Note: Descriptions are shown in the official language in which they were submitted.
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SYRIA WOW LOGGING SWIG ACOUSTIC MULTIPLE DEVICES
This invention relates to well logging in general and more
particularly, to acoustic shear aye well logging.
Bac~round of the Invention -
In acoustic well logging, it is customary to measure the come
pressional wave velocity of earth formations surrounding Berlioz. A
conventional compression Al wave velocity logging system includes a
cylindrical logging Sunday for suspension in a Barlow liquid, a source
connected to the Sunday for generating compression Al waves in the Barlow
liquid, and one or more detectors connected to the Sunday and spaced apart
from the compression Al wave source for detecting compression Al waves in
the Barlow liquid. A compression Al wave in the Barlow liquid generated
by the source is refracted into the earth formation surrounding the
Barlow. It propagates through a portion of the formation and is
refracted back into the Barlow liquid at a point adjacent to the detector
and is then detected by the detector. The ratio of the distance between
the source and detector to the time between generation and detection of
the compression Al wave yields the compression Al wave velocity of the
formation. The distance between source and detector is usually fixed
and known so that measurement of the time between compression Al wave
generation and detection is sufficient to determine the compression Al
wave velocity of the formation. For better accuracy, such distance is
usually much greater than the dimensions of the source or detector.
Information important for production of oil and gas from subterranean
earth formations may be derived from the compression Al wave velocities
of such formations.
When a compression Al wave generated by a compression Al wave
source in the Barlow liquid reaches the Barlow wall, it produces a
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refracted compression Al wave in the surrounding earth formation as
described above. In addition, it also produces a refracted shear wave
in the surrounding earth formation, and guided waves which travel in the
Barlow liquid and the part of the formation adjacent to the Barlow.
S Part of such shear wave is refracted back into the Barlow liquid in the
form of a compression Al wave and reaches the detector in the logging
Sunday. The guided waves are also detected by such detector. Any wave
that is one of the three types of waves detected by the detector may be
called an arrival: the compression Al waves in the Barlow liquid caused
by refraction of compression Al waves in the formation the compression Al
wave arrivals, those caused by refraction of shear waves in the formation
the shear wave arrivals, and those caused by guided waves the guided
wave arrivals. Thus, the signal detected by the detector is a composite
signal which includes the compression Al wave arrival, the shear wave
arrival and the guided wave arrivals. Compression Al waves travel faster
than shear waves and shear waves usually travel faster than the guided
waves. Therefore, in the composite signal detected by the detector, the
compression Al wave arrival is the first arrival, the shear wave arrival
the second arrival, and the guided wave arrivals the last arrivals. In
measuring the compression Al wave velocity of the formation, the time
interval between generation of`compressional waves and detection of the
first arrival detected by the detector gives the approximate travel time
of the refracted compression Al wave in the formation. Hence the later
shear wave and guided wave arrivals do not affect measurement of compress
signal wave velocity of the formation.
In addition to traveling over a vertical distance in the formation approximately equal to the distance between the source and
detector, the compression Al wave also travels over short distances in
the liquid. The extra time required to travel such short distances
introduces errors in the velocity log. To reduce such errors, conventional
logging devices employ at least two detectors spaced vertically apart
along the Barlow from each other. The time interval between detection
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by the two detectors is measured instead of the time interval between
transmission and detection. The ratio between the distance between the
two detectors and such time interval yields the compression Al wave
velocity. Since the compression Al wave travels over approximately equal
short distances in the Barlow liquid before reaching the two detectors,
the time interval between detection by the two detectors is a more
accurate measure of the actual travel time in the formation. Therefore,
using two detectors and measuring the time between detection by the two
detectors yield a more accurate comprsssional wave velocity. Other
spurious effects such as borehole-size changes and Sunday tilt may be
reduced by conventional devices. One such device is described in
Interpretation, Volume 1 - Principles, Schlumberger limited, New York,
NAY. 10017, 1972 Edition, pages 37-38.
It is well known that shear wave velocity logging may also
yield information important for production of oil and gas prom subterranean
earth formations. The ratio between the shear wave velocity and compress
signal wave velocity may reveal the rock lithology of the subterranean
earth formations. The shear wave velocity log may also enable seismic
shear wave time sections to be converted into depth sections. The shear
wave log is useful in determining other important characteristics of
Sarah formations such as shear stress, porosity, fluid saturation and
the presence of fractures. The shear wave log may also be helpful for
determining the stress state around the Barlow which is very important
in designing hydraulic fracture treatments.
The conventional compression Al wave logging source and the
compression Al waves it generates in the Barlow liquid are symmetrical
about the logging Sunday axis. When such compression Al waves are refracted
into the surrounding earth formation, the relative amplitudes of the
refracted shear and compression Al waves are such that it is difficult to
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distinguish the later shear wave arrival from the earlier compression Al
wave arrival and from the reverberations in the Barlow caused by
refraction of the compression Al wave in the formation. Therefore it is
difficult to use a conventional symmetrical compression Al wave source
for logging shear wave velocity. Correlation techniques have been
employed to extract the shear wave arrival from the full acoustic wave
train recorded. Such techniques, however, usually require processing of
data by a computer so that shear wave velocities cannot be logged on
line. It may also be difficult to extract the shear wave arrival if it
is close in time to the compression Al wave arrival.
Asymmetric compression Al wave sources have been developed for
logging shear wave velocity. Using such sources, the amplitude of the
shear wave arrival may be significantly higher than that of the compress
signal wave arrival. By adjusting the triggering level of the detecting
and recording systems to discriminate against the compression Al wave
arrival, the shear wave arrival is detected as the first arrival. It may
thus be possible to determine the travel time of shear waves in the
formation and therefore the shear wave velocity. Such asymmetric sources
each generates in the Barlow liquid a positive compression Al wave in
one direction and a simultaneous negative compression Al wave in the
opposite direction. The interference of the two compression Al waves may
cause the shear wave arrival to be stronger than the compression Al wave
arrival. Asymmetric sources are disclosed by Angina et at, European
Patent Application No. 31989 published 15 July 1981, White, US. Patent
No. 3,593,255, and Kitsunezaki, US. Patent 4,207,961.
Angina et at disclose a bender-type source which comprises two
circular pieæoelectric plates bonded together and attached to a logging
Sunday. When voltage is applied across the two piezoelectric plates, the
plates will bend. The bending of the transducer plates creates a positive
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compression Al wave in one direction and a simultaneous negative compress
signal wave in the opposite direction. White discloses a compression Al
wave source comprising two piezoelectric segments each in the shape of
a half hollow cylinder. The two segments are assembled to form a split
cylinder. The two segments have opposite polarization and electric
voltage is applied to each segment causing one segment to expand radially
and simultaneously causing the other segment to contract radially thereby
producing a positive compression Al wave in one direction and a simultaneous
negative compression Al wave in the opposite direction. In Kitsunezaki,
coils mounted on a bobbin assembly are placed in the magnetic field of a
permanent magnet and current is passed through the coils to drive the
bobbin assembly. The movement of the bobbin assembly ejects a volume of
water in one direction and simultaneously sucks an equivalent volume of
water in the opposite direction, thereby generating a positive pressure
change in one direction and a simultaneous negative pressure change in
the opposite direction.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of an acoustic logging system
illustrating this invention. ,
FIG. 2 is a simplified perspective view of an octopole shear
wave logging device illustrating the preferred embodiment of this invention.
FIG. 3 is a cross-sectional view of the octopole shear wave
logging source of FIG. 2 taken along the line 3-3.
FIG. 4 is a simplified perspective view of the octopole shear
wave logging device of FIGS. 2 and I illustrating the orientation of
the detectors relative to that of the octopole source, and the electrical
connections to the source and detectors.
FIG. 5 is a cross-sectional view of an octopole shear wave
logging source illustrating an alternate embodiment of this invention.
FIG. 6 is a cross-sectional view of an octopole shear wave
logging source illustrating another alternate embodiment of this invention.
FIG. 7 is a cross-sectional view of an octopole shear wave
logging source illustrating still another alternate embodiment of this
invention.
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SYRIA OF TIE I~E~'TION
The method and apparatus of this invention are for logging the
shear wave velocity of an earth formation surrounding a well or Barlow.
The method of this invention comprises transmitting a 2n-pole shear wave
through the earth along the well wherein n is an integer greater than 2,
and detecting the 2n-pole shear wave aureole at at least one point .-
longitudinally spaced along the well from the point of transmission. If
the shear wave arrival is detected at two points, the tire lapse between
the detections at the two points is measured to determine the shear wave
velocity of the earth surrounding the well. If the shear wave arrival
is detected at only one point, the time lapse between transmission and
, detection of the shear wave signal is measured to determine the shear
; wave velocity of the earth. The apparatus of this invention comprises a
housing adapted to be raised and lowered into a well, signal generating
means in the housing for transmitting a 2n-pole shear wave into the
earth formation surrounding the well where n is an integer greater than
2, and signal detecting means in the housing longitudinally spaced along
the well from the signal generating means for detecting the arrival of
such shear wave.
DESCRIPTION OF THE PREFERRED E~lBOnIMENTS
FIG. 1 is a schematic view of an acoustic logging system
illustrating this invention. A logging Sunday 10 is adapted to be raised
and lowered into a well. The Sunday contains a multiple shear wave
source 12 and two detectors, 14, 16. To initiate logging, Sunday 10 is
25 suspended into a liquid 18 contained in a Barlow 20, which is surrounded
by an earth formation 22. Detectors 14, 16 are so connected to Sunday lo
that they are spaced longitudinally along Barlow 20 from each other
and from source 12. Source 12 is connected to a firing and recording
control unit 24. Although the firing and recording control unit is
30 shown in FIG. l as a separate unit from the logging Sunday, the part of
the unit that powers the multiple shear wave source may, for convenience
" of operation, be housed by the logging Sunday. Signals recorded by
detectors 14, 16 are fed to a band pass filter 26, an amplifier 28 and a
time interval unit 30.
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In a manner explained below the firing and recording control
unit is used to fire source 12 which produces a shear wave in formation
22. The shear wave arrival is detected by detectors 14 and 16. Sunday
10 also contains a preamplifier (not shown in FIG. 1) which amplifies
the shear wave arrival detected by detectors 14, 16. The amplified
signals are then filtered by filter 26 and amplified again by amplifier
28. The time interval between the detection of the arrival by detector
14 and its detection by detector 16 is then measured by time interval
unit 30. Such time interval may be stored or displayed as desired.
10 FIG. 2 is a simplified perspective view of an octopole shear
, wave logging device illustrating the preferred embodiment of the invention.
I As shown in FIG. 2, logging Sunday 10 comprises a number of hollow cylinder-
eel sections. The top section 32 contains an octopole shear wave
it logging source snot shown in FIG. 2), and has six windows 42 which allow
the compression Al waves generated by the source to propagate readily
there through into the Barlow liquid. Sections 34, 36, each containing
a detector snot shown), are located below the source and also have
windows 44, 46 as shown in FIG. 2. The combined compression Al waves
generated by the source in section 32 propagate through windows 42 and
Barlow liquid 18 to reach the wall of Barlow 20. A portion of such
combined compression Al waves is refracted into earth formation 22 in the
form of a shear wave. After such shear wave travels a distance through
the formation, portions of it are refracted back into the Barlow, into
Barlow liquid 18, to reach the detectors in sections 34, 36 through
windows 44 and 46, respectively. The time interval between the detections
by the two detectors is then measured as described.
The nomenclature for the multiple is based upon consecutive
powers of two, that is, on, n being an integer, and n = 1, 2, 3 and on
indefinitely. Thus the multiples include the dipole on = 1), the
quadruple (n = 2) and the octopole (n = 3). The nomenclature for
higher order multiples is based upon on with n =-4, 5, 6 and so on
indefinitely.
I. .
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FIG 3 is a cross-sectional view of the octopole shear wave
source of JIG. 2 taken along the line I Six substantially similar
sectors 62, Al 66, I 70, 72 of a radially polarized piezoelectric
hollow cylinder are so spatially arranged that they are substantially
coaxial and that they surround a common axis. Substantially the same
electrical pulse is applied across the cylindrical surfaces of each
sector substantially simultaneously such that the pulses supplied to any
two adjacent sectors are opposite in polarity. This arrangement is
illustrated in FIG. 3. With such an arrangement, if one sector is
lo caused by the electrical pulse to expend radially then the two sectors
adjacent to it will contract radially and vice versa. If the six sectors
are polarized radially outward, then the directions of expansion and
contraction will be as illustrated by hollow arrows in FIG. 3. During
contraction of a sector, its entire inner cylindrical surface will move
inward. During its expansion, its entire outer cylindrical surface will
move outward. The combined compressior.al wave so generated by the
expansion and contraction of the six sectors will refract into the
surrounding earth formation to generate an octopole shear wave. To
detect the octopole shear wave arrival, the detectors may be of construction
similar to the octopole shear wave Lou cues illustrated in FIG. 3, or in
FIG. 5, which will be described later.
The central space between the six sectors is filled by an
annular body of backing material 74 to damp out the reverberations of
the vibrations of the six sectors so that the octopole shear wave generated
will be short in duration. This annular body 74 may be attached to
section 32 by a conventional means such as inserting a mandrel 76 through
the center of body 74, and screwing the two ends of the mandrel onto two
disks that fit snugly into section 32. The six sectors are placed on
the outer cylindrical surface of body 74 and may be wept in place by two
annular rings trot shown in FIG. 3) of elastic backing material fitting
snugly over the six sectors. The six sectors are so positioned in
section 32 that each sector faces one of the six windows 42, as shown in
FIG. 3. The vectorial spaces between the windows and the sectors are
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filled by oil 78. The vibrations of the six sectors will generate
compression Al waves in oil 78 which are transmitted through window 42 to
generate an octopole shear wave in the earth. The vectorial spaces
between the oil filled spaces are filled by backing material 80 for
damping out the reverberations of the vibrations of the six sectors
FIG. 4 is a simplified perspective view of the octopole shear
. wave logging device of FIGS. 2 and 3 illustrating the orientation of
detectors relative to that of the octopole source, and the electrical
connections to the source and the detectors. To detect the compression Al
wave in a Barlow liquid caused by refraction of the octopole shear
wave generated by source 12, detector 14 is preferably also an octopole
detector of construction similar to source 12. The six sectors are
' placed so that they have substantially the same axis as the six sectors
j of source 12, and that they have substantially the same lateral positions
around the common axis as the sectors of source 12 to maximize the
strength of the octopole signal detected. The outer and inner cylindrical
surfaces of the six sectors of the detector are connected to band pass
filter 26 in a manner similar to the connections from the respective
surfaces of source 12 to the firing and recording control unit 24.
20 Detector 16 is similar to detector 14 but is not shown in FIG. 5 for
simplicity. To allow the six sectors of each of the two detectors to
detect the octopole shear wave arrival, sections 34, 36 of FIG. 2 each
will have preferably six windows 44, 46 respectively. While the detector
of FIG. 4 is shown as similar in construction to the source of FIG. 3,
it will be understood that detectors of construction similar to the
sources of FIGS. 5, 6, 7 (described below) may also be used. The six
sectors or plates of each type of detector are preferably aligned laterally
around the common axis, that is, azimuthal) with the six sectors of
the source to maximize the detected signal.
FIG. 5 is a cross-sectional view of an octopole shear wave
. source illustrating an alternate embodiment of this invention. Six
elongated piezoelectric composite plates 82, 84, 86, 88, 90, 92 are so
spatially arranged that they form substantially the parallelograms of a
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hexagonal prism. Each of the Sue composite plates comprises two oppositely
polarized piezoelectric plates bonded together. The six composite
plates arc attached to section I of the logging Sunday by two clamping
plates (not shown in FIG. ;). Each of the two clamping plates his six
slots into which the ones of the six composite plates are fitter snugly.
The two clamping plates are then inserted into and attached to section
I in such position that the elongated composite plates are substantially -
parallel to the logging Sunday assay. The portion of each composite plate
between the two ends will hereinbelow be called the "unclamped portion"
or -the "unattached portion." It will be understood, however, thaw the
six composite plates need not be attached to the Sunday at their ends.
Attachment of one end, or at a location between the two ends, will
suffice. Then the portion of each plate other than the attached part
may be called the "unclamped portion" or the "unattached portion."
Substantially the same electrical pulse is applied across the
flat surfaces of each of the six composite plates substantially simulate-
nuzzle. The pulses applied to any two adjacent composite plates are
opposite in polarity such that if the unattached portion of one composite
plate bends and moves radially outward then the unattached portions of
the two adjacent composite plates will bend and move radially inward.
The directions of the bending movements of the six composite plates are
illustrated by hollow arrows in FIX. 5. The bending motion of each
composite plate will generate a compression Al wave in the Barlow
liquid . The combined compression Al wave generated by the octopole
source will refract into the formation surrounding the Barlow to
produce an octopole shear wave. To detect the octopole shear wave
arrival in the Barlow liquid detector 14 is preferably an octopole
type which may be of construction similar to the octopole sources thus-
treated in FIG. 3 or in FIG. 5. The outer surfaces ox the composite
plates of detector 14 are connected to band pass filter 26 instead of to
the firing and recording control unit 24. The six sectors or plates of
! the detector are preferably aligned azimuthal with the six plates of
the shear wave source of FIG. 5.
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The composite plates comprising a pair of oppositely polarized
piezoelectric plates are readily available commercially. Piezoelectric
composite plates supplied by- the Vernitron Company of Bedford, Ohio,
known as Bender Bimorphs have been satisfactory. The six piezoelectric
sectors of the Tao illustrated in FIG. 3 and of the types illustrated
in FIGS. 6, 7 to be described layer are also supplied by ~ernitron
Company.
IT. 6 is a cross-sectional view of an octopole shear wave
source illustrating another alternate embodiment of the invention. In
FIGS. 3 and 4, the six sectors of the octopole source are polarized
radially. Alternatively, the six sectors may be polarized circumferential
as shown by the polarizations of sectors 102, 104, 106, 108, 110 and 112
in FIG. 6. The six sectors polarized circumferential are in what is
known as the hoop mode. The six sectors may be obtained from a hollow
cylindrical piezoelectric cylinder by cutting out six narrow long-
tudinal sectors. An electrical pulse is applied across the side surfaces
of each of the six sectors so that the resulting electric field in each
sector is substantially parallel to its polarization. The electrical
pulse will cause each sector to expand or contract radially depending
upon the polarity of the pulse If the sectors 102, 106, 110 are
polarized in a circumferential clockwise direction but the electric
fields therein are in circumferential counterclockwise direction as
shown in FIG. 6, the three sectors will contract radially. If the
polarizations of and the electric fields in sectors 104, 108, 112 are
all in the circumferential counterclockwise direction, then the three sectors will expand radially.
FIG. 7 is a cross-sectional view of still another alternate
embodiment illustrating an octopole shear wave source in the hoop mode.
The six sectors, 122, 124, 126, 128, 130, 132 are six of the twelve
longitudinal sectors of a piezoelectric hollow cylinder, each of the
twelve sectors having been polarized circumferential. Adjacent members
have opposite circumferential polarizations. the six sectors 122, 124,
126, 128, 130, 132 are the only sectors of the cylinder which will
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expand and contract and are all polarized in the circumferential clockwise
direction. The connecting edges of adjacent sectors may be coated by
conducting layers snot shown). Electrical pulses are so applied that
the electric field in each of the six sectors is substantially parallel
to its polarization. hit the polarizations of sectors and polarities
of pulses as shown in FIG. 7, sectors 122, 126, 130 will expand radially
while sectors 124, 128, 132 will contract radially. The remaining six
sectors will not expand or contract since no potential difference is
applied across such sectors.
In both the preferred embodiment and the three alternate
embodiments described above, piezoelectric materials are used to construct
the octopole shear wave source, and the source is vibrated by electrical
pulses. It will be understood, however, that other constructions of the
source and other vibrating means may be used. Thus purely mechanical
means may be used to vibrate the six sectors of the preferred embodiment,
and the six plates or sectors of each of the three alternate embodiments.
An octopole shear wave will be venerated so long as the sectors or the
plates are caused to vibrate in the same manner as in the preferred and
alternate embodiments.
The octopole shear wave source of this invention may be used
to log shear wave velocities on line (that is, the shear wave velocities
may be determined without data processing if the shear wave arrival is
significantly greater in amplitude than the compression Al wave arrival.
The shear wave arrival is significantly greater in amplitude than the
compression Al wave arrival only when the frequencies of the octopole
shear wave produced in the earth surrounding the Barlow are within
certain frequency ranges. For any earth formation there is a preferred
frequency range for logging the shear wave velocity so that the shear
wave arrival is significantly stronger than the compression Al wave
arrival. The preferred frequency range varies with the shear wave
velocity of the formation to be logged. Thus if the approximate range
of the shear wave velocities of the formation is known, a preferred
range of frequencies can be chosen For a well with ten inches diameter
the preferred frequency ranges are shown in the table below.
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App~-o~imate Range of Preferred Frequency
Shear Wave Velocities (ft/sec~ Range (kHz)
5000 - 6000 3.7 - 12.6
6000 - 7000 US - 20
7000 owe 3.9 - 26.5
~000 - 9000 4.1 - 33
The approximate range of shear wave velocities of a formation
ma be estimated by conventional methods such as measuring the ccmpres-
signal wave velocities of the formation. The shear wave velocity is
approximately one-half the compression Al wave velocity. From tune measured
compression Al wave velocities the approximate range of shear wale
velocities may be estimated. The preferred frequencies vary inversely
with the diameter of the well. Therefore for a well with diameter d
inches instead of ten inches the preferred frequency ranges are given by
those listed in the table above multiplied by a factor 10/d.
FIGS. 3, 4, 6, 7 illustrate octopole shear wave sources
employing six sectors which are vibrated radially to generate octopole
shear waves in earth formations. The frequencies of the octopole shear
waves so generated vary inversely with the radii of the sectors. For
the frequencies to be within the preferred frequency ranges listed above,
it is preferable that the radii of the sectors be large. Therefore,
their radii are preferably only slightly smaller than the radius of the
logging Sunday. It will be understood that FIGS. 3, 4, 6, 7 are not drawn
to scale.
The higher order multiple sources may be constructed in a
manner similar to the four embodiments of the octopole shear wave source
illustrated in FIGS. 3, 5, 6 and 7. Thus the 16-pole source may be
constructed by spatially arranging 8 elongated pie~oelectric composite
plates to form the 8 parallelograms of an octagonal prism. Substantially
the same electrical pulse is applied to each of the eight composite
i plates with such polarity that adjacent plates vibrate in substantially
opposite phases. An alternative embodiment of the 16-pole source is
constructed if the eight composite plates are replaced by eight substantially
-14-
identical sectors of a radially or circumferential polarized piezoelectric
hollow cylinder. Substantially the same electrical pulse is applied to
each sector such that adjacent sectors vibrate in substantially opposite
phases. Other jays of constructing and vibrating the plates and sectors
may be used so long as the plates and sectors are vibrated in the same
manner. Other higher order multiples may be constructed in a manner
similar to the octopole and 16-pole. Preferably the detectors used to
detect the higher order shear wave arrivals will be of an order what
matches the order of the source.
The number of composite plates or sectors in the embodiments
of the octopole and the 16-pole sources described above does not match
the nomenclature of the octopole and 16-pole sources. Thus the octopole
source comprises 6 plates or sectors end the 16-pole source S plates or
sectors. The 32-pole source comprises 10 plates or sectors. Thus while
the nomenclature of the multiple sources is based on on, n being an
integer, with n = 1, 2, 3...., the corresponding number of plates or
sectors is on. Thus, a dipole (n = 1) source comprises 2 x 1 or 2
plates or sectors. A quadruple (n = 2) source comprises 2 x 2 or 4
plates or sectors. An octopole (n = 3), a 16-pole on = 4) and a 32-pole
on = 5) source comprises 6, 8 and LO plates or sectors respectively.
Therefore, in general, a 2 -pole source will comprise on plates or
sectors, n being an integer, whereon = 1, 2, 3 and so on indefinitely.
The above description of method and construction used is
merely illustrative thereof and various changes in shapes, sizes, materials,
or other details of the method and construction may be within the scope
of the appended claims.
,..
