The intensity distribution of secondary gamma radiation resulting from neutron capture has been measured in simulated drill holes, of various types such as cased and uncased, empty and water-filled. The intensity of neutron-capture gamma rays depends on the hydrogen content of the rock. In a six-inch well, it increases with hydrogen content at points within about 16 inches of the neutron source and decreases at more distant points. The absolute gamma intensity is greatly reduced when non-hydrogenous gamma-ray or neutron absorbers, e.g., lead or boron are introduced between logging probe and formation. The slope of logarithmic gamma ray intensity vs. distance remains virtually constant. It changes with the hydrogen content of the formation and offers a means of quantitatively estimating porosity behind casing despite extraneous absorption. The slope can be measured automatically in the well by two gamma detectors placed in a probe at fixed distances from a neutron source. Hydrogenous material between probe and rock increases the slope. Two or three inches of drilling mud seriously impair the sensitivity. Gamma radiation scattered by the walls of drill holes necessitates proper shielding for detectors used in (n, gamma ) logging with a radium-beryllium source. A shielded detector is predominantly sensitive to the hard neutron-capture gamma rays and tends to discriminate against the softer scattered radiation. The scattered intensity decreases with increasing bulk density of the formation. Under favorable conditions, continuous logs of formation density as a function of depth can be obtained.

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