Karlhede's invariant and the black hole firewall proposal
  The Karlhede invariant is formed from the contraction of the covariant derivative of the Riemann tensor. It is a coordinate invariant that vanishes at the Schwarzschild event horizon $r=2m$. The vanishing of the invariant allows an observer to construct a local measuring device and use it to detect an event horizon while falling into a black hole. Recent proposals postulate the existence of a "firewall" at the event horizon that may incinerate an infalling observer. These proposals face an apparent paradox if a freely falling observer detects nothing special in the vicinity of the horizon. The behavior of Karlhede's invariant raises the possibility that the event horizon is a real physical membrane with measurable properties that are detectable by a freely falling observer. 
J. W. Moffat and V. T. Toth
gr-qc hep-th