In some embodiments, apparatus and systems, as well as methods, may operate to acquire fluid signature data representing a plurality of nuclear magnetic resonance echo trains associated with fluids in a material, to simultaneously invert a plurality of relaxation time models to provide inverted results, and to determine fluid properties using the inverted results. The relaxation time models may be associated with the fluids using the fluid signature data after constraining ratios of T1 relaxation times to T2 relaxation times.