Pulsed neutron well logging techniques with background radiation removal

Claim 3

The method of claim 1 wherein, just prior to each neutron pulse, said background gamma radiation counts are stored in said counter memory means by applying data pulses representative of said background gamma radiation to the ''''up count'''' input terminal of each counter over a predetermined time interval, hence summing said background pulses as a background count in said counter.

Claim 4

The method of claim 2 wherein said subtracting step is performed by routing data pulses of gamma radiation made during each neutron pulse to the ''''down count'''' input terminal of such counter untIl said counter reaches a zero count content and then re-routing said data pulses of gamma radiation made during each neutron pulse to a summing register whereby a cumulative total of corrected counts is stored.

Claim 5

The method of claim 3 wherein the counting interval for background radiation just prior to each neutron pulse and the counting interval for inelastic together with background radiation during each neutron pulse are of approximately the same time duration.

Claim 6

The method of claim 1 wherein said steps are performed for gamma ray pulses corresponding to at least two different energy regions of the gamma ray energy spectrum.

Claim 7

The method of claim 5 wherein the ratio of the corrected counts in the memory register means of the first of said two energy regions and the second of said energy regions is performed and an output ratio signal representative thereof generated when the counts in the memory register means corresponding to said second energy region reaches a predetermined value.

Claim 8

The method of claim 6 wherein said predetermined value is some power of 10.

Claim 9

Apparatus for correcting inelastic gamma ray measurements of earth formations in the vicinity of a well borehole for background gamma rays caused by other than neutron scattering events comprising: means for repetitively irradiation the earth formations surrounding the well bore with relatively short duration pulses of fast neutrons; means for detecting gamma radiations in the well bore and for generating electrical data signal pulses representative thereof; first gating means operative for a short duration beginning and ending just prior to each neutron pulse for routing said data pulses to the up count input terminal of a counter means; second gating means operative for a short duration during each of said neutron pulses for routing said data signal pulses to the down count input terminal of said counter means and to one input terminal of a data blocking gate; means for detecting a zero content condition of said counter means for generating a conditioning signal to a second input terminal of said data blocking gate upon the occurrence thereof, to permit passage of said data pulses therethrough; summing register means for accumulating a count of data pulses passed through said data blocking gate, said pulses being representative of a background corrected inelastic gamma ray event occurrence.

Claim 10

Apparatus for correcting inelastic gamma ray measurements of earth formations in the vicinity of a well borehole for background gamma rays caused by other than neutron scattering events comprising: means for repetitively irradiation the earth formations surrounding the well bore with relatively short duration pulses of fast neutrons; means for detecting gamma radiations in the well bore and for generating electrical data signal pulses representative thereof; first gating means operative for a short duration beginning and ending just prior to each neutron pulse for routing said data pulses to the up count input terminal of a counter means; second gating means operative for a short duration during each of said neutron pulses for routing said data signal pulses to the down count input terminal of said counter means and to one input terminal of a data blocking gate; means for detecting a zero content condition of said counter means for generating a conditioning signal to a second input terminal of said data blocking gate upon the occurrence thereof, to permit passage of said data pulses therethrough; summing register means for accumulating a count of data pulses passed through said data blocking gate, said pulses being representative of a background corrected inelastic gamma ray event occurrence.

Claim 11

The apparatus of claim 8 and further including energy gating means for allowing only data pulses falling within a preselected energy range in the gamma ray energy spectrum to be input to said first and second gating means.

Claim 12

Apparatus as in claim 8 and further comprising plural energy gating means and plural background subtraction circuit means as described together with means for forming a ratio of background corrected counts from selected pairs of said summing register means upon the occurrence of a predetermined number of counts in one of said selected pair of summing register means.

Claim 15

ThE method of claim 11 wherein said correcting step is performed by subtracting all of said stored background counts from said inelastic together with background counts.

Claim 16

The method of claim 11 wherein the step of detecting the background radiation and storing counts representative thereof is performed repetitively during a time interval, just prior to each neutron pulse and of approximately the same duration as the neutron pulses, and each such background detection interval ends prior to the beginning of the succeeding neutron pulse.

Claim 17

The method of claim 11 wherein said steps are performed for gamma ray pulses of background gamma radiation and inelastic together with background gamma radiation in a plurality of energy regions in the gamma ray spectrum in parallel modes of operation for each such energy region.