Abstract: In accordance with an illustrative embodiment of the present invention, a technique is disclosed for determining a characteristic of an earth formation by irradiating the formation with a source of neutrons and measuring the resulting neutron population at two different spacings from the source. Each neutron population measurement is converted into a signal which is related to the formation porosity corrected for environmental effects. The two converted signals are then compared with the porosity derived from the ratio of the two neutron population measurements in order to obtain an indication of the presence of gas in the formation.