U.S. patent number 3,818,324 [Application Number 05/315,383] was granted by the patent office on 1974-06-18 for well logging pad having a flexible electrode structure.
This patent grant is currently assigned to Schlumberger Technology Corporation. Invention is credited to Roger Espinasse.
United States Patent |
3,818,324 |
Espinasse |
June 18, 1974 |
WELL LOGGING PAD HAVING A FLEXIBLE ELECTRODE STRUCTURE
Abstract
In accordance with an illustrative embodiment of the present
invention, an improved electrode carrying pad member suitable for
engaging the wall of a borehole to investigate the adjacent
formations is disclosed. The electrodes carried by the pad member
include a plurality of linear, strip-like concentrically arranged
and rectangular shaped electrodes. To enable the pad to bend to
conform to the curvature of the borehole wall while maintaining
good mechanical and electrical characteristics, portions of the
electrode segments that are transverse to the longitudinal axis of
the borehole are made of flexible spring blades with rigid metal
segments attached thereto.
Inventors: |
Espinasse; Roger (Essone,
FR) |
Assignee: |
Schlumberger Technology
Corporation (New York, NY)
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Family
ID: |
26832827 |
Appl.
No.: |
05/315,383 |
Filed: |
December 15, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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134939 |
Apr 19, 1971 |
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Current U.S.
Class: |
324/374;
324/347 |
Current CPC
Class: |
G01V
3/20 (20130101) |
Current International
Class: |
G01V
3/20 (20060101); G01V 3/18 (20060101); G01v
003/18 () |
Field of
Search: |
;324/1,10 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Strecker; Gerard R.
Parent Case Text
This application is a continuation of Ser. No. 134,939, now
abandoned.
Claims
What is claimed is:
1. In a borehole wall engaging apparatus for use in investigating
earth formations surrounding a borehole, a combination comprising a
borehole wall engaging pad having a relatively flexible member made
of a relatively flexible, electrically insulating material and
having a front face suitable for engaging a borehole wall, a
plurality of electrodes embedded in said member in proximity of the
front fact thereof to make electrical contact with the borehole
wall, at least one portion of at least one electrode comprised of a
flexible relatively thin strip-like element which is offset from
the front face of said flexible member, and at least one rigidly
highly conductive element attached to said flexible strip-like
element and extending outwardly toward the front face of said pad,
whereby said wall-engaging member with electrode embedded therein
will be able to bend to conform to the borehole wall curvature.
2. The apparatus of claim 1 further including a rigid support
member secured to the back, non-borehole wall engaging side of said
flexible member, a central support member adapted for movement
through the borehole and linkage means coupled between said rigid
support member of said flexible member and said central support
means for coupling said flexible member to said central support
member.
3. In a borehole wall-engaging apparatus for use in investigating
earth formations surrounding a borehole, the combination
comprising: a borehole wall engaging pad having a relatively
flexible member made of a relatively flexible electrically
insulating material and having a front face suitable for engaging a
borehole wall, a plurality of electrodes embedded in said member in
the proximity of the front face thereof to make electrical contact
with the borehole wall, said electrodes comprising a central
relatively rigid electrode, and at least one electrode surrounding
said central electrode, said surrounding electrode having a first
portion comprising a relatively rigid linear strip-like element
aligned along the front face of the flexible member parallel to a
given axis, and a second portion aligned substantially
perpendicular to said axis, said second portion comprising a
relatively flexible strip-like element having at least one rigid
highly conductive element attached to said flexible strip-like
element and extending toward the front face of said pad, whereby
said pad can bend around an axis parallel to said given axis.
4. The apparatus of claim 3 wherein said plurality of electrodes
comprises a plurality of electrodes surrounding said central
electrode, said plurality of surrounding electrodes being
concentrically arranged around said central electrode, each of said
surrounding electrodes comprising said first portions aligned
parallel to said axis and said second portions substantially
perpendicular to said axis.
5. The apparatus of claim 4 wherein said flexible member has a
rigid section aligned parallel to said given axis and wherein the
second portion of said electrodes surrounding said central
electrode further comprise a relatively rigid-element located in
the relatively rigid section of said flexible member and wherein
said flexible strip-like elements are located in the relatively
flexible portions of said flexible member.
6. In a borehole wall-engaging apparatus for use in investigating
earth formations surrounding a borehole, the combination
comprising; a borehole wall-engaging pad which includes a
relatively flexible member made of relatively flexible,
electrically insulating material and having a front face suitable
for engaging a borehole wall, a longitudinally extending rigid
support member secured to the backside of said flexible member and
defining a given area on said backside, thus causing a
corresponding portion of said flexible member to be relatively
rigid, a plurality of electrodes embedded in said flexible member
in the proximity of the front face thereof to make electrical
contact with the borehole wall, said electrodes comprising a
relatively rigid central electrode located in the relatively rigid
portion of said flexible support and a plurality of electrodes
surrounding said central electrode and having relatively rigid
portions parallel to said longitudinally extending rigid support
member, those portions of said surrounding electrodes which are
transverse to said longitudinally extending support member having
rigid sections in the relatively rigid portion of said flexible
support and relatively flexible sections having at lease one rigid
highly conductive element attached to said flexible section and
extending toward the front face of said pad member in the non-rigid
portion of said flexible support.
7. The apparatus of claim 6 wherein the flexible sections of the
electrode comprise a strip of flexible, spring-like conductive
material having a front face which faces the front face of said
flexible member, said at least one rigid, highly conductive element
secured to the front face of said strip of flexible metal whereby
said pad will be able to conform to the borehole wall curvature.
Description
This invention relates to tools used in the investigation of
geological formations through which a borehole passes and, in
particular, to those tools whose active parts operate in the
immediate proximity of the walls of the borehole.
These tools have at least one pad member designed to be moved
longitudinally near or against the wall of the borehole. This pad
is mounted on a linkage arrangement which is in turn supported by a
central support member. The central support member is suspended at
the end of a cable which connects the tools electrically and
mechanically to the surface. This linkage arrangement makes it
possible to move the pad outward when desired to apply it against
the wall of the borehole or to retract it to a position along the
body of the tool.
The pads used for the electrical investigation of formations meet
this general definition. Their active parts consist of a set of
electrodes located on the front face of the pad which is designed
to be applied against the wall of the borehole. Through well-known
use of three electrodes, a measurement of the electrical
characteristics (e.g., resistivity) of the formation in the
immediate proximity of the borehole can be made.
All sorts of pad structures have already been proposed. Among the
first pads were those which comprised one-piece electrodes,
buttons, strips or concentric circular rings, set in the front face
of the pad in insulating rubber. Examples of such pads can be found
in U.S. Pat. Nos. 2,712,629 and 2,712,630 granted to H. G. Doll on
July 5, 1955. An example of a typical design comprises electrodes
having a succession of buttons distributed linearly, for example,
in the form of a circle or an ellipse. These buttons are embedded
in a thick plate of insulating material, such as an elastomer,
defining the front face of the pad. The electrical connections
between the buttons or contacts are set in this material, the pad
assembly thus formed being shaped to have a rounded front face and
to conform to the wall, but having only a limited flexibility. An
example of such a pad is found in U.S. Pat. No. 2,712,629 granted
to M. C. Ferre on July 5, 1955.
Pads provided with a small number of electrodes of large dimensions
covering almost the entire front face are in widespread use. In
these pads, the electrodes are insulated from each other by thin
strips, often rectangular, of insulating material. An example of
such a pad can be found in U.S. Pat. No. 3,132,298 granted to H. G.
Doll on May 5, 1964. It has been proposed to provide a degree of
flexbility in electrodes located on opposite sides of a rigid
central portion of the pad so that the electrodes can make better
contact with the walls of the borehole in spite of variations in
the diameter of the hole. A solution proposed in this regard
consists of making the side wings of these pads with a surface
layer of electrically conductive elastomer reinforced by a sheet of
flexible conducting material such as perforated sheet steel set
between the conducting elastomeric material and an insulating
elastomer. This solution may be considered for shoes having
electrodes of large surface area and which can thus transmit a
sufficient current into the formation even if the conductive
elastomer surface layer has a relatively limited conductivity. A
pad of this type can be found in U.S. Pat. No. 3,379,963 granted to
D. F. Saurenman on Apr. 23, 1968. Other types of pads having
flexible side members can be found in U.S. Pat. Nos. 3,379,964 and
3,379,965 granted to F. F. Segesman and D. R. Tanguy et al.,
respectively on Apr. 23, 1968. To use this construction for
electrodes of relatively small dimensions may prove to be somewhat
less than completely desirable because of the inherent resistance a
conductive elastomer must have to allow sufficient cohesion and
mechanical strength when the pad is rubbed against the borehole
wall.
In order to improve the contact between the pad and the wall of the
borehole with different types of electrodes, successful use has
also been made of "hydraulic" pads. Such pads include a
bladder-like pad void of air and partially inflated with a liquid
such as oil. One face of this pad is secured to a linkage apparatus
and the other face constitutes the front face in which are mounted
electrodes consisting of series of metallic buttons or plates of
small dimensions. This type of pad has a very flexible front face
which will conform to the rough features of the borehole, laterally
as well as longitudinally. It thus makes it possible to prevent
leakage of the current transmitted by the electrodes directly into
the mud, without going through the formation. This type of pad is
fragile, and the pad front face is worn during the repeated raising
and lowering of the tool in boreholes which are not always in good
condition and which are capable of collapsing. Moreover, in
formations containing high pressure gases, it sometimes occurs that
this gas gets into the bladder and causes it to burst when the
apparatus is brought to the surface of the earth. Moreover, while
this type of pad is characterized by its flexibility, it is thick
and its lateral parts cannot bend beyond a certain limit, thus
making its use difficult in very narrow holes.
In general, there are cases where pads of large transverse
dimensions are necessary. It is also very desirable to be able to
use the same pad for any size (diameter) borehole. A large pad
capable of being used under these conditions must have very good
flexibility about a longitudinal axis parallel to the borehole
axis. It must in fact be able to conform to the walls of boreholes
of large diameter, for which it is in general preshaped, and those
having small diameters. Moreover, in the case of small-diameter
holes, a large pad rubs against the wall of the hole even in the
retracted position when it is brought to a position close to the
central support member, for example when lowering the tool through
the borehole. Such a pad must thus not only be flexible but also
very rugged to withstand this repeated friction.
The types of pad structures just discussed are, in general, not
able to satisfy all of these requirements. Pads made of solid metal
electrode plates embedded in insulation are in general too rigid to
be able to adapt without difficulty to holes of highly variable
dimensions. This is particularly so when the electrodes are of
large transverse dimensions. However, such problems are overcome to
a considerable degree through use of pads of the type disclosed in
the earlier mentioned Saurenman patent. Pads of this type with
flexible electrodes of large surface areas forming the lateral or
side portions of the structure employ technical principles which
are not easily applicable to large pads which have many electrodes
of limited surface area. With such pads having a number of small
cross-section electrodes, it will be very difficult to provide good
electrical characteristics, and at the same time, sufficient
mechanical strength, if a conductive elastomer constituted the
front wall engaging face of such small electrodes.
Finally, the difficulties inherent in the use of hydraulic pads in
the particular case have been considered. Their deformation upon
contact with the uneven features of the borehole wall is badly
defined and affects the response of the investigating apparatus in
an indeterminate manner, i.e., the coefficient of proportionality
between the actual formation resistivity and the measurement
furnished directly by the measuring appartus may change with
deformation of the pad.
It is therefore an object of the invention to provide a reliable
pad structure which overcomes one or more of the above-discussed
problems.
It is another object to provide an improved pad structure which is
durable under stresses of all kinds encountered while logging
boreholes and, which is also flexible enough to mate easily with
the walls of boreholes having very different diameters.
Another object of the invention is to provide a pad capable of
being applied against the walls of small-diameter boreholes without
requiring excessively great application forces.
Still another object of the invention is to provide a pad structure
whose front face has a plurality of concentrically arranged
electrodes of relatively large dimensions and which, at the same
time, is flexible enough to conform to boreholes having different
borehole diameters.
In accordance with the present invention, apparatus for use in
investigating earth formations includes a borehole wall engaging
pad which includes a flexible member made of a relatively flexible
electrically insulating material, such as neoprene, having a front
face suitable for engaging the wall of a borehole. A plurality of
electrodes are embedded in the flexible member. At least a portion
of one of these electrodes comprises a flexible, relatively thin
element which is offset from the front face of the flexible member
and a plurality of rigid, highly conductive elements attached to
the borehole wall engaging side of the flexible element. By this
arrangement, the pad will be able to bend sufficiently to conform
to the curvature of the borehole wall while still maintaining good
mechanical qualities in spite of the pad rubbing against the
borehole wall, as will be explained in detail later. Moreover,
because of the rigid conductive elements, the electrodes will make
good electrical contact with the formation. In other words, the
resistance between the circuits which are coupled with the
electrodes and the formation to be investigated will be extremely
small.
For a better understanding of the present invention, together with
other and further objects thereof, reference is had to the
following description taken in connection with the accompanying
drawings, the scope of the invention being pointed out in the
appended claims.
FIG. 1 is a perspective view of a pad constructed according to the
invention and mounted on a central support member lowered into a
borehole;
FIG. 2 is a plan view of such a pad, partially cut by the line
II--II of FIG. 3;
FIG. 3 is a cross-section view of this same pad taken along the
line III--III of FIG. 2; and
FIG. 4 is a view similar to that of FIG. 3 of the shoe taken along
the line IV--IV of FIG. 2.
Referring now to FIG. 1, a central support member 1 is lowered into
a borehole 2 on the end of a suspension cable (not shown) which
provides both mechanical and electrical connection of this support
member 1 with suitable, well-known apparatus located at the surface
(not shown).
The cylindrical body of the support member 1 has two longitudinal
grooves 3 into which may fit two pads. In FIG. 1 only one pad 4 is
shown. This pad 4 is connected to the support member by articulated
arms 5 and 6. The arms 5 and 6 are articulated on the body of the
support member 1 at one of their ends. At the other end, they are
articulated on a metallic pad linkage support 8 elongated in the
longitudinal direction (parallel to the support member 1). The pad
4 is mounted on the pad linkage support 8 by means of screws which
are not visible in FIG. 1. The arms 5 and 6, the rib member 8 and
the body of the pad 1 form an arrangement for moving the pad 4
towards and away from the borehole wall and which can be activated
by suitable motorized means (not shown). This device may be
maintained in the closed position, with the pad 4 then being in the
retracted position in the groove 3, or it may be opened so that the
pad 4 will be applied against the wall of the borehole 2 with a
selected force.
The front face of the pad 4 shown in FIG. 1 includes a back plate
12 made of flexible, electrically insulating material, for example
an elastomer such as neoprene. One of the faces of this plate
defines the outer limits of the major part of the front face 13 of
the pad, i.e., of the face which is applied against the wall 10 of
the borehole. This front face 13 is equipped with a set of five
rectangular electrodes 14 to 18 arranged in a concentric (common
center point) fashion from the center to the periphery. Thus, the
four electrodes 15 to 18 are linear or strip-like and are arranged
in close rectangular paths around each other. The electrode 14 is a
rectangular plate at the center of the pad.
The front face 13 has a form which is generally elongated in the
longitudinal direction, i.e., parallel to the axis of the support
member 1. The lateral edges of the pad are parallel to the
longitudinal axis while the upper and lower edges describe a
substantially trapezoidal form which narrows down towards the ends
7 and 9 of this pad.
Along its entire length, the pad is reinforced in its central part
by a back rib 20 consisting of a support covered laterally with
neoprene. This support appears on the back face 19 of the shoe and
may be mounted on the pad linkage support 8. The side wings of the
plate 12 on either side of the back rib 20 are flexible. The back
face 19 of these wings is completely covered with neoprene.
Referring now to FIGS. 2 and 3 which show the pad 4 separate from
the pad linkage support 8 and linkage arms 5 and 6, the support in
the back rib 20 of the pad 4, designated 22, can be clearly seen.
It is a sort of parallelepiped box formed by an elongated narrow
frame defined by two longitudinal uprights 23 and 24, two end walls
25 and 26, and a base plate 28 closing this box on the side of the
front face 13. On the back face 19 of the pad, this support 22 is
hollow and defines an elongated cavity 30. The support 22 may be
made of a rigid material, such as metal or preferably a hard
plastic, or even a more flexible material such as an elastomer. As
shown in FIG. 2, it has a series of tapped holes 32 in reinforced
portions of the uprights 23 and 24 by means of which it may be
bolted or screwed to the pad linkage support 8 (FIG. 1). The pad
linkage support 8 is rigid, and can thus give the support 22 the
rigidity which it may be lacking.
Referring to FIG. 2, three pairs of electrical connection pieces 34
are mounted inside the cavity 30 and provided with plugs 35 which
may be plugged into suitable sockets for connecting the pad
electrically via conductors (not shown) to suitable electrical
circuits in a fluid tight housing (not shown) in the support member
1. The connection pieces 34 are secured by means of a small bracket
36 secured to the upright 23 of this support by means of screws 37.
These connecting pieces are also linked to the electrodes 14 to 18
by conductors such as 38 and 38'. Should the support 22 be molded
in an insulating material, plastic, for example, the necessary
insulation for these connecting elements is inherent. If the
support 22 is a machined metallic part, an additional insulation is
provided for the connection pieces 34 and the conductors 38. When
the support 22 is secured to the pad linkage support 8, the latter
fits in the cavity 30 in which is made the electrical connection
between the pad 4 and the support member 1. In order to protect the
electrical connections, the cavity 30 is packed with grease which
prevents the borehole fluids from penetrating to these connections
and gives them an excellent service life.
In the present example, still viewing FIG. 2, the two central
electrodes 14 and 15 are made of rigid metal pieces. The electrode
14 is a plate mounted on the base plate 28 of the support 22 by a
central screw 40 (see FIG. 3 also). The electrode 15 is a
rectangular metal strip mounted around the electrode 14 and secured
to the base plate 28 (see FIG. 3) by means of two screws such as
41. These two electrodes desirably have a thickness of about 3
millimeters. They are separated by a rectangular strip of neoprene
42.
Referring to FIG. 3, the front face of electrodes 14 and 15 appear
in the front face 13 of the pad 4 about 1 millimeter back from the
front surfaces of the flexible neoprene plate 12 and the neoprene
strip 42. Contrasted with the electrodes 14 and 15 whose transverse
dimensions do not exceed those of the support 22, certain portions
of the electrodes 16, 17 and 18 extend beyond this support. These
electrodes 16, 17 and 18 are mounted on flexible rectangular
frames. Each of these frames possesses two flexible transverse
sides and two rigid longitudinal sides. The frame supporting the
electrode 17 will be described in greater detail with reference to
FIGS. 2 and 4.
The two transverse sides 44 and 45 of this frame are essentially
composed of hard carbon steel spring blades on which are welded at
close intervals conducting pieces in the form of low carbon steel
contacts. These contacts constitute electrode elements, one face of
which appears in the front face 13 of the pad.
In FIG. 4, the transverse side 44 is composed of a spring blade 46
whose central section is mounted on the base plate 28 of the
support 22. Over this central section is welded a conducting strip
48 whose length is substantially equal to the width of the support
22, i.e., about 30 millimeters. A screw 49 secures this strip 48
and the spring blade 46 to the support 22. A plurality of contacts
50, 51, 52 are located on both sides of the support 22 and consist
of small rectangular metal plates welded on the upper face 53 of
the spring blade 46 on the side of the front face 13 of the
pad.
The contacts 50, 51, 52 have a thickness of about 2 millimeters and
their front, wall engaging faces are not covered with the neoprene
of the plate 12. These faces, as well as those of the neoprene
strips which separate them (FIG. 4) are set about 1 millimeter back
from the front face of this plate for reasons well known in the
well logging art. The conductive drilling mud will fill these
recesses thus enabling the electrodes to make electrical contact
with the borehole wall and the formations therebehind.
In the present example, the spring blade 46 has a thickness of
three-tenths of a millimeter and a width of about 5 millimeters,
thereby giving it a very high flexibility perpendicular to its
plane, without any danger of permanent set. The choice of the
thickness and the width of these blades is very important to obtain
a pad having the desired properties. A relatively small increase in
the dimensions of the spring blades quickly leads to insufficient
performance as regards the flexibility of the pad around a
longitudinal axis.
The transverse dimension of the contacts 50, 51 and 52 and their
spacing are also factors influencing the deflection which an
electrode-carrying frame of given width may attain upon
deformation. By way of example, the transverse dimension of the
contacts 51 and 52 is about 5 millimeters and that of the contact
50 about 6 millimeters. Only the portions of the spring blade 46
which are situated between the contacts play a part in the
transverse flexibility of the frame. This is one reason why, if one
wishes to minimize the force used for applying the pad against the
wall, in particular in the case of small-diameter holes, it is very
important to have blades of small thickness and small width. In the
pad presently described, the values of the spacing between contacts
are of the same order of magnitude as the transverse dimension of
these contacts. These spacing values are relatively low because one
endeavors to have electrodes with the highest possible continuity
from the electrical standpoint. This is particularly important for
electrodes which emit current into the formation.
The two parallel spring blades 46 secured to the support 22 on
either side of its transverse axis of symmetry (line III--III in
FIG. 2) are perpendicular to the longitudinal axis of the pad. On
each side of the support 22, their ends are connected by a rigid
connecting strip 60, shown in plan in FIG. 2 and in section in
FIGS. 3 and 4. This strip of 2 millimeter thickness, like the
contacts, is set in the neoprene plate 12, and only its front face
appears slightly set back from the front surface of this plate.
These strips are welded onto the ends of the two spring blades 46
and they close the frame associated with the electrode 17. This
frame is thus rigid in the longitudinal direction through the
combined action of the two rigid strips 60 and the support 22 and
has a transverse flexibility because of the spring blades 46. Owing
to these spring blades 46, there is not only a mechanical
continuity but also an electrical continuity between the parts of
this frame. Its transverse side 45 is connected electrically to the
connection piece 34' by the conductor 38' shown in FIG. 2. The
front surfaces of the contacts 50, 51 and 52 and of the strips 60
constitute the active faces of the electrode proper, distributed
over the front face 13 along a closed linear or strip-like
path.
The frames associated with the electrodes 16 and 18 are formed in a
manner similar to frame 17. Thus, the frame 16 has two transverse
spring blades 62 mounted on the support 22 on either side of the
transverse sides of the electrode 15. The two spring blades 62 are
connected by two longitudinal strips 63 parallel to the axis of the
pad. The frame also has two conducting strips 64 similar to the
strips 48 secured to the support 22. It will be noted in FIG. 2
that spring blades 62 are extended from the support 22 beyond the
strip 63, with their ends 62' providing an additional anchoring of
the electrode frame 16 in the flexible neoprene plate 12.
Likewise, the external electrode frame 18 has two transverse sides
66 and two longitudinal sides 72 (see FIG. 2). The two transverse
sides include spring blades similar to the blades 46 whose central
portions are secured to the support 22. In FIG. 2, the solid
central portions of electrode frame 18 are designated 76. A
succession of contacts 67 to 70 are located on both sides of the
central strips 76. The ends of these blades are connected on both
sides to two strips 72 whose ends 73 are slightly rounded and which
define the longitudinal edges of the pad. FIG. 3 shows that these
strips are slightly thicker than the strips 60 and 63, by about 4
millimeters, and that their external face 74 defines the lateral or
transverse limit of the flexible neoprene plate 12. By this
construction, the side of the flexible plate 12 is protected
against tearing.
As shown clearly in FIG. 2, the frames associated with the
electrodes are mounted concentrically around each other on a
surface substantially parallel to the front face of the shoe. The
side sections (60, 72, 63, etc.) of these electrode frames are
parallel to the longitudinal axis of the pad and the end sections
(66, 44, 64, etc.) are all perpendicular to this longitudinal axis.
The sides and ends of the electrode are symmetrical relative to the
longitudinal axis of the pad (which passes through the point 40 in
FIG. 2) and the transverse axis of the pad (which also passes
through point 40) respectively such that all of the rectangular
electrodes have a common center point. The neoprene plate 12 is
shaped so that the pad face 13 is slightly convex and the neoprene
holds the frames in the desired position. The longitudinal parts of
these frames are rigid thereby requiring the pad to bend
cylindrically about a longitudinal axis of the pad upon pressure
being applied.
The electrode frames mounted on the support 22 give a fragile
appearance owing to their dimensions, particularly for the large
frames such as 17 and 18. The elastomer plate 12 in which they are
held along with the support 22 provides cohesion and structural
integrity of these parts while also insulating the electrodes from
each other. It gives the assembly a resistance to forces tangent to
the front face 13. This resistance is reinforced by the blades in
the flexible plate 12. On either side of the back rib 20, the
thickness of the plate is reduced to 8 millimeters thereby allowing
it to bend in response to a limited force which is compatible with
the wearability of the front surface of the shoe.
The flexible plate 12 covers all the spring blades and the side
faces of the different contacts and strips, with the exception of
the external lateral faces 74 of the strips 72. It also covers the
external walls of the longitudinal uprights 23 and 24 of the
support 22 as shown in FIG. 2 and 3. It adheres to all these parts
because of a suitable treatment during molding.
It is noteworthy that, in spite of their thinness, the spring
blades which support the transverse sides of the flexible frames do
not cut the elastomer during temperature changes owing to the
presence, at close intervals, of contacts which act as
reinforcement along these blades and prevent the internal shearing
by the plate under the effect of the different expansions of the
steel and the neoprene.
To manufacture a pad of the type just described, the following
procedure may be used. On an elongated hollow support 22, suitably
molded and machined, the electrode frames 14 and 18 are secured by
screws. The frames are electrically connected by means of
conductors 38 to the electrical connection pieces 34 which have
been previously mounted on this support. The assembly thus formed
is placed in a mold whose bottom portion has the form of the front
face 13 (see FIG. 3), after all the parts to be held in the
elastomer are covered with adhesive. The front face of the contacts
and the conducting strips are not covered with adhesive and fit
into suitable recesses in the mold. The depth of these recesses
corresponds to the inset of the faces of the electrodes in the
front face of the pad. An elastomer is injected into the mold on
the back face of the pad according to the technique known as
transfer molding. In this technique, the softened elastomer
penetrates into the mold through a multiplicity of openings made in
a plate closing off the mold on the back side of the pad. Under the
action of the elastomer injection pressure, the electrode frames
are thrust to the bottom of the mold against the corresponding
recesses.
As regards the frames proper, they are made by using individual
steel blades and contacts having substantially the same composition
and placed on a welding fixture to be fusion welded in argon. After
this operation, the frames are removed from the fixture and undergo
a heat treatment with tempering intended to give the blades their
optimum elastic properties.
By way of example, a pad constructed as described above has a width
of 136 millimeters between the edges 74 of the strips 72 (see FIG.
3) and a length (along the pad's longitudinal axis) of 300
millimeters. The back rib 20 has a width of 40 millimeters and the
greatest thickness of the pad is about 20 millimeters. The large
electrode 18 is 190 millimeters long and has the same width as the
pad 4. The thickness of the active pieces (those pieces which come
in direct contact with drilling fluid) of the electrodes is about 5
millimeters.
This pad may be easily bent by hand and, when bent, takes on a
cylindrical profile. By way of example, with the dimensions
indicated in the description above, the application force of such a
pad against the walls of a hole of about 15 centimeter diameter is
8 kilograms. About half of this force is balanced by the spring
blades, the other half being balanced by the inherent elasticity of
the neoprene. Tests have shown that this pad was very rugged during
its use in boreholes; that the elastomer does not tear; that the
conducting electrode contacts remain securely in place in the front
face because of their attachment to the spring blades set in the
elastomer; and that they are protected against corrosion and
wear.
The pad described above constitutes an example of an embodiment of
the invention which has actually been developed. The pad so
described is particularly useful with the investigating system
described in copending application Ser. No. 815,265 filed by N. A.
Schuster on Apr. 7, 1969. It is however clear that many variations
are possible without departing from the basic principles or the
scope of the invention. It is particularly evident that these
principles are capable of being applied to other types of pads with
different arrangements, forms and number of electrodes. Similar
arrangements may be adopted for making pads with circular or
elliptical electrodes.
While there have been described what are at present considered to
be preferred embodiments of this invention, it will be obvious to
those skilled in the art that various changes and modifications may
be made therein without departing from the invention, and it is,
therefore, intended to cover all such changes and modifications as
fall within the true spirit and scope of the invention.
* * * * *