Abstract An activation well logging method for determining the presence and quantity of aluminum and magnesium in formations surrounding a borehole. The formations are irradiated with 14 mev neutrons which produce aluminum and magnesium activity with half lives of 20 milliseconds and gamma ray energy of 473 kev. The neutron source is turned on for a period of time sufficient to activate aluminum and magnesium and then turned off for 20 milliseconds. During the first 4 milliseconds while the source is off, no measurement is made. During the remaining 16 milliseconds, pulses are counted which fall in the amplitude range representing gamma ray energies from 450 kev to 500 kev. The rate of occurrence of these pulses is an indication of the aluminum and magnesium content of the formation. To correct for undesired background, the rate of occurrence of pulses in the energy intervals immediately above and below the 450 to 500 kev interval is determined and subtracted from the 450 to 500 kev measurement.
The method of claim 1 wherein the measurement of gamma radiation is made during the time period beginning about 4 milliseconds after cessation of neutron irradiation and the gamma radiation measured results from the capture of fast neutrons.
Claim 2
Dependent Claim
33 words
The method of claim 2 including the steps of: measuring the gamma radiation from said formation in the energy ranges immediately above and immediately below the 450 kev to 500 kev energy range.
Claim 3
Dependent Claim
51 words
The method of claim 3 including the steps of: subtracting a portion of the measurement of gamma radiation in the energy ranges immediately above and immediately below the 450 kev to 500 kev energy range from the gamma radiation measurement in the energy range of about 450 kev to 500 kev.
Claim 4
Dependent Claim
30 words
The method of claim 4 wherein the time period for measurement of gamma radiation in the energy range of about 450 kev to 500 kev extends for about 16 milliseconds.
Claim 5
Dependent Claim
30 words
The method of claim 1 wherein the measurement of gamma radiation consists of a first portion and a second portion, said first portion and second portion being of equal duration.
Claim 6
Dependent Claim
26 words
The method of claim 6 including the step of obtaining the difference between said first portion of the measurement and said second portion of the measurement.
Claim 7
Dependent Claim
17 words
The method of claim 1 including the step of measuring the lifetime of neutrons in the formations.
Claim 8
Dependent Claim
18 words
The method of claim 1 including the step of measuring the neutron capture cross section of the formations.
Claim 9
Dependent Claim
53 words
The method of claim 9 including the steps of measuring radiation from the formations during a time period beginning at least 20 milliseconds after cessation of neutron irradiation as an indication of background radiation and using said measurement to correct the measurement of neutron capture cross section for errors due to background radiation.