Borehole measurement of NMR characteristics of earth formations

Borehole NMR logging apparatus and methods, and methods for the interpretation thereof. A logging tool is provided which produces a strong, static and homogeneous magnetic field B.sub.0 in a Volume of an adjacent formation on one side of the tool to measure nuclear magnetic resonance characteristics thereof. In the preferred embodiment, the tool has an RF antenna mounted on the outside of the metal body of the tool, directing focussed oscillating magnetic fields B.sub.1 at said Volume to polarize or tip the magnetic moments of hydrogen nuclei of fluids within rock pores. The same antenna can be used to receive signals of proton precession in the Volume of interest immediately after transmission of the RF polarizing field B.sub.1. Extremely rapid damping of the antenna between the transmitting and receiving modes of operation is accomplished by a Q-switch disclosed herein. The invention provides for the direct measurement of NMR decay having transverse relaxation time T.sub.2 behavior, and further provides for the fast repetition of pulsed measurements from within a borehole. An additional magnet array may be mounted offset from the first magnet configuration to prepolarize a formation before it is measured in order to pre-align a larger number of protons than the single magnet configuration could do by itself. Additional features of the invention are disclosed which increase the Signal/Noise ratio of the measured data, and improve the quality and quantity of borehole NMR measurements, per unit of time spent. Disclosed interpretation methods determine fluid flow permeability and longitudinal relaxation time T.sub.1 -type parameters by directly comparing the measured decay signals (such as T.sub.2 or T.sub.2 * type decay) to a representation which responds to both the decay time t.sub.dec and the imposed polarization period prior to such decay t.sub.poi. The parameters of amplitude and T.sub.1 are determined and combined with certain preferred methods to generate robust values of formation characteristics such as fluid flow permeability. Other related methods are disclosed.

FIELD OF THE INVENTION 
This invention relates to apparatus and techniques for making nuclear 
magnetic resonance (NMR) measurements in boreholes, and to methods for 
determining magnetic characteristics of formations traversed by a 
borehole. 
BACKGROUND OF THE INVENTION 
Repeated attempts have been made to use the principles of nuclear magnetic 
resonance to log wells in oil exploration over the past several decades, 
with limited success. It was recognized that any particles of a formation 
having magnetic spin, for example atomic nuclei, protons, or electrons, 
have tendencies to align with a magnetic field which is imposed on the 
formation. Such a magnetic field may be naturally generated, as is the 
case with the earth's magnetic field B.sub.E which has an intensity of 
approximately 0.5 gauss in areas of the globe where boreholes are 
typically drilled. Any given particle in a formation is additionally 
influenced by localized magnetic fields associated with nearby magnetic 
particles, other paramagnetic materials, and the layer of ions which 
typically line pore walls of certain types of formations such as shales. 
These localized fields tend to be inhomogeneous, while the earth's 
magnetic field is relatively homogeneous. 
The hydrogen nuclei (protons) of water and hydrocarbons occurring in rock 
pores produce NMR signals distinct from any signals induced in other rock 
constituents. A population of such nuclei, having a net magnetization, 
tends to align with any imposed field such as B.sub.E. 
When a second magnetic field B.sub.1 transverse to B.sub.E is imposed on 
the protons by a logging tool electromagnet, the protons will align with 
the vector sum of B.sub.E and B.sub.1 after a sufficient polarization time 
t.sub.pol has passed. If the polarizing field B.sub.1 is then switched 
off, the protons will tend to precess about the B.sub.E vector with a 
characteristic Larmor frequency .omega..sub.L which depends on the 
strength of the earth's field B.sub.E and the gyromagnetic constant of the 
particle. Hydrogen nuclei precessing about a magnetic field B.sub.E of 0.5 
gauss have a characteristic frequency of approximately 2 kHz. If a 
population of hydrogen nuclei were made to precess in phase, the combined 
magnetic fields of all the protons can generate a detectable oscillating 
voltage in a receiver coil. Since the magnetic moment of each proton 
produces field inhomogeneities, the precessing protons tend to lose their 
phase coherence over time, with a characteristic time constant called the 
transverse or spin-spin relaxation time T.sub.2. Furthermore, field 
inhomogeneities are also produced by other physical phenomena as mentioned 
above, so that the observed dephasing relaxation time T.sub.2 * is usually 
shorter than T.sub.2. Borehole magnetic resonance measurements of the 
above type are commercially available as a part of the NML service 
Schlumberger Technology Corporation, Houston, Tex. ( Mark of 
Schlumberger). This tool is capable of measuring the Free Induction Decay 
of hydrogen nuclei in formation fluids, and to obtain the parameters 
T.sub.1 and T.sub.2 *. It does not measure the transverse relaxation time 
T.sub.2. A description of the basic components, operation and 
interpretation of the commercial logging tool used in the NML service is 
contained in a paper entitled, "An Improved Nuclear Magnetism Logging 
System and its Application to Formation Evaluation", by R. C. Herrick, S. 
H. Couturie and D. L. Best, presented at the 54th Annual Fall Technical 
Conference and Exhibition of the Society of Petroleum Engineers (A.I.M.E., 
Dallas, Tex.) in Las Vegas, Nev., Sept. 23-26, 1979; this paper, appended 
hereto, is incorporated herein by reference. 
Other sequences of magnetic fields can be imposed on a population of 
protons in a formation, to measure other characteristics thereof. For 
example, if a pulse of alternating current having a frequency f is passed 
through a transmitter coil, producing an oscillating polarizing field 
B.sub.1 perpendicular to a static field B.sub.o, a population of protons 
precessing at a Larmor frequency equal to f would tend to align at an 
angle to B.sub.1. At the end of the pulse, when B.sub.1 is removed, the 
aligned protons experience a perpendicular torque, and precess about the 
B.sub.o vector. After a characteristic time called the longitudinal or 
spin-lattice relaxation time T.sub.1, the protons have relaxed to thermal 
equilibrium, wherein a weighted percentage of protons are aligned in the 
direction of B.sub.o. Various other sequences of imposed magnetic fields 
can be used, as is discussed in T. C. Farrar and E. D. Becker, "Pulse and 
Fourier Transform Nuclear Magnetic Resonance", Academic Press, N.Y. 
(1971), Chapter 2, pp. 18-33, which is incorporated herein by reference. 
Although measurements of NMR characteristics of rock samples can be 
accurately made in a laboratory, making comparable measurements in a 
borehole is greatly exacerbated by the hostile environment where 
temperatures may reach several hundred degrees Fahrenheit, pressures reach 
thousands of p.s.i. and all of the equipment must be packed within a 
cylindrical volume of only several inches diameter. 
One of the earliest NMR logging tools is shown in U.S. Pat. No. 3,289,072 
granted Nov. 29, 1966 to N. A. Schuster. A strong electromagnet is used to 
subject a sample of water or oil to a predetermined magnetic field. A RF 
coil produces an oscillating second magnetic field which causes nuclear 
magnetic resonance of protons in the sample and resonance of similar 
protons in the adjacent formation. Schuster proposed the use of a 
multipole electromagnet mounted in a wall engaging pad, or alternatively a 
larger electromagnet mounted within the logging sonde, to produce a static 
magnetic field B.sub.0. Schuster has also proposed other configurations of 
electromagnets and detection RF coils, for example in U.S. Pat. No. 
3,083,335 granted on Mar. 26, 1963, wherein the coil is positioned within 
a gap between two opposite poles of two bar magnets. Here, the magnetic 
field lines of the coil intersect field lines of the bar magnets 
perpendicularly, which is the optimum angle for inducing nuclear magnetic 
precession. 
A more recent U.S. Pat. No. 3,667,035 granted May 30, 1972 to C. P. 
Slichter, shows a similar configuration of two coaxially aligned bar 
magnets and a RF coil positioned within the gap between opposite poles of 
the magnets. The term "bar magnet" is used herein to mean any magnet 
having only one north pole and one south pole, facing opposite directions, 
and may be either a permanent magnet or an electromagnet. Both the 
Slichter design and the Schuster design use electromagnets which require 
inconveniently large D.C. currents to be transmitted to a logging sonde 
through many thousands of feet of electrical cable. 
U.S. Pat. No. 3,528,000 granted Sept. 8, 1970 to H. F. Schwede shows one 
type of NMR logging tool in FIGS. 8 and 9, wherein a permanent magnet 
produces a first magnetic field which is fixed in its intensity, and an 
inductive coil produces an oscillating magnetic field whose frequency is 
varied over a selected range. Since the first magnetic field is produced 
by two opposite magnetic poles (one N and one S) placed side by side, the 
field is not homogeneous and the spatial gradient of the field is 
evidently non-zero at all points in the formation. In addition, since the 
first and second fields intersect not only in the formation, but also 
within the borehole, it is evident that protons constituting water or 
hydrocarbons within the borehole fluid contributes to signals detected by 
the RF coil, and must be removed either electronically or by chemically 
treating the borehole fluid, if a true formation measurement is desired. 
Other NMR logging tools have been proposed which use permanent bar magnets, 
aligned coaxially in a logging sonde with a detection coil positioned in 
the gap between the magnets, for example as shown in U.S. Pat. No. 
3,597,681 granted Aug. 3, 1971 to W. B. Huckabay. 
Another permanent magnet configuration has been proposed in U.S. Pat. No. 
4,350,955 granted Sept. 21, 1982 to J. A. Jackson, wherein two permanent 
bar magnets are coaxially aligned such that the RF detection coil is 
positioned in the gap between two similar poles of the two magnets. 
Similarly, United Kingdom Patent Application No. 2,141,236-A, published 
Dec. 12, 1984, shows a similar configuration of coaxially aligned bar 
magnets with a detection coil positioned in a gap between the magnets. 
This type of configuration produces a toroidal region of homogeneous 
magnetic field wherein nuclear resonance may be measured. However, these 
tools may be adversely affected by signals from the borehole fluid in a 
large or deviated borehole where the tool would tend to lean against one 
side wall of the borehole. If the tool is designed to produce the toroidal 
region far away from the tool body, the produced magnetic field becomes 
much weaker, resulting in a significantly weaker signal. This 
configuration further requires that the detection coil or antenna be 
enclosed by a structure which would not block the oscillating 
electromagnetic waves of the measured signal. For example, fiberglass or 
some other non-metallic material is typically used; unfortunately, this 
structurally weakened link decreases the structural integrity of the tool 
and renders it considerably less useful in rough borehole conditions. 
NMR measurement of particles other than hydrogen nuclei having magnetic 
spin have also been proposed. U.S. Pat. No. 3,439,260, granted Apr. 15, 
1969 to G. J. Benn et al, for example, discloses techniques of measuring 
magnetic resonance of carbon-13 nuclei in earth formations. 
Other representative U.S. patents which have been granted for NMR logging 
tools and techniques include the following: U.S. Pat. Nos. 3,042,855 to R. 
J. S. Brown; 3,508,438 to R. P. Alger et al.; 3,483,465 to J. H. Baker, 
Jr.; 3,505,438 to R. P. Alger, et al.; 3,538,429 to J. H. Baker, Jr.; 
4,035,718 to R. N. Chandler. 
Each of the NMR logging tools which have been proposed or constructed has 
had practical deficiencies. All of them had to deal with the fundamental 
difficulties of making this kind of delicate measurements under severe 
conditions of temperature, pressure, and physical trauma typical of 
logging runs in oil wells. Furthermore, since the concentration of 
hydrogen nuclei within the borehole is much higher than the concentration 
in any rock formation, the undesirable NMR signals arising in a borehole 
are potentially much higher than any signals from surrounding formations. 
In order to alleviate this troubling phenomenon, it has been known in the 
art to treat the borehole fluid with a paramagnetic substance such as 
magnetite, and to circulate the treated fluid throughout the borehole 
before a logging run is made so that the relaxation time of hydrogen 
nuclei in the borehole is shortened by so much that its contribution to 
the NMR measurement is eliminated. Such pretreatment of borehole fluid is 
expensive and time consuming. Pretreatment may also introduce the same 
chemical, via the borehole, into adjacent permeable formations, and thus 
distort measurements. 
It has also been recognized that those NMR logging tools which require 
powerful electromagnets tend to be unreliable because the high power 
currents flowing through the tool inevitably tend to break down various 
electronic components such as switches, especially under the high 
temperature environment in boreholes. The previous tools all required that 
the sonde or pad body be constructed of a non-metallic material such as 
fiberglass, synthetic rubber or teflon to enable detection of A.C. 
signals. These materials are considerably weaker than the alloy metals 
which are normally used in constructing other types of logging tools. The 
inability to use a strong metallic superstructure in constructing NMR 
logging tools has further contributed to their relative unpopularity in 
the industry. 
Previous NMR logging tools typically required approximately 20-30 
milliseconds, called "dead time", after a polarizing field pulse is shut 
and before the transmitting coil is sufficiently damped to permit 
measurements to be taken. During this dead time, considerable information 
of magnetic relaxation is irretrievably lost, and the S/N ratio is 
considerably degraded. 
The commercially available NMR logging tool cannot directly measure the 
spin-spin relaxation time T.sub.2. Instead, the existing commercial tool 
obtains measures of the Free Fluid Index (FFI) and the observable 
dephasing relaxation time T.sub.2 *, also called the free induction decay 
time constant. Various log interpretation techniques may be used to derive 
other useful information as discussed in, e.g. "Applications of Nuclear 
Magnetism Logging to Formation Evaluation" by C. H. Neuman and R. J. S. 
Brown, Journal of Petroleum Technology (December 1982) pp. 2853-2860, and 
in the Herrick et al. paper cited above. 
For many of the above reasons, the prior logging tools have not been 
capable of determining formation characteristics with sufficient accuracy 
and dependability to become fully accepted by the industry. 
Accordingly, it is an object of the present invention to provide improved 
apparatus and methods for determining magnetic characteristics of earth 
formations more accurately and more dependably. 
It is an additional object of the invention to provide apparatus and 
methods to determine the nuclear magnetic relaxation time, the free fluid 
porosity, permeability and related pore fluid characteristics of earth 
formations traversed by a borehole. 
It is also an object of the invention to provide borehole apparatus for 
measuring magnetic resonance which can be constructed of strong metallic 
materials, which can operate repeatedly in borehole conditions with high 
reliability, and which can accurately measure formation characteristics 
without requiring any pretreatment of the borehole fluid with magnetic 
substances. 
It is a further object of the invention to provide a magnet configuration 
for NMR measurements which has a simple, sturdy construction, and which is 
easily and conveniently tested, calibrated and used in logging a borehole. 
It is also an object of the invention to provide a magnetic resonance 
logging tool which directly measures the transverse relaxation time 
T.sub.2 of formations traversed by a borehole. 
In another aspect of the invention, it is an object to provide improved 
methods for determining the permeability and like parameters from measured 
free induction decay signals of NMR logging tools. 
It is also an object of the invention to provide improved methods for 
determining the magnetic relaxation times of a measured population of 
particles in earth formations surrounding a borehole. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, apparatus is provided which 
produces a static and substantially homogeneous magnetic field focussed 
into a formation on one side of the logging tool. By directing and 
configuring the combined magnetic fields of a configuration of magnets, 
applicants have produced a region, remote from the configuration of 
magnets, wherein the spacial field gradient substantially vanishes, 
thereby insuring that the field is highly homogeneous throughout that 
region. In a preferred form, the magnets are mounted within a skid or 
logging pad, the static magnetic field is directed through the face of the 
pad into an adjacent formation, and the region of substantially 
homogeneous field is situated in a volume of formation behind the mudcake 
layer which typically lines borehole walls. A homogeneous magnetic field 
several hundred times stronger than the earth's magnetic field can be thus 
imposed, or "focused", on a volume of formation in situ. 
In one aspect of the present invention, the RF antenna is mounted on the 
outside of the metallic structure of the tool so that the tool body serves 
as a natural shield against any signals which may be generated by resonant 
conditions behind the body, particularly those potentially strong 
resonance signals from borehole fluid. In the preferred form, the antenna 
is configured to focus its signals radially outwardly from the pad face, 
into the volume of formation having the homogeneous field, thereby 
additionally reducing the distortion of measured signals from borehole 
effects. As a consequence of this distinctive feature of the invention, 
the logging apparatus may be constructed of strong metallic alloys, unlike 
prior art tools, and the present measurement technique actually uses the 
shielding effect of a metal sonde to good advantage, to enhance the S/N 
ratio of NMR measurement. Since borehole effects are excluded by the dual 
focussed design, it is no longer necessary to pretreat the borehole fluid 
with paramagnetic chemicals. 
In accordance with a preferred form of the invention, an elongated trough 
antenna is provided on a pad face, parallel to the borehole axis and to an 
elongated volume of substantially homogeneous static magnetic field in the 
adjacent formation. By superposing the geometric shape of the volume of 
homogeneous and static field with the pattern of the RF field from the 
antenna, near optimum resonance conditions can be created. In the 
preferred embodiment the static field is directed radially into said 
volume, while the RF field is circumferentially directed and thus 
perpendicular to the static homogeneous field within the volume of 
investigation. The length of the trough antenna is preferably about equal 
to the length of the volume of investigation. 
In accordance with the present invention, methods and apparatus are 
provided for making fast pulsed measurements of magnetic resonance in 
earth formations surrounding a borehole, particularly the direct 
measurement of spin-spin relaxation time T.sub.2, to determine formation 
characteristics. 
In accordance with a further aspect of the invention, the transmitting 
antenna is also used for receiving magnetic resonance signals, and special 
circuitry is used to very rapidly damp the ringing current which occurs in 
the antenna after a power shut-off. The special circuitry, called a 
Q-Switch, damps the polarizing antenna current about 1000 times faster 
than the previous tool, and enables many pulses to be injected 
successively into a formation in a short period of time. 
By vastly increasing the number of measurement cycles per unit time, the 
present invention enables the logging tool to: (1) increase the S/N of the 
overall measured data set, thereby permitting either a faster logging rate 
or continuous logging, and (2) reduce the NMR measurement time during 
which nuclei may diffuse within rock pores, thereby reducing the 
undesirable magnetic effects of such diffusion. 
In accordance with another aspect of the invention, additional apparatus is 
provided to prepolarize a formation volume of interest before the main 
magnetic configuration reaches proximity to the formation. The 
prepolarization field is preferably much stronger than that of the main 
magnet configuration, and serves to increase the population of protons 
that is aligned in the B.sub.0 vector direction, and thus further 
increases the magnetic precession signal level. 
In accordance with yet another aspect of the invention, small currents are 
introduced in the vicinity of a measuring tool to alter the static field 
during part of the measurement cycle to spoil the signals from these 
localized regions. In the preferred embodiment, the small currents flow 
through a wire preferably configured as a loop covering the antenna 
opening and attached parallel to the wall-engaging face of the tool. This 
configuration serves to significantly reduce or eliminate any resonance 
signals produced by borehole mud or mudcake immediately adjacent to the 
antenna surface, and substantially reduces undersirable signals. The 
spatial extent and magnetic effects of these field inhomogeneities can be 
carefully controlled by selecting the spacing of adjacent sectors of the 
wire, the current, and other relevant dimensions. In a different aspect of 
the invention, the wire or its equivalent can be used to produce magnetic 
field gradients extending into the volume of measured resonance, which 
permits other advantageous magnetic measurements to be made. 
For other objects and advantages of the invention, and to provide a fuller 
understanding thereof, reference is made to the following description 
taken in conjunction with the cited references and the accompanying 
drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to the drawings and particularly FIG. 1 thereof, a borehole 10 is 
shown adjacent to formations 11, 12, the characteristics of which are to 
be determined. Within borehole 10 there is shown a logging tool 13 
connected via a wireline 8 to surface equipment 7. Tool 13 preferably has 
a face 14 shaped to intimately contact the borehole wall, with minimal 
gaps or standoff. The tool 13 also has a retractable arm 15 which can be 
activated to press the body of the tool 13 against the borehole wall 
during a logging run, with the face 14 pressed against the wall's surface. 
Although tool 13 is shown in the preferred embodiment of FIG. 1 as a single 
body, the tool may obviously comprise separate components such as a 
cartridge, sonde or skid, and the tool may be combinable with other 
logging tools as would be obvious to those skilled in the art. Similarly, 
although wireline 8 is the preferred form of physical support and 
communicating link for the invention, alternatives are clearly possible, 
and the invention can be incorporated in a drill stem, for example, using 
forms of telemetry which may not require a wireline. 
The formations 11, 12 have distinct characteristics such as formation type, 
porosity, permeability and oil content, which can be determined from 
measurements taken by the tool. Deposited upon the borehole wall of 
formations 11, 12 is typically a layer of mudcake 16 which is deposited 
thereon by the natural infiltration of borehole fluid filtrate into the 
formations. 
In the preferred embodiment shown in FIG. 1, tool 13 comprises a magnet 
array 17 and an antenna 18 positioned between the array 17 and the wall 
engaging face 14. Magnet array 17 produces a static magnetic field B.sub.0 
in all regions surrounding the tool 13. The antenna 18 produces, at 
selected times, an oscillating magnetic field B.sub.1 which is focussed 
into formation 12, and is superposed on the static field B.sub.0 within 
those parts of formation opposite the face 14. The Volume of Investigation 
of the tool, shown in dotted lines in FIG. 1, is a vertically elongated 
region directly in front of tool face 14 in which the magnetic field 
produced by the magnet array 17 is substantially homogeneous and the 
spatial gradient thereof is approximately zero. 
A prepolarizing magnet 19, shown in dotted lines, may be positioned 
directly above the array 17 in a modified embodiment of the invention 
which will be separately discussed. 
The tool 13 makes a measurement by magnetically tipping the nuclear spins 
of particles in formation 12 with a pulse of oscillating field B.sub.1, 
and then detecting the precession of the tipped particles in the static, 
homogeneous field B.sub.0 within the Volume of Investigation, over a 
period of time. As seen in FIG. 1, this Volume of Investigation does not 
overlap the surface of the wall engaging face 14 as in some previous 
logging tools, and does not overlap the mudcake 16 on the borehole wall. 
In a pulse echo type of measurement, as discussed in detail in the 
previously cited book of Farrar and Becker, for example, a pulse of RF 
current is passed through the antenna 18 to generate a pulse of RF field 
B.sub.1 where the RF frequency is selected to resonate only hydrogen 
nuclei subjected to a static field strength equal to the field B.sub.0 
within the Volume of Investigation. The signals induced in antenna 18 
subsequent to the RF pulse represent a measurement of nuclear magnetic 
precession and decay within the Volume, automatically excluding any 
undesirable contributions from the borehole fluid, mudcake, or surrounding 
formations where the field strength of B.sub.0 is different. 
In devising the preferred embodiment of the invention, applicants have 
sought to optimize the signal-to-noise (S/N) ratio of the measurement 
process. As will be appreciated by those skilled in the art, the following 
dicussion helps to explain the principal parameters to be considered in 
making the preferred embodiments of the invention. 
Referring to FIGS. 2 and 3, in considering the signal strength of a nuclear 
magnetic resonance measurement made by tool 13 in the adjacent formation 
12, it is helpful to treat the interaction of the antenna 18 with the 
magnetic moment of the formation as parts of a single four terminal 
network to which the Reciprocity Theorem can be applied. The network 20, 
having an input impedance Z.sub.o, comprises a lossless matching circuit 
21 and an RF probe or antenna 22 which is shown simply as having a 
resistance and inductance in series. An oscillating current I.sub.1 of 
frequency .omega. flows through RF probe 22, producing an oscillating 
magnetic field B.sub.1 in the formation including the Area within a test 
loop 23. The voltage that is induced in test loop 23 as a result of the 
current I.sub.1 is given by 
EQU V.sub.2 =Z.sub.21 I.sub.1 =-i.omega.A.multidot.B.sub.1 (1) 
where Z.sub.21 is the cross impedance between the antenna current and the 
voltage induced in the test loop 23. 
Now assume that a current I.sub.2 is impressed upon the test loop, and 
induces a voltage V.sub.1 at the terminals of the matching network 20, 
which is 
EQU V.sub.1 =Z.sub.12 I.sub.2 (2) 
Where Z.sub.12 is the cross impedence between the test loop current and the 
voltage induced in the antenna 22. Invoking the Reciprocity Theorem, 
Z.sub.12 =Z.sub.21, we obtain. 
##EQU1## 
Since the magnetic moment of the loop 23 is m=I.sub.2 A, the magnetization 
dM of a volume dV is dM=I.sub.2 A dV. The net signal from the entire 
formation can be derived from the above and represented as 
##EQU2## 
where the integral is taken over the volume in which the resonance 
condition B.sub.0 =.omega./.gamma. is satisfied. In order to optimize the 
measured signal response of the present NMR tool, we may assume that the 
volume of NMR investigation has an area A.sub.R and a length L, and that 
the integrand is constant within this volume. In practice, we have found 
this assumption to be a fairly good approximation. By making the 
substitutions .omega.=.gamma. B.sub.0 and M=.chi.B.sub.0 /.mu..sub.0 in 
the above equation, where .gamma. is the gyromagnetic ratio, .mu..sub.0 is 
the magnetic permeability of free space, and .chi. is the nuclear magnetic 
susceptibility of protons in the formation we derive 
##EQU3## 
Now we want to estimate the size of the area A.sub.R for actual magnetic 
configurations such as that of the magnet array 17 shown in FIG. 1. As 
will be further discussed hereinbelow, magnet array 17 produces a static 
magnetic field Bo having a saddle point at the center of a homogeneous 
field region designated as the volume 9 in FIG. 1. The field strength may 
be approximated by the Taylor Series expansion 
##EQU4## 
Noting that the resonance condition in pulse NMR is met when the deviation 
of the static field from its center value, B.sub.o (x,y)-B.sub.o (0,0) is 
no greater than half the magnitude of the RF field B.sub.1, the area of 
the resonant region can be derived to be approximately 
##EQU5## 
after making the simplifying assumption that the area has a square 
cross-section. This and other approximations are made here to more simply 
illustrate the principles underlying the invention, it being understood 
that other more exact derivations are obviously possible. Making 
substitutions for the above equation (5) for the measured NMR signal, we 
obtain 
##EQU6## 
It is clear from the qualitative relationships of equation (8) that the 
measured NMR signal can be made better or worse by changing the various 
parameters given. However, increasing the signal level is not sufficient, 
and it is highly desirable to keep the thermal noise level as low as 
possible, relative to the signal V.sub.S. The root mean square thermal 
noise is 
EQU V.sub.N =(4.kappa.TZ.sub.o .DELTA.f).sup.1/2 (9) 
where Z.sub.0 is the input impedance of the matching network 20 (nominally 
50 ohms), .kappa. is Boltzman's constant, and .DELTA..function. is the 
measurement bandwidth, which is matched to the bandwidth of the population 
of resonated particles in the formation, .gamma.B.sub.1 /2.pi.. The 
following ratio of the peak signal to the root mean square noise is 
obtained; 
##EQU7## 
In equation (10) the power fed to the antenna, P.sub.1, has been 
substituted for the current using the relationship P.sub.1 =I.sub.1.sup.2 
Z.sub.0 /2. Further, using the conventional definition of the Curie Law 
susceptibility in MKS units, 
##EQU8## 
we obtain the final expression for the signal-to-noise (S/N) ratio 
##EQU9## 
In equation (12), the first bracketed expression above depends on 
environmental parameters such as the proton spin density of the fluid N, 
porosity .phi., and absolute temperature T. Some of the other terms are 
Planck's constant (over 2.pi.).pi., and the nuclear spin I which has a 
value of 1/2 for protons. The second bracketed expression contains design 
parameters which optimize tool performance. For example, it is readily 
seen that a high static field B.sub.0, being a squared term, can immensely 
improve the S/N ratio. 
Furthermore, it is seen that the expression (B.sub.1 
/P.sub.1.sup.1/2).sup.3/2 which is related to the "Q" of the antenna, 
should be made large. However, S/N cannot be indefinitely increased by 
increasing B.sub.1 /P.sub.1.sup.1/2 because equations (10) and (12) hold 
only when the bandwidth is limited by .gamma.B.sub.1 /2.pi., e.g. when 
Q&lt;B.sub.0 /B.sub.1. In contrast, when Q&gt;B.sub.0 /B.sub.1, the bandwidth is 
.DELTA.f=.gamma.B.sub.0 /2.pi.Q, and equation (12) becomes 
##EQU10## 
For a set antenna geometry and measurement point, it is known that the 
quantity B.sub.1 /P.sub.1.sup.1/2 is directly proportional to Q.sup.1/2. 
It is seen from the above that further decreasing the losses of the 
antenna beyond the point where Q&gt;B.sub.0 /B.sub.1 would not increase S/N. 
Referring to FIGS. 1 and 3, the magnet array 17 consists of three samarium 
cobalt permanent magnets 24, 25, 26, which are mounted parallel to each 
other within a metal alloy body 27. Magnets 24, 25, 26 are elongated in 
the direction longitudinally of the borehole, and measure 12 inches in the 
preferred embodiment. The magnetic poles of the magnets are not on the 
smallest faces of the slab, commonly viewed as the ends of a bar magnet; 
instead, the poles appear on the two opposing edges of the slab magnet and 
point to the left and right, respectively, in both FIG. 1 and FIG. 3. 
Thus, within the formation 12, the magnetic field B.sub.0 surrounding the 
magnets remains fairly constant along the longitudinal direction of the 
borehole axis. 
Magnets 24,25,26 should be as strong as practical, and should be capable of 
withstanding physical shock without disintegration. The samarium cobalt 
magnets that have been used, for example, are preferably enclosed in a 
sturdy brass casing to prevent any explosive fragmentations in the event 
the magnet cracks or breaks. These magnets are commercially available, and 
have a residual induction of typically 10,500 gauss. It would be obvious 
to those skilled in the art that other magnets may be substituted for the 
samarium cobalt magnets herein, and the slab magnets can have other 
dimensions than that shown in the preferred embodiment. 
It is preferable to use elongated slab magnets to produce a static field in 
formation 12 which is constant over a substantial distance L along the z 
coordinate parallel to the borehole axis. A large L improves S/N and also 
facilitates continuous logging along the z coordinate. However, the 
magnets should not be so long as to make the tool 13 structurally unwieldy 
or to cause excessive standoff between the face 14 and the borehole wall 
in washed out zones. 
Magnets 24, 26 are symmetrically mounted in the two sides of the body 27 
with the north poles facing the same directions. Magnet 25 is positioned 
parallel to and between the other two magnets, but with its north poles 
facing oppositely from magnets 24, 26. Magnet 25 is also shifted slightly 
away from face 14, relative to magnets 24, 26. As shown in FIG. 3, the 
north poles of magnets 24, 26 point in the direction of the face 14 of the 
tool, while the north pole of magnet 25 is pointed away from the face 14, 
although the configuration obviously may be reversed and still produce a 
similar result. 
Referring to FIGS. 4-6, it can be seen that, by configuring the two N poles 
of magnets 24, 26 to point at the face 14 and the formation 12 lying 
beyond, magnet array 17 would appear at a great distance like a magnetic N 
pole. However, the reversed pole positioning of magnet 25 substantially 
alters the magnetic field at close and intermediate distances into 
formation 12. At intermediate distances, this preferred configuration of 
magnet array 17 produces an interesting and important field anomaly within 
a uniquely defined volume directly in front of the tool face 14. As seen 
in greater detail in FIGS. 5-7 there is a well-defined volume in which the 
magnetic field is substantially constant, and wherein the spatial gradient 
of B.sub.0 substantially vanishes. This is the primary resonance region 
for NMR measurements and is the Volume of Investigation shown in FIG. 1. 
FIG. 8 shows vector arrows which follow field lines within the cross 
sectional area approximately one inch from the tool face 14, with the 
length of each vector arrow proportional to field strength and the 
direction of each arrow following the field lines. It can be seen that the 
field B.sub.o projects radially into the formation and that it is quite 
uniform throughout this area, and has a substantially constant field 
strength of approximately 232 gauss. 
Although the volume of greatest field homogeneity is centered about a point 
approximately 4/5 inch away from the wall engaging face 14, this volume of 
substantially homogeneous field can be shifted either a greater or less 
distance into the formation, depending on the relative positioning, 
spacing, and field strength of magnet 25 with respect to magnets 24, 26. 
It is additionally possible, in other embodiments of the invention, to 
provide for shifting of magnet 25 within the body 27 during operation of 
the tool so as to obtain a variable depth of investigation, depending on 
the circumstances. For example, if a formation having high rugosity or an 
extremely thick mudcake is encountered, the logging engineer may shift the 
position of magnet 25 a predetermined distance to the right, as seen in 
FIG. 3 so that the volume of investigation 9 is shifted further into the 
formation, to avoid obtaining undesirable resonance signals from the 
mudcake. However, in the preferred embodiment shown in the figures, 
magnets 24, 25, 26 are rigidly mounted in position since the resultant 
positioning of the volume 9 already avoids any substantial overlap with 
the relatively thin mudcake layer of typical boreholes. 
Referring again to FIGS. 5, 6 and 7 it can be appreciated that the size of 
the Volume of Investigation 9 can depend on the nature of the measurement 
that is taken and the strength of a RF pulse that is transmitted by the 
antenna 18 as explained hereinbelow. 
The metal body 27 has, on the front face 14 thereof, a semi-cylindrically 
shaped cavity or slot 28 which faces formations engaged by the face 14. 
The cavity 28 is adapted for receiving the RF antenna 18, as will be 
further described below. It is already clearly seen, however, that antenna 
18 is positioned outside of the metal body 27 of the tool, and is 
automatically shielded from electromagnetic communication with regions of 
the borehole which lie behind the body 27, or regions of other formations 
in directions intercepted by the body 27. Antenna 18 is thus responsive 
only to magnetic fields originating in front of the wall engaging face 14, 
e.g. fields originating in the formation 12 or in the mudcake or mud which 
contacts face 14 in the vicinity of the antenna 18. By utilizing the 
relative geometric positioning of the antenna and the metal body 27, it is 
possible to minimize undesirable signal contributions which would 
otherwise be difficult to eliminate by other means. In the preferred 
embodiment, body 27 is made of metal alloy sheathing, rigidly attached to 
interior metal bracing, which envelops most components of the tool other 
than the antenna 18, including the circuitry, the magnet array 17, and the 
hydraulics system of the arm 15. It is also possible for the body 27 to be 
constructed of other combinations of materials such as composite resins, 
steel, etc., as long as the overall structure is sufficiently strong, and 
the magnetic field of the magnet array 17 can penetrate and enter the 
adjoining formation 12. 
Antenna 18 is used both as a RF transmitter to produce a polarizing 
magnetic field in formation 12, and as a receiving antenna to detect 
coherent magnetic signals emanating from precessing protons immediately 
after the polarizing field is terminated. Antenna 18 should be constructed 
of one or more current carrying loops which are highly efficient in 
generating magnetic fields in the formation. It is preferably made of a 
current loop which produces an oscillating field B.sub.1 within the volume 
of investigation which is perpendicular to B.sub.o. Other current loop 
orientations may be useful in other embodiments of the invention having a 
static field B.sub.o differing from that of the preferred magnet array 17. 
Antenna 18 is attached to body 27 and fitted within the slot 28. Its 
efficiency can be ideally maximized when the current density within the 
slot 28 is made uniform. In practice, optimum antenna efficiency is 
difficult to achieve, because of various electromagnetic parasitic effects 
like the "skin effect", the mutual inductive effects between distinct 
current loops, and electrical effects within individual conductors. The 
preferred antenna 18 pursuant to the invention comprises a single current 
loop, in the shape of a trough or slot, as shown in FIG. 9. 
Referring to FIG. 9, the antenna 18 comprises a highly conductive 
semi-cylindrical cavity or trough 29, end plates 30, 31, and antenna 
element 32 which extends from one end plate 30 to the other end plate 31, 
parallel to and centered in the semi-cylindrical trough 29. The trough 29, 
end plates 30, 31 and antenna element 32 are all preferably made of heavy 
gauge copper which has extremely low electrical resistance. Antenna 
element 32 is insulated from end plate 30 by a non-conducting bushing 33 
and is connected to an electrical mounting 34 on the other side of end 
plate 30. Antenna element 32 is attached at its other end to the other end 
plate 31 so that current passes freely between trough 29 and antenna 
element 32 via end plate 31. Electrical mounting 34 is shown in FIG. 9 
schematically as being connected to circuitry including an amplifier 35 
and a detector 36. All connections in antenna 18 are brazed or silver 
soldered, to ensure a suitably low resistive loss. 
RF antenna 18 can be driven by amplifier 35 during specified periods of 
time, during which it serves as an RF antenna transmitter. Alternatively, 
at other specified times, antenna 18 is electronically connected to 
detector 36, during which time it serves as an RF receiving antenna. In 
certain modes of operation, antenna 18 may be called upon to alternately 
function as transmitter or receiver in very rapid succession. 
The space between trough 29 and antenna element 32 is preferably filled 
with a nonconductive material 37 having high magnetic permeability. In 
order to increase the antenna sensitivity Ferrite materials are preferably 
used. Several tuning capacitors 38 are connected between the base of 
antenna element 32 and the trough 29, with the capacitances thereof being 
chosen to produce a LC circuit, with the resonant frequency being the 
Larmor frequency .omega..sub.L =.gamma.B.sub.o. 
Referring to FIG. 10, the relative dimensions of antenna 18 should be 
selected to maximize the antenna efficiency. The slot element radius R 
should be as large as practical, and the spacing R-r should be maximized 
subject to the condition that r must not be so small as to increase the 
antenna impedance excessively. It has been found that for a 12 inch trough 
antenna without ferrite filling, R=0.75 inch and r=0.2 inch produces 
optimum efficiency. A ferrite filled trough antenna having dimensions 
R=0.75 inch and r=0.3 inch has been found to be optimum. The length L of 
the antenna may be the same as the length of the magnet array 17, which is 
12 inches in the preferred embodiment, but antenna 18 is preferably about 
the same length as the resonance region produced by the magnet array 17 in 
the formation, which is approximately 4 to 8 inches long. 
The field B.sub.1 produced by antenna 18 is an oscillating magnetic field 
having a frequency .function. equal to the resonance frequency of hydrogen 
nuclei in the sensitive volume of formation where the static field is 
about 232 gauss. Therefore, f=(.gamma.B.sub.0)/2.pi.=1.0 MHz. The strength 
of B.sub.1 has direct impact on the bandwidth of precessing nuclei which 
are resonated by B.sub.1 in accordance with the bandwidth formula: 
EQU .DELTA.f=(.gamma.B.sub.1)/2.pi. 
Since the static field B.sub.o is about 232 gauss in the preferred 
embodiment, and the antenna generated field strength B.sub.1 at 1 inch is 
3 gauss, the resultant bandwidth of B.sub.o within the desired volume is 
also 3 gauss. 
The strength of the B.sub.1 field in the sensitive volume also affects the 
S/N ratio (equation 12) in accordance with the term B.sub.1 
/P.sub.1.sup.1/2, where B.sub.1 is perpendicular to the static B.sub.o 
field. FIG. 13 shows a plot of the magnitude of B.sub.1 /P.sup.1/2 in 
front of a 12" trough antenna, where P.sub.1 is the power applied to a 50 
ohm impedence matching network of the antenna 18 and B1 is the circularly 
polarized component of the radiated field. It is seen that the field 
strength is quite constant at 1 inch for a longitudinal distance of about 
8 inches. 
Referring to FIGS. 3,10, and 11, antenna 18 is installed in slot 28 and 
covered with a wear plate 39 made of a non-conductive abrasion resistant 
material to protect the ferrite material 37 as well as antenna element 32. 
It is preferred to additionally provide, either under or within the wear 
plate 39, a thin conducting wire 40 which substantially fills the antenna 
opening. The wire 40 is preferably arranged in a loop, with a spacing S 
between wire segments of about 1/2 inch, although this dimension can be 
altered if it is desired to spoil magnetic resonance in a local region of 
greater or lesser thickness. A small D.C. current is passed thru wire 40 
at selected intervals during the measurement cycle, generating local 
inhomogeneous magnetic fields B.sub.2 which extend towards the formation 
12 for a distance approximately equal to the spacing S between segments of 
the wire 40. Within this region of local field inhomogeneity, nuclear 
magnetic precession is disrupted during part of the measurement cycle, and 
any resonant conditions which were otherwise created by the intersection 
of B.sub.o and B.sub.1 are substantially altered. The wire 40 constitutes 
one form of a means for creating localized inhomogeneous fields, and other 
embodiments are clearly possible. For example, multiple wires, coils, or 
conductive grids may be used. 
Spoiling resonance field conditions in the mudcake region is especially 
advantageous for the preferred embodiment shown in FIGS. 1-8 because 
typical mudcake contains a high concentration of hydrogen nuclei which may 
resonate strongly with an applied RF pulse from the antenna 18. The 
mudcake lying adjacent antenna 18 is subjected to a stronger RF field 
B.sub.1 than even the volume of investigation 9 in formation 12, and 
therefore may become strongly polarized by B.sub.1. Furthermore, there 
exists high gradient points within 1/2 inch of the face 14 where the 
B.sub.0 field strength equals the resonant frequency of 232 gauss, as 
shown in FIGS. 6-7. By imposing the inhomogeneous field B.sub.2 within the 
mudcake region, and spoiling resonance conditions therein, any undesirable 
NMR contributions from the mudcake are eliminated. 
The wire 40 produces a magnetic field B.sub.2 having a high spatial 
gradient dB.sub.2 /dx, and can alternatively be used to make field 
gradient type NMR measurements within formation 12. In this case, it would 
be desirable to arrange the relevant dimensions of wire 40 such that the 
region of measured NMR resonance overlaps with the gradient field B.sub.2. 
It is seen that the tool 13 as described measures in a single direction by 
preferentially directing or "focussing" both the static field B.sub.0 and 
the oscillating field B.sub.1, to create the special Volume of 
Investigation 9. By imposing an additional localized field B.sub.2 at 
regions between the Volume 9 and the tool face 14 which spoils resonance 
therein, the measurement effectively excludes signals arising from within 
the mudcake region of the borehole. Furthermore, since the measurement 
range (distance of sensitivity) of the tool 13 is fairly limited, it is 
possible to enclose the tool within a reasonably sized calibration cell 
during testing or calibration of the tool, to exclude magnetic effects of 
the environment. Consequently, the effective use of the tool 13 for 
logging oil wells is facilitated. 
ELECTRONICS 
The electronics requirements for the tool 13 may be mounted in the body 27 
or in a separate cartridge or sonde. The preferred circuitry, shown 
schematically in FIG. 12, operates in three modes: transmitting, damping, 
and receiving. 
In the transmitting mode, the circuit 41 must generate a large power of 
about 1 kilowatt at a frequency on the order of 1 MHz. for a short 
precisely timed period, shut off this current very quickly, within about 
10 microseconds, and then isolate any signals or noise of the power 
circuits from coupling with other detection circuitry within the tool 13. 
Referring to FIG. 12, the transmitting circuitry comprises a 20 MHz. 
oscillator 42 and a synthesizer 43 which generates a sinusoidal signal of 
frequency 20 MHz.+.function., where .function. is the desired frequency of 
operation of the tool. Both oscillators are linked by a clock 44 and kept 
in synchronization at all times. The output of oscillator 42 is fed to a 
phase shifter 45, which is controlled by a timing generator 46. The phase 
shifter 45 can produce shifts of 0, 90, 180, and 270 degrees, as desired 
by the operator of the tool, and in accordance with the requirements of 
various measurement schemes such as the Meiboom-Gill sequence. The phase 
shifted signal passes through a gate 47, and is then combined with the 20 
MHz.+.function. signal from the synthesizer 43 at a mixer 48. The combined 
signal is fed through a low pass filter 49 which allows only the signal of 
frequency .function. to pass. Although the mixer 48 also has other 
frequency components at its output, these higher frequency .function. may 
be turned on and off at any time by appropriate control of the gate 47 by 
the timing generator 46. Timing generator 46 is kept in precise 
synchronization with the clock by a 40 MHz synthesizer. Furthermore the 
oscillators 42, 43, which have much higher frequencies, may be left 
running at all times without risk that they would adversely affect the 
detection circuits which operate at the frequency .function.. 
The .function. signal, which retains information of the shifted phase, is 
passed to the amplitude modulator 50 which adjusts the amplitude to change 
the signal into a desired pulse shape. Both modulator 50 and gate 47, as 
well as other components in circuit 41, are controlled by timing generator 
46 which is in turn controlled by a computer 52. Referring now to FIGS. 
14-15, a typical pulse fashioned by the modulator 50 and gate 47 has a 
first short time interval t(1) where the amplitude has been increased by 
the modulator 50, and a second short time interval t(2) during which the 
amplitude is not increased, and a third short time interval t(3) during 
which the amplitude is increased and the phase of the signal is reversed. 
The third period of phase reversal may not be necessary where the shaped 
pulse is already adequately damped by the Q-switch described hereinbelow. 
The increased amplitude during t(1) helps to decrease the time it takes to 
ring up the RF antenna 18, while the reversed-phase signal during t(3) 
helps to kill the ringing in antenna 18 at the end of the pulse. 
Therefore, the resultant pulse of magnetic field B.sub.1 that is radiated 
into the formation 12 much more closely resembles a square pulse. 
The pulse signal from amplitude modulator 50 is amplified by power 
amplifier 35 which is capable of out putting approximately 1.2 kilowatts 
without distorting the signal shape. The signal then passes through an 
extender 53 which prevents low level noise on the order of ten volts or 
less from leaking out of amplifier 35 when it is not activated, during the 
receiving mode. The amplified pulses are then fed to the RF antenna 18, 
radiating a pulse of magnetic field B.sub.1 to resonates nuclear spins in 
the formation. In between transmitting pulses, the RF antenna 18 receives 
oscillating magnetic signals of nuclear spin precession. 
As previously mentioned, the receiving system of RF probe and matching 
circuit is designed to have a high Q to maximize the S/N ratio. In such a 
high Q system, the antenna tends to ring for an undesirably long time, and 
causes undesirable magnetic spin tipping in the formation. If the antenna 
is permitted to ring uncontrollably, the transmitted magnetic field pulse 
bandwith may be substantially reduced. In order to minimize the antenna 
ringing problem, a Q switch 54 is connected to the line between extender 
53 and antenna 18 as a preferred means for damping antenna ringing very 
quickly at the end of a transmitted pulse. The Q switch 54 closes a 
circuit at the appropriate time, which changes the impedence of the RF 
probe system (including RF antenna 18) so that the system is critically 
damped, and the ringing energy quickly dissipated. 
During the receiving mode of operation, Q switch 54 is switched off, and 
signals from precessing nuclei are received by RF antenna 18 and passed 
through a duplexer 55 to a receiver amplifier 56. Duplexer 55 protects the 
receiver amplifier 56 from the high power pulses which pass from extender 
53 to the RF antenna 18 during the transmitting and damping modes. During 
the receiving mode, duplexer 55 is effectively just a 50 ohm cable 
connecting the antenna 18 to receiver amplifier 56. The detected and 
amplified signal is then passed to a dual phase sensitive detector 57 
which also receives a reference signal that controls the frequency of 
sensitivity of the detector 57. 
The reference signal, also having frequency .function., is generated by 
combining the 20 MHz and 20 MHz+.function. outputs of the oscillator 42 
and synthesizer 43 in a second mixer 58, and extracting the lower 
frequency component .function. via a low pass filter 59. The output of low 
pass filter 59, a sinusoidal wave form of frequency .function., is used as 
a reference signal for both the detector 57 and a pulse generator 60. The 
pulse generator 60 generates an appropriate pulse shape in response to 
control and triggering signals from the computer 52, and triggers the 
timing generator 46. 
The detector 57, in response to the reference signal and the formation NMR 
signal from receiver amplifier 56, obtains a measured NMR resonance signal 
of frequency .function. from the desired volume of investigation, and 
passes it to the digitizer 82. The digitized signal is then forwarded to 
the computer 52 for processing and/or formatting as desired by the 
operator. 
Q-SWITCH 
Referring to FIG. 16, the Q Switch 54 comprises two symmetric circuits 61 
and 62, shown on the top and bottom, respectively of the figure. The 
purpose of the Q switch 54 is to introduce a resistive element into the 
antenna network, in parallel with the capacitors 38 (see FIG. 9), so as to 
critically damp the network, in accordance with the formula R.sub.C= 
1/2(L/C).sup.1/2. For example, where L=1.1.times.10.sup.-7 H., 
C=2.3.times.10.sup.-7 F. and assuming an operating frequency of 1 MHz, the 
needed additional resistance to critically damp antenna 18 would be 
R.sub.C =0.36 ohm. 
Q switch 54 utilizes two field effect transistors (FET's) 63, 64 connected 
back to back, to provide approximately the resistance of 0.36 ohms, when 
they are closed. Two FET's are needed since each one is effective in 
developing a resistance to a high voltage therein of only one polarity due 
to an effective internal diode 67, and the ringing of the antenna 18 is a 
bipolar oscillating voltage. If the resistive value needed for critically 
damping an antenna 18 is greater than the resistance of the FET's 63,64, 
additional resistive elements may be inserted in series. 
When the FET's 63, 64 are switched open, the circuit is an open circuit and 
has no effect on the antenna network. Thus, Q switch 54 is open during the 
transmitting mode and during the receiving mode of the circuitry (FIG. 
12), and closed during the damping mode which is shown in FIGS. 14-15 as 
the time period t(3). 
The remaining circuitry in FIG. 16 comprise means for gating the FET's 63, 
64 on and off with minimum noise being introduced into the antenna 
network. Since the circuits 61,62 are essentially identical, only one of 
them will be described below. 
In the transmitting mode, during the RF pulse, a 20 nF capacitor 65 and the 
collector of an intermediate stage transistor 66 are charged through the 
line comprising diode 67, resistor 68 (51 ohms) and diode 69, where diode 
67 is part of the FET 64 of the lower symmetric circuit 62. The base of 
transistor 66 is connected to an optical coupler 70 which is controlled by 
a signal on line 71 from the timing generator 46. The optical coupler 70 
is preferably used with the NMR logging tool because it ensures isolation 
between the switching signals and the high voltage on the antenna. 
The emitter of intermediate stage transistor 66 is connected via diode 77 
to the gate 72 of the FET 63. The gate 72 is also connected to a 1 k 
resistor 73 and a 8.2 nF capacitor 74, which are in turn connected to the 
source line 75, to constitute a R-C combination high pass filter between 
gate 72 and source line 75. This R-C filter ensures that the 
source-to-gate voltage never reaches excessive levels during the 
transmission of an RF pulse and also provides a self-turnoff time 
constant. A zener diode 76 connected in parallel with the 20 nF capacitor 
65 to prevent excessively large voltages from damaging the optical coupler 
70. 
During the damping mode, a signal from the timing generator 46 is passed 
via line 71, to activate the optical coupler 70 and charge the base of the 
intermediate stage transistor 66. The transistor 66 is turned on, causing 
a voltage to be applied to gate 72, making the FET 63 conductive, and 
providing damping of the antenna network. 
During operation of the tool 13, the operator enters into the computer 52 
information respecting the type of measurement sequence to be taken. 
Computer 52 then sets the sequence of electronic steps needed for the 
equipment to implement the measurement sequence. The computer 52 controls 
the timing generator 46 which in turn sends control signals to the various 
components of circuit 41 to control the polarizing pulse height, length, 
frequency, relative phases of sequential pulses, receiving mode period and 
frequency, and the timing of all of the above. 
Because the high Q antenna 18 can be rapidly damped after transmitting a 1 
kilowatt RF pulse, the tool 13 is capable of resonating a targeted 
formation 12 with many successive pulses in a short time. The deadtime 
between a transmitted pulse and the commencing of the receiving mode, 
about 25 microseconds, is about 1000 times shorter than the deadtime of 
the previous commercially available logging tool. Using peak power of 100 
W, the pulses can have a duration of about 40 .mu.sec. and it is possible 
to have as many as 1000 pulses within a measurement cycle lasting 1 
second. 
In addition to the NMR measurements heretofore known to be performed by the 
existing commercial logging tool discussed in the previously cited 
references, a Carr-Purcell type measurement may also be made to measure 
the transverse relaxation time T.sub.2. This sequence is also commonly 
known as a 180.degree.-90.degree. sequence, where the angles refer to the 
degree of tipping undertaken by the precessing protons during the 
measurement process. Other measurement sequences which can be undertaken 
with the present apparatus include the Meiboom-Gill sequence, as described 
in the cited Ferrar and Becker textbook, or a 90.degree.-.tau.-90.degree. 
sequence of the type described in G. G. McDonald and J. S. Leigh, Jr., "A 
New Method for Measuring Longitudinal Relaxation," Journal of Magnetic 
Resonance, Vo. 9, pp. 358-362 (1973), which measures the longitudinal 
relaxation time T.sub.1. It is contemplated that additional types of 
borehole NMR measurements may be devised in accordance with the invention 
to advantageously investigate the magnetic properties of earth formations. 
PREPOLARIZATION 
When the tool 13 is used to make continuous logs, without stopping the tool 
for each measurement, an alternative embodiment may be preferred, to 
further improve the S/N beyond what has already been discussed. Referring 
to FIG. 1, a prepolarizing magnet 19 is installed within the tool 13 above 
the position of the main magnet array 17, to magnetically polarize the 
formation 11 before the magnet array 17 has reached proximity to it for 
measurement. The field of the prepolarizing magnet 19 should be similar to 
that of the magnet array 17 in orientation, but preferably much stronger, 
so as to polarize a much larger population of nucleii. As the tool is 
moved up the borehole, magnet array 17 comes into proximity of formation 
11, and radiates it with RF pulses. However, because of the 
prepolarization, a larger number of nuclei are aligned with the field 
B.sub.0 of the magnet array 17 within the volume of substantially 
homogeneous filed, and a correspondingly larger signal is produced. 
The prepolarization magnet is preferably an array magnets in a 
configuration such as that shown in FIG. 17, comprising magnets 90, 91 92, 
aligned with similar poles facing in the same direction to maximize the 
field strength that is produced in the formation. Obviously, other 
combinations of magnets can produce a similar field, and a single magnet 
may be used instead of an array as shown. Referring to FIGS. 18-19, the 
magnetic field B.sub.P of the prepolarizing magnet 19 can be substantially 
less homogeneous than the field of the magnet array 17, without adversely 
affecting the bandwith or S/N of the NMR measurement which depends only on 
the existing static field B.sub.0 at the time of the measurement. 
The preferred embodiments may obviously be modified in various ways without 
departing from the spirit of the invention, as would be clear to persons 
skilled in the art. For example, the prepolarization magnet need not be 
constructed of the configuration of magnets as shown in FIG. 17, but may 
be a single magnet or some other arrangement. Slab magnets have been used 
because the slim profile tends to minimize demagnetization effects, and 
they are relatively easy to assemble. Since they are large and have very 
high energy density, the ease the handling and assembly of the magnets are 
significant considerations, and the simple configuration discussed herein 
has been found to be advantageous from many standpoints. Nevertheless, 
other types of magnets may be used, in accordance with the invention. 
If stationary measurements are desired, there is the possibility that 
differential sticking and other dynamic borehole effects would cause the 
tool 13 to become stuck. Thus, if such stationary measurements are to be 
made, it is preferable to provide means for prying tool face 14 away from 
the borehole wall after the completion of a measurement cycle at depth. 
For example, two push-off pistons 93, 94, shown in FIG. 1 in dotted lines, 
may be hydraulically actuated to force the tool 13 away from the borehole 
wall after the arm 15 has been retracted. 
It has been described and illustrated herein novel apparatus and methods 
for measuring and interpreting magnetic characteristics of formations 
traversed by a borehole. Those skilled in the art will recognize that 
numerous variations and modifications may be made without departing from 
the scope of the present invention. Accordingly, it should be clearly 
understood that the forms of the invention described hereinabove are 
exemplary, and are not intended as limitations on the scope of the present 
invention, which should be defined only by the claims appended hereto.