Oxygen in a borehole fluid in the vicinity of a reference region, such as an earth formation of known fluid permeability, is activated with a logging instrument by being bombarded with neutrons of 14 MEV energy level to form the unstable isotope nitrogen 16 giving rise to gamma radiation. Flow of the activated fluid is then caused by an electrohydraulic flow generator which forms an intense pressure pulse in the fluid. A gamma ray detector in the logging instrument senses the amount of flow of the activated fluid within the borehole in the reference region. The logging instrument is then moved to a formation whose liquid permeability is to be tested. Borehole fluid opposite the formation to be tested is activated by neutron bombardment, again giving rise to gamma radiation, and fluid flow in the formation under test is caused by the electrohydraulic flow generator. The gamma ray detector reading of activated fluid flow in the borehole opposite the formation under test, when compared with the flow reading in the borehole opposite the reference region, is a measure of relative fluid permeability in the formation being tested.