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.

Parent Case Text

This application is a continuation of Ser. No. 134,939, now abandoned.

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.

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.