Nonreciprocal effects in neutron logging of air-filled boreholes

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