In-situ assaying for uranium in rock formations

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 at a rate of at least about one pulse per second ,
• 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.

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 at a rate of at least one cycle per second 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.

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 at a rate of at least one cycle per second 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.

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.

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.

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.