| 摘要 |
An apparatus (sonde) and method of measuring density, or gravel pack quality, in a cased well borehole using a fast neutron source and one or more thermal neutron detectors is described. In one embodiment, a neutron source creates a fast neutron flux which reacts primarily with the material within the borehole casing while a collocated neutron detector counts the number of backscattered thermal neutrons. A novel means of obtaining azimuthal measurement discrimination is provided by a rotating neutron shield. In one instance the shield is quite substantial, creating a narrow measurement window. In another instance, the shield only marginally screens the detector, creating a large measurement window. In an alternative embodiment, a second thermal neutron detector is spaced distally from the neutron source and first detector. This second detector is used to provide a measurement of the borehole's background, or environmental neutron activity, and can be used to improve the quality of the sonde's gravel pack density measurement.
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