Method and apparatus for determining neutron detector operability using gamma ray sources
First Claim
1. A method of determining the operations of a plurality of neutron detectors in a neutron well logging tool, each said neutron detector generating an output pulse in response to the detection of a gamma ray or a neutron, comprising the steps of:
- (a) establishing first and second voltage thresholds;
(b) exposing each said detector to a source of gamma rays;
(c) comparing said detector pulses to said first and second threshold levels thereby to obtain event signals representative of detected gamma rays or neutrons for each said detector, in which (i) detector pulses greater in amplitude than said second threshold represent detected neutrons, and (ii) detector pulses greater in amplitude than said first threshold but less than said second threshold represent detected gamma rays;
(d) determining the event signal rate of detected gamma rays for each said detector; and
(e) comparing the determined rate of detected gamma rays for each said detector to an expected value to determine if each said neutron detector is operational.
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Accused Products
Abstract
The invention relates to an apparatus and method for verifying the operability of neutron detectors and associated pulse generating circuits in a neutron logging tool prior to committing the tool to the well borehole. The method comprises the steps of exposing the neutron detectors to a source or sources of gamma rays, determining the rate of detected gamma rays by counting detector output pulses, and comparing the detected rate to an expected value thereby to verify the operations of the detectors. In the apparatus in accordance with the invention, the gamma ray source(s) may be contained inside the tool near or mounted on the detectors themselves. In an alternate embodiment, the gamma ray source(s) may be externally applied to the tool at the surface during verification procedures. Counting of detected gamma rays is based on detection of detector pulser amplitudes falling in a range defined by first and second thresholds where gamma rays are those pulses falling in the range, while pulses outside the range, i.e., greater than the second threshold, represent detected neutrons.
68 Citations
27 Claims
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1. A method of determining the operations of a plurality of neutron detectors in a neutron well logging tool, each said neutron detector generating an output pulse in response to the detection of a gamma ray or a neutron, comprising the steps of:
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(a) establishing first and second voltage thresholds; (b) exposing each said detector to a source of gamma rays; (c) comparing said detector pulses to said first and second threshold levels thereby to obtain event signals representative of detected gamma rays or neutrons for each said detector, in which (i) detector pulses greater in amplitude than said second threshold represent detected neutrons, and (ii) detector pulses greater in amplitude than said first threshold but less than said second threshold represent detected gamma rays; (d) determining the event signal rate of detected gamma rays for each said detector; and (e) comparing the determined rate of detected gamma rays for each said detector to an expected value to determine if each said neutron detector is operational. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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11. A neutron well logging tool comprising:
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(a) a plurality of neutron detectors each capable of detecting both neutrons and gamma rays; (b) at least one gamma ray source positioned proximate said detectors for exposing said detectors to gamma rays; (c) means connected to each said detector for determining the rate of detected neutrons and gamma rays by each said detector; and (d) means for comparing the rate of detected gamma rays for each said detector to an expected value to determine if the detectors are operational. - View Dependent Claims (12, 13, 14, 15, 16, 17, 18, 19)
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20. A method for verifying the operability of the neutron detectors in a neutron logging tool prior to insertion into a well, comprising the steps of:
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(a) exposing each of said neutron detectors to a known source of gamma rays, each said detector outputting a pulse in response to a detected gamma ray or a detected neutron; (b) determining the rate of detected gamma rays by each said detector; and (c) comparing the determined rate of detected gamma rays for each detector to an expected value to determine if said detectors are operational. - View Dependent Claims (21, 22, 23, 24, 25, 26, 27)
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Specification