NONRECIPROCAL EFFECTS IN NEUTRON LOGGING OF AIR-FILLED BOREHOLES

In neutron measurements involving a point neutron source and a detecto r, if the detector output is the same when the source and the detector are interchanged, the measurement is reciprocal. The measurement is n onreciprocal if the detector output is not the same upon interchange I n neutron logging when a Fast neutron source and a slow neutron detect or are used, and there is an interface between the source and detector , the measurement is nonreciprocal. Experimental data are presented sh owing this effect in using a neutron-thermal neutron tool inside a 10- cm diameter borehole through a 70-cm thick water zone bounded above an d below by air. This nonreciprocal effect can be used to create an int erface log by using similar detectors equally spaced above and below a neutron source, depth shifting one or both, and subtracting the two i ndividual logs. Another result is that, in general in neutron logging, the apparent depth of an interface is dependent on whether the detect or is above or below the source.