Geovista introduces very small diameter sondes to log heat exchange pipes in geothermal boreholes. Range includes Density, Temperature, and Verticality to log geothermal boreholes in Germany. The short, 16mm diameter tools include a flexible joint to allow them past bends in coiled tubes.
http://geovista.co.uk/product-listing/ultra-slim-tools-for-geothermal-wells/