Abstract: In accordance with an illustrative embodiment of the present invention, a technique is disclosed for determining the porosity of an earth formation by irradiating the formation with a source of neutrons and measuring the resulting neutron population at a given spacing from the source. The neutron population measurement is converted into a signal which is related to the formation porosity in accordance with the relationship:N = A[P+.alpha.(1-P)].sup.-.sup.x,where N is the measured neutron population; A is the neutron population measured in fresh water; P is the porosity; .alpha. is an empirically derived coefficient for which the variation of log N as a function of log [P+.alpha.(1-P)] is represented by a substantially straight line; and x is a coefficient representing the slope of this straight line.