Pulsed neutron logging method using cumulative count curves

Claim 1

A method of measuring earth formation characteristics, comprising in combination: repetitively irradiating the earth formations with pulses of neutrons; then detecting gamma rays emanating from the irradiated formations; and cumulatively counting for a preselected counting period following each pulse the total gamma rays detected in a plurality of separate time bins, and adding the total count from each preceding bin to the count from a next bin, the cumulative counts for each pulse defining points on a cumulative count versus time curve which has a mathematical expression; and computing from the cumulative counts of the time bins variables in the mathematical expression which correspond to characteristics of the earth formations.

Claim 2

A method of measuring earth formation characteristics, comprising in combination: repetitively irradiating the earth formations with pulses of neutrons; then detecting gamma rays emanating from the irradiated formations; and cumulatively counting for a preselected counting period following each pulse the total gamma rays detected in a plurality of separate time bins, and adding the total count from each preceding bin to the count from a next bin, the cumulative counts for each pulse defining points on a cumulative count versus time curve which has a mathematical expression; and computing from the cumulative counts of the time bins variables in the mathematical expression which correspond to characteristics of the earth formations.

Claim 3

A method of measuring earth formation characteristics, comprising in combination: repetitively irradiating the earth formations with a number of pulses of neutrons, the pulses being of a selected duration; then detecting gamma rays emanating from the irradiated formations; after a selected interval following each pulse and before a next pulse, cumulatively counting for a preselected counting period the total gamma rays detected in a plurality of separate time bins, adding the total count from each preceding bin to the count from a next bin, the cumulative counts for each pulse defining points on a cumulative count versus time curve which has a mathematical expression; then after a selected depth increment transmitting the total counts for each time bin to a processor; dividing with the processor the total counts for each time bin by the number of pulses fired during the depth increment to determine an average cumulative counts for each time bin; and computing with the processor based on the average cumulative counts for the time bins, variables in the mathematical expression which correspond to characteristics of the earth formations.

Claim 4

A method of measuring earth formation characteristics, comprising in combination: repetitively irradiating the earth formations with a number of pulses of neutrons, the pulses being of a selected duration; then detecting gamma rays emanating from the irradiated formations; after a selected interval following each pulse and before a next pulse, cumulatively counting for a preselected counting period the total gamma rays detected in a plurality of separate time bins, adding the total count from each preceding bin to the count from a next bin, the cumulative counts for each pulse defining points on a cumulative count versus time curve which has a mathematical expression; then after a selected depth increment transmitting the total counts for each time bin to a processor; dividing with the processor the total counts for each time bin by the number of pulses fired during the depth increment to determine an average cumulative counts for each time bin; and computing with the processor based on the average cumulative counts for the time bins, variables in the mathematical expression which correspond to characteristics of the earth formations.

Claim 7

A method of measuring earth formation characteristics, comprising in combination: drawing a pulsed neutron source and detector through a borehole; repetitively irradiating the earth formation with pulses of selected duration of fast neutrons from the neutron source, creating thermal neutrons due to collisions in the formations, which subsequently are captured by the formation; detecting gamma rays emanating from the irradiated formations as a result of the capture of thermal neutrons; after a selected time following each pulse and before a next pulse, cumulatively counting for a preselected counting period the total gamma rays detected in a plurality of separate time bins, cumulatively adding each bin's count to the preceding bin, the cumulative counts for each pulse defining points on a cumulative count versus time curve which has a mathematical expression with three unknown variables; after a selected depth increment, transmitting the total counts for all of the pulses for each time bin to a processor at the surface; dividing with the processor the total counts for each time bin by the number of pulses fired during the depth increment to determine average cumulative counts for each time bin; computing the three variables with the processor using iterative fit with the average cumulative count from each time bin, and calculating a formation thermal decay time; then varying the duration of the pulses and the time between pulses for a next depth increment in proportion to the thermal decay time computed for the preceding depth increment.

Claim 8

A method of measuring earth formation characteristics, comprising in combination: drawing a pulsed neutron source and detector through a borehole; repetitively irradiating the earth formation with pulses of selected duration of fast neutrons from the neutron source, creating thermal neutrons due to collisions in the formations, which subsequently are captured by the formation; detecting gamma rays emanating from the irradiated formations as a result of the capture of thermal neutrons; after a selected time following each pulse and before a next pulse, cumulatively counting for a preselected counting period the total gamma rays detected in a plurality of separate time bins, cumulatively adding each bin's count to the preceding bin, the cumulative counts for each pulse defining points on a cumulative count versus time curve which has a mathematical expression with three unknown variables; after a selected depth increment, transmitting the total counts for all of the pulses for each time bin to a processor at the surface; dividing with the processor the total counts for each time bin by the number of pulses fired during the depth increment to determine average cumulative counts for each time bin; computing the three variables with the processor using iterative fit with the average cumulative count from each time bin, and calculating a formation thermal decay time; then varying the duration of the pulses and the time between pulses for a next depth increment in proportion to the thermal decay time computed for the preceding depth increment.