US3795141A

US3795141A - Well logging probes

Info

Publication number: US3795141A
Application number: US00256107A
Authority: US
Inventors: J Planche
Original assignee: Schlumberger Technology Corp
Current assignee: Schlumberger Technology Corp
Priority date: 1971-05-24
Filing date: 1972-05-23
Publication date: 1974-03-05
Anticipated expiration: 1991-03-05
Also published as: DE2223735C3; DE2223735A1; FR2138335A1; CA950824A; DE2223735B2; FR2138335B1

Abstract

In the representative embodiment of the invention disclosed herein, an elongated wall-engaging pad carrying one or more formation-investigating devices is operatively mounted along one side of a well logging tool and arranged for tilting about an axis perpendicular to a plane of movement including the longitudinal axes of the tool body and the pad as the pad is urged against an adjacent borehole wall. To insure that the pad member is free to tilt as may be required for following irregular borehole walls, forwardly-urged positioning members are uniquely arranged on the tool above and below the pad for spatially positioning that side of the tool away from the adjacent borehole wall. One or more rearwardly-urged positioning members are also arranged on the opposite side of the tool and adapted to be pressed against the opposite borehole wall with only sufficient force to retain the spaced positioning members engaged with the adjacent borehole wall above and below the pad member.

Description

17 5] Inventor; Jean Planche, liHay-les-Roses,

[ Mar. 5, 1974 United States Patent Planche WELL LOGGING PROBES Primary Examiner.lerry W. Myracle William R. Sherman; Stewart F. Moore France [57] ABSTRACT In the representative embodiment of the invention disclosed herein, an elongated wall-engaging pad carrying one or more formation-investigating devices is op- [73] Assignee: Schlumberger Technology Corporation, New York, NY.

[22] filed: May 1972 eratively mounted along one side of a well lggging tgol [21] Appl. No.: 256,107 and arranged for tilting about an axis perpendicular to a plane of movement including the longitudinal axes to of the tool body and the pad as the pad is urged [30} Foreign Application Priority Data against an adjacent borehole all. To insure that the Ma 24 France 71 18636 pad member is free to tilt as may be required for foly lowing irregular borehole walls, forwardly-urged positioning members are uniquely arranged on the tool 73/ 5 5 2 5 5 above and below the pad for spatially P sitioning that [58] Fieid S '250/83 6 side of the tool away from the adjacent borehole wall.

166/241; 324/5, 6, 7; 181/5 BE 5 P One or more rearwardly-urged positioning members adapted to be pressed against the opposite borehole References Cited wall with only sufficient force to retain the spaced po- UNITED STATES PATENTS sitioning members engaged with the adjacent borehole wall above and below the pad member. 2,960,608 11/1960 Armlstead 250/83.6 W X 3,448,373 6/1969 Hahn 73/152 X 15 Claims, 1 Drawing Figure Att0rney,Agent, 0r Firm-Ernest R. Archambeau, llr;

are also arranged on the opposite side of the tool and WELL LoGGiNG PROBES In recent years, various well logging tools have been developed which obtain one or more formation measurements by pressing a formation-investigating instru- 'ment against the borehole wall. Typically, as shown in US. Pat. No. 3,023,508, such logging tools include an elongated wall-engaging pad carrying one or more formation-investigating devices which is pivotally mounted on the outer ends of extendible paralleled linkage arms and appropriately arranged for limited tilti'figmovement about an axis which is perpendicular to a selected plane of movement including the longitudinal axes of the tool body and the pad. To urge the pad into contact with one wall of the borehole, one or more spring-biased positionirTg arms are typically arranged on the opposite side of the logging tool and adapted to be urged against the opposite borehole wall with sufficient force to retain the face of the pad in contact with the adjacent borehole wall.

With these typical borehole tools, the forward face of the tool body will follow only the general contours of the borehole and the pad member will hopefully move inwardly and outwardly in relation to the tool body as well as tilt about its mounting axis to closely follow irregular borehole surfaces. It will be recognized, however, that unless the tool is centralized as shown in US. Pat. No. 3,500,683. the body of this tool will ordinarily be so close to the borehole wall that frequently the pad will be so closely confined between the borehole wall and the tool body that it will lack sufficient freedom of movement to accurately follow the borehole contours at all times. Thus, since the accuracy of the logging measurements are largely dependent upon the face of the pad remaining in contact with the borehole wall, there will be occasional interruptions or discrepancies in the logging records as the tool traverses borehole intervals in which the pad is restricted from moving freely.

Accordingly, it is an object of the present invention to provide new and improved logging tools having a laterally-extendible wall-engaging measuring pad which is cooperatively arranged for closely following the contours of a well bore wall.

This and other objects of the present invention are attained by providing well logging tools having first positioning means operatively arranged on the body of the tool for urging one side thereof away from an adjacent well bore wall with a first force of at least a predetermined magnitude and second positioning means contacting the opposite well bore wall are adapted for urging the tool body against the first positioning means with a second force less than the first force, Formationinvestigating means are movably arranged to be urged outwardly from the one body side by biasing means operatively arranged for urging the formationinvestigating means against the adjacent well bore wall with a third force less than the second force.

The novel features of the present invention are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may be best understood by way of the following description of exemplary apparatus employing the principles of the invention as illustrated in the accompanying drawing.

Turning now to the drawing, a new and improved logging tool incorporating the principles of the present invention is illustrated as it will appear while suspended in a borehole 11 by a typical logging cable 12 which is carried over a pulley 13 at the surface and spooled in the usual fashion on a winch (not shown). As is typical, the electrical conductors in the logging cable 12 are coupled to appropriate surface equipment 14 operatively arranged for controlling the operation of the logging tool 10 as well as processing the logging signals provided thereby.

As illustrated, the preferred embodiment of the new and improved logging tool 10 includes a pair of enclosed

housings

15 and 16 arranged on its upper end and dependently supporting paralleled, laterallyspaced body members, as at 17, which are joined at their lower ends by a transverse spacer 20. The logging tool 10 includes formation-investigating means including electrical circuitry in the housing 15 and one or more typical electrical, acoustic or radioactivity measuring elements or monitoring devices, as at 18 and 19, which are operatively arranged on a selectivelyextendible wall'engaging pad 341. As will subsequently be explained, the pad 34 is adapted to be moved between a retracted position within or adjacent to the forward side of the spaced body members 17 and its illustrated extended position for placing the

formation measuring elements

18 and 19 into the proximity of the nearby wall of the borehole 11. The logging tool 10 further includes first tool-positioning means such as a pair of selectively-

extendible arms

31 and 57 carrying wallengaging members or

rollers

30 and 61 on their outer ends which are respectively arranged on the body 17 above and below the extendible pad 34. By pivotally mounting the

arms

31 and 57 as at 32 and 60, the

rollers

30 and 61 are adapted for movement in a longitudinal plane between retracted positions and their respectively-illustrated extended positions for spacing the body 17 at a selected distance away from the wall of the borehole 11 so that the pad member 34 is free to move as required for accurately following the contours of the borehole wall. The first positioning means further include biasing means, such as a pair of springs 33 and 62, for urging the

arms

31 and 57 outwardly. Second tool-positioning means are also operatively ar ranged on the body 17 and, in the preferred embodiment of the logging tool 10, include a pair of selectively-extendible, longitudinally-spaced wall-contacting members, as at 21 and 50, which are respectively pivoted, as at 22 and 51, to the body for movement in the aforementioned longitudinal plane. Accordingly, as will be subsequently explained, upon concerted movement of the

arms

21 and 50 from their retracted positions adjacent to the rearward side of the body 17 and their respective extended positions, the tool body will be shifted laterally (to the right as viewed in the drawing) for insuring that the

rollers

30 and 61 are engaged in rolling contact with the borehole wall above and below the pad member 34.

In keeping with the objects of the present invention, it is of paramount importance that the pad 34 be free to move as required for closely following the adjacent irregular surfaces of the borehole 11 as the logging tool 10 is moved through the borehole during a logging operation. Accordingly, in the preferred embodiment of the logging tool 10, the pad 34 is movably coupled to the tool body 17 by a pair of

paralleled arms

36 and 41 each having their outer ends pivotally coupled to the pad member as at 35 and 40. The lower arm $1 is pivotally coupled to the body by a transverse pivot 42; and the inner end portion of the upper arm 36 is provided with an elongated slot 37 which is slidably disposed over a second transverse pivot which is also attached between the paralleled body members 17. it will be recognized, therefore, that upon outward movement of the

pivoted arms

36 and 41, the pad 34 will be moved out wardly from its retracted position between the paralleled body members 17 to its illustrated extended position so as to place the forward face of the pad into contact with the adjacent wall of the borehole 11. By virtue of the pivot 35 and the slot 37 in the arm 36, the pad member 34 will, of course, be free to tilt about the central pivot 411 with a sufficient degree of freedom to enable the pad to closely follow the contours of the ad jacent borehole wall. To urge the pad 34 against the adjacent borehole wall, biasing means are provided such as an arcuate spring 43 which is secured to the body 17 and has its free end resiliently engaged with the pad member.

As previously mentioned, the tool body 17 is spatially positioned at a selected distance from the adjacent borehole wall by the rollers 311 and 61. Accordingly, to insure that the outwardly-disposed rollers 311 and 61 are fully extended into rolling contact with the adjacent wall of the borehole 11, the positioning arms 21 and 511 are cooperatively arranged to be extended outwardly from the rear side of the tool body 17 and urged into contact with the wall of the borehole opposite the wall on which the pad member 34 is engaged against. To accomplish this, biasing means, such as

springs

23 and 52, are cooperatively arranged on the body 17 for urging the

arms

21 and 50 outwardly with sufficient force to shift the body laterally (to the right as viewed in the drawing) for placing the extended rollers 31) and 61 into engagement with the borehole wall. It will be recognized, of course, that if the biasing forces collectively supplied by the

springs

23 and 52 exceed the opposing combined biasing forces supplied by the springs 33 and 43, extension of the

arms

21 and 50 would tend to cause the

rollers

30 and 61 to be retracted inwardly in relation to the tool body 17 and thereby reduce the clearance space in which the pad 34 can freely move. Thus, to prevent this from occurring, the springs 33 and 62 are selected in conjunction with the effective length of the

arms

31 and 57 for urging the

rollers

30 and 61 outwardly with a combined biasing fogce which is at least somewhat greater than the combined biasing forces imposed by the

springs

23 and 52 on the extend

ible arms

21 and 50 respectively. In this manner, whenever the

arms

21 and 50 are extended into contact with the rear wall of the borehole 11, the tool body 17 is shifted laterally to the right as viewed in the drawing until the extended rollers 31) and 61 are in rolling contact with the adjacent or forward wall of the borehole. Then, by virtue of the greater biasing forces collectively supplied by the springs 33 and 62, the

rollers

30 and 61 will be retained in their maximum extended position; and, as the logging tool passes through the borehole 11, the

arms

21 and 50 will shift inwardly and outwardly as is necessary for the continued passage of the logging tool through the borehole. Therefore, since the

rollers

30 and 61 will remain in their maximum extended position, the pad 34 will be free to tilt freely about its pivot 40 as well as to be shifted laterally as is required to follow the irregular contour of the wall of the borehole 11. The spring 43 is selected to be weaker than the

springs

23 and 52 so that rearward movements of the pad 34 will not cause the arms 21 and to move inwardly toward the body 17.

it will, of course, be appreciated that for the logging tool 111 to be readily lowered into the borehole 11, provisions should also be made for retraction of the wallcontacting

members

21 and 50 as well as the

rollers

30 and 61 and the pad member 34. Accordingly, in the preferred embodiment of the new and improved logging tool 111, selectively-operable actuatingfieans, such as a typical hydra u li pressure-devel'c$ing system, are disp sed in the housin 16 and operatively coupled to each of the several

extendible arms

21, 31, 41, 50 and 57. in the p r eferred manner of mging this hydraulic actuating means, an electric motor 65 coupled to a suitable hydraulic pump 66 are mounted in the housing 16 and fluidly coupled as by a hydraulic conduit 63 to typical piston actuators as at 26, 45 and 55. As illustrated, the actuator 26 preferably includes a pair of

opposed pistons

25 and 27 which are respectively adapted fowtward movement into engagement with a crank arm 24 on the end of the positioning arm 21 and the roller support arm 31 upon application of an increased hydga ulic pressure in the piston chamber between the opposed pistons. Similarly, the piston actuator 55 is comprised ofa second pair of

opposed pistons

54 and 56 which, upon an increase of hydraulic P13551116 in the chamber therebetween, are cooperatively arranged for movement into engagement with a crank arm 53 on the arm 50 and the roller support arm 57 respectively. A single piston 44 is cooperatively arranged in the chamber of the piston actuator 45 for engaging a crank arm 51" on the inner end of the lever 41 whenever an increased hydraulic pressure is applied thereto. Accordingly, it will be recognized that whenever there is an increase of hydraulic pressure in the fluid conduit 63, the

piston actuators

26, 45 and 55 will simultaneously be actuated for retracting the

extendible arms

21, 31, 51) and 57 in unison.

To maintain the hydraulic system in equilibrium with the hydrostatic pressure of the borehole fluids, the housing 16 is arranged to provide a reservoir 64 for the hydraulic fluid and a piston 73 is movably disposed in an opening 72 between the reservoir and the exterior of the tool 14). It will be appreciated, therefore, that the piston 73 will serve to maintain the pressure of the hydraulic fluid in the reservoir 64 equal to the hydrostatic pressure of the borehole fluids. To pressure balance the

piston actuators

26, 45 and 55, the pump 66 is arranged to take suction from the reservoir 64 and the discharge conduit 63 is normally opened to the reservoir as by a normally-open solenoid valve 67. Thus, so long as the solenoid valve is open, the fluid in the hydraulic reservoir 64 will be at hydrostatic pressure and the hydrostatic pressure of the borehole fluids will be acting on both ends of the

several pistons

25, 26, 44, 54 and 56 so that the

arms

24, 31, 58, 53 and 57 will be free to be moved by the

several springs

23, 33, 43, 52 and 58. To operate the pump 65 and the solenoid valve 67, conductors and 71 are connected therefrom to the circuitry in the housing 15.

In operation of the new and improved logging tool 10, the hydraulic pump 66 and the solenoid valve 67 are initially energized so that the

piston actuators

26, 45 and 55 will be respectively actuated for retaining the positioning

arms

21, 31, 511 and 57 as well as the pad 34 retracted against the body 17. This will, of course,

allow the logging tool it) to be lowered on the cable 12 into the borehole 1 ll until the tool has reached its initial depth from which a logging operation is to be commenced. Once the tool is at its initial depth, the pump 66 and the solenoid valve 67 are de-energized so that the positioning

arms

21, 31, 50 and 57 as well as the pad 34 are free to be urged outwardly to their respective extended positions by the

several biasing springs

23, 33, 43, 52 and s2.

With the tool MD now positioned within the borehole ill and the pad 34 in engagement with the adjacent borehole wall, the winch (not shown) is operated as, for example, required to withdraw the tool from the borehole so as to investigate the earth formations thereabove. It will, of course, be appreciated that since the rollers 34) and 61 are fuliy extended, there will be an ample clearance space between the forward face of the tool body 17 and the adjacent wall of the borehole llll. Thus, should the measuring pad 34 carrying the investigating devices l8 and 19 pass a protrusion on the borehole wall as illustrated in the drawing, the pad will be free to first tilt rearwardly as it initially encounters the borehole protrusion and then tilt forwardly as shown as the protrusion is passed. Similarly, should the pad 34 meet a washout or enlarged-diameter section in the borehole ill, it will still be free to tilt as required to remain in contact with the borehole wall as far as is permitted by the respective dimensions of the pad member and the discontinuity in the borehole wall.

Accordingly, it will be appreciated that the present invention has provided new and improved logging tools which are particularly suited for logging well bores having irregular contours. By cooperatively arranging the first and second tool positioning means on the tool body in the unique fashion disclosed herein, the body of the tool is always spatially disposed away from a borehole wall so as to provide a sufficient clearance for a pad-mounted formation-investigating device to operate more efficiently and obtain more-effective logging records.

While a particular embodiment of the present invention has been shown and described, it is apparent that changes and modifications may be made without departing from this invention in its broader aspects; and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What is claimed is:

l. Well-logging apparatus adapted for passage through a well bore for investigating earth formations penetrated thereby and comprising: a body; first positioning means adapted for spacing one side of said body away from one wall of a well bore and including upper and lower wall-engaging members movably mounted on said body and respectively adapted for movement between a retracted position and an extended position away from said one side of said body, and first biasing means operatively arranged for urging said wallengaging members respectively against said one well bore wall with a combined first force of at least a predetermined magnitude; second positioning means operatively arranged on said body and including at least one wall-contacting member movably mounted on said body and adapted for movement between a retracted position and at least one extended position away from the opposite side of said body, and second biasing means operatively arranged for urging said wallcontacting member against the opposite wall of a well bore with a second force less than said predetermined magnitude; formation'investigating means adapted to be pressed against said onewell bore wall movably coupled to said body and adapted for movement between a retracted position adjacent to said one side of said body and extended positions against said one well bore wall; and third biasing means operatively arranged for urging said formation-investigating means against said one well bore wall with a third force less than said second force.

2. The well-logging apparatus of claim ll further including actuating means on said body and operatively coupled to said members and said formationinvestigating means for selectively moving said members and said formation-investigating means between their respective retracted and extended positions.

3. The well-logging apparatus of claim 1 wherein said first, second and third biasing means normally urge said members and said formation-investigating means toward their respective extended positions; and further including actuating means on said body and operatively coupled to said members and said formationinvestigating means for selectively moving said members and said formation-investigating means from their respective extended positions to their respective retracted positions.

4. The well-logging apparatus or claim 1 wherein said formation-investigating means include a wall-engaging pad member, and at least one pivoted arm operatively coupling said pad member to said body for moving between said retracted and extended positions of said formation-investigating means.

5. The well-logging apparatus of claim 1 wherein said upper and lower wall-engaging members are respectively above and below said formation investigating means.

6. Well'logging apparatus adapted for passage through a well bore for investigating earth formations penetrated thereby and comprising: a body; first and second positioning members movably mounted at spaced intervals along one side of said body and respectively adapted for movement between a retracted position adjacent thereto and an extended position at a selected distance therefrom for engaging one wall of a well bore; formation-investigating means on said body and including a wall-engaging member, means on said wall-engaging member adapted for measuring at least one formation characteristic upon engagement of said wall-engaging member with said one well bore wall, and support means movably coupling said wallengaging member to said body for movement of said wall-engaging member between a retracted position adjacent to said one side of said body and extended positions in engagement with said one well bore wall; at least one third positioning member movably mounted on said body and' adapted for movement between a retracted position adjacent to the opposite side of said body and extended positions in engagement with the opposite wall of a well bore; first and second biasing means for normally urging said first and second positioning members respectively to their said extended positions and against said one well bore wall with first and second forces having a combined magnitude of at least a predetermined amount; third biasing means for normally urging said third positioning member toward its said extended positions and against said opposite well bore wall with a third force less than said combined first and second forces; fourth biasing means for normally urging said wall-engaging member toward its said extended positions and against said one well bore wall with a fourth force less than said third force; and retracting means on said body and operatively coupled to said extendible members for selectively returning said extendible members to their respective retracted positions.

7. The well-logging apparatus of claim 6 wherein said wall-engaging member is located between said first and second positioning members.

ti. The well-logging apparatus of claim 6 wherein said support means include first and second parallel arms movable in a longitudinal plane defined by the longitudinal axes of said body and said wall-engaging member and respectively pivotally coupled to said wall engaging member and to said body for allowing said wall-engaging member to tilt about a transverse axis perpendicular to said longitudinal plane for following irregular contours on said one well bore wall.

9. The well-logging apparatus of claim 6 wherein said first and second positioning members respectively include a laterallymovable arm member having its inner end portion pivotally coupled to said body for movement in a longitudinal plane defined by the longitudinal axes of said body and said wall-engaging member, and a roller member journalled to the outer end portion of said arm member for rotation about a transverse axis perpendicular to said longitudinal plane.

it). The well-logging apparatus of claim 9 wherein said support means include first and second parallel arms movable in said longitudinal plane and respectively pivotally coupled to said wall-engaging member and to said body for allowing said wall-engaging member to tilt about a selected transverse axis perpendicular to said longitudinal plane.

lll. Well-logging apparatus adapted for passage through a well bore for investigating earth formations penetrated thereby and comprising: a body; upper and lower wall-engaging members movably mounted on said body and respectively adapted for movement between retractcd and extended positions, and first biasing means operatively arranged for urging said wallengaging members respectively in a first direction against a well bore wall with a combined first force of at least a predetermined magnitude for spacing one side of said body away from one wall of a well bore; second positioning means operatively arranged on said body and including at least one wall-contacting member movably mounted on said body and adapted for movement between retracted and extended positions, and second biasing means operatively arranged for urging said wall-contacting member in a second direction against another wall of a well bore with a second force less than said predetermined magnitude for spacing said body away from said other well bore wall; formation-investigating means adapted to be pressed against said one well bore wall movably coupled to said body and adapted for movement between a retracted position adjacent to said body and extended positions against said one well bore wall; and third biasing means operatively arranged for urging said formationinvestigating means against said one well bore wall with a third force less than said second force.

12. The well-logging apparatus of claim 11 further including actuating means on said body and operatively coupled to said members and said formationinvestigating means for selectively moving said members and said formation-investigating means between their respective retracted and extended positions.

T3. The well-logging apparatus of claim llll wherein said first, second and third biasing means normally urge said members and said formation-investigating means toward their respective extended positions; and further including actuating means on said body and operatively coupled to said members and said formationinvestigating means for selectively moving said members and said formation-investigating means from their respective extended positions to their respective retracted positions.

14. The well-logging apparatus of claim 11 wherein said formation-investigating means include a wallengaging pad member, and at least one pivoted arm operatively coupling said pad member to said body for moving between said retracted and extended positions of said formation-investigating means.

l5. The well-logging apparatus of claim ll wherein said upper and lower wall-engaging members are respectively above and below said formationinvestigating means.

Claims

1. Well-logging apparatus adapted for passage through a well bore for investigating earth formations penetrated thereby and comprising: a body; first positioning means adapted for spacing one side of said body away from one wall of a well bore and including upper and lower wall-engaging members movably mounted on said body and respectively adapted for movement between a retracted position and an extended position away from said one side of said body, and first biasing means operatively arranged for urging said wall-engaging members respectively against said one well bore wall with a combined first force of at least a predetermined magnitude; second positioning means operatively arranged on said body and including at least one wall-contacting member movably mounted on said body and adapted for movement between a retracted position and at least one extended position away from the opposite side of said body, and second biasing means operatively arranged for urging said wall-contacting member against the opposite wall of a well bore with a second force less than said predetermined magnitude; formation-investigating means adapted to be pressed against said one well bore wall movably coupled to said body and adapted for movement between a retracted position adjacent to said one side of said body and extended positions against said one well bore wall; and third biasing means operatively arranged for urging said formationinvestigating means against said one well bore wall with a third force less than said second force.

2. The well-logging apparatus of claim 1 further including actuating means on said body and operatively coupled to said members and said formation-investigating means for selectively moving said members and said formation-investigating means between their respective retracted and extended positions.

3. The well-logging apparatus of claim 1 wherein said first, second and third biasing means normally urge said membErs and said formation-investigating means toward their respective extended positions; and further including actuating means on said body and operatively coupled to said members and said formation-investigating means for selectively moving said members and said formation-investigating means from their respective extended positions to their respective retracted positions.

5. The well-logging apparatus of claim 1 wherein said upper and lower wall-engaging members are respectively above and below said formation-investigating means.

6. Well-logging apparatus adapted for passage through a well bore for investigating earth formations penetrated thereby and comprising: a body; first and second positioning members movably mounted at spaced intervals along one side of said body and respectively adapted for movement between a retracted position adjacent thereto and an extended position at a selected distance therefrom for engaging one wall of a well bore; formation-investigating means on said body and including a wall-engaging member, means on said wall-engaging member adapted for measuring at least one formation characteristic upon engagement of said wall-engaging member with said one well bore wall, and support means movably coupling said wall-engaging member to said body for movement of said wall-engaging member between a retracted position adjacent to said one side of said body and extended positions in engagement with said one well bore wall; at least one third positioning member movably mounted on said body and adapted for movement between a retracted position adjacent to the opposite side of said body and extended positions in engagement with the opposite wall of a well bore; first and second biasing means for normally urging said first and second positioning members respectively to their said extended positions and against said one well bore wall with first and second forces having a combined magnitude of at least a predetermined amount; third biasing means for normally urging said third positioning member toward its said extended positions and against said opposite well bore wall with a third force less than said combined first and second forces; fourth biasing means for normally urging said wall-engaging member toward its said extended positions and against said one well bore wall with a fourth force less than said third force; and retracting means on said body and operatively coupled to said extendible members for selectively returning said extendible members to their respective retracted positions.

8. The well-logging apparatus of claim 6 wherein said support means include first and second parallel arms movable in a longitudinal plane defined by the longitudinal axes of said body and said wall-engaging member and respectively pivotally coupled to said wall-engaging member and to said body for allowing said wall-engaging member to tilt about a transverse axis perpendicular to said longitudinal plane for following irregular contours on said one well bore wall.

9. The well-logging apparatus of claim 6 wherein said first and second positioning members respectively include a laterally-movable arm member having its inner end portion pivotally coupled to said body for movement in a longitudinal plane defined by the longitudinal axes of said body and said wall-engaging member, and a roller member journalled to the outer end portion of said arm member for rotation about a transverse axis perpendicular to said longitudinal plane.

10. The well-logging apparatus of claim 9 wherein said support means include first and second parallel Arms movable in said longitudinal plane and respectively pivotally coupled to said wall-engaging member and to said body for allowing said wall-engaging member to tilt about a selected transverse axis perpendicular to said longitudinal plane.

11. Well-logging apparatus adapted for passage through a well bore for investigating earth formations penetrated thereby and comprising: a body; upper and lower wall-engaging members movably mounted on said body and respectively adapted for movement between retracted and extended positions, and first biasing means operatively arranged for urging said wall-engaging members respectively in a first direction against a well bore wall with a combined first force of at least a predetermined magnitude for spacing one side of said body away from one wall of a well bore; second positioning means operatively arranged on said body and including at least one wall-contacting member movably mounted on said body and adapted for movement between retracted and extended positions, and second biasing means operatively arranged for urging said wall-contacting member in a second direction against another wall of a well bore with a second force less than said predetermined magnitude for spacing said body away from said other well bore wall; formation-investigating means adapted to be pressed against said one well bore wall movably coupled to said body and adapted for movement between a retracted position adjacent to said body and extended positions against said one well bore wall; and third biasing means operatively arranged for urging said formation-investigating means against said one well bore wall with a third force less than said second force.

12. The well-logging apparatus of claim 11 further including actuating means on said body and operatively coupled to said members and said formation-investigating means for selectively moving said members and said formation-investigating means between their respective retracted and extended positions.

13. The well-logging apparatus of claim 11 wherein said first, second and third biasing means normally urge said members and said formation-investigating means toward their respective extended positions; and further including actuating means on said body and operatively coupled to said members and said formation-investigating means for selectively moving said members and said formation-investigating means from their respective extended positions to their respective retracted positions.

14. The well-logging apparatus of claim 11 wherein said formation-investigating means include a wall-engaging pad member, and at least one pivoted arm operatively coupling said pad member to said body for moving between said retracted and extended positions of said formation-investigating means.

15. The well-logging apparatus of claim 11 wherein said upper and lower wall-engaging members are respectively above and below said formation-investigating means.