\rm NPHI = \frac{Count_{near}}{Count_{far}}

\rm NPHI = \frac{Count_{near}}{Count_{far}}

\Sigma = (1-\phi) \, \Sigma_m  + \phi \, ( \Sigma_w \, s_w + \Sigma_o \, s_o +\Sigma_g \, s_g)

\Sigma_m = \sum_k \Sigma_k

\Sigma_m = (1-V_{sh}) \, \Sigma_{snd} + V_{sh} \, \Sigma_{sh}

\Sigma_m = (1-V_{sh}) \, \Sigma_{lms} + V_{sh} \, \Sigma_{sh}