1. A computer-implemented method of obtaining sigma values from pulsed neutron capture decay curve, the method comprising: using a pulsed neutron instrument to collect one or more spectra, the pulsed neutron instrument disposed down a borehole traversing a rock formation, the spectra including capture interactions and inelastic interactions; selecting one of the spectra collected by the pulsed neutron instrument; and modeling, by a processor in communication with a non-transitory, computer-readable medium, the non-transitory, computer-readable medium comprising 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, wherein the non-linear model comprises a simulated annealing model.