  In this paper we introduce a new approach to the study of the effects that an
impulsive wave, containing a mixture of material sources and gravitational
waves, has on a geodesic congruence that traverses it. We find that the effect
of the wave on the congruence is a discontinuity in the B-tensor of the
congruence. Our results thus provide a detector independent and covariant
characterization of gravitational memory.
