Note: Descriptions are shown in the official language in which they were submitted.
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NON-POL.ARIZING EI.ECTRODE SYSTEM ~OR GEOPHYSICAL
_ROSPECIING AND THE LIKE
BACKGROUND OF THE INVENTION
Electrical methods of geophysical prospecting
have been known for a considerable period of time.
15 Leaving aside self-potential methods, the systems ordi-
narily employ either an alternating current or direct cur-
rent power source to apply electric curren-t to -the ground
between two spaced electrodes. The reswltant poten-tial
drop is measured either between these electrodes or
20 between other electrodes spaced from them.
Generally speaking, when a solid conductor such
as a metal stake or the like is applied to the ear-th, the
chemical make-up of the earth at the particular point and
the concentration and kind of ions in the wa-ter in pores
25 in the earth cause varying potentials to be generated a~
the elec-trode contacts, and also affect the resistance
between the conductor and the earth. These two phenomena
are usually called polarization. It was found years ago
that if the earth were energized with alternating curren-t
30 instead of direct current, the amount of polariza-tion at
the contact of electrodes with the ground was very much
reduced. However, if the spacing between the electrodes
was greater than a few feet, the phenomenon known as "skin
effect" was present~ ~hat is, the inductive effects of the
35 alternating current flowing to the earth were such that
there was considerable modification of the paths of cur-
rent flow, differing widely from that found when direct
current was used. In general, skin effect caused the
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major path or average path of current Elow -through the
earth to be much more limited -to -the direct line path
between the power electrodes than was the case when direct
currents were employed.
As various investigators discovered that i~ was
possible to cope with the polarization prohlern if one
wished to obtain the deep penetration with direct current
electrode arrangements by allowing polarization to occur
at -the power electrodes, and by employirlg what are ter~led
10 "porous pot electrodes" at the contact points between
which the potential was to be measured. This arrangemen-t
has been described well in a number of applied geophysics
textbooks, for example in that of J. J. Jakosky, "Explora-
tion Geophysics," published by Times-Mirror Press,
15 Los Angeles, California (1940). Basically one employed a
permeable ceramic po-t, usually in the shape of a right
cylinder with an open top. The inside of the pot held a
saturated solution of the salt of some metal, and an elec-
trode of that particular metal was immersed in the solu-
20 tion, giving as much surface contact as possible. Understeady state conditions, these electrodes could be plan-ted
on the earth's surface with relatively negligible gener-
ated polarization vol-tage between them. Of course, they
could not be employed for ordinary electric well logging,
25 since the hydrosta-tic pressure of the well fluids sur-
rounding such an electrode would force the well liquids
into the pot. In well logging, accordingly, ordinary
procedure is to use relatively close spacing between power
and potential electrodes, metal electrodes, and al-ter-
30 nating potential, the frequency of which usually was abovethat of the 50-60 Hz power range, but ordinarily only a
few hundred hertz, say 40Q Hz.
Aside from the nonpolarizing porous electrodes
described above, I have been unable to find in a careful
35 search of the literature (including patents dealing with
electric surface prospecting and wel~ logging) any variety
of nonpolarizing electrode system.
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SUM~IA~Y OF THE INVENTIO
Basically my invention comprises the use of
pairs of solid conductors between which a biasing a:Lter-
nating current Elows upon contact with the earth, as an
5 individual electrode for elec~rical geophysica.L pros-
pecting or well logging. I prefer to employ clirect cur-
rent for the measurement current, altho-ugh very low fre-
quency alternating current, for example, not above 20 or
30 Hz can be employed. The biasing alternating current of
10 each electrode pair is a-t least of the order of magni-tude
of the direct current or very low frequency alternating
current employed in making the measurements, and prefer-
ably is somewhat greater than such current. The frequency
of the biasing alternating current should be considerably
15 different from that employed in -the measuring circuit for
simplicity in measurement, i.e., for insuring that the
biasing current does not affect the measuring instruments
employed. It may, for example, conveniently be of the
order of power frequency (50 to 60 Hz) or a few hundred
20 hertz, for example, ~00 to 600 Hz but preferably not above
approximately 1000 Hz.
BRIEF DESCRIPTION OF THE DRAWINGS
A complete understanding of the present inven-
tion and of the advantages thereof may be gained from a
25 consideration of the description of -two embodiments of the
invention in connection with which the accompanying draw-
ings have been made. These drawings form a part of this
specification and are -to be read in conjunction therewith.
In these drawings like identifying numbers correspond to
30 like elemen-ts, and:
FIG. 1 is a diagrammatic view of a well survey
method conducted in accordance with one embodiment of my
invention.
FIG. 2 is a schematic diagram of the wiring con-
35 nections employed in the well logging arrangement~ ~hownin FIG. 1.
FIG. 3 shows partly in isometric -form a symbolic
representation of a second embodiment of my invention
. applied in surface electric prospecting.
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DETAILED DESCRIPTION
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FIG. 1 shows in highly diagrammatic form an
arrangement of appara-tus in accordance wi.th my i.nvention
applied in the field of electric well logging o~ a type in
5 which -the nonpolarizing electrode sys~em shown has ~pecia~L
advantage. It was well-known that the electrode s~pacirlg
in most varieties of electric resist-ivity wel.:L logging was
of -the order of inches to as much as but rarely exceed:ing
2 or 3 feet. Since the average depth of pene-tration of
10 the current employed in making the measurement is gener-
ally speakin~ (depending somewhat on the electrode
configuration) of -the order of the electrode spacing to 2
or 3 times this value, it is seen that the electric
effects measured at the potential electrodes could not
15 expect to -take into accoun-t m-uch of the adjacent rock for-
mations beyond a distance of the order of 3 to perhaps 10
feet. In the petroleum industry, it is occasionall~
desirable to use a system of well logging in which the
effective penetration of the prospec-ting sys-tem is greatly
20 in excess of this, particularly when one is trying -to
investigate perpendicular to the axis of -the wellbore at a
depth of 100 to 1000 feet or so. This, -for example, is
the case when one is attempting to locate the flanks of a
salt dome in the vicini-ty. In this case, one wishes to
25 employ ultra-long spaced electric logging ~ULSEL) in which
the spacing between the power electrode and the first
potential elec~rode is of the order of 100 to 1000 feet or
more. With reference -to FIG. 1 which illustrates such an
ULSEL system, the uncased portion 11 of a well 12 is being
30 surveyed using an electric cable 13 supporting a current
electrode 14 at the bot-tom of the cable, above which at a
distance Ll is located a first potential electrode 15,
and a distance L2 above this is located a second poten-
tial electrode 16. The second current electrode 18 is
35 located somewhere in the vicinity of the wellhead, for
example, buried in ground at the surface 17 (see location
of electrode 18). The electric cable 13 is a m~lticon-
ductor cable with the conductors well insulated from each
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other and from possible penetration by the well fluids in
the region 1~ within the bore of well 11, as -is well-known
in the art. It is also well understood that when -wsing
this arrangement, ordinarily the length L2 is made sev-
5 eral times the length Ll, -for example, 5 to 50 times
Ll. In this case, the difference of po-tential between
electrocles 15 and 16 clue to the c~rrent :EIowing ~etween
electrodes ll~ and 18 can be consiclered to a Eirst approxi-
mation -to be affected chiefly by the resistive nature o~
10 the formation in a sphere centerecl at electrode 1~ and a
radius of length somewhere between Ll and 2Ll.
In accordance with this inven-tion, each of the
electrodes in this arrangement physically comprises two
solid conductor portions (for example, 21 and 22 of elec-
15 trode 15) separated electrically by a central insula-tor
23.
~ s can be better seen in the schematic diagram
of FIG. 2, the pairs of solid conductor elec-trodes forming
the electrodes 14, 15, 16, and 18 each have applied to
20 them an alternating potential from the individual secon-
daries 24 to 27 of a power transformer 28 suitably fed
from an A.C. power source 2~. Since -these elec-trodes are
in contact with the earth which does not possess infinite
electric resis-tance, an alternating current or biasing
25 current will flow between the two conductors forming elec-
trode 14, and similar alterna-ting currents will flow
between the pairs forming electrodes 15, 16, and 18.
The magnitudes of the individual biasing cur-
rents are not particularly of importance. In my inven-
30 tion, I have found that it is highly desirable that thealternating current (biasing current) of -the curre.nt
flowing from one solid conductor forming a part of elec-
trode 15, for e~ample, should be a-t least of -the order of
magnitude of the measuring current (and perferably
35 somewhat greater than such current) at the electrode. In
the embodiment shown in FIG. 2, the measuring power source
is shown to be a DC source 30 (shown also in FIG. 1) which
is applied to the electrodes by being applied between the
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cen-ter taps 31 and 32 of secondaries 2~ and 27,
respectively. I have found that under these circumstances
that the pair of solid conduc-tors forming elec-trode 1~l ancl
those forming elec-trode 18 each act as an essentially non-
5 polarizing electrode very sa-tis-factor:ily conveying the
direct current due to -the DC power source 30 between elec-
-trodes 1~ and 18. The di-rection oE polarity o~ the powe~
source 30 becomes of no particuLar importance uncler this
circumstance.
When one of my electrode pair arrangements is
employed as a potential-measuring electrode ra-ther than as
one applying measuring current to the ground, I wan-t the
peak of the A.C. biasing potential -to be at least of the
order as the difference in potential being measured and
15 preferably have the rms biasing potential at least as
great as the measured potential. Thus, in FI&. 1 and FIG.
2 the potential across transformer secondaries 25 and 26
should have a peak value at least of the order of the d.c.
potential between the center taps of these secondaries and
20 more desirably the rms voltage of each of these secon-
daries should be as great as the d.c. potential.
Similarly, center taps 33 and 34 of secondaries
25 and 26 of transformer 28 which supply -the AC biasing
current to electrodes 16 and 15 respectively form the
25 electrodes between which the DC poten-tial in this well
logging embodiment is measured. Accordingly, these center
taps are connected to the DC strip chart voltage recorder
35. As is well known in this art, the strip chart on
which the record of the potential drop of electrodes 15
30 and .L6 is recorded for observation is ordinarily moved in
synchronism with the motion of the electrodes in -the well.
For example, one passes the cable 13 over a measuring
sheave 35 which deflects the cable to the take-up reel 37
A mechanical take-off from the axle of the sheave 36 is
; 35 then employed as the s-trip chart drive. This is shown as
broken line 38, FIG. 1.
The ends of -the conductors making up cable 13
are brought into the secondaries of the power -transformer
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28 by means of slip rings ~mostly not shown) on the
take-up reel 37, again as is very well-known in this art.
FIG. 1 shows this only schernatically; the electr:ical con-
nections are more clearly ind:icatecl in FIG. 2.
It is to 'be understood that one could employ a
very low frequency alternating potentia'l power source
instead of the DC power source 30, althollgh there :is
little advantage in doing -this. In that case, the ~C
strip chart :recorder 35 would be replaced with a recordin~
10 AC poten-tiometer operative at that frequency.
In any case, the power source 30 and recorder 35
are operated at a frequency much 'below that of -the AC
power source 2~. As has been mentioned above, for
example, the AC power source 29 may be at a fre~uency of
15 400 to 1000 Hz, while the power source 30 is preferably DC
and, if AC, is not over about 30 Hz.
It is to be understood that since the alter-
nating current bias is applied to relatively closely
spaced electrodes (I prefer that the insulator 23 sepa-
20 rating electrode components 21 and 22, for example, be atmost of -the order of a foot or so and preferably of the
order of only a few inches), very low power is required to
supply bias current of the same magnitude as -that of -the
applied di.ect current. With the arrangement shown, ordi-
25 narily a power supply of only a few watts at most is
required. It is seen that the spacing between the e]ec-
trode components for each pair is of -the order of 1/50 or
less the spacing between the adjacent electrodes, for
example, the spacing Ll.
In FIG. 3 is shown a second embodiment of the
invention, applied to an otherwise conventional surface
electrical prospecting arrangement. Here the swrface of
the earth represented by reference number 40 is a boundary
of current flow 'between two pairs 41 and 42 of solid, con-
35 ducting, closely-spaced electrod~s which have been driven
into the surface of the earth 40. Two other pairs of
similar electrodes 43 and 44 are employed to pick up the
potential between -the center third of the spacing between
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electrodes 41 and 4~. As shown in ~IG. 3, the secondaries
45, ~6, ~7, and 48 of 4 transformers 49 to 52 con-nect res-
pectively -the electrode pa:irs ~ 13 a 4~ ~ and ~. The
primaries of these transEormers are connected in parallel
5 across an A.C. source 5~ so that an alternating current
bias flows 'between the electrodes maklng up each e'lectrode
pair. ~s already stated, i~ is clesirecl to have th:is
'biasing current of t'he same orde-r of magni~ude or gr~ater
-than the amplitude of the measuring current which in ~'his
l0 case would be the current applied to the center taps 55
and 58 of the transformer secondaries 45 and 48. This is
the current I measured by the ammeter 59 shown in series
with the D.C. dynamo 60. The strength of the D.C. signal
may be adjusted or regulated by rheostat 61. ~ poten-tiom-
15 eter or other accurate means of determining potential 62is connec-ted between center taps 56 and 57. This is an
arrangement for surface electric prospecting called the
Wenner or Gish-Rooney configuration. The electrodes of my
invention are, of course, capable of employment with any
20 other type of configuration for surface prospecting.
As with the earlier embodiment, it is desirable
that the frequency of the biasing potential obtained from
the A.C. source 54 be high compared -to that of the dynamo ''
used for the measuring current. If, as shown in FIG. 3,
25 D.C. is employed in the measuring circuit, the ~.C. source
54 may be of power frequency, for example 50 to 60 Hz. On
the other hand~ if one employs a low-frequency alterna-ting
current source instead of a D.C. dynamo for source 60,
(for example, with frequency of the order of l0 to 30 Hæ),
30 it is desirable that the frequency of the A.C. source 54
be at least l00 Hz and preferably in the order of ~00 to
l000 Hz.
In practice the electrode pairs may be physi-
cally made up of two rods bound together in a sort of
35 "sandwich," with an insulating strip between them, so tha-t
one can plant this composite or "sandwich" rod with one
operation, rather than separately driving two stakes as
shown in FIG. 3.
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In all events, the arrangement shown in FIG, 3
is an embodiment of an electric prospecting system in
which nonpolariæing electrodes are used for the applica-
tion of the electric current I into the earth. As -in the
5 previous embodiment, it is preferred to have cl,ose ~paci,ng
between the two electrodes making the electrc~cle pair com-
pared to the spacing between acljacent eLectrocles. t
prefer to have the space be-tween the -two electrodes ~1
preferab:Ly not over 2/~ oE the distance from electrode ~1
10 to electrode 43, for example. ~lso, while I have shown in
both embodiments of the invention, the measuring circuits
applied to the center taps of the secondaries of the res-
pective transformers energizing the bias current, it is to
be understood -that it is not essential that a center tap
15 be used, as long as the measuring circuit is coupled -to
the electrode pair.
There are a number of varia-tions in arrangemen-ts
of apparatus physically equivalent to -that which has been
already shown and described. It is to be understood that
20 these are considered to be within the scope of the broad
invention, which is best defined in the appended claims.
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