Note: Descriptions are shown in the official language in which they were submitted.
~0~
Background of Invention
This invention relates to radiological well logging
methods and apparatus for investigating the lithological
characteristics of subsurface earth formations traversed
by a borehole and, more particularly, relates to improved
neutron-gamma ray logging methods and apparatus.
It is well known that oil and gasare more likely to
be found in commercially recoverable quantities in those
earth formations which are relatively porous and permeable
than in the more highly consolidated formations. It is
also well known that an oil or gas-filled strata may be
located by passing a neutron source through the borehole
and mea8urlng the intensity of secondary gamma radiations
which are produced at various depths in the borehole. A
chlorine nucleus has a thermal neutron capture cross
section which is much higher than that of nuclei of most
of the other elements which are found in greatest abundance
in the earth, and thus a salt water filled limestone or
sandstone layer will have a greater macroscopic thermal
neutron capture cross section than will an oil saturated
layer. Accordingly, this difference can be observed by
measuring either chlorine capture gamma rays or the life-
time of the thermal neutron population in the layer.
Although thege logging technique6 have long been used,
and although a great many oil or gas-bearing formations
have been found in this manner, they have al8o produced a
great many spurious indications. This is because many
porous earth formations contain low salinity water, which
is indistinguishable from oil using these method8.
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104~
Thus, the intensity of the capture gamma radiation which
is detected at various borehole depths is an indication
of fluid salinity and porosity, and is not necessarily
conclusive evidence that an oil-bearing formation has been
discovered. An inelastic gamma ray spectrum, however, is
independent of salinity since chlorine has a small
inelastic cross section.
The carbon nuclei in the oil will, to a limited extent,
also engage in capture interactions with bombarding neutrons,
although the thermal neutron capture cross section of carbon
is extremely low, and this is also true for oxygen nuclei.
However, the inelastic scattering reaction cross section is
appreciable for both car~on and oxygen if the collision
energy of the neutron is sufficiently high. Furthermore,
the initial energies of the gamma rays resulting from carbon
are distinctively different from that of gamma rays resulting
from oxygen when this reaction occurs. Accordingly~ it has
long been assumed that a measurement of inelastic scattering
gammas could provide the basis for a technique for detecting
and identifying an oil or gas-bearing earth formation as
opposed to a water-bearing formation as such.
Many attempts have been made to employ this concept in
well logging. Thus far, however, none of the methods and
apparatus which utilize this concept have been reliable.
One of the principal reasons for this lack of success
is that carbon is one of the most common elements in the
earth's crust. Moreover, a limestone formation is largely
composed of calcium car~onate, and thus a water-bearing
limestone formation will frequently emit more carbon gammas
3 than will an oil-filled sand or shale.
~04~:~1'7
Another problem is that a gamma ray tend~ to readily
engage in scattering reactions itself and further tends
to lose energy to some extent with each scattering. The
initial energy of a gamma which results from the inelastic
scattering of a neutron by an oxygen nucleus is only a
little higher than the initial energy of a gamma resulting
from inelastic scattering of a neutron by a carbon nucleus.
Thus, many of the gammas which emanate from fast neutron
bombardment of a water-bearing formation will frequently
have declined in energy by the time they are actually
detected, whereupon they may be mistaken for non-degraded
gamma rays which have emanated from carbon nuclei.
Not all oxygen-emitted gamma rays will be degraded
before they are detected, however, and thus it has been
proposed to log a well with the detector signal being
applied to a "two-window" analyzer. More particularly, one
window is set to accept only pulses attributable to detected
gammas having terminal energies which approximate the
initial energies of oxygen-emitted gamma rays, and the other
window is set to accept only pulses comparable to gammas
with terminal energies corresponding to carbon-emitted
gammas. Thus, the counting rates of the two windows may be
compared to provide an indication of whether a particular
formation contains oil or water.
Although such a log has been performed with some succe8s,
it nevertheless is often unreliable and has therefore never
been universally accepted by the petroleum industry. A
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principal reason for its lack of reliability is, again, the
fact that gamma rays tend to quickly lose energy, and this
is especially true when the gamma radiation encounters a
relatively dense medium. Thus, a high count rate for the
lower of the two windows may actually be due to the fact that
most of the oxygen-emitted gamma rays have become degraded
before detection, by reason that the gammas were required to
pass through the formation, the liquid-filled borehole, the
sonde case, and possibly a cemented casing before they could reach
the detector in the logging tool. Furthermore, most oil-bearing
formations also contain at least an appreciable amount of water,
and the oil/water interface is extremely difficult to detect
merely by a qualitative measurement of the number of carbon-
emitted gammas which manage to reach the detector.
Many attempts have been made to improve the foregoing
technique. For example, the degrading effect of the drilling
fluids in the borehole has been reduced by decentralizing
the logging instrument in the borehole. Also, the size of
the phosphor used in the detector has been increased
substantially in order to sense a greater proportion of the
gamma rays sought to be detected, whereby the measurement
has been improved from a statistical standpoint. Although
most if not all of these changes have been of some benefit,
no change has been found which would make any log fully
acceptable to the petroleum industry which is based princi-
pally on this concept.
. .
These and other disadvantages of the prior art are
overcome with the present invention, however, and novel well
logging methods and apparatus are provided for simultaneously
and correlatively measuring the gamma radiation resulting
from the inelastic scattering of neutrons by carbon and
oxygen nuclei as a function of the lithological characteristics
of a subsurface earth formation.
Summary of Invention
O In one embodiment of the present invention, a well
logging tool is employed which contains a conventional source
of fast neutrons and at least one gamma ray detector. Any
neutron source capable of producing inelastic interactions
with carbon and oxygen may be used for this purpose, such as
an encapsulated mixture of radium and beryllium, plutonium and
beryllium, or the like. All alpha-emitters such as radium or
plutonium will also emit gamma rays in large numbers, however,
and thus an especially suitable neutron source is the well-
kn~wn deuterium/tritium accelerator or the like since such a
) source can be operated to produce timed bursts of neutrons and
other capabilities as will hereinafter be referred to in detail.
The detector which is most appropriate to the purposes
of the present invention is one that provides an indication
of the terminal energy of each detected gamma ray, whereby
those gammas which originate from carbon or oxygen nuclei
may be identified and distinguished from gamma radiation
originating from other sources such as silicon nuclei
~(~4Z~17
or from the neutron source itself. Accordingly, an
especially suitable detector is a conventional scintillation
counter having a large thallium-activated crystal which is
composed of sodium iodide or cesium iodide or the like.
The neutron source and detector are preferably spaced
apart from each other within a fluid-tight instrument
housing which is preferably urged continually against one
side of the casing or borehole wall, whereby neutrons from
the source may bombard the earth formation without having to
first pass through the drilling fluid and other liquids which
usually accumulate in the borehole or casing, and whereby
the gamma rays emanating from the neutron-irradiated
formation may travel to the detector without first having
traveled through such liquid.
A conven~ional scintillation counter operates to provide
an electrical pulse upon the occurrence of each gamma ray
detected by the crystal and to provide each such pulse with
an amplitude which is a function of the terminal energy of
the gamma ray to which it corresponds. Thus, other
conventional electrical circuitry may be included within
the instrument housing for eliminating noise pulses and
--~ other spurious signals from the train of pulses generated by
the detector and for suitably amplifying the genuine pulses,
whereby they can be transmitted to the surface of the earth
by way of the logging cable.
In addition to the various signal processing and
recording equipment which is conventionally employed at the
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.
surface, a multichannel pulse height analyzer having four
or more windows may be included for the purpose of
separating the incoming detector pulses according to four
preselected energy ranges. More particularly, two of the
four windows will preferably be set to pass only those pulses
which correspond to carbon and oxygen gammas, respectively,
and the other two windows will preferably be set to pass
those background pulses which have heretofore been accepted
as carbon and oxygen gammas but which are actually primarily
degraded gammas, capture gammas, gammas originating in the
instrument housing, or gammas from other elements in the
formation.
Accordingly, window No. 1 i preferably set to pass
only pulses having amplitudes in the range of 3.0 Mev. to
4.7 Mev., window No. 2 to pass only pulses in the range of
4.7 Mev. to 5.0 Mev., window No. 3 to pass only pulses in the
range of 5.0 Mev . to 6.5 Mev., and window No. 4 to pass only
pulses in the range of 6.5 Mev. to 7.5 Mev. amplitude. The
counting rate of the pulses from each of these windows may be
~ determined by conventional circuitry and may be indicated by
signals C, CB, O, and OB, respectively, whereby signals
(C-CB) and (O-OB) will represent the carbon gamma and oxygen
gamma counting rate, respectively, and whereby signals Cg and
OB will represent the two background counting rates, respectively.
As hereinbefore stated, the carbon and oxygen signals
heretofore derived for the purpose of determining the ratio
of carbon to oxygen are actually composites or summations
~ 7
of signals due to the carbon and its hackground, and of
signals due to the oxygen and its background, respectively.
- Accordingly, it will now be readily apparent that signals
(C-CB) and (0-OB~ are more precise indications of the carbon
and oxygen content of the formations being irradiated~
Further, the background signals CB and OB can be used to
provide a better signal-to-background ratio.
As will hereinafter be apparent, the slope of the
entire gamma ray spectrum is dependent upon the composition
as well as the po~osity of the formation. Thus, a better
indication of the carbon-to-oxygen ratio will be obtained
by the ratio C : 0 , where C is the difference between C
and CB, and where 0 is the difference between 0 and OB,
since C and 0 are obviously iess dependent on the intensity
of the background gamma rays which are sought to be eliminated
from the carbon and oxygen measurements. Even if C' and 0
are treated merely as qualitative indications or representa-
tions of the carbon and oxygen in the formations of interest,
fractional changes in the hydrocarbon content of these forma-
tions will produce greater and more easily observed variations
in C and 0 , and the ratio C : 0 will be less dependent
on the porosity and type of formation.
As hereinbefore explained, a water-bearing
limestone c o n t a~ins a substantial amount of carbon, due
to the calcium carbonate character or composition of the
formation matrix. Nevertheless, the output signals from the
aforementioned four windows in the pulse height analyzer
provides a clear basis for distinguishing between increases in
the carbon : oxygen ratio which are due to the presence of
oil or gas and those increaseswhich occur when the logging
instrument passes from a sandstone matrix to a limestone matrix.
10~
Since the nuclear interactions which occur in a
sandstone matrix are necessarily different from those which
occur in a limestone matrix, these differences are usually
distinctive in the slopes of their secondary gamma ray
spectra because of different capture and inelastic
scattering cross sections of calcium and silicon. Thus,
the ratio of OB : CB will usually provide a sufficient
indication of whether the formation matrix is limestone or
sandstone.
The inelastic scattering reactions in the matrices
occur substantially only as long as the neutrons are "fast";
however, whereas most of the capture reactions occur when
the neutrons have slowed to thermal energy. If the neutron
source is an accelerator, and if the logging tool is provided
with suitable detector gating circuitry, the accelerator may
be pulsed so as to generate fast neutrons in discrete bursts.
Thus, the output signal from the detector may be correlatively
gated to select only the pulses which occur during each
neutron burst from the accelerator, and during which a
maximum number of inelastic scattering reactions will occur.
A second time-dependent portion may also be selected, after
termination of each such neutron burst from the accelerator,
and preferably also after all or most of the thermal neutrons
in the borehole have been captured, to provide a second
signal which will be heavily dependent on gammà radiation
resulting from neutron capture in the irradiated formation.
rne use of the aforementioned four windows and only one gate will
generally provide a simultaneous measurement of both the type of formation
matrix and the C' : 0' ratio for the formation. If additional information is
needed, however, capture gamma rays in the second time dependent interval
might also be utilized for lithology and porosity determination.
It will be recognized that changes in the porosity of the adjacent
earth materials will produce changes in the C' : 0' ratio measurement, even
in those instances when the actual carbon and oxygen concentrations have not
changed, and thus it is preferable to also derive a supplemental indication
of porosity which may be used to better inte~pret the C' : 0' ratio signal.
Also, porosity measurements are desirable in determining water saturation. A
measurement which is indicative of porosity is the count rate of either or
both of the two background signals, and preferably (because of statistics) a
summation of the signals from all four windows of the analyzer.
In accordance with the present invention there is provided a method
of investigating subsurface earth formations traversed by a borehole compris-
ing the steps of repeti~ively irradiating earth formations in the vicinity of
a borehole with pulses of fast neutrons which engage in inelastic scattering
interactions with formation nuclei and which are thereafter slowed to thermal
energy and engage in further neutron capture interactions with formation
nuclei; detecting, during a first discrete time interval associated with said
fast neutron pulses, energy dependent distributions of gamma radiations att-
ributable to the inel stic scattering of fast neutrons by the elements carbon
and oxygen in the formations and generating signals representative of carbon
inelastic gamma rays and oxygen inelastic gamma rays; detecting, during a
second, later discrete time interval, energy dependent distribution of gamma
radiations attributable to the capture of thermal neutrons by the elements
calcium and silicon in the formations and generating signals representative
of calcium capture gamma rays and silicon capture gamma rays; generating a
first ratio signal comprising a ratio of said carbon and oxygen inelastic
gamma ray representative signals; generating a second ratio signal comprising
a ratio of said calcium and silicon capture gamma ray representative signals;
C
~ - lQ -
104;~11~
and recording said first and second ratio signals as a function of borehole
depth.
These and other advantages and ~eatures of the present invention
will become apparent from the following detailed description wherein reference
will be made to the figures in the accompanying drawings.
Figure 1 is a simplified and substantially functional representation
of one form of apparatus which is especially
- lQa -
1 0~
suitahle for the purposes of the present invention.
Figure 2 is a graphic representation of a suitable
operating sequence for the apparatus depicted in Figure 1,
and more particularly illustrating how the appropriately
time-dependent detection of gamma radiation in the borehole
will provide a more accurate and dependable measurement of
the carbon:oxygen ratio for an irradiated earth formation.
Figure 3 is a graphic representation of the rate of
occurrence of radiations detected at various energies
throughout the composite inelastic spectrum of such radiations
and providing an illustrative representation of the variations
in intensity which may appear at measurable energy levels
for radiation resulting from certain preselected nuclear
reactions of interest in the present invention.
Figure 4 is a graphic representation of the composite inelastic
radiation spectrum illustrated in Figure 3, and showing
in particular the relative proportions of unwanted
background raaiation which is necessarily obtained when an
energy-dependent measurement is made of variations in
intensity of gamma raliations emanating from the irradiated
formation depicted in Figure 1.
Figure 5 is a graphic representation of the irradiation
and detection intervals obtained by use of an operating
sequence other than that illustrated in Figure 2, and more
particularly depicting how a subsequent time-dependent
radiation measurement can be obtained which is`functionally
related to the porosity of the formation matrix.
.. .. . _ _ . _ _
i(J~ll'~
Detailed Description
Referring now to Figure 1, there may be seen a simplified
functional and partly pictorial representation of the basic
features of a well-logging system which is illustrative of
features of the present invention. More particularly, the
system may be seen to be composed of a subsurface probe or
sonde 2 which is suspended at one end of a conventional
logging cable 18 and which provides data in the form of
electrical signals tc surface instrumentation which is
connected to the other or upper end of the cable 18.
Referring to the system in greater detail, the sonde 2
is illustrated as being composed of a fluid-tight elongated
steel housing 2, which is adapted to be passed longitudinally
through a borehole 4 in the earth 3, and which contains a
neutron source 6 and a radiation detector which, for present
purposes, is preferably a scintillation counter 10. As
previously explained, the function of the neutron source 6 is
to bombard adjacent sections of the earth 3 and borehole 4
with high energy neutrons as the sonde 2 is lifted through
the borehole 4 by the cable 18, and the function of the
scintillation counter 10 is to detect a representative
number of the gamma rays emanating from the earth 3 as a
result of such neutron bombardment. Accordingly, a radiation
shield 9 of suitable composition is preferably interposed
between the scintillation counter 10 and the n`eutron source
6 to prevent direct irradiation of the scintillation counter
10 by the source 6.
104'~
As also previously explained, the neutron source 6 may
be any suitable means for generating neutrons of sufficient
energy to produce inelastic scattering reactions in the
adjacent earth 3. Accordingly, the neutron source 6 may be
an encapsulated quantity of material such as radium or
plutonium mixed with a light metal such as beryllium or
lithium. A neutron source 6 also produces a substantial
amount of unwanted gamma radiation which is detected by the
scintillation counter 10, however, and is also less desirable
for present purposes since neutron emission from this type
of source 6 cannot conveniently be interrupted. Furthermore,
neutrons produced by a capsule-type source 6 tend to be
emitted at various energies which is also detrimental to the
meaningfulness of the measurements sought to be obtained.
Accordingly, for present purposes the neutron source 6 which
is preferred is a st~tic ion accelerator 7 of the type which
employs the well known deuterium-tritium reaction to generate
a substantially gamma-free supply of 14.4 Mev. neutrons. In
addition, an actuating circuit 8 of conventional design is
preferably included whereby the accelerator 7 may be
~electively activated and inactivated on command.
Referring agair to the structure generally illustrated
in Figure 1, the scintillation counter 10 may be of
conventional design and thus may include a phosphor or
crystal 11 which is preferably optically couplèd to the appro-
priate end of an end-window photomultiplier tube 12, and
which is also preferably composed of a suitable inorganic
~o~
material such as thal~ium-activated sodium or cesium iodide
or the like. As is well known, each gamma ray stopped by
the crystal 11 tends to create a momentary light flash or
scintillation within the crystal 11 of an intensity
substantially proportional to the terminal energy of the
stopped gamma ray. The photomultiplier tube 12 is adapted
to produce a voltage pulse for each such scintillation
which is of an amplitude substantially proportional to the
intensity of the scintillation, and thus the output signal
which is generated by the photomultiplier tube 12 is a
train of pulses tending to indicate the rate of occurrence
and the terminal energies of the gamma radiation existing
within the adjacent section of the borehole 4.
Not all of the gamma rays which occur in the borehole
4 a ri s e because of fast neutron interactions within the
adjacent earth 3, of course, and thus the crystal 11
te ~ to produce a certain proportion of unwanted scintilla-
tions which are due tc the occurrence of thorium and other
naturally radioactive substances which tend to accumulate
in shales and other like strata. In addition, some of the
voltage pulses which are generated in the photomultiplier
tube 12 are merely due to circuit noise. Most of these
spurious or unwanted signals are relatively low voltage,
however, and thus the output signal from the photomultiplier
tube 12 may be conveniently "cleaned up" by coupling it to
a pulse height discriminator 13 having its trigger level
set to pass only pulses of a preselected minimum voltage.
This discriminator might also assist in alleviàting any
pulse pile-up problems in the transmission of signals over
the logging cable 18.
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104'~
The sonde 2 may be required to be positioned deep in
the earth 3, and thus it may be necessary to employ a
logging cable 18 which is several thousand feet long to deliver
the output pulses from the discriminator 13 to the surface of
the earth 3. Accordingly, a conventional voltage amplifier
14 is preferably included within the sonde 2, which has
its input coupled to receive pulses from the discriminator
13, and which may have its output coupled to a suitable
conductor 17 in the cable 18. Alternatively, a conventional
cable driving circuit 15 may be coupled between the amplifier
14 and the cable 18 as indicated in Figure 1.
As previously indicated, it is a principal feature of
the present invention to detect and measure those gamma rays
which arise from inelastic neutron scattering reactions with
carbon and oxygen nuclei in the earth materials surrounding
the sonde 2. The fast neutron which is inelastically
scattered al.æo t e n d s to be slowed to thermal energy and
thereafter captured, however, and this -also generates a
gamma ray which may be detected. If the neutron source 6
depicted in Figure 1 is an encapsulated neutron-emitter, this
c re at e B a relati~ely steady outflow of fast neutrons which
are constantly and continuously being elastically and
inelastically scattered in the materials surrounding crystal
11, and which are also being continuously slowed and captured
by such materials. Consequently, the radiation surrounding
the crystal 11 i8 composed o~ gamma rayæ being continuouæly
produced by capture reactions as well as inelastic scattering
104;~11 7
reactions, and since capture-produced gamma rays have initial
energies within the same general range as those resulting
from inelastic scattering, this is a principal reason why
it has heretofore been substantially impossible to use a
continuously-emitting neutron source to make a meaningful
measurement of the carbon and oxygen adjacent a borehole 4.
On the other hand, if the neutron source 6 is an
accelerator 7, as indicated in Figure 1, and if the
actuating circuit 8 is arranged and adapted to cause the
accelerator 7 to sequentially and intermittently generate the
fast neutrons in discrete bursts, a time-dependent gamma ray
measurement may be made which emphasizes the gamma rays
arising from fast neutron processes and which de-emphasizes
the gamma radiation produced by slow or thermal neutron capture.
Each fast neutron burst produced by the accelerator 7
will, if properly timed, tend to create a corresponding discrete
fast neutron population in the materials surrounding the crystal
11, and each discrete fast neutron population thus tends to
decline away at a rate which is dependent upon the macroscopic
inelastic and elastic scattering cross section of such
materials. Bach disappearing fast neutron population is
replaced by a discrete thermal neutron population, however,
which thereafter declines at a rate which is dependent upon
the macroscopic capture cross section of the surrounding
material. Accordingly, if the accelerator 7 is actuated to
generate each fast neutron burst at a rate which i8 such that
iU4;~11 ~
each resulting thermal neutron population substantially
disappears before the next succeeding fast neutron burst
is produced, it will be apparent that there will be a
measurable time interval during which a much heavier proportion
of the gamma radiation at the detector 4 will be attributable
to fast neutron reactions in the materials surrounding the
crystal 11 ~nd housing 5.
'rhe actuating circuit 8 depicted in the sonde 2 may be
arranged and adapted to independently energize the accelerator
7 according to a predetermined operating sequence. As
indicated in Figure 1, however, the surface equipment may
conveniently include a pulse generator 24 of suitable design
which is arranged to be triggered or activated by a variable
timing circuit 23, and which provides a suitable trigger
pulse 21 to an appropriate conductor 16 in the cable 18.
As will be apparent from the foregoing, the measurement
sought to be obtained is preferably time dependent as well
as energy d~pendent, and thus it is only the gamma rays
which occur during a particular portion of the system
operating cycle which are of principal importance in the
practice of the presert invention. A convenient technique
for making a time dependent radiation measurement is to
activate and inactivate the scintillation counter 10 in cyclic
relationship to the operation of the accelerator 7, and
circuitry is available in the prior art for suitably
performing this function, either independently according to a
predetermined sequence, or in synchronism with the opera~tion
117
of the accelerator 7. ~n especially convenient technique
for making a time aependent radiation measurement, however,
is to synchronously gate the output signal from the amplifier
14 substantially concurrently with the operation of the
actuating circuit 8, for the reason that most of the fast
neutron processes will occur while the accelerator 7 is
actuated to produce neutrons.
As indicated in Figure 1, however, the surface equip-
ment may conveniently include a gating circuit 25 having its
input 20 coupled to the cable 18 to receive the pulse train
arriving uphole from the sonde 2, and having it~ output 20A
coupled to the input of a suitable multichannel pulse height
analyzer 26 of conventional design. ~he gate 25 is, of course,
arranged and adapted to be normally closed and to open only
in conjunction with the production of neutron bursts by the
accelerator 7. Accordingly, the timing signal 22 which is
generated by the timirg circuit 23, and which is applied
to the input 22B of the pulse generator 24, is also simul-
taneously applied to the input 22A of the gate 25.
As herelnafter explained in detail~ the pul~e
height analyzer 26 is preferably adjusted to establish four
different energy ranges or "windows," and thus pulse
received from the output 20A of the gate 25 will be sorted
into four different outputs 26A-D. As will also be explained
in detail, output 26A will be composed of pulses with
-~ amplitudes commensurate with gamma rays produced by carbon
nuclei, and output 26B will include pulses with amplitudes
: -
-18-
104;~
corresponding to carbon background signals which are only a
little higher than the upper level of the so-called "carbon
window." Similarly, output 26C will include pulses
corresponding to gamma rays generated by inelastic
scattering reactions with oxygen nuclei, and output 26D will
include pulses with amplitudes only a little higher than
the upper level of the so-called "oxygen window." Accordingly,
signal 26B will derive from the "carbon background window" in
the analyzer 26, and signal 26D will derive from its "oxygen
background window."
As may further be seen in Figure 1, outputs 26A-D are
each connected to a different one of four separate count
rate meters 32-35. It is a feature of the present invention
to provide a measurement of the sum of these four count rates,
and thus the four outputs 26A-D are also preferably coupled
to a fifth count rate meter 31. It is also a function of the
present invention to derive a corrected carbon : oxygen ratio,
and thus a difference circuit 28 is preferably included to
provide an electrical voltage 28A which is functionally
related to the difference between the carbon count rate 32A
and the carbon background count rate 33A. Similarly,
another difference circuit 29 may be included to provide a
voltage 29A which is functionally related to the difference
between the oxygen count rate voltage 34A and the oxygen
background count rate voltage 35A.
A conventional ratio circuit 30 may be included to
derive a ratio signal 30A from the two difference voltages
-19--
~04i~
28A and 29A, and the ratio signal 30A may be conveniently
applied to one of the inputs of a suitable chart recorder
27 or the like. The summation signal 31A from the count
rate meter 31 may also be applied simultaneously to the
recorder 27.
It is desirable that both signals 31A and 30A be
recorded in correlation with an indication of the depth of
the sonde 2 in the borehole 4, as well as in correlation
with each other. This may be accomplished in any of several
well known ways, however, such as by connecting the driving
mechanism of the chart recorder 27 to a sheave wheel l9
which is rotated by movement of the cable 18.
As hereinbefore stated, it is a feature of the present
invention that a more representative measurement of the
carbon and oxygen content of the earth 3 be provided by making
a time-dependent measurement of the radiation emanating from
- the earth 3 as a result of the neutrons generated by the
accelerator 7. The fast neutron population which is produced
by each actuation of the accelerator 7 will tend to disappear
very quickly, since a fast neutron tends to slow to thermal
energy within a very short time interval. Thus, it is
preferable that the actuator 8 be adapted to "turn on" the
accelerator 7 for an interval which is sufficient to extend
through the average lifetime of the fast neutron population
but which is preferably terminated before the resulting slow
or thermal neutron population grows to appreciable size.
-20-
104'~
Referring now to Figure 2, there may be seen a simplified
functional illustration of the operating cycle 59 of the
system depicted in Figure 1 and, more particularly, an
illustration of a preferred operatiny sequence for both the
neutron source 6 and the gate 25. Specifically, the
accelerator 7 ls preferably actuated to produce a series or
discrete fast neutron bursts 55 which each tends to create
a discrete fast neutron cloud or population 56.
For example, the accelerator 7 may be turned on for an
irradiation interval cf about 5-10 microseconds to produce
the fast neutron population 56 which permeates the section
of borehole 4 and earth 3 immediately surrounding the section
of the sonde 2 which contains the accelerator 7 and crystal
11. As indicated in Figure 2, the rise time of the fast
neutron population 56 may be almost instantaneous if the
accelerator 7 is operating effectively.
The irradiation or activation interval 55 should be
long enough to produce an adequate number of fast neutrons.
However, the interval 55 should otherwise be kept as short
as is reasonably possible since the average slowing time of a
fast neutron in the earth 3 is very short and is even shorter
for those fast neutrons which are in the fluids in the
borehole 4. This is illustrated in Figure 2 by the fact
that the rise rate of the thermal neutron population 57 is
only a little slower than the rise rate or timè of the fast
neutron population 560
-21-
1(;)4;~
It should be understood that, as indicated in Figure
2, the rise rates or times for both the fast and slow
(thermal) neutron populations 56 and 57 will be the same
whether the accelerator or irradiation interval 55 is long
or short in duration. This is because the fast neutron
population 56 tends to reach an equilibrium once the rate
of decline of the fast neutrons tends to equal the output
rate of the accelerator 7. As further indicated in Figure
2, however, the irradiation interval 55 of the system
operating sequence 59 may be terminated before the thermal
neutron population 57 reaches its peak, whereupon the
majority of the neutrons which are present in the borehole
4 and earth 3 adjacent the sonde 2 during the irradiation
interval 55 will be fast neutrons.
As hereinbefore explained, the fast neutrons in the
fast neutron population 56 undergo fast neutron reactions
with the nuclei in the borehole 4 and earth 3, since this is
why these fast neutrons are slowed to thermal energy to create
the thermal neutron population 57. Thus, there will be a
corresponding population of gamma rays created in the borehole
4 and earth 3 as a result of these fast neutron processes,
and this gamma ray population (not depicted in Figure 2)
will be substantially coincident in rise and decline rate
with the rise and decline of the fast neutron population 56.
The slow or thermal neutron population 57`will also
decline away during a measurable interval due to capture
reactions in the borehole 4 and earth 3, albeit over a much
104;~
longer time interval than that required to accomplish the
disappearance of the fast neutron population 56. These
capture reactions are also evidenced by the production of
gamma rays, and thus there will be a capture gamma ray
population (not depicted) created which will be
substantially coincident in rise and decline with the rise
and decline of the thermal neutron population 57. Thus, it
will be apparent that there will be a measurable interval
during which the gamma ray population in and around the
sonde 2 will be substantially composed of gamma rays
arising from inelastic scattering of fast neutrons,
provided that, as indicated in Figure 2, each irradiation
interval 55 is scheduled to commence only after the preceding
thermal neutron population 57 (and capture gamma population)
has substantially disappeared.
It will be recognized that 14.4 Mev. neutrons are
capable of producing many threshold-type reactions other
than inelastic scattering reactions with carbon and oxygen
nuclei. In particular, many oxygen nuclei will be transformed
to the unstable nitrogen-16 isotope which tends to emit
gamma rays as it reverts to its stable form. As is well
known, nitrogen-16 has a half-life of about 8 seconds.
Accordingly, even if the accelerator 7 is actuated for only
about 5-10 microseconds out of each one-thousand microseconds
of system operating time, it will nevertheless`be apparent
that a substantial amount of gamma radiation will soon be
existent in the borehole 4 which is due neither to capture
-?3
_
or inelastic scattering of these 14.4 Mev. neutrons.
The number of neutron events will at all times be
dependent upon the distance from the neutron source 6 in the
sonde 2. It is also customary to log any borehole 4 in an
upward direction in order to compensate for cable stretch
in determining depth in an accurate manner. Accordingly,
the neutron source 6 is conventionally located below the
scintillation counter 10 or other detector, and in the
lower end of the sonde 2, whereby the scintillation counter
10 will tend to move from and not toward these ~ctl~ated
nuclei. This, in turn, tends to reduce the proportion of
the gamma ray population which is attributable to nuclei
which have been activateddurlng the irradiation interval 55.
Referring again to Figure 2, it will be seen that the
gate 25 depicted in Figure 1 may be opened and closed to
provide for a detection interval 58 which is substantially
coincident with the occurrence and duration of the irradiation
interval 55, whereby the gamma rays which bombard the crystal
11 during this period will have primarily resulted from
inelastic scattering of fast neutrons generated by the
accelerator 7. A detection interval 58 which is only 5-10
microseconds in duration will permit only a relatively small
number of gamma rays to be counted, however, and thus it is
desirable to employ as large a crystal 11 as is practical,
and also to establish a reasonably close spacing between the
neutron source 6 and the crystal 11, in order to maximize the
number of gamma rays which are detected in this manner.
-24-
10~
As further indicated in Figure 2, it may al80 be desirable
to even extend the detection interval 59 a little beyond
the termination of the irradiation interval 55, since some
fast neutrons will still be present in the earth 3 duxing
the 2 or 3 microseconds following termination of the
irradiation interval 55.
Referring now to Figure 3, there may be seen a
simplified representation of a typical composite spectrum
40 of the gamma ray population which permeates the borehole
4 and earth 3 as a result of each actuation of the
accelerator 7. As indicated, gamma ray counting rate tends
to increase as the trigger level of the analyzer 26 is
decreased, which is due not only to the fact that more
gamma rays are initiated with lower rather than higher
energies, but also because the gamma rays having higher
initial energies tend to be slowed by Compton scattering
reactions before disappearing.
It is well Xnown that a gamma ray which is produced
by an inelastic scattering reaction may have almost any
initial energy. Nevertheless, there are predictable
resonances in the counting rates for gamma rays emanating
from specific typ~s of nuclei, and if such nuclei are
present in an irradiated material in sufficient amounts,
these resonances will manifest themselves as "peaks" on
the spectrum 40 to indicate the presence of an excessive
or abnormal number of such nuclei.
-~5
104;~117
For example, the composite spectrum of gamma radiation
emanating from an irradiated mass or quantity of pure
carbon will show characteristic gamma ray and escape peaks
at 3.4 Mev., 3.9 Mev., and at 4.4 Mev. values. Similarly,
the gamma ray spectrum from an irradiated quantity of pure
oxygen will show characteristic gamma ray and escape peaks
at 5.1 Mev., 5.6 Mev., and 6.1 Mev. values.
Referring again to Figure 3, it will be seen that the
composite spectrum 40 of the gamma rays which are detected
during the detection interval 58 exhibits six characteristic
peaks 46-51 which, for purposes of illustration, may be
attribured to the two characteristic gammas at 4.4 and 6.1
Mev. Thus, the height of these peaks 46-51 may be taken
as a functional representation of the proportion of carbon
and oxygen which are present in the borehole 4 and earth 3
immediately surrounding the sonde 2.
Not all gamma rays which are detected with a terminal
energy of 3.4 Mev. will, of course, have issued from a
carbon nucleus which has engaged in an inelastic scattering
reaction with a fast neutron, and the total count rate at
the 3.4 Mev. energy level will obviously include radiations
which had higher initial energies. Nevertheless, if the
carbon peaks 46-48 are predominant in size, this can only be
interpreted as an indication that an abnormal amount of
carbon has been encountered by the sonde 2 at that particular
depth in the borehole 4.
~he shape of the composite spectrum 40 will be
-26_
_
1 0~ 7
affected to a ~ nsiderable degree, of course, by whether the
gamma radiation which is detected is predominantly composed of
gamma rays arising from inelastic scattering reactions, or
whether a substantial number of capture gamma rays are also
present. In other words, if the accelerator 7 is permitted to
operate steadily rather than intermittently, as hereinbefore
explained, the shape d the composite spectrum 40 may be
altered to the extent that the carbon peaks 46-48 may not be
discernible even though the sonde 2 is kept at the same
depth in the borehole 4.
It will be apparent, therefore, that it is an important
feature of the present invention that the accelerator 7 be
actuated to produce discrete bursts of neutrons as hereinbefore
explained. It is also important that the gate 25 be actuated
or opened selectively as explained, since the pulses in the
detector signal 20 which arrive within the period immediately
following each irradiation interval 55 will, as illustrated
in Figure 3, be almost entirely attributable to gamma rays
occurring for reasons other than inelastic scattering of fast
neutrons.
It will also be apparent that even if the system is
operated according to the sequence 59 illustrated in Figure 2,
and even if the composite spectrum 40 is composed substantially
of inelastic scattering gamma radiation, much if not most of
this radiation is unwanted insofar as the pùrposes of the
present invention are concerned. Accordingly, it is preferable
that the pulse height analyzer 26 be adjusted to establish a
104;~117
so-called "carbon window" which will encompass the portion of
the spectrum 40 which is attributable to gamma rays issuing
from carbon nuclei, and also a similar "oxygen window" to en-
compass the portion of the spectrum 40 attributable to oxygen
gamma rays. In the prior art techniques which have been sought
to be used for this purpose, the carbon window has been
established between the energy levels 41 and 43 indicated in
Figure 3, and the oxygen window has been established between
levels 43 and 45. More particularly, the lower energy level
41 of the carbon window has usually been established at about
3.0 Mev., and the upper level 43 at or about 5.0 Mev.
Similarly, the upper and lower levels 45 and 43 chosen for the
oxygen window have usually been established at or about 7.0
Mev. and 5.0 Mev., respectively.
In both the carbon and oxygen windows, and especially in
a relatively narr~w energy range at or near the upper portions
of each of these two windows, much of the detector signal 20
is attributable to unwanted radiation arising from thermal
neutron capture and from fast neutron processes such as
inelastic scattering by nuclei of the housing 5, to Compton
degraded carbon and oxygen gamma radiation from the surrounding
earth 3, and to inelastic gammas from other elements in the
earth 3. Although these components of the detection
signal 20 can be useful in the determiDation of
. ~
-~8-
1 0~ ~ 1 i 7
the character of the matrix of the adjacent earth 3, as will
hereinafter be explained, a measurement of capture and
inelastic gamma radiation originating in the borehole 4, in
the wall of the housing 5, or from earth elements other than
carbon and oxygen, has little or no relationship to whether
oil is present in the earth 3. Thus, such radiation
constitutes an unwanted factor in any measurement based on
the ratio of carbon and oxygen. Accordingly, if the outputs
from the carbon and oxygen windows in the analyzer 26 can be
adjusted so as to delete or at least neutralize these
unwanted signal factors, it will be apparent that a
determination of the ratio of carbon to oxygen which
incorporates such adjustment will be less dependent on the
porosity and composition of the matrix of the adjacent earth
3, and will accordingly be more a function of the oil content
of such matrix.
In addition, it will be apparent that the shape of
the composite spectrum 40 is dependent to a considerable
extent on the hydrogen content of the irradiated earth 3, and
therefore on the porosity of the matrix, and this is
especially true if the gate 25 is opened for a longer rather
than a shorter period, and if a larger rather than a smaller
proportion of the detected gamma radiation is compose~ o~
capture gamma radiation. Deletion of this special background
component from the signals obtained from the carbon and
oxygen windows will obviously increase resolution of the
remaining portions of the composite spectrum 4 and will
therefore clearly improve or enhance any measurement of the
carbon:oxygen ratio which may be obtained in this manner. I
Accordingly, the pulse height analyzer may be further
-29-
adjusted to establish a minimum carbon background trigger level
42, and a similar minimum oxygen background trigger lével 44,
whereby four separate windows will be provided. llle trigger
level 42 should be chosen so that it is low enough to encompass
as much carbon background as is practical, but also high enough
so that valid carbon gamma pulses wi 11 not be included within
the background component. The highest energy oxygen peak 51 is
found at or about 6.1 Mev., as hereinbefore stated, and thus
the lower oxygen background trigger level 44 may appropriately
be set at or about 6.5 Mev.
Referring again to Figure 1, it may be seen that if the
pulse height analyzer 26 is set with four different windows as
hereinbefore proposed, signal 26A will correspond to the pulses
falling within the carbon window defined by the energy range of
3.0 Mev.-4.7 Mev. as indicated in Figure 3 by trigger levels
41-42. Thus, signal 26B will be composed of the pulses falling
within the carbon background window defined by the energy range
of 4.7 Mev.-5.0 Mev. as indicated by trigger levels 42-43
depicted in Figure 3. Similarly, output signal 26C will be
20 - composed of the pulses falling within the oxygen window defined
by the energy range 5.0 Mev.-6.5 Mev. as indicated by trigger
levels 43-44, and the out put signal 26D will constitute those
pulses in signal 20 which fall within an energy range of 6.5
Mev.-7.5 Mev. which is the oxygen background window defined in
Figure 3 by trigger levels 44-45.
It will readily be apparent from the foregoing
explanation that the ratio of signal 26A to signal 26C
-~0 -
l O~ i 7
will provide an accurate representation of the carbon:oxygen
ratio in the matrix of the earth 3, and this is especially
true when the accelerator 7 is actuated as indicated in
Figure 2 and the gate 25 is cycled to provide a time dependent
measurement as hereinbefore explained. Accordingly, signals
26B and 26D may be discarded (except for purposes of deriving
lithology information, and the summation obtained by count
rate meter 31), and an accurate and dependable measurement
of carbon:oxygen may be obtained by merely coupling signals
26A and 26C directly to the inputs of a third ratio meter 230.
This ratio meter 230 produces an output signal 230A which is
directly proportional to the uncorrected carbon:oxygen ratio.
As hereinbefore stated, a gamma ray is highly
susceptible to loss of energy as a result of experiencing
Compton scattering, and thus at least a portion of the
pulses in signals 26A and 26C will be caused by degraded
gamma radiation. Another portion will be caused by capture
gamma rays and gammas from the formation matrix and borehole
casing. The number of unwanted pulses in signals 26A and 26C
will be greater or lesser depending upon whether the pulse
rate of signals 26B and 26D, respectivPly, is greater or
lesser. Accordingly, it can be assumed that the unwanted
component of signal 26A is proportional to the size of
signal 26B, and the unwanted component of signal 26C is also
proportional to the size or magnitude of signal 26D.
-31
10~ 7
Referring now to Figure 4, there may be seen a
representation of the composite spectrum 40 depicted in
Figure 3, but wherein horizontal lines 52 and 53 have
been drawn between trigger levels 41-42 and 43-44,
respectively. The purpose and vertical location of line
52 is to define the number of pulses (above line 52)
which wo~ld have been counted if no carbon background
signals were present in signal 26A, and to define below
line 52 the number of carbon background signals which
nonetheless are present and must be deleted. Similarly,
the vertical location of line 53 defines the number of
oxygen background pulses (below line 53) which have been
included in signal 26C.
A realistic basis for determining the appropriate
vertical locations of lines 52 and 53 in Figure 4, and
therefore the number of unwanted pulses in signals 26A
and 26C~ is to merely assume that the unwanted pulses in
signal~ 26A and 26C are equal to the magnitude6 of signals
26B and 26D, respectively. Accordingly, and as indicated
in Figure 1, signals 26A and 26B are preferably coupled
to a difference circuit 28, which provlde~ a di~-
ference in signal 28A which is applied to one of the
inputs of the aforementioned ratio meter 30. Similarly,
signal6 26C and 26D are coupled to a difference circuit
29 having its output 29A coupled to the other input of
the ratio meter 30. In thiæ manner, the ratio signal 30A
which is applied to the recorder 27 (assuming statistics
to be adequate) provid~ an extremely accurate representation
-
~ 7
of the ratio of the carbon and oxygen surrounding the
crystal 11 in the sonde 2. This is because the signal
30A is now much less dependent on the amount of back-
ground radiation which may be present. Furthermore,
fractional variations in the carbon:oxygen ratio will be
more apparent with signal 30A.
The gamma ray which appears when a fast neutron is
inelastically scattered by a carbon nucleus in an oil
molecule is indistinguishable from the gamma which appears
when the scattering carbon nucleus is in the matrix of a
limestone formation. Thus, the type of formation matrix
which is present is important to the present invention,
not only insofar as the slope or shape of the composite
spectrum 40 is concçrned, but also insofar as the size
(and therefore the significance) of the carbon peaks
46-48 are concerned.
Since limestone and sandstone formations have
basically different chemical compositions, it may be
expected that they will also have different characteristics
10~
for nuclear interactions. Further, they may be reasonably
expscted to have different composite secondary gamma spectra.
It is known that when a fast neutron is inelastically
scattered by a silicon nucleus, the resulting radiation may
be expected to have an energy distribution with a higher
mean value than a corresponding distribution from calcium.
Accordingly, the counting rate of the pulses composing
signal 26B may reasonably be expected to be significantly
higher when the formation in question has a limestone matrix
than when it is composed of sandstone. Similarly, the
so-called "oxygen background" signal 26D may include a
larger number of radiations resulting from inelastic
scattering of neutrons by silicon nuclei than by calcium.
It will be apparent from the foregoing that a measurement
of the ratio of the carbon background radiation to the oxygen
background radiation may provide a basis for determining
whether the formation in question is sandstone or limestone,
and that such a measurement is therefore especially useful
in correlation with an improved carbon:oxygen log according
to the principles of the present invention. Referring again
to Figure 1, therefore, it may be seen that the output signals
33A and 35A may also be conveniently coupled to the input
side of another ratio meter 130 having its output signal 130A
separately but correlatively coupled to the recorder 27 along
with signals 30A and 3lA to provide this information.
It should be noted, however, that the magnitude of
-34~
1 0~ 7
signals 30A and 130A will also be dependent upon the porosity
of the formation in question. The inelastic gamma ray counting
rate is to some degree dependent on the hydrogen content of
the formation, since, with more hydrogen present, gamma rays
originate further from the detector. Count rates are
therefore lower in high porosity formations, and thus the
composite inelastic scattering spectrum 40 or, as an approxima-
tion, the sum of signals 26A, 26B, 26C, and 26D from the
formation in question can be employed, signal 3LA, to indicate
the porosity of the formation. Thus, it is desirable to
correlate the aforementioned carbon:oxygen signal 30A, and
the background ratio signal 130A, with a measurement of the
sum of the inelastic gamma radiation 31A which results from
each actuation of the accelerator 7.
Referring now to Figure 5, there may be seen a representa-
tion of a different system operating sequence or cycle 60,
wherein the accelerator 7 is pulsed during the same or
substantially the same irradiation interval 55 as illustrated
in Figure 4. However, in the case of this sequence 60,
there is in addition to a first detection interval 58A,
which is directed to the inelastic scattering gamma
radiation, a second cetection interval 58B following
each irradiation interval 55. Moreover, as further illustrated
in Figure 5, this second detection interval 58B is preferably
timed to commence only after the inelastic scattering gamma
rays have substantially all been absorbed, whereby the gamma
-35-
104~117
rays occurring duxing this second later detection interval
58B will largely be composed of gamma rays resulting from
capture of the neutrons in the slow or thermal neutron
population 57.
Although the duration of this second detection interval
58B is indicated in Figure 5 to be the same as that of the
first detection interval 58A, the second interval 58B may
be of any duration provided it does not extend into the next
succeeding irradiation interval 55. Of course, it should be
appreciated that not all of the fast neutrons produced by the
accelerator 7 during each irradiation interval is
inelastically scattered, and that many will translate stable
nuclei into unstable nuclei having a measurable half-life.
Moreover, gamma radiaton from naturally occurring radioactive
materials can always be anticipated to be present in the
earth 3, and this radiation is detectable at any time.
Accordingly, it is frequently preferable to terminate the
second detection interval 58B before the thermal neutron
population 57 declines to insignificant proportions, whereby
the gamma rays which are detected durin~ the second interval
58B a re predominately those which result from thermal
neutron capture.
It should also be noted that most wells are normally
filled with fluids such as drilling mud, brine, and even oil.
~ydrogen is well known to be much more efficient at slowing
down neutrons than any other element, and thus the time
-36-
_ _ . . .....
~ 7
required to slot7 down neutrons in the borehole to thermal
energy will usually be much less than that of any fluid-
bearing formation in the earth 3. In addition, the hydrogen
and chlorine in the borehole are more efficient at capturing
thermal neutrons than the elements in most typical forma-
tions. Accordingly, the portion of the thermal neutron
population 57 which occupies the borehole 4 may be expected
to disappear earlier than the portion of such population 57
which permeates the earth 3.
It is the capture gamma radiation which emanates
from the earth 3 which is of interest and not the capture
gamma radiation which originates in the borehole 4. Accordingly,
if the initiation of each of the later detection intervals 58B
is delayed until the thermal neutrons in the borehole 4 have
substantially disappeared, the capture gamma radiation which
is thereafter counted may reasonably be attributed entirely
to the lithological characteristics of the earth 3, and the
effect of the borehole 4 will thereby be substantially
eliminated.
The second detection interval 58B may be obtained
by causing the timer circuit 23 in Figure l to actuate twice,
whereby the gate 25 will be caused to open twice during each
operating cycle. The four windows in the pulse height
analyzer 26 will still pass only pulses as defined by the
window however, and thus the signals 30A and 130A which would
appear as a result of the second detection interval 58B would not
-37-
only be meaningless but would tend to confuse the meaning
of the signal 30A and 130A generated during the first
interval 55A, and should not be allowed to contribute to
signals 30A and 130A.
Referring again to the system depicted in Figure 1,
there may be seen certain additional components and circuitry
which have not previously been discussed but which are
included for the purpose of making another porosity
measurement in addition to 31A, in correlation with the
measurements represented by signals 30A and 130A. In
particular, there may be seen a second gate 39 having its
input side connected to receive the detection pulses 20
directly from the caDle 18, and which has its output 39A
coupled to another count rate meter 36. It will further be
seen that a suitable delay circuit 37 is included which has
its input 22C coupled to receive the timing signal 22 from
the timing circuit 23, whereby it will transmit a suitable
actuating signal 38 to the second gate 39 at an appropriate
time following receipt of the timing signal 22C. Accordingly,
the second gate 39 will now open for the predetermined second
detection interval 58B following the initiation of each
irradiation interval 55 by the timing signal 22.
As thus arranged, the count rate meter 36 will receive
all of the pulses which occur in the detection signal 20
during the second detection interval 58B. Accordingly, the
voltage signal 36 which is preferably recorded by the recorder
27 separately but in correlation with the ratio signals 30A
and 130A, as well as the summation porosity signal 31A, will
provide a functional representation of the count rate for the
capture gamma ray population generated as a result of each
irradiation interval 55. This infor~ation 36A will be used as a
porosity indicator.
-38-
ln4;~l7
Moreover, it may be desired to provide a signal indicative
of lithology which is derived from therma~ neutron capture
data to go along with the thermal capture porosity indicator
signal 36A whose derivation has just been described. To this
end the output signal 39A from the thermal time gate 39 is
provided to another pulse height analyzer 40 for breakdown
into another energy spectral analysis similar to that previously
described with respect to the pulse height analyzer 26. In
this instance, however, it is sought to obtain an indicator
of the relative amounts of silicon, Si, and calcium, Ca, to
aid in distinguishing the lithology as being either basically
sandstone or basically limestone.
It is known, for example, that silicon exhibits a thermal
capture gamma ray peak at approximately 3.54 Mev. It is also
known that calcium exhibits a thermal neutron capture gamma
peak at approximately 6.41 Mev. Discrete energy windows to
measure the relative magnitude of these two known peaks, in
pulse height analyzer 40, however, are preferably not chosen
symmetrically about these energies. Interference from capture
gammas from other formation and borehole elements has led to
the choice of an energy window in the 2.5 to 3.2 Mev. range
to detect the Si peak and an energy window in the 5.2 to 6.25
Mev. range to detect the Ca peak.
The output of the counts in each of these respective
energy windows from pulse height analyzer 40 are supplied to
a ratio meter 43 which produces an output signal 43A
proportional to the Si/Ca ratio based on this two energy
window measurement. The lithology signal 43A may then be
recorded on a recorder 27 in depth correlation with the
previously discussed signals 30A, 130A, 31A and 36A.
-39-
104;~ 7
Various other modifications may be made in the methods
and apparatus hereinbefore discussed without significantly
departing from the essential concept of the present invention.
Accordingly, it should be clearly understood that the
structures and techniques which are described herein and
depicted in the accompanying drawings are illustrative only
and are not intended as limits on the scope of the invention.
-40-
