System for measuring the natural gamma radiation of surface and subsurface formations

A natural gamma radiation measuring system includes a gamma ray detector and a multichannel analyzer for recording a full gamma-ray energy spectrum. An artificial source of gamma radiation that is temperature insensitive provides a known level of gamma radiation which is utilized by the multichannel analyzer to provide gain stabilization for the full energy spectrum output of the gamma ray detector.

BACKGROUND OF THE INVENTION 
This invention relates to a system for measuring the natural gamma 
radiation of surface and subsurface formations. 
Various methods and apparatus have been utilized in the well logging art to 
study the radioactive properties of subsurface formations, both where the 
radioactivity is natural and where it is artifically induced. Logs of such 
properties aid in the study of the nature of the subsurface formations, 
particularly in exploration for minerals and hydrocarbon deposits. Certain 
elements in the subsurface formations exhibit distinctive properties which 
are measurable in situ. Of the many elements that occur, potassium (K), 
uranium (U), and thorium (Th) are important natural sources of gamma 
radiation. Each of these elements either contains or radioactively decays 
to, radioactive isotopes which emit gamma radiation at characteristic 
energy levels. The neutral gamma-ray spectrum of a given formation 
therefore exhibits peaks of intensity at energies corresponding to the 
potassium, uranium, and thorium content of the formation. 
In U.S. Pat. No. 3,940,610 to Dennis et al, a borehole logging system 
employs a gamma-ray detector for measuring the total natural gamma 
radiation within the borehole. Three energy band selectors discriminate 
this measurement of the gamma-ray detector into potassium, uranium, and 
thrium energy band signals centered about the energy levels at which 
potassium, uranium, and thorium exhibit peak gamma radiation intensities. 
These potassium, uranium, and thorium energy band signals are then applied 
to a stripping unit having potassium and uranium channels. The potassium 
channel strips the potassium gamma radiation measurement, as represented 
by the potassium energy band signal, of the influence from the gamma 
radiation from uranium and thorium, Further, the uranium channel strips 
the uranium gamma radiation measurement, as represented by the uranium 
energy band signal, of the influence from the gamma radiation from 
thorium. 
For a further discussion on the applications of field gamma-ray 
spectrometry as a geological mapping and exploration tool reference may be 
had to an article entitled "Techniques of field gamma-ray spectrometry" in 
the Mineralogical Magazine, December 1981, Vol. 44. 
SUMMARY OF THE INVENTION 
The present invention is directed to a system for measuring natural gamma 
radiation of potassium, uranium and thorium. Such system includes a 
gamma-ray detector and a multichannel analyzer for recording the full 
energy spectrum output from the gamma-ray detector. An artificial source 
of gamma radiation provides a gamma-ray energy output that is temperature 
insensitive and does not interfere, with the energy peaks from the 
measured potassium, uranium and thorium gamma radiation. The artificial 
source of gamma radiation is positioned so as to permit measurement of the 
artificial gamma radiation by the detector at the same time as the 
measurement of the natural gamma radiation. The multichannel analyzer 
records the full energy spectrum output of said detector and gain 
stabilizes the full energy spectrum on the energy level of the artificial 
source of gamma radiation. 
The artificial source of gamma radiation has an energy level peak below the 
energy range of the natural gamma radiation. Both the natural and 
artificial gamma radiation measurements are amplified and applied to the 
multichannel analyzer which examines the channels on either side of the 
energy peak from the artificial source and adjusts the gain of the 
gamma-ray detector to provide a gain stabilized full energy spectrum for 
said natural gamma radiation. 
In a further aspect of the invention the presence of potassium, uranium and 
thorium is identified by performing a channel-by-channel least squares fit 
of the measured full energy spectrum from the multichannel analyzer to a 
standard energy spectrum recorded from known amounts of potassium, uranium 
and thorium.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, a gamma-ray detector 10 measures the natural gamma 
radiation emitted by the formation 11. The gamma-ray detector includes a 
scintillation type detector 12 (e.g. a crystal of sodium iodide activated 
with thallium) which cooperates with power supply and pre-amplifier 13. 
The gamma-ray spectrum recorded by detector 10 is applied to the 
multichannel analyzer 15 which records the entire gamma-ray spectrum from 
200 Kev to 3.5 Mev, such a recording being illustrated in FIG. 2. 
Gamma-ray energy peaks are illustrated for potassium, uranium and thorium 
centered about 1.46 Mev, 1.76 Mev and 2.62 Mev respectively. Centered 
about 356 Kev is a barium-133 energy peak provided by means of the 
barium-133 gain stabilization source 16 located in the detector 10. This 
barium source products a known level of gamma radiation that is recorded 
by detector 10 and is utilized by the multichannel analyzer 15 to provide 
gain stabilization to the detector 10. More particularly, gamma radiation 
output of detector 10 is responsive to temperature changes in the field. 
Such temperature changes move the energy peak outputs of detector 10 for 
potassium, uranium and thorium. The barium-133 source is, however, not 
temperature sensitive and therefore produces artificial gamma radiation 
with a stabilized energy peak under variable environmental conditions. The 
multichannel analyzer 15 looks at channels on either side of the 
barium-133 energy peak and adjusts the gain of the pre-amplifier 13 to 
maintain a gain stabilized energy spectrum over the full energy spectrum. 
Barium-133 is one example of an energy source which is useful for gain 
stabilization purposes. Any suitable energy source that is not temperature 
sensitive and does not interfere with the natural radioactivity spectrum 
from potassium, uranium and thorium within a formation may be used. In 
this manner the full natural gamma radiation energy spectrum is recorded 
for processing in contrast to single channel recording (with its inherent 
stripping requirement) for each of the natural gamma radiation components 
of potassium, uranium and thorium as shown in the aforementioned U.S. 
patent to Dennis et al. 
The recorded full energy spectrum of the field natural gamma radiation 
measurement is applied from the multichannel analyzer 15 to the computer 
17 which fits this measured spectrum to a standard full energy spectrum 
recorded from known amounts of potassium, uranium and thorium gamma 
radiation by a least squares fit. This least squares fit may be carried 
out use of the ND6600 Computer and the ND6600 NAI Data Reduction Program 
Package supplied by Nuclear Data, Inc. This program package determines the 
activities of potassium, uranium and thorium over the full energy 
spectrum. A channel by channel comparison is carried out between the field 
measured full energy spectrum and a standard full energy spectrum. It 
determines gamma radiation contributions of standard spectra of known 
radiation sources by least square resolution of the measured data. This 
program is based on the Alpha M program developed at Oak Ridge National 
Laboratory. 
In a preferred embodiment for surface formation measurements the detector 
12 is a Bicron Corp., Model 3MT3/3 NaI(Tl) scintillation detector. The 
pre-amplifier 13 is a Video Optics, Inc., Model RPS 2000 battery powered 
high-voltage power supply and pre-amplifier. The artificial energy source 
16 is a 5 .mu.Ci.sup.133 Ba source. The multichannel analyzer 15 is a 
Nuclear Data, Inc., Model ND6-256, channel analyzer system. The detector 
12, pre-amplifier 13 and artificial energy source 16 are contained in a 
plexiglass housing 19. The field measured natural gamma radiation and 
artificial radiation from source 16 are applied from detector 10 to 
multichannel analyzer by way of the coaxial signal connection 20. 
In applying the method of the present invention to borehole measurements, 
the full pulse-height spectra are digitized down hole, and telemetered to 
the surface where the full spectra are recorded. Potassium, uranium and 
thorium contents of the subsurface formations are calculated from the 
spectra using the ND6600 program package as described above. 
When calibration is carried out for formations or formation models 
containing a mixture of two or more of the subject radionuclides K, U, or 
Th, an additional calculation must be performed using the contribution of 
those standards reported by the ND6600 program package. For each 
radionuclide, the contribution of each standard is multiplied by its 
radionuclide content and summed over all of the standards employed. This 
sum represents the radioelement content of the unknown formation based on 
the fitted contributions of standard spectra containing a mixture of 
potassium, uranium and thorium. 
Having now described the present invention in connection with a preferred 
embodiment, it is to be understood that various modifications and changes 
may be made without departing from the spirit and scope of the invention 
as set forth in the appended claims.