[en] A new improved apparatus for determining rock formation porosity was developed which is substantially independent of the formation salinity. The improvements achieved by using differing gating intervals for the two detectors. The rock formations surrounding the earth borehole are first pulse-irradiated with discrete bursts from a high-energy neutron source. The radiations at two different points in the formation are detected and electrical signals are generated. The electrical signals from the first point are gated for a shorter time interval than those from the second point. The gated first and second electrical signals are combined to determine the porosity of the formations. (DN)