Total sum of depth intervals of a given formation unit with porosity above the porosity cut-off:
| h_{\rm eff} = \sum_k h_k |_{\phi_k > \phi_0} = NTG \cdot h_{\rm total} |
It's closely related to concept of NTG Thickness:
| h_{\rm eff} = NTG \cdot h_{\rm total} |
where h_{\rm total} is total thickness of formation unit.
See also
Petroleum Industry / Upstream / Subsurface E&P Disciplines / Petroleum Geology