In-situ assaying for uranium in rock formations

Abstract

The specification discloses a technique for assaying for delayed fission neutrons from uranium to obtain a quantitative measure of uranium ore grade. In one embodiment, a pulsed neutron source of 14-Mev neutrons and a neutron detector are located in a borehole at the level of a formation of interest. The source is operated cyclically to irradiate the formations with bursts of fast neutrons, and the resulting neutrons from the formations are detected. Measurements are made of those neutrons detected between neutron bursts and indicative of delayed neutrons emitted as a result of neutron fission of uranium. Measurements also are obtained in a nonore-bearing formation to record the count of delayed neutrons emitted from oxygen when irradiated. These measurements are compared with those obtained in the ore-bearing formations of interest to correct for the effect of the oxygen background.

What is claimed is: 
     
       1. A method of assaying for uranium in the formation traversed by a borehole and to obtain information indicative of the interfering effect of oxygen in said formations, comprising the steps of: locating a pulsed neutron source and a thermal neutron detector at the level of a nonuranium-bearing formation,   said source being productive of fast neutrons having energies of about 14 Mev,   operating said source to periodically irradiate said nonuranium-bearing formation with bursts of fast neutrons spaced in time,   detecting neutrons resulting from the irradiation of said nonuranium-bearing formation and indicative of delayed neutrons emitted by oxygen as a result of irradiation by neutrons,   locating said source and detector at the level of a formation of interest suspected of containing uranium,   operating said source to periodically irradiate said formation of interest with bursts of fast neutrons spaced in time, and   detecting neutrons resulting from the irradiation of said formation of interest to detect for delayed neutrons emitted as a result of neutron fission of uranium.   
     
     
       2. The method of claim 1 comprising the step of: recording the quantity of neutrons detected, at each of said formations, between neutron bursts within a time period when neutrons from said source have disappeared but while delayed neutrons from oxygen and delayed fission neutrons from uranium, respectively, may be emitted.   
     
     
       3. A method of assaying for uranium in the formations traversed by a borehole, comprising the steps of: locating a pulsed neutron source and a neutron detector at the level of a formation of interest suspected of containing uranium,   operating said source to periodically irradiate said formation of interest with bursts of fast neutrons spaced in time .Iadd.at a rate of at least about one pulse per second.Iaddend.,   the time between each neutron burst being sufficient to allow neutrons from said source to disappear but being long enough to allow delayed neutrons emitted as a result of neutron fission of uranium to appear at said detector,   detecting neutrons with said detector as a result of the irradiation of said formations with bursts of fast neutrons, and   obtaining measurements of the quantity of neutrons detected between neutron bursts at a time period when neutrons from said source have disappeared but while delayed fission neutrons from uranium may be emitted.   
     
     
       4. The method of claim 3 comprising the steps of: locating said source and detector at the level of a nonuranium-bearing formation,   operating said source to periodically irradiate said nonuranium-bearing formation with bursts of fast neutrons spaced in time, and   detecting neutrons resulting from the irradiation of said nonuranium-bearing formation and indicative of neutrons emitted by oxygen as a result of irradiation by fast neutrons for comparison with said neutrons detected and recorded upon irradiation of said formation of interest.   
     
     
       5. A method of assaying for uranium in formations traversed by a borehole, comprising the steps of: locating a borehole tool containing a source of neutrons and a neutron detector at the level of a formation of interest suspected of containing uranium,   operating said source cyclically .Iadd.at a rate of at least one cycle per second .Iaddend.to irradiate with neutrons a zone of said formation of interest,   detecting neutrons resulting from the irradiation of said zone of said formation of interest, and   recording the counts of neutrons detected within each cycle of operation when delayed neutrons resulting from neutron fission of uranium are expected to be detected by said detector.   
     
     
       6. A method of locating and assaying for uranium in the formations traversed by a borehole, comprising the steps of: obtaining a natural gamma-ray log of the formations traversed by said borehole and, from said natural gamma-ray log, identifying formations of interest suspected of containing uranium,   locating a borehole tool containing a source of neutrons and a thermal neutron detector at the level of a formation of interest suspected of containing uranium,   operating said source cyclically .Iadd.at a rate of at least one cycle per second .Iaddend.to irradiate with neutrons a zone of said formation of interest,   detecting thermal neutrons resulting from the irradiation of said zone of said formation of interest, and   recording the counts of thermal neutrons detected within each cycle of operation when delayed neutrons resulting from neutron fission of uranium are expected to be detected by said detector.   
     
     
       7. The method of claim 6 comprising the steps of: locating said borehole tool at the level of a nonuranium-bearing formation,   operating said source to periodically irradiate said nonuranium-bearing formation with bursts of fast neutrons spaced in time, and   detecting neutrons resulting from the irradiation of said nonuranium-bearing formation and indicative of neutrons emitted by oxygen as a result of irradiation by fast neutrons for comparison with said neutrons detected and recorded upon irradiation of said formation of interest.   
     
     
       8. A method of assaying for natural uranium-bearing rock, comprising the steps of: locating a pulsed neutron source productive of fast neutrons having energies greater than 7.93 Mev and a thermal neutron detector adjacent a nonuranium-bearing rock formation,   operating said source to periodically irradiate said nonuranium-bearing formation with bursts of fast neutrons spaced in time,   detecting neutrons resulting from the irradiation of said nonuranium-bearing formation and indicative of delayed neutrons emitted by oxygen as a result of irradiation by neutrons,   locating said source and detector adjacent a rock formation of interest suspected of containing uranium,   operating said source to periodically irradiate said formation of interest with bursts of fast neutrons spaced in time, and   detecting neutrons resulting from the irradiation of said formation of interest to detect for delayed neutrons emitted as a result of neutron fission of uranium. .Iadd. 9. A method of assaying for uranium in the formations traversed by a borehole, comprising the steps of:   locating a pulsed neutron source and a neutron detector at the level of a formation of interest suspected of containing uranium,   operating said source to periodically irradiate said formation of interest with bursts of fast neutrons spaced in time,   the time after each neutron burst being sufficient to allow neutrons from said source to disappear but being long enough to allow delayed neutrons emitted as a result of neutron fission of uranium to appear at said detector,   detecting neutrons with said detector as a result of the irradiation of said formations with bursts of fast neutrons at a time following each burst to emphasize the shorter-lived delayed fission groups, and   recording the counts of neutrons detected following each neutron burst when delayed neutrons resulting from neutron fission of uranium are expected to be present..Iaddend..Iadd. 10. The method of claim 9 in which the detection of neutrons takes place during the first half life of the delayed fission neutrons identified in the Table..Iaddend. .Iadd. 11. A method of assaying for uranium in the formations traversed by a borehole in the presence of the interfering effect of oxygen in said formations, comprising the steps of:   locating a pulsed neutron source and a thermal neutron detector at the level of a formation of interest suspected of containing uranium,   said source being productive of fast neutrons having energies of about 14 Mev,   operating said source to periodically irradiate said formation of interest with bursts of fast neutrons spaced in time,   detecting neutrons resulting from the irradiation of said formation of interest to detect for delayed neutrons emitted as a result of neutron fission of uranium as well as neutrons indicative of delayed neutrons emitted by oxygen as a result of irradiation by neutrons, and   recording the detected neutrons..Iaddend.