  We revisit the definition of transverse frames and tetrad choices with
regards to its application to numerically generated spacetimes, in particular
those from the merger of binary black holes. We introduce the concept of local
and approximate algebraic Petrov types in the strong field regime. We define an
index $\mathcal{D}=\sqrt{12/I}\left(\Psi_2 - \Psi_3^2/\Psi_4\right)$ able to
discriminate between Petrov types II and D and define regions of spacetime of
those approximate types when used in conjunction with the speciality invariant
$S=27J^2/I^3$. We provide an explicit example applying this method to
Brill-Lindquist initial data corresponding to two nonspinning black holes from
rest at a given initial separation. We find a doughnut-like region that is
approximately of Petrov type II surrounded by an approximately Petrov type D
region. We complete the study by proposing a totally symmetric tetrad fixing of
the transverse frame that can be simply implemented in numerically generated
spacetimes through the computation of spin coefficients ratios. We provide an
application by explicitly deriving the Kerr-perturbative equations in this
tetrad.
