Method for correcting spectrum shift

A method which evaluates at least one subject element includes detecting radiation from a plurality of elements including the subject element. Data pulses are provided which are representative of the detected radiation. A spectrum is derived from the data pulses and a shift of the spectrum is determined. The subject element is evaluated in accordance with the energy level of data pulses occurring in a region of interest associated with the subject element which are derived from with the spectrum and the determined shift of the spectrum.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates to the detection of radiation in general and 
more particularly to those radiation detection systems utilizing a pulse 
height analyzer. 
SUMMARY OF THE INVENTION 
A method which evaluates at least one subject element includes detecting 
radiation from elements including the subject element. Data pulses are 
provided which are representative of the detected radiation. A spectrum is 
derived from the data pulses and a shift of the spectrum is determined. 
the subject element is evaluated in accordance with the energy level of 
data pulses occurring in a region of interest associated with the subject 
element derived from the spectrum and the determined shift of the 
spectrum. 
The object and advantages of the invention will appear more fully 
hereinafter from a consideration of the detailed description which 
follows, taken together with the accompanying drawings wherein one 
embodiment of the invention is illustrated by way of example. It is to be 
expressly understood, however, that the drawings are for illustration 
purposes only and not to be construed as defining the limits of the 
invention.

DESCRIPTION OF THE INVENTION 
With reference to FIG. 1, a production stream analyzer or a radioactivity 
well logging sonde 3 provides reference pulses and data pulses to an 
amplifier 5. The data pulses correspond to detected radiation. By way of 
example, a production stream analyzer is described and disclosed in U.S. 
Pat. No. 4,200,789 while a radioactivity well logging sonde is described 
and disclosed in U.S. Pat. No. 4,220,851. It should be noted that in 
regard to the radioactivity well logging sonde, there may be multiple 
radiation detectors involved in which case there would necessarily be 
additional circuitry to handle the additional detectors. 
Amplifier 5 amplifies the reference pulses and the data pulses. The 
amplified data pulses are provided to a multi-channel pulse height 
analyzer 14 which counts the number of pulses occurring and provides a 
corresponding output to a computer 20. Computer 20 provides an output 
which is recorded by recorder 22. The output from computer 20, when used 
with a production stream analyzer, would be representative of the content 
of an element of the production stream such as a percentage volume of 
water or salt in a production stream which is derived from the counts of 
pulses in the different channels of analyzer 14. When used with a the 
radioactivity well logging sonde, the output of computer 20 would be 
representative of characteristics of the earth formation normally 
associated with the exploration for hydrocarbons. 
All of the foregoing, as can be seen from the cited patents, represents the 
prior art. However, due to the variation of the temperature surrounding 
the nuclear detector and the gain drifting of the analog signal amplifier, 
the pulse height analyzer 14 spectrum data are often found shifted as is 
shown in FIG. 2, where the dashed line represents the shifted spectrum. 
Because of the drifting of the specturm, regions of interest which for the 
production stream analyzer would be hydrogen, sulfur, and chlorine 
required for determining the salt content of a production stream of crude 
oil are changed. Unless the pulse height analyzer 14 is calibrated for 
those three elements, the salt and sulfur result will be deviated. 
Heretofor, the calibration was done manually to stabilize the spectrum 
data from pulse height analyzer 14. 
With reference to FIG. 3, the hydrogen region of interest in the spectrum 
data from pulse height analyzer 14 is between 2.05 MEV and 2.50 MEV, the 
sulfur region of interest in the spectrum data is between 5.00 MEV and 
5.74 MEV, while the chlorine region of interest in the spectrum data is 
between 5.76 MEV and 8.00 MEV. The method of the present invention 
utilizing computer 20 and starts with hydrogen, the first boron (first 
escape), and the second boron (primary) peak searches. It should be noted 
that due to the statistical nature of the data, the nuclear data spectrum 
is not smooth and as a result there are several maximum points around the 
hydrogen, first boron and second boron peaks. By definition the maximum 
point is a point on a curve at which there exists a horizontal tangent 
line at which maximum data count around this neighborhood. The spectrum 
data are smoothed by averaging until only three maximum points appearing 
in the area shown in FIGS. 4A and 4B. The effect of the smoothing 
operation may be shown in FIGS. 4A and 4B where FIG. 4A is the spectrum 
data prior to smoothing and FIG. 4B shows the spectrum after smoothing. 
Thus the peak is the mean of several channels around the maximum point. 
The energy levels of levels E1, E2 and E3, corresponding to the hydrogen 
peak, the first boron peak B1 and the second boron peak B2, respectively, 
are know. The relationship between channel number and energy level can be 
derived by the least square fit of the three peaks H, B1 and B2 as shown 
in FIG. 5. Since the energy levels of the hydrogen, sulfur and chlorine 
regions of interests are known, the channel numbers for the shifted 
regions of interest can be determined by the computer from the least 
square fit line. 
With reference to FIG. 6, a simplified flow diagram for the programming of 
Computer 20 includes the start block 30 where data from pulse height 
analyzer 14 is entered into Computer 20. The next step is to point to the 
middle of the pulse height analyzer 14 spectrum as represented by block 
33. In this operation the operator picks a centrally located channel. For 
example, for a pulse height analyzer 14 having 1,024 channels, the 
programmers point for channel No. 512. 
The spectrum data is then compared with the B2 threshold to find the right 
edge of the spectrum as represented by block 35. The spectrum data is 
compared with the Hydrogen threshold to find the left edge of the 
spectrum, by block 38. Computer 20 is then queried as to whether the right 
and left edges can be found (block 40). If the answer is no, block 42 
requires the setting of a low Hydrogen count flag which also results in 
the end of the routine represented by block 44. If the answer is yes then, 
as required by block 48, the low Hydrogen count flag is cleared. 
The next step is to store that portion of the spectrum occurring between 
the edges as represented by block 50. The stored spectrum is averaged in 
block 54 and a search for peaks and valleys is conducted by computer 20, 
see block 57. Computer 20 is then queried, "are there more than 6 peaks 
and valleys in the area" as shown in block 62. If the answer is yes, the 
data is returned for further processing by block 54 and further searching 
by block 57. This procedure keeps up until the answer to the querie of 
block 62 is that there are not more than 6 peaks and valleys in the area. 
At that time, peak channel numbers are calculated as per block 65. Block 68 
requires that the channel numbers for those peak channels be stored. Block 
69 provides for the adjustment of the three peaks. Based on probabilities, 
Computer 20 calculates the peaks as required in block 72. Block 75 states 
that a parabolic curve fit to a finite H peak is performed using the 
several data channels around Hydrogen peak. A least square fit line is 
determined as per block 78. Based on the least square fit line, the 
Hydrogen, Chlorine and Sulfur regions of interest are determined as per 
block 82. 
Computer 20 is then queried, "Is the left boundary of the Hydrogen region 
of interest greater than 150", as shown in block 86. If the answer is no, 
block 87 requires the setting of the FSTAB flag which results in the end 
of the program 44. The FSTAB flag signifies that the unit has failed 
stabilization and the data currently presented should not be used. If the 
answer to the querie of block 86 is yes, Computer 20 is then queried, to 
see if the right boundary of the chlorine region of interest is greater 
than 1000. If the answer to that query is yes, then we proceed to the 
setting of the FSTAB flag as per block 87. 
If the answer is no, block 91 requires the calculation of the Hydrogen, 
Sulfur and Chlorine using 5 minute integral counts, using the regions of 
interest. Again Computer 20 is queried as per block 94. "Does any integral 
count equal O"? If the answer is yes, the FSTAB flag is set. If the answer 
is no we proceed to the next step which is to calculate thirty minute 
integral counts by averaging 6 five minute counts as set forth in block 
96. 
Computer 20 is again queried, "Does any 30 minute integral count equal 0?" 
as shown in block 100. If the answer is no this simply means that the data 
is good and ends that portion of the program dealing with the adjustments 
to pulse height analyzer 14 data. Spectrum block 105 requires the 
calculation of the salt and sulfur in the production stream as provided 
for in the aforementioned U.S. Patent using the Salt-in-Crude monitor. 
After this calculation, Computer 20 is then queried. "Is Calfail set", as 
provided for in block 107, Calfail stands for calibration failed. If the 
answer is yes, the FSTAB flag is again set. If the answer is no, the next 
step as provided for by block 110 is to clear the FSTAB flag. Proceed to 
the end of routine 44. 
As noted, the aformentioned flow diagram of FIGS. 6A, 6B and 6C is 
applicable to the system of FIG. 1 where the production stream analyser is 
used. A similar program would be developed for a radioactivity well 
logging system. 
When the method of the present invention is used in radioactive well 
logging, the method is the same but the elements for yielding the these 
peaks are different. For example, with a well logging system described and 
disclosed in U.S. Pat. No. 4,032,778; 4,032,780 and 4,035,640. The peaks 
searched for are the TL-208 peak (2.615 MEV) is from the Thorium source 
inside the detector, the primary oxygen peak (6.13 MEV), and the Oxygen 
first escape peak (5.62 MEV). The two oxygen peaks are derived from gamma 
radiation from water flowing behind the casing. 
The present invention as herein before described is method of automatically 
correcting far spectrum drift of the spectrum provided by a pulse height 
analyzer receiving pulses from a radiation detector.