1. A computer-implemented method of obtaining sigma values from pulsed neutron capture decay curve, the method comprising: selecting spectra collected by pulsed neutron instrument disposed down a borehole traversing a rock formation, the spectra including capture interactions and inelastic interactions; and modeling, by a processor in communication a computer-readable medium including instructions that when executed cause the processor to select the spectra using a nonlinear model to obtain sigma values relating to one or more reservoir properties.