LIAG
 

Research borehole 2014


Scientific questions

Leaning Tower of Bad Frankenhausen and drilling rig (photo: H. Buness, 2014).

More tilted than the Leaning Tower of Pisa is the Leaning Tower of Bad Frankenhausen in Thuringia. Subrosion – the cause of this tilt and abundant sinkholes in entire Thuringia – is a phenomenon that describes solution of material in the subsurface. It takes places for instance in salt, but also in other soluble rocks. After comprehensive geophysical and geological site surveys in the urban district, the borehole was located close to the former church called Oberkirche.

Geophysical identification and characterisation of subrosion phenomena and processes are in the scientific focus of this research borehole using the example of Bad Frankenhausen for investigation of a subsidence area down to 400 m depth.

The following questions are addressed:

  • recovery of drill cores for the determination of rock properties in different layers,
  • characterisation of the subsurface and existence of disaggregation zones or voids,
  • borehole geophysics for petrophysical parameter determination and fracturing,
  • geophysical transmission experiments between the new and old boreholes,
  • setup of a 3-D subsurface model and spatio-temporal changes.

Results

Drill core with Staßfurt rock salt recovered between 370-400 m depth (photo: TLUG, 2015).

The research borehole commenced early October 2014 and was finished in April 2015.  It will let open as long as possible for geophysical investigations.  LIAG works closely together with the Thuringian Geological Survey (TLUG) who owns a considerable amount of geoscientific data for Bad Frankenhausen.

The borehole was drilled down to 457.5 m depth.  The upper 163 m contain anhydrite, salt, carbonates, and different breccia.  Down to the main fault plane of the Kyffhäuser South Fault, that was penetrated at 347.7 m depth, older strata of Permocarboniferous age (e.g., sandstone, conglomerate) overlie younger Zechstein layers (e.g., anhydrite, salt).  The prominent Stinkschiefer (limestone, smelling of bitumen) allows a first estimate of the fault offset reaching ca. 350 m at the fault.