FIELD OF INVENTION
This invention relates to gain stabilization of a natural gamma ray tool.
BACKGROUND
5 Understanding the structure and properties of geological formations
can reduce the cost of drilling wells for oil and gas exploration.
Measurements made in a borehole (i.e., downhole measurements) are
typically performed to attain this understanding, to identify the composition
and distribution of material that surrounds the measurement device
10 downhole. To obtain such measurements, gamma ray detectors are often
used to measure naturally-occurring gamma radiation downhole. However,
the gain of some gamma ray detectors may fluctuate due to environmental
conditions downhole. These fluctuations can cause changes in the apparent
energy level detected by the gamma ray detector, thereby leading to
15 inaccuracies in the measurements reported by gamma ray measurement
tools.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a gamma ray measurement tool in
20 accordance with some embodiments.
FIG. 2 is an example gamma spectrum such as can be generated based
on values provided by a gamma ray measurement tool in accordance with
some embodiments.
FIG. 3 is an example of modeled spectra for various radioactive
25 isotopes.
FIG. 4 is an example of centroids calculated from modeled spectra of
FIG. 3 in accordance with some embodiments.
FIG. 5 is a block diagram of a logging system according to some
embodiments.
-2-
5
10
FIG. 6 is a flowchart showing an embodiment of a method for
adjusting gain of a gamma ray detector.
FIG. 7 is a diagram of a wireline system embodiment.
FIG. 8 is a diagram of a drilling rig system embodiment.
DESCRIPTION OF INVENTION w.r.t. DRAWINGS
To address some of the challenges described above, as well as others,
systems, apparatus, and methods are described herein for stabilizing net gain
of gamma ray detectors.
FIG. 1 is a schematic diagram of a gamma ray measurement tool100 in
accordance with some embodiments. The gamma ray measurement tool100
includes a downhole gamma ray detector 102, electronics unit 104, and a
processing unit 106. The gamma ray detector 102 may be part of a drilling
assembly, for logging while drilling (LWD}, or measuring while drilling (MWD}
15 operations, or may be a wireline tool for logging an existing well as described
later herein with reference to FIGs. 7 and 8. The gamma ray measurement
tool 100 can include a plurality of azimuthal gamma ray detectors. Each
gamma ray detector 102 counts gamma ray energy emitted naturally
subsurface, but could also be used for other sources of gamma rays in the
20 wellbore. The azimuthal gamma ray detectors may be near an exterior of a
logging tool and be spaced about a circumference of the logging tool. While a
plurality of gamma ray detectors may be included, only one is explicitly
presented here.
The gamma ray detector 102 provides signals that scale with the
25 energy deposited by the gamma rays in the gamma ray detector 102. The
gamma ray detector 102 includes one or more scintillator crystals 108 for
receiving the gamma rays that then create light emissions that influence an
adjacent, optically-coupled photodetector 110, e.g., a photomultiplier tube.
The gamma ray detector 102 is electrically coupled to an electronics unit 104.
- 3 -
The electronics unit 104 may include an amplifier 112, a variable high voltage
supply unit 114, and an analog-to-digital (A/D) converter 116. The high
voltage supply unit 114 is coupled to and powers one or more of the gamma
ray detectors 102. One high voltage supply unit 114 may be used to power
5 multiple gamma ray detectors. The high voltage supply unit 114 may be
configured so that the output voltage can be adjusted by an external
controller or processing unit 106.
The electronics unit 104 also includes one or more amplifiers 112 to
modify the amplitude of the signals coming from the one or more gamma ray
10 detectors 102. The amplifier 112 may be configured to be adjusted by the
processing unit 106. The electronics unit 104 further includes the analog-todigital
(A/D) converter 116 to convert voltage signals to digital signals to be
passed to the processing unit 106. The electronics unit 104 converts and
processes the signals by, for example, adjusting the signal amplitude or
15 adjusting the voltage supplied to the gamma ray detector 102.
The processing unit 106 includes memory 118 associated with one or
more processors 120. Memory 118 when coupled with the processor 120
can execute code to accomplish functionalities including the methods for gain
control described later herein. Memory 118 can store measurements of
20 formation parameters or parameters of the gamma ray measurement tool
100 such as gain parameters, calibration constants, identification data, etc.
The memory 118 therefore may include a database, for example a relational
database. The processor 120 can control the output voltage of the high
voltage supply unit 114 or amplifier 112. The gamma ray measurement tool
25 100 can also include a battery or other power source (not shown in FIG. 1).
The electronics unit 104 and processing unit 106 are operable, inter
alia, to sort the digital signals from the downhole gamma ray detector 102
into channels according to the amplitude of the digital signals and store the
channels as a gamma spectrum. The gamma ray measurement tool 100 can
30 be a gross counting gamma ray detector, wherein the process of determining
- 4-
the gross count involves developing counts over a plurality of channels
arranged in a spectrum. Whether accomplished in logging while drilling (FIG.
8) or on a wireline (FIG. 7), the gamma ray measurement tool 100 develops
count data over n-channels. Each channel represents a range of energy levels,
5 wherein the energy levels can be measured in units such as kilo electron volts
(keV). The number of channels may vary for different applications; for
example, n may be 10, 16, 20, 50, 64, 100, 128, 150, 200, 256, 400 or more, or
any number in between.
In various embodiments, the processing unit 106 receives an energy
10 spectrum from the emitted gamma radiation and records the spectrum across
a spectrum of n channels (where n equals the number of channels), and the
processing unit 106 determines the total count above a threshold. The
processing unit 106 sets the threshold to be at least greater than a noise level
but low enough to measure all of the gamma rays that enter the gamma ray
15 detector 102. Initially, the processing unit 106 may set the threshold based
on, for example, an actual or predicted noise level, historical data, etc.
The gain of the gamma ray detector 102 varies at times with certain
variables, e.g., temperature, equipment limitations, high voltage of the
photomultiplier in the gamma ray detector 102, etc. These variations will
20 affect the counts stored in the various channels. To obtain a gross count that
is not affected by these variations, the processing unit 106 stabilizes the
system gain of each gamma ray detector 102 of a gamma ray measurement
tool100 independently.
The system gain can be adjusted in one of several ways. The
25 adjustment can be carried out by adjusting the hardware gain. Alternatively,
the spectrum can be scaled in memory. In addition, the thresholds used to
compute count ranges in an energy window can be adjusted.
A controller such as the processing unit 106 can adjust the gain of
each gamma ray detector 102 by adjusting the output of the high voltage
30 supply unit 114, the gain of the amplifier 112, or by adjusting both the output
- 5 -
of the high voltage supply unit 114 and the gain of the amplifier 112. In this
way, a stabilized gross count may be obtained by summing counts in the same
channels. The processing unit 106 can assemble spectra for a period of time
and then analyze the spectra to determine the amount by which the gain is to
5 be adjusted, if any.
The processing unit 106 may perform gain adjustment after the
spectra contain a minimum amount of counts. Alternatively, or in addition,
the processing unit 106 may implement algorithms to determine gain
adjustment periodically, or after a minimum period of time has passed with
10 · the accumulated counts in each gamma ray detector 102 exceeding a
minimum number of counts. The intervals between gain adjustments may
vary as the magnitudes of the counts stored in the spectra vary. The time
between adjustments should be sufficiently long so that the processing unit
106 can make a statistically significant adjustment, yet short enough so that
15 the gamma ray detector 102 can respond to gain variations. In some
embodiments, the processing unit 106 may perform gain adjustment based
on received diagnostics information for the gamma ray measurement tool
100. Diagnostics information can include indicators, such as a flag, for
indicating whether the gamma ray detector(s) 102 have excessive high or low-
20 energy noise, for example.
FIG. 2 is an example gamma spectrum 200 such as can be generated
based on values provided by a gamma ray measurement tool 100 in
accordance with some embodiments. In the illustrate example of FIG. 2,
each channel in the x-axis represents 2 keV, and the count rates measured in
25 each channel are plotted on the y-axis. The gamma spectrum 200 includes a
maximum 202 or gamma peak, and a noise portion or segment 204. The
desired threshold 206 is set above the maximum noise level, yet low enough
so that as many gamma rays as possible will be detected. Assuming that
gamma spectrum 200 was obtained at a desired nominal system gain and that
- 6 -
the noise is the maximum expected, then a choice for threshold 206 may be
at about channel 28.
According to embodiments, an identifiable, stable point of the gamma
spectrum 200 is desired for use as a reference for adjusting the gain,
5 threshold, or hardware. In embodiments, the location of a maximum value of
the first derivative of a curve defining the gamma spectrum 200 with respect
to channel number can be used as this identifiable, stable point, because the
maximum of the first derivative is located on the rising edge of the peak of
the raw data set, and may be insensitive to variations in density of the
10 formation being measured. For example, the threshold may be set at half the
channel number of the location of the maximum of this first derivative.
Some systems compute the first derivative of the gamma spectrum
200 at a point based on channels above and below each computation point.
This computation is often done at many points in the gamma spectrum 200 in
15 order to determine the location of the maximum value. However, statistical
uncertainties in the gamma spectrum 200 can result in an uncertainty in the
computed location of the maximum of the first derivative. Statistical
uncertainties can be reduced or eliminated by summing the spectra over
longer periods of time, but this can result in slow response of gain
20 stabilization, in particular when the gamma ray measurement tool 100 first is
powered on. Embodiments reduce or eliminate the effect of statistical
fluctuations.
In accordance with some embodiments, the processing unit 106 can
determine whether the gain of each gamma ray detector 102 is close to a
25 nominal value. In some examples, the processing unit 106 can determine this
by computing a centroid of the spectrum 200 and then checking to see if the
centroid is within a certain threshold distance (e.g., within 10-15%) of a
nominal value. If the gain is not close, the processing unit 106 can perform a
gross adjustment of the system gain based on the centroid of the gamma
30 spectrum 200. Otherwise, the processing unit 106 can perform a fine
- 7 -
adjustment of the system gain based on the location of the maximum of the
first derivative of a portion of the gamma spectrum 200.
In cases where system gain is to be adjusted based on the centroid,
the processing unit 106 adjusts the gain of the each gamma ray detector 102
5 to put the centroid at the nominal location for the centroid. The nominal
location for the centroid can be set when the gamma ray measurement tool
100 and gamma ray detector(s) 102 are first characterized or initialized. For
example, the nominal locations may be chosen as channel 90 of a 256-
channel spectrum, where the typical centroid range is 50 to 150. Processing
10 time for these centroid computations can be relatively quick, and the
statistical uncertainties are small. Methods according to at least these
embodiments produce accurate estimates for gain adjustment because the
spectral shape of detected gamma rays does not vary significantly with the
formation properties or the radioactive isotopes that generate the gamma
15 rays. Because curve shapes for natural-gamma-ray spectra are very similar,
gain adjustment calculations based on centroids can be relatively accurate
regardless of the formation properties or percentage or identity of radioactive
isotopes therein.
FIG. 3 is an example of modeled spectra for various radioactive
20 isotopes. It can be appreciated upon examining FIG. 3 that the spectral
shapes do not vary significantly. The curves of FIG. 3 represent the count rate
observed in each channel, and each channel represents 5 keV. The amplitude
of the curves have been scaled for purposes of demonstrating that the curves
have the same shape. In FIG. 3, counts that would normally appear in
25 channels above 254 are added to those that naturally occur in channel 254.
The spectra A-H represent the cases where the natural gamma radiation in
the formations is all potassium (spectra A and E), all uranium (spectra B and
F), all thorium (spectra C and G), or a mixture of the three in the proportions
found in an American Petroleum Institute (API) test formation (spectra D and
30 H). Spectra for two different formation densities are shown.
- 8-
5
The processing unit 106 can generate a centroid for the spectrum
according to:
upper channel
L: i · spectrum(i)
Centroid= i=lowerchannel
upperchannel
(1)
L: spectrum(i)
i =lowerchannel
where spectrum(i) is the count rate measured in channel i.
FIG. 4 is an example of centroids calculated from modeled spectra of
FIG. 3 in accordance with some embodiments. The centroids can be
calculated by, for example, the processing unit 106 or by a surface system as
described with reference to FIGs. 6 and 7 later herein.
FIG. 4 .illustrates two cases: a "Full Spectrum" case A that uses all the
10 data, and a "100 keV Threshold" case B that uses only the data above 100 keV.
The variations in centroid values are essentially the same for the two. For
example, it can be demonstrated on inspection of FIG. 4 that the centroids are
all within 9% of channel 63.4 for case A and within 9% of 70.8 for case B.
In most situations, it can be accurately predicted that noise will
15 manifest itself as an increase in counts in the low-energy subset of the
spectrum, as can be seen upon examination of FIG. 2. Accordingly, noise will
typically only distort the centroid computed using the full spectrum, while
noise will have little or no effect on the centroid computed from the data
above 100 keV (case A in FIG. 4). Consequently, the processing unit 106 will
20 use the centroid computed from a first subset of the data, for example the
data above 100 keV, (e.g., the data excludes the low-energy subset of the
spectrum) in the stabilization algorithm. In some embodiments, if the gain
change is greater than a predefined number, for example, if the gain change is
greater than 20% of the original gain, the processing unit 106 may recompute
25 the gain change using a second, larger subset of the data.
- 9-
As mentioned earlier herein, if substantial gain adjustments are not
necessary, the processing unit 106 can fine-tune the system gain based on the
first derivative of the gamma spectrum 200. To find the first derivative, in at
some embodiments, the processing unit 106 will fit a curve to a portion that
5 includes a range of data points of the gamma spectrum 200, where the
processing unit 106 selects the range of channels that includes the expected
or desired location of the maximum of the first derivative of the gamma
spectrum 200. This range of data can be set by the processing unit 106 to be
within a tolerance set based on the centroid test described earlier herein,
10 which can help assure that the location of the maximum of the first derivative
will be within the range that the processing unit 106 fits to the cubic
equation. In some examples, the processing unit 106 can select a range
centered on a reference channel (e.g., channel 50), with a multipliers being
used to calculate the lower limit of the range and an upper limit of the range
15 based on the reference channel.
The processing unit 106 can obtain the location of the maximum of
the first derivative from the curve parameters obtained in the fit. In some
example embodiments, the processing unit 106 can fit a cubic polynomial,
expressed with respect to channel number, to the data. The cubic polynomial
20 can be represented by:
{2)
where co, c1, c2, and C3 are the coefficients of the polynomial.
However, embodiments are not limited thereto, and the processing
unit 106 can use other equations such as, for example, a fourth-order
25 polynomial. It will be understood by those of ordinary skill in the art that the
- 10-
location of an extreme value X extreme of the first derivative of Equation
{2) is located at: X extreme (3)
Assuming that care is taken to ensure that the correct portion of the
gamma spectrum 200 is being fit, then the extreme value Xextreme will
5 be the maximum of the first derivative and not the minimum.
This method when implemented in accordance with embodiments
allows a large region of the spectrum to be scanned at once, while reducing
or eliminating statistical fluctuations. The processing unit 106 may use a
cubic equation for the curve fitting because a cubic equation can describe a
10 large portion of the gamma spectrum 200, while still not using significant
computational power to find the first derivative. Furthermore, processing
units 106 may use cubic equations for the curve fitting because cubic
equations have a unique location of the maximum value of the first
derivative, which simplifies the process of selecting this location for use in
15 setting the gain of the gamma ray detector 102. The statistical uncertainty
related to the actual location of the maximum of the first derivative of the
curve will be reduced as the number of points to which the processing unit
106 fits the curve is increased. In the case of a cubic polynomial, statistical
uncertainty in the location of the maximum of the first derivative is reduced
20 as the number of points used exceeds four, because four points is the
minimum number required to compute cubic parameters.
Table 1 is example pseudocode for computing a gain change based on
the maximum of the first derivative of Equation (2). However, it will be
understood that embodiments are not limited to any particular
25 implementation for finding this maximum and the algorithm can include
other operations such as error checking, range checking, etc.
- 11 -
1: a(l,l)=fit_upper _channel- fit_lower _channel+l
fit_ upper_ channel
2: a(l,2) = :L i
i= fit _lower_ channel
fit_ upper_ channel
2
5 3: a(1,3) = I i
i= fit _lower_ channel
fit_ upper_ channel
3
4: a(l,4) = I i
i=fit _lower_ channel
· fit_ upper_ channel
4
5: a(2,4) = I i
i= fit _lower_ channel
10
fit_ upper_ channel
5
6: a(3,4) = I i
i= fit _lower_ channel
fit_ upper_ channel
6
7: a( 4,4) = I i
i =fit _lower_ channel
15 8: a(2,2) = a(1,3)
9: a(2,3) = a(l,4)
10: a(3,3) = a(2,4)
11: a(2,1) = a(1,2)
12: a(3,1) = a(1,3)
20 13: a(3,2) = a(2,3)
14: a(4,1) = a(1,4)
15: a(4,2) = a(2,4)
16: a(4,3) = a(3,4)
- 12-
17:
fit_ upper_ channel
b(l) = I spectrum(i +derivative location_ nom)
i= fit _lower_ channel
18:
5
fit_ upper_ channel
b(2) = 2.:: i · spectrum(i _derivative _location_ nom)
i= fit _lower_ channel
19:
fit_ upper_ channel
b(3) = 2.:: i 2 · spectrum(i +derivative _location_ nom)
i= fit _lower_ channel
10 20:
fit_ upper_ channel
b( 4) = I i3 · spectrum(i +derivative _location_ nom)
i= fit _lower_ channel
21: det4=determinant(b, 4)
22: det3=determinant(b, 3)
15 23: maximum=-det'¥'(3*det4)
24: limit the computed maximum to be within a range
25: gain_change_compute=ref_channel/(maximum + ref_channel)
Table 1: pseudocode for computing gain change.
In lines 1-20, the processing unit 106 computes the coefficients of the
20 matrix equation ax=b that defines the parameters of a cubic equation that
fits the input spectrum between fit_lower_channel and fit_upper_channel,
where xis a vectorthat represents the four coefficients of the cubic equation
and derivative location nom is the nominal location of the maximum - -
derivative. Referencing position to the nominal location of the maximum
25 derivative lowers the precision to which the calculations must be performed.
In lines 21-22, the processing unit 106 computes two of the
determinants that can be used to solve the equation for the four coefficients,
wherein determinant (b,j) represents the determinant of a modified matrix a,
where the modification is performed by replacing the/h column of a with the
30 vector b. In line 23, the processing unit 106 finds the location of the
- 13-
maximum of the first derivative of the cubic equation defined by the matrix
equation ax=b with matrix coefficients specified in lines 1-20, wherein the
location is referenced to the nominal location of the maximum derivative.
This method allows the location of the maximum of the first derivative to be
5 computed without fully computing the coefficients of the cubic equation,
since the value computed in line 23 is mathematically equivalent to the value
of the location of the extreme given in Equation (3). In line 24, the processing
unit 106 limits the maximum of the first derivative to a location within a
predetermined range, relative to the reference channel for example or based
10 on centroids as described earlier herein.
In line 25, the processing unit 106 computes the gain change based on
the maximum. In some examples, the processing unit 106 may provide larger
gain adjustments upon powering up the gamma ray measurement tool 100
than would have been provided after the gamma ray measurement toollOO
15 has been operating for longer periods of time.
FIG. 5 is a block diagram of a logging system 500 according to various
embodiments. The logging system 500 can receive count measurements or
other data from the gamma ray measurement tool 100 (FIG. 1) and provide
gain stabilization for one or more gamma ray detectors 102 of the gamma ray
20 measurement tool 100. The logging system 500 includes gamma ray
measurement tool 504 operable in a wellbore.
The processing unit 106 can couple to the gamma ray measurement
tool 504 to obtain measurements from the gamma ray measurement tool
504 as described earlier herein regarding FIG. 1. The processing unit 106 can
25 perform gain stabilization on the gamma ray measurement tool 504 as
described herein. In some embodiments, a logging system 500 comprises
one or more of the gamma ray measurement tool 504, as well as a housing
(not shown in FIG. 5) that can house the gamma ray measurement tool504 or
other electronics. The housing might take the form of a wireline tool body, or
30 a downhole tool as described in more detail below with reference to FIGs. 7
- 14-
and 8. The processing unit 106 may be part of a surface workstation or the
processing unit 106 can be packaged with the gamma ray measurement tool
504 as described earlier herein regarding FIG. 1 or attached to the housing.
The logging system 500 can additionally include a controller 525, an
5 electronic apparatus 565, and a communications unit 540. The controller 525
and the processing unit 106 can be fabricated to operate the gamma ray
measurement tool 504 to acquire measurement data such as counts as the
gamma ray measurement tool 504 is operated.
Electronic apparatus 565 can be used in conjunction with the
10 controller 525 to perform tasks associated with taking measurements
downhole with the gamma ray measurement tool 504. The communications
unit 540 can include downhole communications in a drilling operation. Such
downhole communications can include a telemetry system.
The logging system 500 can also include a bus 527, where the bus 527
15 provides electrical signal paths among the components of the logging system
500. The bus 527 can include an address bus, a data bus, and a control bus,
each independently configured. The bus 527 can also use common
conductive lines for providing one or more of address, data, or control, the
use of which can be regulated by the controller 525. The bus 527 can include
20 instrumentality for a communication network. The bus 527 can be
configured such that the components of the logging system 500 are
distributed. Such distribution can be arranged between downhole
components such as the gamma ray measurement tool 504 and components
that can be disposed on the surface of a well. Alternatively, various of these
25 components can be co-located such as on one or more collars of a drill string
or on a wireline structure.
In various embodiments, the logging system 500 includes peripheral
devices that can include displays 555, additional storage memory, or other
control devices that may operate in conjunction with the controller 525 or
30 the processing unit 106. The display 555 can display diagnostic information
- 15 -
for the gamma ray measurement tool 504 based on the signals generated
according to embodiments described above.
In an embodiment, the controller 525 can be realized as one or more
processors. The display 555 can be arranged to operate with instructions
5 stored in the processing unit 106 (for example in the memory 118 (FIG. 1)) to
implement a user interface to manage the operation of the gamma ray
measurement tool 504 or components distributed within the logging system
500. Such a user interface can be operated In conjunction with the
communications unit 540 and the bus 527. Various components of the
10 logging system 500 can be integrated with the ganima ray measurement tool
504 or associated housing such that processing identical to or similar to the
processing schemes discussed with respect to various embodiments herein
can be performed downhole.
In various embodiments, a non-transitory machine-readable storage
15 device can comprise instructions stored thereon, which, when performed by
a machine, cause the machine to perform operations, the operations
comprising one or more features similar to or identical to features of
methods and techniques described herein. A machine-readable storage
device, herein, is a physical device that stores data represented by physical
20 structure within the device. Examples of machine-readable storage devices
can include, but are not limited to, memory 118 in the form of read only
memory (ROM}, random access memory (RAM}, a magnetic disk storage
device, an optical storage device, a flash memory, and other electronic,
magnetic, or optical memory devices, including combinations thereof.
25 The physical structure of such instructions may be operated on by one
or more processors such as, for example, the processing unit 106. Executing
these physical structures can cause the machine to perform operations
according to methods described herein. The instructions can include
instructions to cause the processing unit 106 to store associated data or
30 other data in the memory 118.
- 16-
FIG. 6 is a flowchart showing an embodiment of a method 600 for
adjusting gain of a gamma ray detector 102. The example method 600 is
described herein with reference to elements shown in FIGs. 1 and 5. Some
operations of example method 600 can be performed in whole or in part by a
5 processing unit(s) 106 and memory 118 (FIG. 1), or any component of system
500 (FIG. 5) or gamma ray measurement tool 100 (FIG. 1), although
embodiments are not limited thereto.
The example method 600 begins with operation 602 in which the
processing unit 106 receives gamma ray measurements from the gamma ray
10 detector 102.
The example method 600 continues with operation 604 in which the
processing unit 106 generates a spectrum based on the gamma ray
measurements. The spectrum can be similar to spectra described earlier
herein with reference to FIGs. 2 and 3. The spectrum can include several
15 channels with corresponding count rates, wherein a channel number of a
channel corresponds to energy values of the received gamma rays.
The example method 600 continues with operation 606 in which the
processing unit 106 fits a curve to a portion of the spectrum. The curve can
be similar to that described above with reference to at least Equation (2)
20 although embodiments are not limited thereto. For example, an equation to
describe the curve can include a cubic polynomial, a fourth-order polynomial,
etc.
The example method 600 continues with operation 608 in which the
processing unit 106 determines a location of the maximum of the first
25 derivative of the curve that was generated in operation 606. In order to
execute operation 608, the processing unit 106 may implement code similar
to pseudocode described earlier herein with reference to Table 1, although
embodiments are not limited thereto.
The example method 600 continues with operation 610 in which the
30 processing unit 106 adjusting a gain of at least one gamma ray detector 102
- 17 -
based on the location of the maximum of the first derivative of the curve.
The processing unit 106 will continue to monitor the location of the
maximum of the first derivative of the curve based on gamma ray
measurements received subsequent to adjusting the gain. The processing
5 unit 106 may trigger a gain readjustment process to include any or all
operations of the example method 600 if the location shifts by more than a
threshold amount. The processing unit 106 may perform other operations
such as centroid computations or other operations in the event that gross
gain adjustments are necessary, as decided periodically, on power up, or
10 according to other criteria as described earlier herein.
As described earlier herein, gamma ray measurement tools can be
used in a logging-while-drilling (LWD) assembly or a wireline logging tool.
FIG. 7 illustrates a wireline system 764 embodiment of the invention, and FIG.
8 illustrates a drilling rig system 864 embodiment of the invention. Thus, the
15 systems 764, 864 may comprise portions of a wireline logging tool body 770
as part of a wireline logging operation, or of a downhole tool 824 as part of a
downhole drilling operation. Thus, FIG. 7 shows a well during wireline logging
operations. In this case, a drilling platform 786 is equipped with a derrick 788
that supports a hoist 790.
20 Drilling oil and gas wells is commonly carried out using a string of drill
pipes connected together so as to form a drilling string that is lowered
through a rotary table 710 into a wellbore or borehole 712. Here it is
assumed that the drilling string has been temporarily removed from the
borehole 712 to allow a wireline logging tool body 770, such as a probe or
25 sonde, to be lowered by wireline or logging cable 774 into the borehole 712.
Typically, the wireline logging tool body 770 is lowered to the bottom of the
region of interest and subsequently pulled upward at a substantially constant
speed.
During the upward trip, at a series of depths the instruments (e.g., the
30 gamma ray measurement tool100 shown in FIG. 1) included in the tool body
- 18 -
770 may be used to perform measurements on the subsurface geological
formations adjacent the borehole 712 (and the tool body 770). The
measurement data can be communicated to a surface logging facility 792 for
storage, processing, and analysis. The logging facility 792 may be provided
5 with electronic equipment for various types of signal processing, which may
be implemented by any one or more of the components of the gamma ray
measurement tool 100. Similar formation evaluation data may be gathered
and analyzed during drilling operations (e.g., during LWD operations, and by
extension, sampling while drilling).
10 In some embodiments, the tool body 770 comprises a gamma ray
measurement tool for obtaining and analyzing gamma ray field
measurements in a subterranean formation through a borehole 712. The
tool is suspended in the wellbore by a wireline cable 774 that connects the
tool to a surface control unit (e.g., comprising a workstation 754, which can
15 also include a display). The tool may be deployed in the borehole 712 on
coiled tubing, jointed drill pipe, hard wired drill pipe, or any other suitable
deployment technique.
Turning now to FIG. 8, it can be seen how a system 864 may also form
a portion of a drilling rig 802 located at the surface 804 of a well 806. The
20 drilling rig 802 may provide support for a drill string 708. The drill string 708
may operate to penetrate the rotary table 710 for drilling the borehole 712
through the subsurface formations 814. The drill string 708 may include a
Kelly 816, drill pipe 818, and a bottom hole assembly 820, perhaps located at
the lower portion of the drill pipe 818.
25 The bottom hole assembly 820 may include drill collars 822, a
downhole tool 824, and a drill bit 826. The drill bit 826 may operate to create
the borehole 712 by penetrating the surface 804 and the subsurface
formations 814. The downhole tool 824 may comprise any of a number of
different types of tools including MWD tools, LWD tools, and others.
- 19-
During drilling operations, the drill string 708 (perhaps including the
Kelly 816, the drill pipe 818, and the bottom hole assembly 820) may be
rotated by the rotary table 710. Although not shown, in addition to, or
alternatively, the bottom hole assembly 820 may also be rotated by a motor
5 (e.g., a mud motor) that is located downhole. The drill collars 822 may be
used to add weight to the drill bit 826. The drill collars 822 may also operate
to stiffen the bottom hole assembly 820, allowing the bottom hole assembly
820 to transfer the added weight to the drill bit 826, and in turn, to assist the
drill bit 826 in penetrating the surface 804 and subsurface formations 814.
10 During drilling operations, a mud pump 832 may pump drilling fluid
(sometimes known by those of ordinary skill in the art as "drilling mud") from
a mud pit 834 through a hose 836 into the drill pipe 818 and down to the drill
bit 826. The drilling fluid can flow out from the drill bit 826 and be returned
to the surface 804 through an annular area 840 between the drill pipe 818
15 and the sides of the borehole 712. The drilling fluid may then be returned to
the mud pit 834, where such fluid is filtered. In some embodiments, the
drilling fluid can be used to cool the drill bit 826, as well as to provide
lubrication for the drill bit 826 during drilling operations. Additionally, the
drilling fluid may be used to remove subsurface formation cuttings created by
20 operating the drill bit 826.
Thus, it may be seen that in some embodiments, the systems 764, 864
may include a drill collar 822, a downhole tool 824, and/or a wireline logging
tool body 770 to house one or more gamma ray measurement tools 100,
similar to or identical to the gamma ray measurement tool 100 described
25 above and illustrated in FIG. 1. Components of the system 500 in FIG. 5 may
also be housed by the tool 824 or the tool body 770.
Thus, for the purposes of this document, the term "housing" may
include any one or more of a drill collar 822, a downhole tool 824, or a
wireline logging tool body 770 (all having an outer wall, to enclose or attach
30 to magnetometers, sensors, fluid sampling devices, pressure measurement
-20-
devices, transmitters, receivers, acquisition and processing logic, and data
acquisition systems). The tool 824 may comprise a downhole tool, such as an
LWD tool or MWD tool. The wireline tool body 770 may comprise a wireline
logging tool, including a probe or sonde, for example, coupled to a logging
5 cable 774. Many embodiments may thus be realized.
Thus, a system 764, 864 may comprise a downhole tool body, such as
a wireline logging tool body 770 or a downhole tool 824 (e.g., an LWD or
MWD tool body), and one or more gamma ray measurement tools 100
attached to the tool body, the gamma ray measurement tool 100 to be
10 constructed and operated as described previously.
Any of the above components, for example the gamma ray
measurement tools 100, processing units 106, etc., may all be characterized
as "modules" herein. Such modules may include hardware circuitry, and/or a
processor and/or memory circuits, software program modules and objects,
15 and/or firmware, and combinations thereof, as desired by the architect of the
gamma ray measurement tool 100 and systems 500, 764, 864 and as
appropriate for particular implementations of various embodiments. For
example, in some embodiments, such modules may be included in an
apparatus and/or system operation simulation package, such as a software
20 electrical signal simulation package, a power usage and distribution
simulation package, a power/heat dissipation simulation package, and/or a
combination of software and hardware used to simulate the operation of
various potential embodiments.
It should also be understood that the apparatus and systems of
25 various embodiments can be used in applications other than for logging
operations, and thus, various embodiments are not to be so limited. The
illustrations of gamma ray measurement tool 100 and systems 500, 764, 864
are intended to provide a general understanding of the structure of various
embodiments, and they are not intended to serve as a complete description
- 21 -
of all the elements and features of apparatus and systems that might make
use of the structures described herein.
Applications that may include the novel apparatus and systems of
various embodiments include electronic circuitry used in high-speed
5 computers, communication and signal processing circuitry, modems,
processor modules, embedded processors, data switches, and applicationspecific
modules. Some embodiments include a number of methods.
It should be noted that the methods described herein do not have to
be executed in the order described, or in any particular order. Moreover,
10 various activities described with respect to the methods identified herein can
be executed in iterative, serial, or parallel fashion. Information, including
parameters, commands, operands, and other data, can be sent and received .
in the form of one or more carrier waves.
Upon reading and comprehending the content of this disclosure, one
15 of ordinary skill in the art will understand the manner in which a software
program can be launched from a computer-readable medium in a computerbased
system to execute the functions defined in the software program. One
of ordinary skill in the art will further understand the various programming
languages that may be employed to create one or more software programs
20 designed to implement and perform the methods disclosed herein. For
example, the programs may be structured in an object-orientated format
using an object-oriented language such as Java or C#. In another example,
the programs can be structured in a procedure-orientated format using a
procedural language, such as assembly or C. The software components may
25 communicate using any of a number of mechanisms well known to those
skilled in the art, such as application program interfaces or interprocess
communication techniques, including remote procedure calls. The teachings
of various embodiments are not limited to any particular programming
language or environment. Thus, other embodiments may be realized.
-22-
In summary, using the apparatus, systems, and methods disclosed
herein may provide increased stability in the gain of gamma ray
measurement tools in the presence of electronics drift, temperature
extremes, or other environmental or design factors relative to conventional
5 mechanisms. These advantages can significantly enhance the value of the
services provided by an operation/exploration company, while at the same
time controlling time-related costs.
10
Further examples of apparatuses, methods, a means for performing
acts, systems or devices include, but are not limited to:
Example 1 is a method for adjusting gain of a gamma ray detector, the
method comprising: receiving gamma ray measurements from the gamma
ray detector; generating a spectrum based on the gamma ray measurements,
the spectrum including a plurality of channels and count rates for the
plurality of channels, wherein a channel number of a channel corresponds to
15 energy values of the received gamma rays; fitting a curve to a portion of the
spectrum; determining a location of the maximum of the first derivative of
the curve; and adjusting a gain of the gamma ray detector based on the
location of the maximum of the first derivative of the curve.
Example 2 may include or use, or may optionally be combined with
20 the subject matter of Example 1 can further include wherein the curve is
defined by a cubic polynomial.
In Example 3, the subject matter of Examples 1-2 can further include
generating a centroid for the spectrum; and fitting the curve to a portion of
the spectrum if the centroid is within a threshold distance of a nominal value,
25 and adjusting the gain to generate a revised gain value for the gamma ray
detector based on the centroid otherwise.
In Example 4, the subject matter of Example 3 can further include
wherein the centroid is generated based on a first subset of the spectrum.
-23-
In Example 5, the subject matter of Example 4 can further include
wherein the first subset excludes low-energy channels where noise is
predicted to be present.
In Example 6, the subject matter of Example 5 can further include
5 wherein, if a difference between the gain and the revised gain value ·exceeds
a threshold, the method further comprises: revising the gain based on a
second subset of the spectrum larger than first the subset.
In Example 7, the subject matter of Examples 1-6 can further include
monitoring the location of the maximum of the first derivative of the curve
10 based on gamma ray measurements received subsequent to adjusting the
gain; and triggering a gain readjustment process if the location shifts by more
than a threshold amount.
In Example 8, the subject matter of Example 7 can further include
performing a drilling operation based on gamma ray measurements captured
15 subsequent to adjusting the gain.
Example 9 is an apparatus, which can include means for performing
any of Examples 1-8, comprising: a gamma ray detector to detect gamma rays
reflected from materials in a wellbore; and a processor to receive gamma ray
measurements from the gamma ray detector; generate a spectrum of the
20 gamma ray measurements, the spectrum including a plurality of channels
corresponding to energy values of the received gamma rays and count rates
for the plurality of channels; fit a curve to a portion of the spectrum;
determine a location of the maximum ofthe first derivative of the curve; and
adjust a gain of the gamma ray detector based on the location of the
25 maximum of the first derivative of the curve.
In Example 10, the subject matter of Example 9 can further include
wherein the processor is further configured to generate a centroid based on a
first subset of the spectrum that excludes low-energy channels of the
spectrum; and fit the curve to a portion of the spectrum if the centroid is
-24-
within a threshold distance of a nominal value, and adjust the gain of the
gamma ray detector based on the centroid otherwise.
In Example 11, the subject matter of Examples 9-10 can further
include an amplifier and a voltage supply unit, and wherein the processor is
5 configured to adjust the gain by adjusting an input to at least one of the
amplifier and the voltage supply unit.
In Example 12, the subject matter of Examples 9-11 can further
include wherein, if a difference between the gain and a revised gain value
exceeds a threshold, the processor is further configured to: revise the gain
10 based on a second subset of the spectrum larger than the first subset.
In Example 13, the subject matter of Examples 10-12 can further
include a memory to store the spectrum and data representative of the
curve, the gain, and the centroid.
In Example 14, the subject matter of Examples 9-13 can further
15 include wherein the curve is a third order polynomial.
In Example 15, the subject matter of Examples 9-14 can further
include wherein the processor is further configured to: monitor the location
of the maximum of the first derivative of the curve based on gamma ray
measurements received subsequent to adjusting the gain; and trigger a gain
20 readjustment process ifthe location shifts by more than a threshold amount.
Example 16 is a system, which can include means for performing any
of Examples 1-8, comprising: a logging tool, including a housing to house a
gamma ray measurement tool, the gamma ray measurement tool including a
gamma ray detector for detecting gamma radiation at a plurality of energy
25 levels and for generating detector output signals each representing a
detected count of gamma radiation; and a processor to receive gamma ray
measurements from the gamma ray detector; generate a spectrum of the
gamma ray measurements, the spectrum including a plurality of channels
corresponding to energy values of the received gamma rays and count rates
30 for the plurality of channels, fit a curve to a portion of the spectrum,
-25-
determine a location of the maximum of the first derivative of the curve; and
adjust a gain of the gamma ray detector based on the location of the
maximum of the first derivative of the curve.
In Example 17, the subject matter of Example 16 can further include
5 communication circuitry to communicate signals from the gamma ray
measurement tool; and a surface system to receive the signals from the
gamma ray measurement tool over the communication circuitry.
In Example 18, the subject matter of Examples 16-17 can further
include a display to display diagnostic information for the gamma ray
10 measurement tool, based on the signals.
In Example 19, the subject matter of Examples 16-18 can further
include wherein the processor is configured to fit a third order polynomial
curve to the portion ofthe spectrum.
The accompanying drawings that form a part hereof, show by way of
15 illustration, and not of limitation, specific embodiments in which the subject
matter may be practiced. The embodiments illustrated are described in
sufficient detail to enable those skilled in the art to practice the teachings
disclosed herein. Other embodiments may be utilized and derived
therefrom, such that structural and logical substitutions and changes may be
20 made without departing from the scope of this disclosure. This Detailed
Description, therefore, is not to be taken in a limiting sense, and the scope of
various embodiments is defined only by the appended claims, along with the
full range of equivalents to which such claims are entitled.
Such embodiments of the inventive subject matter may be referred to
25 herein, individually and/or collectively, by the term "invention" merely for
convenience and without intending to voluntarily limit the scope of this
application to any single invention or inventive concept if more than one is in
fact disclosed. Thus, although specific embodiments have been illustrated
and described herein, it should be appreciated that any arrangement
30 calculated to achieve the same purpose may be substituted for the specific
- 26-
embodiments shown. This disclosure is intended to cover any and all
adaptations or variations of various embodiments. Combinations of the
above embodiments, and other embodiments not specifically described
herein, will be apparent to those of skill in the art upon reviewing the above
5 description.
Although specific embodiments have been illustrated and described
herein, it will be appreciated by those of ordinary skill in the art that any
arrangement that is calculated to achieve the same purpose may be
substituted for the specific embodiments shown. Various embodiments use
10 permutations or combinations of embodiments described herein. It is to be
understood that the above description is intended to be illustrative, and not
restrictive, and that the phraseology or terminology employed herein is for
the purpose of description. Combinations of the above embodiments and
other embodiments will be apparent to those of ordinary skill in the art upon
15 studying the above description.

We Claim:
1. A method for adjusting gain of a gamma ray detector, the method
comprising:
receiving gamma ray measurements from the gamma ray detector;
5 generating a spectrum based on the gamma ray measurements, the spectrum
including a plurality of channels and count rates for the plurality of
channels, wherein a channel number of a channel corresponds to
energy values of the received gamma rays;
fitting a curve to a portion of the spectrum;
10 determining a location of the maximum of the first derivative of the curve;
and
adjusting a gain of the gamma ray detector based on the location of the
maximum ofthe first derivative ofthe curve.
15 2. The method of claim 1, wherein the curve is defined by a cubic
polynomial.
3. The method of claim 1, further comprising:
generating a centroid for the spectrum; and
20 fitting the curve to a portion of the spectrum if the centroid is within a
threshold distance of a nominal value, and adjusting the gain to
generate a revised gain value for the gamma ray detector based on
the centroid otherwise.
25 4. The method of claim 3, wherein the centroid is generated based on a
first subset of the spectrum.
5. The method of claim 4, wherein the first subset excludes low-energy
channels where noise is predicted to be present.
30
- 28-
5
6. The method of claim 5, wherein, if a difference between the gain and
the revised gain value exceeds a threshold, the method further comprises:
revising the gain based on a second subset of the spectrum larger
than first the subset.
7. The method of claim 1, further comprising:
monitoring the location of the maximum of the first derivative of the curve
based on gamma ray measurements received subsequent to adjusting
the gain; and
10 triggering a gain readjustment process if the location shifts by more than a
threshold amount.
8. The method of claim 7, further comprising:
performing a drilling operation based on gamma ray measurements captured
15 subsequent to adjusting the gain.
9. An apparatus comprising:
a gamma ray detector to detect gamma rays reflected from materials in a
well bore; and
20 a processor to
25
30
receive gamma ray measurements from the gamma ray detector;
generate a spectrum ofthe gamma ray measurements, the spectrum
including a plurality of channels corresponding to energy
values of the received gamma rays and count rates for the
plurality of channels;
fit a curve to a portion of the spectrum;
determine a location of the maximum of the first derivative of the
curve; and
adjust a gain of the gamma ray detector based on the location of the
maximum ofthe first derivative ofthe curve.
-29-
10. The apparatus of claim 9, wherein the processor is further configured
to:
generate a centroid based on a first subset of the spectrum that excludes
5 low-energy channels of the spectrum; and
fit the curve to a portion of the spectrum if the centroid is within a threshold
distance of a nominal value, and adjust the gain of the gamma ray
detector based on the centroid otherwise.
10 11. The apparatus of claim 10, further comprising an amplifier and a
voltage supply unit, and wherein the processor is configured to adjust the
gain by adjusting an input to at least one of the amplifier and the voltage
supply unit.
15 12. The apparatus of claim 11, wherein, if a difference between the gain
20
and a revised gain value exceeds a threshold, the processor is further
configured to:
revise the gain based on a second subset of the spectrum larger than
the first subset.
13. The apparatus of claim 10, further comprising:
a memory to store the spectrum and data representative of the curve,
the gain, and the centroid.
25 14. The apparatus of claim 13, wherein the curve is a third order
polynomial.
15. The apparatus of claim 9, wherein the processor is further configured
to:
- 30-
5
10
15
20
25
30
monitor the location of the maximum of the first derivative of the curve
based on gamma ray measurements received subsequent to adjusting
the gain; and
trigger a gain readjustment process if the location shifts by more than a
threshold amount.
16. A system comprising:
a logging tool, including a housing to house a gamma ray measurement tool,
the gamma ray measurement tool including
a gamma ray detector for detecting gamma radiation at a plurality of
energy levels and for generating detector output signals each representing a
detected count of gamma radiation; and
a processor to
receive gamma ray measurements from the gamma ray
detector;
generate a spectrum of the gamma ray measurements, the
spectrum including a plurality of channels
corresponding to energy values of the received gamma
rays and count rates for the plurality of channels;
fit a curve to a portion ofthe spectrum;
determine a location of the maximum of the first derivative of
the curve; and
adjust a gain of the gamma ray detector based on the location
oft he maximum ofthe first derivative of the curve.
17. The system of claim 16, further comprising:
communication circuitry to communicate signals from the gamma ray
measurement tool; and
a surface system to receive the signals from the gamma ray measurement
tool over the communication circuitry.
- 31 -
5
10
15
18. The system of claim 16, wherein the surface system further
comprises:
a display to display diagnostic information for the gamma ray measurement
tool, based on the signals.
19. The system of claim 16, wherein the processor is configured to fit a
third order polynomial curve to the portion of the spectrum.