Movement monitoring is obviously a critical issue on such a sensitive scheme. There is a total of 170 precise level monitoring points, while two transducers in each of the four mines measure the hydrological pressures in the brine.
Eighteen extenso meters in bore holes from the 2001-2002 site investigation give belowground readings which can show patterns of movement at eight levels, down to mine roof levels at about 90m.
Wrekin is also measuring water levels in site investigation boreholes in and around the town. Some are up to 3km away and will show if changes in water levels are localised or are occurring over a wider area.
About 60 sonar survey holes are included in the scheme, to show mine roof and floor positions as well as pillar locations. On top of this, researchers from Keele University are using geophones to check for creaking and mine collapse.