Apparatus and method for the suppression of microphonic noise in proportional counters for borehole logging-while-drilling

An improved neutron counting arrangement for a neutron detection assembly in a logging-while-drilling tool having at least two neutron detectors at a common axial position of said tool which produce pulses in response to radiation returning to said tool from formations surrounding a borehole, such tool being subjected to forces which produce microphonic noise pulses from said neutron detectors, and wherein the outputs of said two neutron detectors are connected to respective negative and positive inputs of a differential amplifier,

• wherein the improvement comprises
• first and second comparator means connected to the output of said differential amplifier for producing first and second output pulses responsive to pulses applied to said negative and positive inputs of said differential amplifier which are respectively greater than or more negative than predetermined amplitude levels;
• means responsive to said first and second output pulses for measuring the time lengths of said first and second output pulses, and
• means for counting said respective first and second output pulses as neutron counts only if their respective time lengths are less than a predetermined time length.

A method for eliminating shock induced pulses from a sequence of radiation induced pulses while counting pulses from a neutron detector comprising the steps of

• providing a neutron detector assembly in a logging-while-drilling tool including two neutron detectors at a common axial position of said tool with each detector producing pulses in response to radiation returning to said tool from formations surrounding a borehole, said tool being subject to forces which produce microphonic noise pulses in the output of said neutron detectors,
• applying the outputs of said two neutron detectors to respective negative and positive inputs of a differential amplifier,
• providing first and second comparator means connected to the output of said differential amplifier for producing first and second output pulses responsive to pulses applied to said negative and positive inputs of said differential amplifier which are respectively greater than or more negative than predetermined amplitude levels,
• measuring the time lengths of said first and second output pulses, and
• counting said respective first and second output pulses as neutron counts only if their respective time lengths are less than a predetermined time length.