?

To our delight the answer is 3.9 deg. C/100m - even higher than we found in Weardale! This is a phenomenal result, and it suggests that the bottom hole temperature at 1821m will be closer to 80 deg. C than the 70's we were previously hoping for. That's great news. Frustratingly, though, we now need to mobilise some different gear to clear a small blockage in the borehole at 969m, where some shale beds have tipped across the hole since we flushed the mud out. It'll take us a bit of planning before we can do that, but with a fantastic geothermal gradient like this, we have every encouragement to get back in there as soon as possible and take the next steps towards harnessing this increasingly exciting resource.