Nuclear magnetic resonance apparatus and method

A nuclear magnetic resonance apparatus including a magnet generating a static magnetic field in a first region containing materials to be analyzed. The magnet generates zero static magnetic field in a second region. The magnet has generally homogeneous magnetization along a longitudinal axis and is magnetized substantially perpendicular to the axis. The apparatus includes means for generating a radio frequency magnetic field in the first region for exciting nuclei of the materials. The means for generating the radio frequency magnetic field includes an antenna disposed within the second region. The apparatus includes means for receiving a nuclear magnetic resonance signal from the excited nuclei. The means for receiving also provides an output indication of properties of the materials to be analyzed. In a preferred embodiment, the means for generating and receiving include an antenna at least partially disposed within the second region. In a specific embodiment, the antenna consists of wire coils wound in planes perpendicular to the longitudinal axis. A high permeability ferrite is disposed inside the wire coils.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is related to the field of Nuclear Magnetic Resonance 
(NMR) sensing apparatus and methods. More specifically, the present 
invention is related to NMR well logging apparatus and methods for NMR 
sensing within earth formations surrounding a wellbore. The present 
invention also relates to methods for using NMR measurements to determine 
properties of the earth formations surrounding the wellbore. 
2. Description of the Related Art 
The description of the present invention and the background thereof are 
approached in the context of well logging because well logging is a known 
application of NMR measurement techniques. It is to be explicitly 
understood that the present invention is not limited to the field of well 
logging. 
NMR well logging instruments can be used for determining properties of 
earth formations including the fractional volume of pore space and the 
fractional volume of mobile fluid filling the pore spaces of the earth 
formations. Methods of using NMR measurements for determining the 
fractional volume of pore space and the fractional volume of mobile fluid 
are described, for example, in Spin Echo Magnetic Resonance Logging: 
Porosity and Free Fluid Index Determination, M. N. Miller et al, Society 
of Petroleum Engineers paper no. 20561, Richardson, Tex., 1990. 
NMR oil well logging instruments known in the art typically make 
measurements corresponding to an amount of time for hydrogen nuclei 
present in the earth formations to substantially realign their spin axes, 
and consequently their bulk magnetization, with an applied magnetic field. 
The applied magnetic field is typically provided by a permanent magnet 
disposed in the NMR well logging instrument. The spin axes of hydrogen 
nuclei in the earth formation, in the aggregate, align with the magnetic 
field applied by the magnet. 
The NMR instrument also typically includes an antenna, positioned near the 
magnet and shaped so that a pulse of radio frequency (RF) power conducted 
through the antenna induces an RF magnetic field in the earth formation. 
The RF magnetic field is generally orthogonal to the field applied by the 
magnet. This RF pulse, typically called a 90 degree pulse, has a duration 
and amplitude predetermined so that the spin axes of the hydrogen nuclei 
generally align themselves perpendicularly both to the orthogonal magnetic 
field induced by the RF pulse and to the magnetic field applied by the 
magnet. After the 90 degree pulse ends, the nuclear magnetic moments of 
the hydrogen nuclei gradually "relax" or return to their original 
alignment with the magnet's field. The amount of time taken for this 
relaxation, referred to as T1, is related to petrophysical properties of 
interest of the earth formation. 
After the 90 degree pulse ends, the antenna is typically electrically 
connected to a receiver, which detects and measures voltages induced in 
the antenna by precessional rotation of the spin axes of the hydrogen 
nuclei. The precessional rotation generates RF energy at a frequency 
proportional to the strength of the magnetic field applied by the magnet, 
this frequency being referred to as the Larmor frequency. The constant of 
proportionality for the Larmor frequency is known as the gyromagnetic 
ratio (.gamma..sub.0). The gyromagnetic ratio is unique for each different 
chemical elemental isotope. The spin axes of the hydrogen nuclei gradually 
"dephase" because of inhomogeneities in the magnet's field and because of 
differences in the chemical and magnetic environment within the earth 
formation. Dephasing results in a rapid decrease in the magnitude of the 
voltages induced in the antenna. The rapid decrease in the induced voltage 
is referred to as the free induction decay (FID). The rate of FID is 
typically referred to by the notation T2*. The FID decay rate consists of 
a first component, referred to as "true T2", which is due to internal 
molecular environmental effects, and a second component resulting from 
microscopic differences in magnetic field gradients and inhomogeneities in 
the earth formation. The effects of the second component can be 
substantially removed by a process referred to as spin-echo measurement. 
Spin echo measurement can be described as in the following discussion. 
After a predetermined time period following the FID, another RF pulse is 
applied to the antenna. This RF pulse has an amplitude and duration 
predetermined to realign the spin axes of the hydrogen nuclei in the earth 
formation by an axial rotation of 180 degrees from their immediately 
previous orientations, and is therefore referred to as a 180 degree pulse. 
After the end of the 180 degree pulse, hydrogen nuclear axes that were 
precessing at a slower rate are then positioned so that they are "ahead" 
of the faster precessing spin axes. The 180 degree reorientation of the 
nuclear spin axes therefore causes the faster precessing axes to be 
reoriented "behind" the slower precessing axes. The faster precessing axes 
then eventually "catch up" to, and come into approximate alignment with, 
the slower precessing axes after the 180 degree reorientation. As a large 
number of the spin axes thus become "rephased" with each other, the 
hydrogen nuclear axial precessions are again are able to induce measurable 
voltages in the antenna. The voltages induced as a result of the rephasing 
of the hydrogen nuclear axes with each other after a 180 degree pulse are 
referred to as a "spin echo". 
The spin echo induced voltage is typically smaller than the original 
voltage generated after cessation of the first RF pulse, because the 
aggregate nuclear axial alignment, and consequently the bulk 
magnetization, of the hydrogen nuclei at the time of the spin echo is at 
least partially realigned with the magnet's field and away from the 
sensitive axis of the antenna. The spin echo voltage itself decays by FID 
as the faster precessing nuclear axes quickly "dephase" from the slower 
precessing nuclear axes. 
After another period of time, typically equal to two of the predetermined 
time periods between the initial 90 degree RF pulse and the 180 degree 
pulse, another RF pulse of substantially the same amplitude and duration 
as the 180 degree pulse is applied to the antenna. This subsequent RF 
pulse causes another 180 degree rotation of the spin axis orientation. 
This next 180 degree pulse, and the consequent spin axis realignment again 
causes the slower precessing spin axes to be positioned ahead of the 
faster precessing spin axes. Eventually another spin echo will occur and 
induce measurable voltages in the antenna. The induced voltages of this 
next spin echo will typically be smaller in amplitude than those of the 
previous spin echo. 
Successive 180 degree RF pulses are applied to the antenna to generate 
successive spin echoes, each one typically having a smaller amplitude than 
the previous spin echo. The rate at which the peak amplitude of the spin 
echoes decays is related to petrophysical properties of interest of the 
earth formations. The number of spin echoes needed to determine the rate 
of spin echo amplitude decay is related to the properties of the earth 
formation; in some cases as many as 1,000 spin echoes may be needed to 
determine the amplitude decay corresponding to the properties of the earth 
formation which are of interest. The rate at which the peak amplitude of 
the spin echo measurements decays is directly related to the true T2. True 
T2 is related to parameters of interest in the earth formation. 
One type of NMR well logging apparatus is described, for example in U.S. 
Pat. No. 4,350,955 issued to Jackson et al. The apparatus disclosed in the 
Jackson et al '955 patent includes permanent magnets configured to induce 
a magnetic field in the earth formations which has a toroidal volume of 
substantially uniform magnetic field strength. A particular drawback to 
the apparatus disclosed in the Jackson et al '955 patent is that the 
thickness of the toroidal volume is very small relative to typical rates 
of axial motion of well logging tools. Well logging tools, in order to be 
commercially useful, typically must be able to be moved axially through 
the wellbore at rates not less than ten feet per minute. The length of 
time needed to make a typical NMR spin-echo measurement set can be as long 
as several seconds. The NMR logging tool is therefore likely to move a 
substantial distance during a measurement cycle. Measurements made by the 
apparatus disclosed in the Jackson et al '955 patent are therefore subject 
to error as the apparatus is moved during logging operations, because the 
antenna would no longer be positioned so as to be sensitive to the same 
toroidal volume which was magnetized at the beginning of any measurement 
cycle. 
Another drawback to the apparatus disclosed in the Jackson et al '955 
patent is that it does not eliminate NMR signal originating within the 
fluid filling the wellbore. 
A still further drawback to the apparatus disclosed in the Jackson et al 
'955 patent is that the toroidally shaped static magnetic field is subject 
to changes in field strength as the instrument is subjected to changes in 
ambient temperature and variances in the earth's magnetic field. The 
antenna in the Jackson et al '955 apparatus is tuned to a single 
frequency. If the field strength of the static magnetic field in the 
toroidal volume changes, the antenna may no longer be sensitive to NMR 
signals originating from within the toroidal volume. Using the apparatus 
in Jackson et al '955, it is impractical to compensate the frequency of 
the RF magnetic field for changes in the static magnetic field strength 
within the toroidal volume. 
An apparatus disclosed in U.K. patent application no, 2,141,236 filed by 
Clow et al and published on Dec. 12, 1984 provides improved 
signal-to-noise ratio when compared with the apparatus of Jackson et al 
'955 by including a high magnetic permeability ferrite in the antenna. 
However, the apparatus disclosed by Clow et al is subject to similar 
limitations and drawbacks as is the Jackson et al '955 apparatus. 
Another NMR well logging apparatus is described, for example in U. S. Pat. 
No. 4,710,713 issued to Taicher et al. The apparatus disclosed in the 
Taicher et al '713 patent includes a substantially cylindrical permanent 
magnet assembly which induces a static magnetic field having substantially 
uniform field strength within an annular cylindrical volume. 
The apparatus disclosed in the Taicher et al '713 patent is subject to 
several drawbacks. First, because the antenna is located within the 
strongest part of the magnet's field, when RF electrical pulses are 
applied to the antenna acoustic waves can be generated in the antenna by 
an effect known as the "Lorenz force". The antenna returns to its original 
shape in a series of damped mechanical oscillations in a process referred 
to as "magnetoacoustic ringing". Ringing can induce large voltages in the 
antenna which interfere with the measurement of the voltages induced by 
the NMR spin echoes. Additionally, the magnet is located in the highest 
strength portion of the RF magnetic field. The magnet can be deformed by 
magnetostriction. When each RF power pulse ends, the magnet tends to 
return to its original shape in a series of damped mechanical 
oscillations, in a process referred to as "magnetostrictive ringing", 
which as magnetoacoustic ringing, can induce large voltages in the antenna 
making it difficult to measure the spin echoes. 
A further drawback to the apparatus in the Taicher et al '713 patent is 
that the antenna induces an RF magnetic field in the formations 
surrounding the tool which decreases in strength as the square of the 
radial distance from the axis of the magnet. Moreover, a significant 
portion of the RF energy can be lost in an electrically conductive fluid 
in the wellbore. Because the signal-to-noise ratio of NMR measurements 
made in a gradient magnetic field is typically related to the strength of 
the RF magnetic field, the apparatus disclosed in the Taicher et al '713 
can have difficulty obtaining measurements having sufficient 
signal-to-noise ratio at radial distances which are likely to be outside a 
zone within the earth formations known as the "invaded" zone. The invaded 
zone is typically formed by introduction, under differential pressure, of 
the liquid phase of a fluid called "drilling mud" which is used in the 
process of drilling the wellbore. The liquid phase displaces native fluids 
within the pore spaces of the earth formations proximal to the wellbore, 
making near-wellbore measurements unrepresentative of the native fluid 
content of the earth formations. 
Still another drawback to the apparatus disclosed in Taicher et al '713 is 
that the antenna length is related to the vertical resolution required by 
the system designer. Typically, the vertical resolution is preferred to be 
very short. If the antenna in Taicher et al '713 is not made substantially 
longer than the diameter of the sensitive volume within the earth 
formation, the strength of the RF magnetic field can decrease faster than 
the square of the radial distance from the axis of the antenna. Lines of 
equal RF magnetic field strength can then become substantially 
elliptically shaped, which does not match the lines of equal strength of 
the static magnetic field. This drawback can significantly limit the 
ability of the apparatus in Taicher et al '713 to make measurements 
outside the invaded zone. 
Another drawback to the apparatus of the Taicher et al '713 patent is that 
the antenna must be connected to complicated, difficult to build tuning 
circuitry in order to establish an operating frequency for the RF pulses 
and to receive the spin-echo emitted energy at that same frequency. It can 
be desirable to operate the antenna at a plurality of substantially 
different frequencies in order to measure properties of the earth 
formation at a plurality of radial distances from the axis of the NMR 
logging tool. Operating the antenna of the apparatus in the Taicher et al 
'713 patent at substantially different frequencies can be difficult and 
expensive, as the antenna cannot be returned to a different frequency 
during operation except by connection to different transmitter and 
receiver circuits each having different tuned electrical characteristics. 
Another NMR logging apparatus, known as the Combinable Magnetic Resonance 
(CMR) logging tool, is described in U.S. Pat. No. 5,055,787 issued to 
Kleinberg et al. The CMR logging tool includes permanent magnets arranged 
to induce a magnetic field in the earth formation having substantially 
zero field gradient within a predetermined sensitive volume. The magnets 
are arranged in a portion of the tool housing which is typically placed in 
contact with the wall of the wellbore. The antenna in the CMR tool is 
positioned in a recess located external to the tool housing, enabling the 
tool housing to be constructed of high strength material such as steel. A 
drawback to the CMR tool is that its sensitive volume is only about 0.8 cm 
away from the tool surface and extends only to about 2.5 cm radially 
outward from the tool surface. Measurements made by the CMR tool are 
therefore subject to large error caused by, among other things, roughness 
in the wall of the wellbore, by deposits of the solid phase of the 
drilling mud (called "mudcake") onto the wall of the wellbore in any 
substantial thickness, and by the fluid content of the formation in the 
invaded zone. 
All of the prior art NMR well logging instruments described herein 
typically have antennas for generating the RF magnetic field and for 
receiving the NMR signals which are substantially the same length as the 
axial extent of the static magnetic field. A drawback to prior art NMR 
apparatus having such antenna dimensions is that measurements made which 
the instrument is moving are subject to significant error. The first 
source of error is that the RF magnetic field may be generated in a region 
different from that which is completely "prepolarized" by the static 
magnetic field. A second source of error is that the receiving antenna may 
be sensitive to an axial region which is different from the axial region 
in which the NMR signal is likely to originate, as the instrument is 
axially moved during measurement. 
Accordingly, it is an object of the present invention to provide an NMR 
well logging apparatus which provides more accurate measurements while the 
apparatus is moved axially through the wellbore. 
It is another object of the present invention to provide an NMR well 
logging apparatus which has substantially reduced effects of 
magnetoacoustic and magnetostrictive ringing. 
It is yet another object of the present invention to provide an NMR well 
logging apparatus which includes selectable RF pulse frequencies to 
generate NMR measurements at a plurality of preselected radial distances 
into the earth formation from the axis of the tool. 
SUMMARY OF THE INVENTION 
The present invention is a nuclear magnetic resonance sensing apparatus. 
The apparatus comprises a magnet for generating a static magnetic field in 
a first region containing materials which are to be analyzed. The magnet 
generates substantially zero static magnetic field within a second region. 
The magnet has generally homogeneous magnetization along a longitudinal 
axis and is magnetized substantially perpendicular to the longitudinal 
axis. The apparatus also includes means for generating a radio frequency 
magnetic field within the first region for exciting nuclei of the 
materials which are to be analyzed. The means for generating the radio 
frequency magnetic field is disposed within the second region. The 
apparatus includes receiving means for receiving a nuclear magnetic 
resonance signal from the excited nuclei. The means for receiving also 
provides an output indicative of properties of the materials which are to 
be analyzed. 
In a preferred embodiment of the invention, the means for generating and 
receiving comprise an antenna which is at least partially disposed within 
the second region. 
In a specific embodiment of the invention, the antenna includes wire coils 
which are wound in planes substantially perpendicular to the longitudinal 
axis of the magnet. A high magnetic permeability ferrite is included 
inside the wire coils to increase efficiency of the antenna. The antenna 
includes a frequency control coil wound around the ferrite to change the 
magnetic permeability of the ferrite, thereby changing the tuning 
frequency of the antenna.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
1. Configuration of the Apparatus 
FIG. 1 shows a well logging apparatus disposed in a wellbore 22 penetrating 
earth formations 23, 24, 26, 28 for making measurements of properties of 
the earth formations 23, 24, 26, 28. The wellbore 22 in FIG. 1 is 
typically filled with a fluid 34 known in the art as "drilling mud". A 
"sensitive volume", shown generally at 58 and having generally cylindrical 
shape, is disposed in one of the earth formations, shown at 26. The 
sensitive volume 58 is a predetermined portion of the earth formations 26 
in which nuclear magnetic resonance (NMR) measurements are made, as will 
be further explained. 
A string of logging tools 32, which can include an NMR apparatus according 
to the present invention, is typically lowered into the wellbore 22 by a 
means of an armored electrical cable 30. The cable 30 can be spooled and 
unspooled from a winch or drum 48. The tool string 32 can be electrically 
connected to surface equipment 54 by an insulated electrical conductor 
(not shown separately in FIG. 1) forming part of the cable 30. The surface 
equipment 54 can include one part of a telemetry system 38 for 
communicating control signals and data to the tool string 32 and computer 
40. The computer can also include a data recorder 52 for recording 
measurements made by the apparatus and transmitted to the surface 
equipment 54. 
An NMR probe 42 according to the present invention can be included in the 
tool string 32. The tool string 32 is preferably centered within the 
wellbore 22 by means of a top centralizer 56 and a bottom centralizer 57 
attached to the tool string 32 at axially spaced apart locations. The 
centralizers 56, 57 can be of types known in the art such as bowsprings. 
Circuitry for operating the NMR probe 42 can be located within an NMR 
electronics cartridge 44. The circuitry can be connected to the NMR probe 
42 through a connector 50. The NMR probe 42 is typically located within a 
protective housing 43 which is designed to exclude the drilling mud 34 
from the interior of the probe 42. The function of the probe 42 will be 
further explained. 
Other well logging sensors (not shown separately for clarity of the 
illustration in FIG. 1) may form part of the tool string 32. As shown in 
FIG. 1, one additional logging sensor 47 may be located above the NMR 
electronics cartridge 44. Other logging sensors, such as shown at 41 and 
46 may be located within or below the bottom centralizer 57. The other 
sensors 41, 46, 47 can be of types familiar to those skilled in the art 
and can include, but are not limited to, gamma ray detectors, formation 
bulk density sensors or neutron porosity detectors. Alternatively, parts 
of the NMR electronics may be located within electronic cartridges which 
form part of other logging sensors. The locations of the other sensors 41, 
46, 47 shown in FIG. 1 are a matter of convenience for the system designer 
and are not to be construed as a limitation on the invention. 
FIG. 2 shows the NMR probe 42 in more detail. The NMR probe 42 preferably 
comprises a generally cylindrical, permanent- or electro-magnet assembly 
60. The magnet assembly 60 can include at least one permanent magnet 62, 
which preferably has a substantially circular cross section and is 
generally elongated along a magnet axis 80. The magnet axis 80 is 
preferably positioned coaxially with the longitudinal axis 76 of the 
wellbore (22 in FIG. 1). Alternatively, a plurality of permanent magnets 
may be used to make up the magnet assembly 60. For clarity of the 
description of the invention, the one or more permanent magnets 62 will be 
considered together and referred to as permanent magnet 62, and their 
common axis 80 and the collocated axis of the wellbore (22 in FIG. 1) will 
be jointly identified herein as the longitudinal axis, shown at 78. 
The permanent magnet 62 preferably has substantially uniform magnetization 
along the longitudinal axis 78. The direction 82 of magnetization of the 
magnet 62, shown at 82 is preferably perpendicular to the longitudinal 
axis 78. The permanent magnet 62 should have an overall length along the 
longitudinal axis 78 which is greater than twice the dimension of the 
permanent magnet 62 perpendicular to the longitudinal axis 78. The overall 
length of the permanent magnet 62 should also generally be greater than 
twice the diameter of the sensitive volume 58, as will be further 
explained. 
The permanent magnet 62 preferably comprises a main permanent magnet 61, a 
top end magnet 63 located above the main permanent magnet 61 and a bottom 
end magnet 64 located below the main permanent magnet 61. The end magnets 
63, 64 are provided to reduce axial asymmetry of the static magnetic field 
generated by the permanent magnet 62 within the sensitive volume 58. 
The main permanent magnet 61 is preferably formed into an annular cylinder 
having a hole 83 of substantially circular cross section. The axis 81 of 
the magnet hole 83 is preferably parallel to the longitudinal axis 78. 
Details of the static magnetic field imparted by the permanent magnet 62 
within the sensitive volume 58 and within the magnet hole 83 will be 
further explained. It is to be understood that the cylindrical shape of 
the permanent magnet 62 and the hole 83 are preferred but not essential. 
An essential feature of the magnet 62 is that the direction of the static 
magnetic field induced by the magnet 62 be substantially perpendicular to 
the longitudinal axis 78 within the sensitive volume 58. If the shape of 
the magnet 62 is other than cylindrical, for example, elliptical, the hole 
83 should have the same general shape and the same ratio of long axis to 
short axis as the magnet 62 in order that the static magnetic field inside 
the hole 83 be substantially equal to zero, as will be further explained. 
The main permanent magnet 61 can be made from a ferrite magnet material 
such as that sold under the trade name "Spinalor" and manufactured by 
Ugimag, 405 Elm St., Valparaiso, Ind., or another material sold under the 
trade name "Permadure" and manufactured by Philips, 230 Duffy Ave., 
Nicksville, N.Y. The permanent magnet material of the main permanent 
magnet 61 should be electrically nonconductive, so that an antenna used to 
generate a radio frequency magnetic field can be located in the hole 83, 
as will be further explained. 
The top end magnet 63 and the bottom end magnet 64 may be formed from the 
same or similar ferrite permanent magnet material as is the main permanent 
magnet 61. Alternatively, the end magnets 63, 64 may be formed form 
magnetically stronger material such as a neodymium-iron-boron magnet alloy 
sold under the trade name "Ugistab" and manufactured by Ugimag, 405 Elm 
St., Valparaiso, Ind., or another material sold under trade name "Vacodym" 
and manufactured by Vacuumschmelze GMBH, 9/7 Rhenaniastrasse St., Berlin, 
Germany. Alternatively, the top end magnet 63 and the bottom end magnet 64 
may be formed from samarium-cobalt permanent magnet material such as one 
sold under trade name "Recoma" and manufactured by Ugimag, 405 Elm St., 
Valparaiso, Ind., or another sold under trade name "EEC" and manufactured 
by Electron Energy Corp., 924 Links Ave., Landsville, Pa. The material 
forming the top end magnet 63 and the bottom end magnet 64 need not be 
electrically non-conductive. 
The NMR probe 42 further includes the previously described transceiver 
antenna 67, which can comprise one or more coil windings 66 preferably 
arranged inside the hole 83 in the main permanent magnet 61. The coil 
windings 66 are preferably arranged so that each coil winding 66 lies 
substantially in a plane perpendicular to the longitudinal axis 78. Radio 
frequency alternating current passing through the coil windings 66 
generates an RF magnetic field in the earth formation 26 in FIG. 1). The 
RF magnetic field generated by the current flow in the coil windings 66 
has field directions substantially parallel to the longitudinal axis 78 
within the sensitive volume 58. 
The coil windings 66 have should have an overall length parallel to the 
longitudinal axis 78 which is about equal to the diameter of the sensitive 
volume 58. The overall length of the coil windings 66 parallel to the 
longitudinal axis 78 should also be substantially shorter than the overall 
length of the main permanent magnet 62 along the longitudinal axis 78, as 
will be further explained. 
Preferably, the coil windings 66 are formed around a soft ferrite rod 68. 
The soft ferrite rod 68 can be formed from a material such as one sold 
under trade designation "F6" and manufactured by MMG-North America, 126 
Pennsylvania Ave., Paterson, N.J., or another material sold under trade 
designation "3C2" and manufactured by Philips, 230 Duffy Ave., Nicksville, 
N.Y. The ferrite rod 68 preferably is positioned parallel to the 
longitudinal axis 78. The overall length of the ferrite rod 68 along the 
longitudinal axis 78 should be substantially less than the length of the 
permanent magnet 62 along the longitudinal axis 78. Alternatively, a 
plurality of coils and a plurality of ferrite rods may be employed. The 
assembly of coil windings 66 and soft ferrite rod 68 will be referred to 
hereinafter as the transceiver antenna 67. The ferrite rod 68 has the 
particular function of increasing the field strength of the RF magnetic 
field generated by the transceiver antenna 67. Using the ferrite rod 68 
particularly enables the transceiver antenna 67 to have a relatively small 
external diameter so that it can be located in the hole 83. Having a small 
external diameter particularly enables the transceiver antenna 67 of the 
present invention to be sized so that the apparatus of the present 
invention can be used in smaller diameter wellbores. 
The transceiver antenna 67 also can include a frequency control coil 101, 
which can be another wire coil wound around the ferrite rod 68. As will be 
further explained, a control voltage selectable by the system operator can 
be applied to the frequency control coil 101 to change the resonant 
frequency of the transceiver antenna 67. The purpose of changing the 
resonant frequency, and the source of the control voltage will be further 
explained. 
The NMR probe 42, can also comprise one or more additional receiver coils, 
such as the one shown generally at 70 (only the lead-in wires are shown in 
FIG. 2 for clarity) which are arranged about the exterior surface of the 
permanent magnet 62. Each turn (not shown in FIG. 2) of additional 
receiver coil 70 should lie in a plane substantially parallel to a plane 
containing both the magnetization axis 82 of the permanent magnet 62 and 
containing the longitudinal axis 78. Preferably the additional receiver 
coil 70 has an overall length parallel to the longitudinal axis 78 which 
is less than the overall length of the transceiver antenna 67. As a 
consequence, the overall length of the additional receiver coil 70 
parallel to the longitudinal axis 78 should be substantially shorter than 
the length of the permanent magnet 62 along the longitudinal axis 78. 
Alternatively, a plurality of additional receiver coils 70 may be included 
in the NMR probe 42. A particular property of the additional receiver coil 
70 arranged as described herein is that it is substantially orthogonal to, 
and consequently substantially insensitive to, the direct RF magnetic 
field generated by the transceiver antenna 67. This insensitivity to the 
direct RF field enables the additional receiver coil 70 to provide the 
apparatus of the present invention with very short "dead time", while the 
current flowing through the transceiver antenna 67 decays to zero, as will 
be further explained. 
Details of the synthesis of the RF magnetic field in the sensitive volume 
58 using the transceiver antenna 67, and details of detecting an induced 
NMR signal using the transceiver antenna 67 and/or the additional receiver 
coil 70 will be further explained. 
The permanent magnet 62, the transceiver antenna 67 and the additional 
receiver coil 70 are preferably housed within a non-conductive, 
non-ferromagnetic protective housing 43. Such housings and additional 
components (not shown) for excluding the drilling mud under high 
hydrostatic pressure, are familiar to those skilled in the art. 
FIG. 4 shows, in general form, the NMR probe 42 and a functional block 
diagram of the NMR well logging apparatus according to the present 
invention. A transmitter/receiver (T/R) matching circuit 45 can be 
disposed within the housing 43. The T/R matching circuit 45 typically 
includes a series of resonance capacitors (not shown separately), a 
transmitter/receiver switch (not shown separately) and both 
"to-transmitter" and "to-receiver" matching circuitry. The T/R matching 
circuit 45 can be coupled both to a radio frequency (RF) power amplifier 
74 and to a receiver preamplifier 73. While shown as located inside the 
housing 43 the T/R matching circuit 45, the RF power amplifier 74 and the 
receiver preamplifier 73 alternatively may be located outside the housing 
43 within the top centralizer (56 in FIG. 1) or within the NMR electronics 
cartridge (44 in FIG. 1 ). The locations of the T/R matching circuit 45, 
the RF power amplifier 74 and the receiver preamplifier 73 are not to be 
construed as a limitation on the invention. 
Part of the control circuitry for the NMR logging apparatus includes a 
down-hole computer 92, which among other functions provides control 
signals to a pulse programmer 91. The computer 92 and the pulse programmer 
91 may also be located within the top centralizer 56 or in the NMR 
electronics cartridge 44. The pulse programmer 91 controls the timing and 
operation of the variable frequency RF signal source 93. The RF driver 94 
receives an input from the variable frequency RF source 93 and provides an 
output to the RF power amplifier 74. The RF power amplifier 74 provides a 
high power signal to drive the transceiver antenna 67 for generating an RF 
magnetic field in the sensitive volume (58 in FIG. 1). The RF power 
amplifier 74 can be electrically connected (typically by the switch in the 
T/R matching circuit 45) to the transceiver antenna 67 during transmission 
of RF power pulses. 
During reception of the induced NMR signal, the transceiver antenna 67 
and/or the additional receiver antenna 70 can be electrically connected to 
the receiver preamplifier 73 by means of the switch in the T/R matching 
circuit 45. The output of the RF receiver preamplifier 73 is provided to 
an RF receiver 89. The RF receiver 89 also receives a phase reference 
input from a phase shifter 98. The phase shifter 98 receives a primary 
phase reference input from the variable frequency RF source 93. The RF 
receiver 89 may include quadrature detection. The RF receiver 89 provides 
an output to an A/D converter 96. The A/D converter 96 output can be 
stored in a buffer 97 until required for use by the down-hole computer 92. 
Alternatively, the buffer 97 contents can be conducted directly to a 
downhole part of the telemetry unit 99 for transmission to the surface 
equipment (54 in FIG. 1). 
The downhole computer 92 typically preprocesses the data from the buffer 97 
and transfers the preprocessed data to the downhole portion of the 
telemetry system, shown generally at 99. The downhole portion of the 
telemetry system 99 transmits the preprocessed data to the telemetry unit 
(38 in FIG. 1) in the surface equipment (54 in FIG. 1). The telemetry unit 
38 transfers the data to the surface computer (40 in FIG. 1) for 
calculating and presenting desired well logging output data for further 
use and analysis as is understood by those skilled in the art. 
All of the elements described herein and as shown in FIG. 4, except the 
transceiver antenna 67, the magnet assembly (60 in FIG. 2) and the 
additional receiver antenna 70, at the convenience of the system designer 
may be disposed within the housing 43, the top centralizer (56 in FIG. 1) 
or the NMR electronics cartridge (44 in FIG. 1). These same elements may 
alternatively be located at the earth's surface, for example in the 
surface equipment 54 using the cable (30 in FIG. 1) for transmission of 
electrical power and signals to the transceiver antenna 67 and the 
additional receiver antenna 70. 
FIG. 5 illustrates the static magnetic field and the RF magnetic field 
created by the NMR well logging apparatus of the present invention. The 
direction of the static magnetic field generated by the permanent magnet 
(62 in FIG. 2) is shown by arrows 110. Nuclear magnetic moments in the 
material to be analyzed (the earth formation located within the sensitive 
volume 58) are substantially aligned in the direction of the static 
magnetic field. In the preferred embodiment of the invention, the 
direction of the RF magnetic field, denoted by arrows 120, within the 
sensitive volume 58 is substantially perpendicular to the static magnetic 
field at any point within the sensitive volume 58. Such a magnetic field 
arrangement is conventional for NMR experiments. 
Although the static magnetic field direction is not symmetrical about the 
longitudinal axis 78 (the field direction undergoes two rotations for each 
circumlocution of the longitudinal axis 78), the static magnetic field 
magnitude is symmetric about the longitudinal axis 78. The static magnetic 
field has an amplitude gradient which is also symmetrical about the 
longitudinal axis 78 and is directed substantially radially inwardly 
towards the longitudinal axis 78. As a result there is generally only one 
substantially cylindrical surface external to the permanent magnet 62 
which has a particular static magnetic field amplitude (ignoring end 
effects of the magnet). It follows from this particular feature of the 
static magnetic field that stray resonance signals from diverse materials 
such as the drilling mud (34 in FIG. 1), which originate outside of the 
sensitive volume 58 do not seriously affect the NMR measurements if 
appropriate RF frequencies are selected. 
As previously explained, the transceiver antenna 67 can include the 
frequency control coil 101. A DC voltage having a magnitude selectable by 
the system operator can be applied to the frequency control coil 101 to 
partially magnetize the ferrite rod 68. Circuitry for providing the 
selectable DC voltage to the frequency control coil 101 is well known in 
the art and is not shown in FIG. 5 for clarity of the illustration. The DC 
source (not shown) can be directly controlled by the system operator, or 
can be controlled by the down hole computer (92 in FIG. 4) in response to, 
among other things, the radial depth at which the sensitive volume (58 in 
FIG. 2) is positioned. Changes in the radial depth of the sensitive volume 
58 can occur, for example, as a result of environmental changes in the 
static magnetic field induced by the magnet (62 in FIG. 2). Partially 
magnetizing the ferrite rod 68 changes its magnetic permeability, and as a 
consequence, changes the inductance of the transceiver antenna 67. 
Changing the inductance of the transceiver antenna 67 changes its resonant 
frequency so that it can be tuned to nearly any frequency within a wide 
range. The RF power pulse generated by the RF source 93 and the RF power 
amplifier 73 can then be efficiently converted by the transceiver antenna 
67 into a strong RF magnetic field within the sensitive volume 58. As is 
understood by those skilled in the art, because the static magnetic field 
has a non-zero magnitude gradient with respect to radial distance from the 
longitudinal axis 78, changing the RF field frequency will change the 
radius of the sensitive volume 58. A particular advantage of the using the 
control coil 101 according to the present invention is the ability to 
change the RF frequency very easily while substantially maintaining the 
amplitude distribution of the RF field. 
Undesired static magnetic field end effects may be substantially eliminated 
by making the transceiver antenna 67 somewhat shorter along the 
longitudinal axis 78 than the permanent magnet 62, so as not to excite 
materials at the extreme longitudinal ends of the static magnetic field. 
To reduce the required length of the permanent magnet 62, the end magnets 
63 and 64 may be utilized, as previously explained. 
When RF power pulses are conducted through the transceiver antenna 67, the 
antenna 67 generates an RF equivalent magnetic dipole 87 centered at the 
origin and directed along the longitudinal axis 78. The equivalent 
magnetic dipole 87 generates an RF magnetic field of substantially equal 
amplitude within the sensitive volume 58, directed opposite to the dipole 
direction. Since the RF magnetic field direction is parallel to the 
longitudinal axis 78, the bulk nuclear magnetization, denoted in FIG. 5 by 
arrows 130, at any point in the sensitive volume 58 rotates in planes 
perpendicular to the longitudinal axis 78. The free precession of the 
nuclear magnetic moments, however, is around the static magnetic field 
direction at any point within the sensitive volume 58, and the free 
precession is always in phase along the longitudinal axis 78. The free 
precession will therefore induce an RF signal in the transceiver antenna 
67. The induced magnetic moment in the transceiver coil 67 is shown in 
FIG. 5 as arrows 140. 
Those skilled in the art of nuclear magnetic resonance measurements will 
readily comprehend that the free precession of the bulk nuclear 
magnetization about the static magnetic field will also induce an RF 
signal in the additional receiver coil 70, this signal shown in FIG. 5 as 
arrows 150. The signal induced in the additional receiver coil 70 is 
directionally rotated 90 degrees (orthogonal) with respect to the signal 
which is induced in the transceiver coil 67. Because the transceiver coil 
67 is substantially orthogonal to the additional receiver coil 70, during 
transmission of the RF pulse, there is substantially zero signal directly 
induced the additional receiver coil 70. As a result, the dead time of the 
whole receiving system may be reduced significantly with respect to prior 
art NMR apparatus having only a single transceiver antenna. 
FIG. 3A shows an embodiment of the transceiver antenna 67 and the 
additional receiver 70 which further improves the performance of the 
apparatus of the present invention. The transceiver antenna 67 includes 
lead-in wires 267 and 367 which are connected, as previously explained to 
the T/R matching circuit (45 in FIG. 4). Similarly the additional receiver 
coil 70 includes lead-in wires 270 and 370 to connected to the T/R 
matching circuit 45. Small wire loops, shown generally at 167 and 170 can 
be positioned, respectively, in either of the lead in wires for the 
transceiver antenna 67 and the additional receiver coil 70. The wire loops 
167, 170 are preferably adjusted by passing RF current through the 
transceiver antenna 67 while observing the voltage on the additional 
receiver coil 70. The wire loops 167, 170 should be adjusted to 
substantially eliminate any voltage being induced in the additional 
receiver coil 70 by the current passing through the transceiver antenna 
67. The step of adjusting the wire loops 167, 170 is preferably performed 
while the NMR probe 42 is suspended in air. 
Orthogonal transmission and reception of the RF signals has an additional 
advantage when permanent magnets are employed and the system dead time has 
to be as short as possible. Prior art NMR logging tools typically suffer 
high levels of magnetoacoustic and magnetostrictive ringing. The means by 
which the present invention reduces such ringing will be further 
explained. 
Another particular advantage of the present invention is the presence of a 
substantially constant static magnetic field amplitude and static field 
amplitude gradient in the materials to be analyzed within the sensitive 
volume 58. This feature can be used for direct measurement of the 
diffusion coefficient of liquid present in the material to be analyzed, as 
explained for example in C. P. Slichter, Principles of Magnetic Resonance, 
Appendix G, Springer Verlag Berlin Heidelberg, New York, 1980. The 
amplitude gradient of the static magnetic field can be used to generate a 
diffusion measurement particularly by adjusting the frequency of the RF 
magnetic field, as previously explained, to first generate the sensitive 
volume 58 where the static magnetic field has a gradient which exceeds 
internal magnetic field gradients of the materials to be analyzed. A 
static field gradient which will perform according to this aspect of the 
invention can be about 30 Gauss/cm. The NMR signal can be received from 
this same sensitive volume 58 at the same frequency. The sensitive volume 
58 can then be moved by adjusting the RF magnetic field frequency to be 
positioned where the static magnetic field is generally less than the 
internal gradients in the materials to be analyzed, generally 
corresponding to a static field gradient of about 5 Gauss/cm. 
The gradient of the static magnetic field can also be utilized to perform 
radial fluid flow measurements by exciting the nuclei using RF pulses 
having a first frequency, and receiving the induced NMR signal at a second 
frequency. This is equivalent to exciting the nuclei at one radial 
distance from the wellbore 22 and receiving the signal therefrom at 
another radial distance from the wellbore 22. 
2. Design Parameters for the Preferred Embodiment 
In the preferred embodiment of the invention, the signal-to-noise ratio 
(S/N) for the NMR measuring process is sought to be optimized. The 
following discussion is intended to explain how certain principal 
parameters affect the S/N. The principal parameters typically include the 
geometries of the permanent magnet (62 in FIG. 2) and the transceiver 
antenna (67 in FIG. 2), the power of radio frequency (RF) pulses used to 
energize the transceiver antenna 67, and the quality factor of the 
transceiver antenna 67. 
Using the transceiver antenna 67 constructed as previously described in the 
present embodiment of the invention, the magnitude of an NMR signal, S, 
induced in the transceiver antenna 67 is typically related to the 
magnitude of an RF electromagnetic field, B.sub.1, by the Reciprocity 
Theorem and can be described as in the following expression: 
EQU S=.omega.mA.sub.sv (B.sub.1 /I.sub.1)l (1) 
where m and A.sub.sv, respectively, represent the nuclear magnetization and 
the cross sectional area of the sensitive volume (58 in FIG. 1), I.sub.1 
represents the magnitude of the current flowing in the transceiver antenna 
67, the oscillating frequency of the current is represented by .omega. and 
l represents the effective length of the transceiver antenna 67. For 
simplicity of the discussion, m and B.sub.1 are assumed to be 
substantially homogeneous within the sensitive volume 58. 
By substituting m=.chi.B.sub.0 /.mu..sub.0 ; where .chi. represents the 
nuclear magnetic susceptibility of hydrogen nuclei within the sensitive 
volume 58, .omega.=.gamma.B.sub.0, where B.sub.0 represents the static 
magnetic field generated by the permanent magnet (62 in FIG. 2) and 
described in equation (1), it is therefore possible to derive the 
following expression for S: 
EQU S=(.gamma..chi./.mu..sub.0)B.sub.0.sup.2 (B.sub.1 /I.sub.1)A.sub.sv l(2) 
The NMR signal thus acquired is therefore directly proportional to the 
sensitive volume 58 in the earth formation (26 in FIG. 1). The geometry of 
the sensitive volume 58 is determined by the existence of a resonance 
condition. In pulsed NMR, the resonance condition is typically met when 
the deviation of the static magnetic field magnitude B.sub.0 (R) from its 
value B.sub.0 (R.sub.sv), corresponding to the central frequency of the 
current energizing the transceiver antenna 67 (B.sub.0 
(R)=.omega./.gamma.), is no greater then half the magnitude of the RF 
magnetic field B.sub.1 induced by passing current through the transceiver 
antenna 67, expressed as shown in equation (3): 
EQU B.sub.0 (R)-B.sub.0 (R.sub.5).ltoreq.B.sub.1 /2 (3) 
The static magnetic field B.sub.0 (R) at the excitation radius R.sub.sv may 
also be described in the form of a Taylor expansion as: 
EQU B.sub.0 (R)=B.sub.0 (R.sub.sv)-(.differential.B.sub.0 
/.differential.R)(R-R.sub.sv) (4) 
where (.differential.B.sub.0 /.differential.R) represents the static 
magnetic field gradient at radius R=R.sub.sv. From equation (3): 
EQU B.sub.0 (R.sub.0)-B.sub.0 (R.sub.i ).ltoreq.B.sub.1 (5) 
where R.sub.0 and R.sub.i represent, respectively, the outer and inner 
radii of the sensitive volume 58. As a practical matter R.sub.0 -R.sub.i 
&lt;&lt;R.sub.exc. 
EQU A.sub.sv =2.pi.R.sub.sv B.sub.1 /(.differential.B.sub.0 /.differential.R)(6 
) 
EQU B.sub.0 =A.sub.m B.sub.r /2.pi.R.sub.sv.sup.2 (7) 
where A.sub.m represents the permanent magnet 62 cross sectional area. From 
equations (6) and (7): 
EQU A.sub.sv =(B.sub.1 /B.sub.0).pi.R.sub.sv.sup.2 (8) 
The current flowing in the transceiver antenna 67 may be expressed as 
I.sub.1 =(P.sub.1 /r).sup.1/2, where P.sub.1 represents the peak power of 
the RF pulse energizing the antenna 67, r represents the active part of 
the antenna 67 impedance. Therefore: r=.omega.L/Q=.gamma.B.sub.0 L/Q. 
Substituting for equation (2) yields the expression: 
EQU S=(.pi..chi./.mu..sub.0)(.gamma.B.sub.0).sup.1/2 (P.sub.1 Q/L).sup.1/2 
(B.sub.1 /I.sub.1).sup.2 R.sub.sv.sup.2 l (9) 
As is understood by those skilled in the art, the root-mean-square (RMS) 
thermal noise can be described by the expression: 
EQU N.sub.rms =(4kT.DELTA..function.r).sup.1/2 (10) 
where .DELTA..function. represents the receiver bandwidth. The bandwidth is 
typically about .gamma. B.sub.1 /2.pi. for a matched receiver; k 
represents Boltzmann's constant; and T represents the absolute 
temperature. 
Then substituting for equations (9) and (10) yields the following 
expression for S/N: 
EQU S/N=(2kT).sup.-1/2 .pi..sup.3/2 (.chi./.mu..sub.0)(B.sub.0 
/.gamma.).sup.1/4 R.sub.sv.sup.2 !(B.sub.1 /I.sub.1).sup.3/2 
P.sub.1.sup.1/4 (Q/L).sup.3/4 l! (11) 
The first bracketed expression in equation (11), for a given proton spin 
density and absolute temperature, depends only on the static magnetic 
field parameters and the radius of the sensitive volume 58. The second 
bracketed expression in equation (11) describes parameters used in the 
design of the transceiver antenna 67, as will be further explained. 
Synthesis of the Radio Frequency Magnetic Field 
The following description is provided to assist in developing the design 
parameters for the transceiver antenna (shown as 67 in FIG. 2). In the 
present description the transceiver antenna 67 can be described as a pair 
of magnetic charges placed at the ends of the transceiver antenna 67. The 
longitudinal component of an RF magnetic field generated in the center 
plane of the transceiver antenna 67, created by passing RF power through 
the transceiver antenna 67, can be described by the following expression: 
EQU B.sub.1 =q.sup.m (l/4.pi.)/R.sup.2 +(l/2).sup.2 !.sup.3/2 (12) 
wherein q.sup.m =.mu..sub.0 .mu..sub.rod (.pi.d.sup.2 /4)I.sub.1 n/l. In 
equation (12), q.sup.m represents the effective magnetic charge, 
.mu..sub.0 represents the magnetic permeability of free space, 
.mu..sub.rod represents the magnetic permeability of the ferrite rod 
(shown as 68 in FIG. 2); d represents the diameter of the ferrite rod 68, 
I.sub.i represents the current flowing in the transceiver antenna 67, n 
represents the number of coil turns in the transceiver antenna's 67 coil 
windings (66 in FIG. 2), l represents the transceiver antenna 67 length, 
and R represents the radius of the sensitive volume (shown as 58 in FIG. 
2). 
It is to be noted that the proportionality to antenna length (l) in 
equation (12) suggests improvement in S/N with respect to increasing l, 
until l is limited by the vertical resolution requirements of the 
apparatus. 
In the absence of the ferrite rod 68 inside the antenna coil (66 in FIG. 
2), .mu..sub.rod =1, and for a fixed value of n: 
EQU B.sub.1 /I.sub.1 .varies.l.sup.-3 1+(2R/l).sup.2 !.sup.-3/2(13) 
Without the ferrite rod (68 in FIG. 2) inside the transceiver antenna (67 
in FIG. 2), the result indicated by equation (13) indicates that the 
transceiver antenna 67 would have low efficiency. 
For a high permeability (.mu.) ferrite rod 68 material, .mu..sub.rod is 
mainly determined by the length-to-diameter ratio of the transceiver 
antenna 67. For those skilled in the art it should be apparent that 
.mu..sub.rod .apprxeq.1/D, where D represents a "demagnetizing factor" of 
the ferrite rod 68. FIG. 6 shows in graphic form the dependence of 
.mu..sub.rod on the ratio of l/d, based on D values described by R. M. 
Bozort, "Ferromagnetism", D. Van Nostroud Company, Inc. New York, 1951. A 
simple approximation of this dependence for large length-to-diameter 
ratios can be described by the following expression: 
EQU .mu..sub.rod .apprxeq.0.35(l/d).sup.2 (14) 
Substituting for equation (12) yields the expression: 
EQU B.sub.1 /I.sub.1 .apprxeq.0.18.mu..sub.0 nl.sup.-1 1+(2R/l).sup.2 
!.sup.-3/2 (15) 
The approximation shown in equation (15) demonstrates that the ratio 
B.sub.1 /I.sub.1 is not dependent on d and the ratio has relatively 
constant values within a range for l comprising 2R&lt;l&lt;5R. A weak maximum in 
the ratio occurs at l=2.sqroot.2R. Because the aperture of the transceiver 
antenna 67 which is required for use in a well logging tool does not 
typically exceed a value of 5R, the reduction in RF field strength, 
expressed as B.sub.1 /I.sub.1, with respect to increasing l, may be 
substantially neglected when the antenna 67 includes the ferrite rod 68. 
Such behavior of the antenna 67 makes the antenna 67 (referred to as a 
longitudinal dipole antenna) including the ferrite rod 68 similar in 
electromagnetic response to the transversal RF dipole antennas employed in 
the prior art, from the standpoint of signal accumulation by using an 
antenna of maximum possible length. A transversal dipole antenna, for 
comparison, is described in U.S. Pat. No. 4,710,713 issued to Taicher et 
al. 
For a typical two-dimensional transversal RF dipole antenna as described in 
the Taicher et al '713 patent, supra, the ratio B.sub.1 /I.sub.1 can be 
described by the following expression: 
EQU B.sub.1 /I.sub.1 =.mu..sub.0 nR.sub.a /4R.sup.2 (16) 
where R.sub.a represents the transceiver antenna 67 dipole radius, which 
radius is primarily restricted to the radius of the wellbore (22 in FIG. 
1). 
It can be determined by reviewing equations (15) and (16) that the rate of 
reduction in the RF field strength for transverse dipole antenna is much 
greater than for the longitudinal dipole antenna (the transceiver antenna 
67 in FIG. 2) according to the present invention. This gives the present 
invention the particular advantage of making possible NMR measurements at 
increased radial depth of investigation into the earth formation (26 in 
FIG. 1) over the prior art using the transversal dipole type antenna. 
Moreover, the RF magnetic field generated by the longitudinal dipole 
transceiver antenna (67 in FIG. 2) of the present invention, which 
includes the ferrite rod 68, is substantially independent of the diameter 
of the wellbore 22. By contrast, the RF magnetic field generated by a 
transverse dipole antenna, as in the prior art, depends linearly on 
R.sub.a. The longitudinal dipole antenna (transceiver antenna 67 in FIG. 
2) of the present embodiment of the invention is therefore particularly 
suitable for use in small diameter wellbores. 
The inductance L of the coil windings (66 in FIG. 2) can be calculated from 
the expression: L=.mu..sub.0 .mu..sub.rod (.pi.d.sup.2 /4)n.sup.2 /l. Then 
substituting .mu..sub.rod as defined in equation (14) yields the 
expression for inductance: 
EQU L=(0.35.mu..sub.0 .pi./4)n.sup.2 l (17) 
Substituting equations (17) and (5) into equation (16) yields the following 
expression for S/N: 
EQU S/N.varies.l.sup.-5/4 1+(2R/l).sup.2 !.sup.-9/4 (18) 
In the preceding discussion one simplifying assumption is that the Q of the 
transceiver antenna 67 does not depend on l of the transceiver antenna 67. 
Equation (18) typically has a maximum at l=(2.sqroot.2.6)R.sub.sv which 
should be taken into account in construction of the transceiver antenna 67 
according to the present embodiment of the invention. 
Referring once again to FIG. 2, the relative dimensions of the transceiver 
antenna 67 should be selected in order to optimize S/N. The ratio of 
antenna length l to the radius of the sensitive volume 58 should be in a 
range of approximately 3-5. The diameter of the ferrite rod 68 should no 
be so large as to ensure that .rho..sub.rod &lt;&lt;.mu.. For typical values of 
.mu., which can be in the range from 1500-2000, the l/d ratio of the 
ferrite rod 68 should generally not exceed 40. The ferrite rod 68 diameter 
is approximately limited to the diameter of the hole 83 in the permanent 
magnet 62. It is also important to note that the diameter of the ferrite 
rod 68 should be as large as practical within the limits of the diameter 
of the hole 83 to minimize magnetic flux density in the ferrite rod 68 and 
consequently to minimize specific power loss (maximize the Q of the 
transceiver antenna 67) when the RF pulses are conducted through the 
transceiver antenna 67. Magnetization dynamics in ferrite materials 
causing power loss in oscillating fields are discussed, for example in A. 
Reiderman, Magnetic Characterization of Recording Media, UB Ac. Sc. USSR, 
Part 1, p. 37, 1990. 
The number of turns, n, used in the coil windings 66 is preferably selected 
to simplify transmitter/receiver matching. In the preferred embodiment of 
the invention wherein the apparatus (32 in FIG. 1) is to be used in a 6 
inch diameter wellbore (22 in FIG. 1) and to have a 12 inch diameter 
sensitive volume 58, the ferrite rod 68 dimensions are typically 40 cm and 
1.5 cm for l and for d, respectively, with 3 turns (n=3) on the coil 66. 
FIG. 8 shows a graphic representation of the RF field distribution at the 
radius of the sensitive volume 58 (this radius being about 6 inches). 
Synthesis of the Static Magnetic Field 
Referring once again to FIG. 2, the magnet assembly 61 including the magnet 
hole 83 are shown. The magnet assembly 61 also typically includes the end 
magnets 63 and 64. The magnet assembly 61 produces a substantial magnetic 
field within the sensitive volume 58, but produces substantially zero 
magnetic field inside the magnet hole 83, where the transceiver antenna 67 
is preferably placed. The magnet assembly 61 prepolarizes nuclei in the 
formation (26 in FIG. 1) to ensure a steady state nuclear magnetization 
measurement even while the NMR probe 42 is moving through the wellbore (22 
in FIG. 1). 
Assuming first that the magnet assembly 61 is long enough so that end 
effects may be neglected, the magnetostatic analysis may be reduced to a 
two-dimensional problem. For those skilled in the art of magnetic field 
synthesis from permanent magnet sources, it should be apparent that there 
is substantially zero magnetic field inside a circular cylindrical hole in 
a circular cylindrical permanent magnet which is magnetized uniformly 
perpendicular to the cylindrical axis. For example, in Manlio G. Abele, 
Structure of Permanent Magnets, John Wiley & Sons, pp. 42-66, 1993 it is 
shown that the field inside a permanent magnet cylinder magnetized 
perpendicularly to its axis is uniform and is equal to B.sub.1 /2, where 
B.sub.r represents the remanence magnetization of the permanent magnet 
material. The hole 83 in the main permanent magnet 61 may be represented 
as a superposition of two permanent magnet cylinders of the same magnet 
material being magnetized to the same value of B.sub.r but having opposite 
magnetization directions. Since each of the superimposed magnets in this 
representation produces the same magnetic field strength, equal to B.sub.r 
/2, but in opposite directions, there exists substantially zero magnetic 
field strength inside the hole 83. Furthermore, when the permanent magnet 
cylinder 62 and the hole 83 are coaxial with each other, the magnetic 
field direction outside the permanent magnet 62, having the hole 83 as 
shown in FIG. 2 is the same as for a solid cylindrical permanent magnet. 
Only the field strength is reduced in proportion to reduction of the cross 
sectional area of the magnet assembly 61 by including the hole 83. 
To keep the length of the magnet 61 as short as is practical, it is 
preferable to compensate end effects by using the end magnets 63, 64 as 
shown in FIG. 2 and previously described herein. FIG. 8 shows a graphic 
representation of the effect of the end magnets (63, 64 in FIG. 2) on the 
magnetic field inside the magnet hole (83 in FIG. 2) as well as outside 
the magnet (62 in FIG. 2) at a 12 inch diameter sensitive volume (58 in 
FIG. 2). The graph of FIG. 8 represents the magnetic field generated by 
the magnet assembly 61 which is especially suitable for use in slim bore 
holes. The permanent magnet 62 for use in slim wellbores can have a 6.6 cm 
diameter and 100 cm length. The permanent magnet 62 can be formed from 
ferrite permanent magnet material such as sold under trade name "Spinalor" 
and manufactured by Ugimag, 405 Elm St., Valparaiso, Ind., or sold under 
trade name "Permadure" and manufactured by Philips, 230 Duffy Ave., 
Nicksville, N.Y. The magnet material described herein has 0.42 T remanence 
induction. The top end magnet 63 and the bottom end magnet 64 can also be 
6.6 cm diameter cylinders about 18.5 cm in length and placed at a distance 
of 3.5 cm from the ends of the main magnet 62. The end magnets 63, 64 can 
be made from a permanent magnet material such as neodymium-iron-boron 
material sold under trade name "Ugistab" and manufactured by Ugimag, 405 
Elm St., Valparaiso, Ind. or sold under trade name "Vacodym" and 
manufactured by Vacuumschmelze GMBH, 9/7 Rhenaniastrasse St., Berlin, 
Germany. The neodymium-iron-boron material typically has a remanence 
induction of about 1 T. 
3. Magnetoacoustic and magnetostrictive ringing 
As is understood by those skilled in the art, determination of properties 
of interest of the earth formations (such as 26 in FIG. 1) require that an 
NMR well logging instrument be able to measure short duration values of a 
magnetic resonance parameter referred to as T2. Some nuclear magnetic 
resonance phenomena decay in amplitude very quickly, as is understood by 
those skilled in the art. In order to measure these short duration events, 
the NMR well logging apparatus should have as short "dead time" as is 
practical. Dead time of an NMR logging system is affected by, among other 
things, magnetoacoustic interaction which may produce an unwanted signal 
in the transceiver antenna (such as 67 in FIG. 2). This section of the 
description of the preferred embodiment will explain how the NMR logging 
apparatus of the present invention reduces the effects of magnetoacoustic 
interaction to reduce the dead time. 
Different types of magnetoacoustic interaction may produce a parasitic 
signal in the NMR antenna. Antenna wiring and other metal parts of the NMR 
probe (42 in FIG. 2) can be affected by the permanent magnet's (62 in FIG. 
2) magnetic field and the RF field generated by passing RF pulses through 
the transceiver antenna 67. These fields can produce spurious "ringing" 
which is well known to those skilled in the art as "coil disease". This 
type of ringing is excited by the Lorenz force. As explained in E. 
Fukushima et. al., Spurious Ringing in Pulse NMR, J. Magn. Res. v.33, pp. 
199-203, 1979, the efficiency of conversion of RF radiation into 
acoustical waves, and vice versa, is directly proportional to square of 
the static magnetic field intensity at the location of the antenna. 
In the present invention, the RF transmitting antenna (referred to as the 
transceiver antenna and shown at 67 in FIG. 2) is positioned in the magnet 
hole (83 in FIG. 2), wherein there is substantially zero static magnetic 
field from the permanent magnet 62. This type of magnetoacoustic ringing 
is substantially eliminated by the transceiver antenna 67 configuration of 
the present invention. 
Another source of magnetoacoustic interaction is magnetostrictive ringing. 
Magnetostrictive ringing is typically caused when non-conductive magnetic 
material, such as magnetic ferrite are used in the antenna. The 
magnetoelastic interaction in the magnetically soft ferrite rod (68 in 
FIG. 2) used in the transceiver antenna 67 and the hard ferrite used in 
the permanent magnet 62 are different from each other. 
Magnetostrictive ringing of the magnetically soft ferrite rod (68 in FIG. 
2) in the antenna 67 is removed if cessation of the RF power pulse leaves 
the ferrite 68 completely demagnetized. This magnetization condition is 
met within the magnet hole 83. 
A spurious signal generated by the permanent magnet 62, which continues to 
vibrate upon cessation of the RF pulse is a direct consequence of the 
inverse effect of magnetostriction. Two features of the present invention 
substantially reduce ringing of the magnet 62. First, the radial 
dependence of the RF field strength, as previously explained herein, is 
relatively small when compared with that of prior art NMR logging 
instruments. The relatively small radial dependence is a result of the use 
of the longitudinal dipole antenna with the ferrite rod (67 and 68 in FIG. 
2). Second is the use of an orthogonal receiver coil. In the present 
invention, the additional receiver coil (70 in FIG. 2) is substantially 
orthogonal to the transceiver coil 67 and so meets this requirement. FIG. 
9 shows a graph of the radial dependence of the RF field strength for the 
longitudinal (transceiver antenna 67), at curve 9-1, and for the 
transversal dipole (additional receiver coil 70) antenna at curve 9-2, 
from which it is apparent that the RF field affecting the permanent magnet 
62 does not significantly exceed B.sub.t at radius R=R.sub.sv. That 
magnitude of RF magnetic field is typically not sufficient to effectively 
excite acoustic waves. The orthogonal receiver antenna (additional 
receiver coil 70) in the preferred embodiment of the invention 
substantially removes coupling of the additional receiver coil (70 in FIG. 
2) with parasitic magnetic flux due to the inverse effect of 
magnetostriction. 
Thus, the total magnetoacoustic ringing in the NMR probe (42 in FIG. 1) of 
the present invention is reduced significantly compared with NMR logging 
instruments of the prior art. 
4. Considerations for Making Measurements While Moving the NMR Probe Within 
the Wellbore 
FIG. 3 shows a sectional view of the apparatus of FIG. 1 taken in a plane 
parallel to the axis of the permanent magnet 62 (indicated by lines II--II 
in FIG. 1). The sectional view in FIG. 3 more clearly illustrates the 
relative dimensions of the permanent magnet 62, the transceiver antenna 67 
and the additional receiver antenna 70. In well logging practice there are 
two particularly common sets of wellbore conditions which should be 
accounted for in building the NMR logging apparatus according to the 
present invention. In the first set of conditions the nominal diameter of 
the wellbore (22 in FIG. 1) is within a range of 7 inches to 12 inches. 
The external diameter of the NMR probe 42 for use in this range of 
wellbore diameters can be about 6 inches. 
The present invention has the capability of selectively varying the RF 
frequency which enables illustration of a particular advantage of the 
present invention. The advantage will be illustrated by the following 
example: assume the wellbore 22 diameter to be 8 inches and the sensitive 
volume (58 in FIG. 1) diameter selected to be 20 inches and 36 inches. The 
permanent magnet 62 axial length can be about 40 inches. This axial length 
for the permanent magnet 62 can provide about 30 inch axial length having 
substantially equal axial strength static magnetic field in the earth 
formation (26 in FIG. 1). The static magnetic field strength decreases 
monotonically with increasing radial distance from the longitudinal axis 
78. In the preferred embodiment of the invention the hydrogen nuclei in 
the sensitive volume 58 are prepolarized by the static magnetic field 
almost at equilibrium. The transceiver antenna 67 has axial length of 
about 24 inches and generates an adequate strength RF magnetic field for 
NMR experiments along a 24 inch long cylindrical volume. The transceiver 
antenna 67 can be positioned in the magnet hole 83 so that the cylindrical 
volume of the RF field can be positioned near the lowermost part of the 
static magnetic field's cylindrical volume. The receiving antenna can be 
about 18 inches long and is positioned to receive the NMR signal mainly 
from a cylindrical volume which can be located near the lowermost part of 
the static magnetic field volume. The present embodiment of the invention 
provides a static magnetic field long enough so that the NMR probe 42 may 
move a significant axial distance while still applying an RF magnetic 
field which is disposed entirely within the region of the earth formation 
which is prepolarized by the static magnetic field. The receiver antenna 
aperture of the present invention is such that the NMR probe 42 may move a 
significant axial distance while enabling the receiver antenna to receive 
NMR signals only from those volumes which have been completely energized 
by the RF field. The present invention is therefore capable of performing 
a proper steady state Carr-Purcell-Meiboom-Gill (CPMG) measurement 
sequence run entirely within in a cylindrical volume 18 inches long. It is 
to be understood that the relative axial positions of the permanent magnet 
62, transceiver antenna 67 and additional receiver antenna 70 are intended 
only as an example for a probe intended for use in more common well 
logging applications in which the measurements are made while the probe is 
withdrawn from the wellbore (22 in FIG. 1). It is to be understood that 
the axial length and positions of the magnet 62 and antennas 67, 70 could 
as easily be adapted for logging while the instrument is lowered into the 
wellbore 22 by reversing the relative axial positions of the magnet 62 and 
antennas 67, 70. 
The second set of conditions includes wellbores having nominal diameters 
between about 4 inches and 7 inches. The NMR probe 42 external diameter in 
this example can be about 3 3/8 inches. The present example includes a 
wellbore 22 having a diameter of about 5 inches, and sensitive volume 58 
diameters of about 7 inches and 12 inches. The permanent magnet 62 axial 
length in this example can be about 80 cm. This axial length for the 
permanent magnet 62 provides about 45 cm length of axially equal magnetic 
field strength. Referring now to FIG. 10, an arrangement of a high 
vertical resolution RF antenna is presented. The main part 168 of the 
transceiver antenna 67 can be about 15 cm length and 1 cm diameter. The 
prepolarizing part 69 of the antenna 67 can be about 7.5 cm length and 1 
cm diameter and is typically placed at a distance about 1 cm from the main 
part 168. A compensating receiver coil 71 serves to compensate of the 
magnetizing effect of the main part 168 on the prepolarizing part 69. FIG. 
11 shows a graph of the spatial distribution of the effective RF field 
(orthogonal to static magnetic field component of RF field) and the 
antenna receiving sensitivity function which is presented in the form of 
the RF field distribution. The effect of compensating the receiver coil 66 
is also illustrated. 
DESCRIPTION OF AN ALTERNATIVE EMBODIMENT 
As is understood by those skilled in the art, the wellbore (22 in FIG. 1) 
can sometimes have a large enough diameter, due to "washouts" or similar 
effects known in the art to cause the sensitive volume (58 in FIG. 1) of 
the first embodiment of the invention to be positioned within the wellbore 
22 itself rather than wholly within the earth formation (such as 26 in 
FIG. 1). An alternative embodiment of the present invention particularly 
suited for use in such situations can be better understood by referring to 
FIG. 12. The permanent magnet 62A, which in the first embodiment of the 
invention (62 in FIG. 2) includes a magnet hole (83 in FIG. 2), in the 
present embodiment includes a magnet hole 83A which is radially displaced 
towards the outer surface of the magnet 62A. A transceiver antenna 67A, 
which can be substantially the same in design as the transceiver antenna 
(67 in FIG. 2) of the first embodiment, can include coil windings 66A in 
planes substantially perpendicular to the longitudinal axis 78, a ferrite 
rod 68A inside the coil windings 66A, and optionally a frequency control 
coil 101A wound on the ferrite rod 68A. The transceiver antenna 67A can be 
disposed generally in the center of the magnet hole 83A. An additional 
receiver antenna 70A can be disposed on the outer surface of the magnet 
62A as shown in FIG. 12 and is generally centered about an axis 103 which 
intersects the longitudinal axis 78 and the center of the magnet hole 83A. 
The axis 103 is typically perpendicular to the magnetization direction 105 
of the magnet 62A. 
FIG. 13 shows a cross-sectional view of the arrangement shown in FIG. 12 to 
better explain the relative placement of the components of the present 
embodiment of the invention. The magnet 62A is shown generally eccentered 
in the wellbore 22A so as to be impressed against the wall of the wellbore 
22A. The sensitive volume 58A is typically selected, by appropriate 
selection of RF frequency for the power pulses conducted through the 
transceiver antenna 67A, to be at a depth into the earth formation 26A of 
about 5 cm from the wellbore wall. Geometrical considerations in selection 
of appropriate frequency include first that the sensitive volume radius 
(R.sub.sv) should exceed the quantity 2R.sub.bh -R.sub.m wherein R.sub.bh 
represents the wellbore radius and R.sub.m represents the radius of the 
magnet 62A. Second, the effective diameter d.sub.a of the additional 
receiver antenna 70A can be approximately equal to the quantity 2R.sub.sv 
-R.sub.m . Axial length considerations for the magnet 62A, the transceiver 
antenna 67A and the additional receiver antenna 70A can be substantially 
the same as in the first embodiment of the invention. 
FIG. 14 shows the a graph of the strength of the static magnetic field 
generated by the magnet 62A of the present embodiment. As can be observed 
in FIG. 14, asymmetry in the static field near the surface of the magnet 
62A is largely absent at the radial depth selected for the sensitive 
volume 58A. 
FIG. 15 shows an X-Y coordinate contour graph of the radial distribution of 
magnitude of the RF magnetic field generated by the transceiver antenna 
67A. As can be observed in FIG. 15, the RF field is substantially 
symmetric about the axis (103 in FIG. 13). 
FIG. 16 shows an X-Y coordinate contour graph of the sensitivity of the 
additional receiver antenna (70A in FIG. 12). The sensitivity of the 
additional receiver antenna 70A is substantially symmetric about the axis 
103. 
FIG. 17 shows a graph of the radial sensitivity function for the additional 
receiver antenna 70A (which was plotted in FIG. 16 in X-Y coordinate 
contour form) as a function of angular deviation from the axis 103. The 
graph of FIG. 17 shows that the sensitivity of the additional receiver 
antenna 70A is substantially confined to a "window" subtending an angle of 
about 120 degrees. 
It will be readily appreciated by persons skilled in the art that the 
present invention is not limited to what has been particularly shown and 
described herein. Rather the scope of the present invention should be 
limited only by the claims which follow.