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