US2842678A

US2842678A - Shielding in irradiation well logging

Info

Publication number: US2842678A
Application number: US379414A
Authority: US
Inventors: Silverman Daniel
Original assignee: Pan American Petroleum Corp
Current assignee: Pan American Petroleum Corp
Priority date: 1953-09-10
Filing date: 1953-09-10
Publication date: 1958-07-08
Anticipated expiration: 1975-07-08

Description

y 3, 1953 D. SILVERMAN 2,842,678

SHIELDING IN IRRADIATION WELL LOGGING Filed-Sept. 10, 1953 4 Sheets-Sheet 1 III! INVENTOR:

N DANIEL SILVERMAN BY -FIG. I gm rroflzsr July 8, 1958 D. SILVERMAN SHIELDING m IRRADIATION WELL LOGGING Filed Sept. 10 1953 4 Sheets-Sheet 2 FIG. 5

FIG. 4

FIG. 6

INVENTOR.

DANIEL. SILVERMAN WW ATTORNEY July 8, 1958 D. SILVERMAN 2,842,678

, SHIELDING I N IRRADIATION WELL LOGGING Filed Sept. 10, 1953 4 Sheets-Sheet 3 .p-bm: 0-0

FIG.|2

FIG. 9

INVENTOR:

. DANIEL SILVERMAN BY pm A TTOR/VEY D. SILVERMAN 4 Sheets-Sheet 4 F IG. l4

FIG. I6

INVENTOR.

BY DANIEL SILVERMAN ATTORNEY SHIELDING IN IRRADIATION WELL LOGGING Filed Sept. 10 1953 July 8, 1958 FIG. l5

United States Patent 'O- SHIELDING 1N IRRADIATION WELL LOGGING Daniel Silverman, Tulsa, Okla., assignor to Pan American Petroleum Corporation, a corporation of Delaware Application September 10, 1953, Serial No. 379,414

8 Claims. (Cl. 250-108) This invention relates to the logging of well formations by irradiating them with a source of penetrating radiations or particles .and detecting, at a distance from the source of irradiation, the resulting radiations or particles after travel through a substantialy thickness of the formation. More specifically, the invention is directed to an improved method and apparatus for logging the density of well formations by the absorption and scatter ing of gamma rays from a concentrated source thereof.

A substantial degree of success has now been attained in logging well-formation densities by the absorption and scattering of gamma rays, and neutron well logging has now been commercially practiced for a number of years. A major difficulty with both types of logging, however, which has only recently been generally recognized for neutron logging, is the variation in detector response due to variations in the bore-hole diameter. This is particularly noticeable in gamma-ray absorption logging where knowledge of or some correction or compensation for' bore-hole diameter variations is absolutely essential to preventmisleading errors in determining formation densities. The diameter-variation effect is further accentuated by sensitivity of the logging device to roughness of the bore-hole Walls or to abrupt changes in the diameter. Furthermore, gamma-ray absorption logging measurements are also sensitive to the density of the fluids in the well bore, which density has no significance in determining formation density.

It is accordingly a primary object of my invention to provide an improved method and apparatus for reducing or substantially eliminating the effect of bore-hole diameter variations in irradiation well logging. Another object of the invention is to provide a method and apparatus to reduce the adverse effects of bore-hole diameter variations and/or wall roughness on the gamma-ray absorption logging measurements. A further object" is to provide a method and apparatus to reduce the influence of varying density of'the bore-hole fluids on the log of formation density. A still further object is to provide a method and apparatus substantially eliminating the need for measuring variations in bore-hole diameter to correct for their effect. Other objects, 'uses, and advantages of the invention will become apparent description proceeds.

Briefly stated, the essence of my invention by which the foregoing and other objects area'ccomplished comprises an arrangement of shielding material around and between the source and the detector of a logging instrument, which arrangement is particularly adapted to well-bore diameter, particularly on the side of the' inafl strument which is pressed against these formations, and especially when used in cooperation with certain fixed as the 2,842,678 Ice l atented July 8, 195 8 shielding at the source and detector, is highly effective to reduce or even substantially'eliminate response variacontact between the irradiation source and the formations Furthermore, with this shielding, the need for tight contact between the irradiation source and the formation and between the detector and the formations is greatly reduced. It is not completely eliminated, however, as any well fluids interposed between the source or the detector and the well wall act as absorbing media in the path of the significant gamma rays which travel chiefly through the formations. i

This will be better understood by reference to the accompanying drawings forming a part of this application and illustrating certain embodiments and modifications of my invention. In these drawings: a l Figure 1 is a cross-section of a portion of a well bore and an elevation view of an'embodiment of the invention in logging position therein; a v

Figure 2 is an enlarged cross-section of the-instrument of Figure 1 along the line 2 2;

Figure 3 is an enlarged cross-section of-the well bore and instrumentof Figure 1 along the line 3-3; a

Figure 4 is an enlarged cross-section view of a preferred mounting for the irradiation source; 1 E

FigureS is'a diagrammatic view of surface record ing equipment used with the instrument shown inFig ure 1; V p 0 7 Q Figure'6 is an enlarged cross-section of a well bore and of a mo'dification of the shield members of Figures land 3; i

t Figure 7 is a cross-section of Figure along the line of a further embodiment of the invention;

Figure 10 is a Ill-10; V

Figure 11 is a cross-section view of one of the flexible shield units of Figure 9; r Figure 12 is a view similar to Figure lshowing an alternative embodiment of the invention; V Figure 13 is a view similar to Figure 1 but showing, in cross-section, an instrument embodying .a further modi-i fication of the invention; Figure 14 is a'view similar to Figure l of a still fur ther embodiment of the invention; 1 Figure 15 is a .plan view of the shielding elements of Figure 14; and Figure 16 is a cross-section view of a portion of an, instrument body showing the means of mounting and operating the shielding elements of Figures 14 and 15.

Referring now to these drawings in detail and particularly to Figure 1 thereof, alogging instrument 20:v is shown in measuring position in a well bore 21, shown.

cross-section of Figure, 9 along the line in cross-section, the instrument 20 comprising generally an elongatedcylindrical housing. At the lower end'of instrument 20 is a gamma-ray source 23 substantially in contact with the wall of well 21, and at a spacing of the order of fifteenfjto thirtyinches above source 23 is a. detector 24. Source 23 is a capsule or pellet of radioactive material emittinghigh-energy gamma rays, such as radium, in theamount of about to 500 milli-E grams, and its equilibrium products, or radioactive cobaltv or the like emitting approximately equivalent gamma radiations. Within the upper portion 25 of instrument housing 20 are located the amplifier, power supply, and.

other equipment necessary to amplify and transmit signals from detector 24*to. a recorder at the ground surface; I "Instrument 20.is held against one side of the well ll by an elongated bow spring 27, the lower end of which is pass through the formations outside of well bore 21 and return to detector 24. It is found, however, that a major portion of the response of detector 24 is to radiation which travels within the well bore 21. This is accentuated particularly if the bore-hole walls are rough, or if poor contact is maintained between the source 23 and/or the detector 24 on the one. hand and the wall 21 on the other. In accordance with my invention, the radiation traversing the bore. hole is substantially reduced or eliminated by interposing, at an intermediate position between source 23 and detector 24, a plurality of flexible shielding elements 33 capable of substantially attenuating the well bore radiations. These shielding elements 33 are not only flexible so as to enter the minor wall depressions constituting the Wall roughness, but preferably are also of variable diameter so that, as the well-bore diameter changes, contact between the shielding material 33 and the well wall is maintained. As long as the shielding of elements 33 remains eflective, the adverse effects of the source 23 or detector 24 moving away from wall 21 are very markedly reduced.

The exact positioning of the group of elements 33 between the source 23 and detector 24-that is, exactly midway between them, or substantially closer to one than to the other--is not at all critical, being of much less importance than the attribute of flexibility so as to extend nearly completely across the bore hole, or at least to maintain contact with the well wall on a line between the points where the wall is contacted by the source 23 and the detector 24. The term, intermediate position used herein accordingly means anywhere within a major portion of the space interval between the source and detector.

. These elements 33 may be constructed as shown more clearly in Figure 3, as wire brush-like elements extending generally outwardly from a hub 35, but preferably of tangential or spinal form rather than radial,.in order to increase the flexibility and ease of adaption to different well diameters. A steel rod 34 extending between'the housing for detector 24 and the housing for source 23 supports the hubs 35 of the shielding wires 33. In order to avoid a tendency to cause rotation of instrument 20, the spiral of each element '33 may be oriented in the opposite sense from the spiral of each adjacent element or elements, or the elements 33 may be free to rotate on rod 34.

"In conjunction with the attenuation offered by the normal drilling fluids in the well bore, these wire shielding elements 33 are effective to reduce the well-bore radiation to a low level. Still further improvement is noted,jhow-- ever, if the side of detector 24 which is not in contact with the well wall .21 is also shielded. The manner of accomplishing this is shown in Figure 2 in which the detector 24 comprising, for example, an ionization chamber with electrodes 36 and 37, between which is a highly compressed gaseous atmosphere, is eceentrically located within the cylindrical housing 20. Lead shielding 38 fills that portion of housing 20 nearest the center of the well bore, extending around one-half to three-quarters or more of the circumference of detector 24, as well as for some distance above and below the ends of detector 24.

Similarly, as shown in Figure 4, the source 23 is preferably mounted in or on a housing consisting of "a steel shell filled with lead 41, the source 23 preferably being mounted on a leaf spring 42, the ends of which are at-' tached to shell 40, with the source capsule Zitacingthe 4 well formations. The flexibility of spring 42 maintains source 23 in contact with the well formations and allows it to enter small depressions which the shell is too large to enter. The shielding 41 effectively prevents gamma rays leaving source 23 in a backward direction-that is, away from the formations or toward the center of the well bore-from penetrating any substantial distance.

In Figure 5 is shown surface equipment suitable for recording the indications of detector 24. At ground surface 45 the suspending cable 30 passes over a measuring wheel '46 and is Wound on a reel 47. The insulated conductor of cable 30 is brought out to a slip ring 48, contacted by a stationary brush 49, and is thereby connected to an amplifier 50. The signals from detector 24, as amplified by amplifier 50, are applied to a recorder 51 which produces a trace 52 by means of a pen 53 on a moving chart 54 drawn by a take-up roller 55 from a supply roller 56, the take-up roller 55 being coupled by a flexible connection 57 to the depth-measuring wheel 46. As the construction and operation of this equipment may be entirely conventional, no further description of this portion of the apparatus or its operation is believed necessary, other than simply to point out that the deflections of trace 52 are proportional to the variations in response of detector 24, which are inversely related to the formation density.

A modification of the wire-brush shielding elements 33 of Figures 1 and 3 is shown in Figures 6 and 7. Here the mounting hubs 35, shown in cross-section in Figure 6, and to which the wires 33 are attached, are of substantial thickness and of increased diameter, occupying a substantial fraction of the cross-sectional area of the normal well bore. Adjacent hubs 35 are separated by spacers 58 and are provided with openings 59a or 5%, the two groups of openings being offset from each other on adjacent hubs as indicated by the solid and dotted circles respectively in Figure 7. These openings allow well fluids to pass freely through the shield structure as indicated by the curved arrows of. Figure 6 which designate the flow paths during lowering through a well bore.

A detail of a preferred shielding wire construction is shown in Figure 8. In order to increase the shielding efiectiveness at the bore-hole Wall, particularly for large well diameters when the shielding material is at maximum extension, each individual wire 33 is coated with a flexible absorbing material 60 such as a flexible plastic material impregnated or filled with a heavy gamma-ray absorbing medium such as lead or iron oxide powder. By a spraying or multiple-dipping technique, the thickness of coating 60 may be graduated along the length of wire 33 to make it greatest at the outer end for maximum shielding and least near the point of attachment to hub 35 where maximum flexibility is desired.

An alternative embodiment of a flexible shield structure for use in an intermediate position between the radiation source and detector is shown in Figures 9, l0, and 11. Surrounding the support rod 34 are resilient disks 61, separated by absorbing spacers 62 of lead or steel, the disks 61 having radially projecting flexible fingers 43 of gamma-ray absorbing material such as rubber weighted or compounded with a substantial portion of lead. Preferably the solid disk portion 61 of each shielding unit is formed of tough, flexible rubber or the like to which the less flexible fingers, 43 are permanently attached by vulcanizing. As flexing of the fingers 43 occurs chiefly at the base 44 near their point of attachment to the disk 61, less force is required to produce bending in accommodating well-bore irregularities than if the flexibility were distributed along the entire finger length.

It will be understood that the fingers 43 of adjacent units are staggered so as to close off any direct radiation path through the spaces between fingers of any one unit. Well-bore. fluids, however, pass easily through these spacesas the logging instrument moves along the bore.

It is preferred that the fingers 43 be inclined upwardly.

at a small angle as indicated by Figure 11. Less flexing of fingers 43 occurs and accordingly less pressure is exerted against a well wall 21 during lowering of the logging instrument 20 through a well, while the opposite occurs during raising of the instrument, insuring more positive contact of fingers 43 with the well wall 21.

In Figure 12 is shown an alternative embodiment of the invention in which the source housing 40 is attached through a pivot or hinge connection 63 to the housing of detector 24. The pivot connection 28 of the bow spring 27 is preferably about mid-way between the pivot 63 and the lower end of the housing 40 of source 23. This arrangement permits the source23 and the detector 24 to maintain contact with the well wall 21 more or less independently of each other, whereas a rigid construction omitting the pivot 63 might at times result in the source 23 or detector 24 being held away from contact with the wall.

Even with this extra degree of freedom of the source and detector, however, 'in passing abrupt variations in hole diameter or other irregularities of the well wall, there are times when the source 23 and, the detector 24 are aligned with each other in such a way that radiations can pass directly through the well fluids along the well wall from the source to the detector without having to penetrate the formations. For the specific purpose of preventing this bypassing, the flexible shielding members 64 are provided which, due to their flexibility, are able at all times to maintain contact with and to penetrate the irregularties of the well wall on the side contacted by the logging instrument.

These shielding members 64 may comprise wire-brush elements similar to those of Figures 1 and 3 except that they extend only partially around the circumference of the well bore. Alternatively, they may comprise small hinged fingers of steel or other high-density absorbing medium or flexible fingers of rubber, as in Figure 9, compounded or weighted with a substantial portion of lead to provide the desired gamma-ray absorption. To the extent that they do not interfere with the action of spring 27, the shielding members 64 may extend around the in: strument through any desired angle greater than that shown on the drawing.

Since these shielding members 64 do not prevent passage of radiation up the well bore except close to what is normally the line of contact between the instrument and the bore-hole wall, the upper housing may be enlarged around the detector 24 as indicated at 65 to provide additional shielding thickness on that side of detector 24 away from the borehole wall. This additional shielding 65, like shielding 38 of Figure 2, is effective to prevent reception of most of the gamma rays traveling by paths other than through the well formations.

In Figure 13 is shown an alternative embodiment of the invention which, like that shown in Figure 1, prevents substantially all transmission of gamma radiation along the bore hole between source 23 and detector 24. In this embodiment the housing 40 of source 23 with-its shielding material 41 and the housing of detector 24 with its shielding 38 are connected by a hollow tubular member 66, placed somewhat eccentrically with respect to the axis of the source and detector housings, so as to leave a space between the tubing 66 and the well wall in all directions.

Completely surrounding and attached to the central portion of tubular member 66 is a hollow toroidal or doughnut-shaped resilient packer or sleeve 67 filled with a heavy gamma-ray absorbing liquid 68 such as mercury. This shape for the rubber or other resilient material of sleeve 67 allows substantial change in its outside diameter without excessive stretching of the material at any point. Further support and protection for the sleeve 67 during running through a well bore is provided by a number of bow springs 69 and 70 substantially surrounding and pivoted to the housing of detector 24 at the pivot ring 71 and slidably engaging the source housing 40 at a retaining ring 72. By providing the springs 70 ontlie side of the instrument 20 contacting the well wall 21 with less strength or less arch than the springs 69 on the opposite side, the latter springs 69 tend to urge the instrument in a direction to place source 23 and detector 24 in contact with the well wall. Preferably also the sleeve 67 is bonded to the

springs

69 and 70 at the center points indicated by the

arrows

73 and 74 so'that these springs help to expand the sleeve outwardly when the hole diameter increases. Support is also thus given to the sleeve 67 so that rubbing against the well walls does not cause vertical'displacement of the sleeve on tubular member 66-.

To facilitate running in liquid-filled bore holes,

ports

75 and 76 in tubular member 66, located respectively above and below the sleeve 67 allow bore-hole fluids to pass through the tube 66 from one side of sleeve 67 to the other as the instrument passes through a well bore and sleeve 67 maintains contact with the well-bore walls. This arrangement is applicable and often advantageous also in the embodiment of Figures 1 and 3, where the ported tube 66 may be substituted for the solid rod 34;

In Figure 14 is shown a still further embodiment of my; invention in which the flexible shielding members intermediate between the source 23 and detector 24 perform the dual function of reducing gamma-ray transmission through the bore-hole fluids and urging the instrument 20 against the well-bore wall. As is shown in this figure, each of these shielding members, typically three in number, comprises a plurality of pivoted, curved plates or petals, 80 which together substantially block off the entire cross-section of the well bore regardless of its diameter, within certain limits. The shielding assembly as a Whole is thus flexible even though each plate is a rigid body. Projecting from the lower end of the instrument 20 is an operating rod 8'1 by which the shielding elements 80, held retracted during running into awell bore, are released at the bottom of the'well and allowed to expand.

The arrangement of these shielding members 80 is better shown in Figure 15. The solid circle 20 represents the outer periphery of the housing of the instrument 20. The dotted circle 21'r'epresents a minimum diameter for the well bore while the solid outline '82 represents the maximum extension of the shielding plates 80, and,thus the maximum diameter of well bore in which the shield will be fully effective. The shielding plates 80 lettered A in this figure are thosepivoted closest to the instrument 20, and during running into a well bore are folded flat against the outside of the instrument. The plates designated by the letter B form the nextlayer of plates, and during running into a well are folded downwardly flat against the first layer of plates A. Similarly, the plates lettered C form a third layer, lying flat against plates B when retracted. As indicated in Figure 15, each of the plates fills a segment of the bore hole without appreciable overlapping of other plates at the maximum extension. A't lesser extensions, of course, there will be some overlapping of the adjacent plates, but this only adds to their shielding eifectiveness.

Typically, the diameter of the instrument 20 may be about three and one-half inches, the thickness of each shielding plate 80 may be about one-quarter inch, the minimum wel-l diameter 21 is about five and one-half inches, and the maximum well diameter 82 is about ten inches. Of course, by utilizing additional layers of such plates 80 a larger maximum diameter than ten inches may be accommodated. Three decks of such plates 80 as indicated in Figure 14 provide a minimum of about threequarters of an inch of steel as shielding, which is effective to reduce the undesired gamma radiation traversing the bore hole by a large factor.

Figure 16 shows the mounting and actuating means for a typical plate 80 of the embodiment of Figures 14 and 15. The plate 80 is pivoted at the point 84 to the instrument housing wall 20 and is provided with an inwardly extending arm. .85 passing through an opening 86in the wall'20. A't'ensionfspring 87 attached betweemend of the arm .85 and afixed point inside housing 20 urges the plate outwardly and .up,--pivoting it around the point.84. For running into a well, a hook 89 near the lowerend of the plate 80 extends through an opening .90 of wall 20 and is engaged by a downwardly open cup 91 attached to :the release rod 81. Movement of the .rod 81 and .(iup -91.upwardly relative to wall 20 by bumping against the well-bore bottom thus releases the hook 89 and permits the plate 80 to swingv outwardly and up under the force of spring 87. As a practical matter, only the C plates require the hooks 89, as they form the outer layer and hold down the A and B plates by overlapping them. In the logging of wells using neutrons, some variation in the detector response is noted due to the varying amount and disposition of the hydrogen present in the well-bore liquids, where the well diameter is varying, or the walls are rough. As this is somewhat analogous to the bypassing effect of the bore-hole liquids in gammaray absorption logging of formation densities, means such as the flexible shields described herein mounted between the neutron source and detector are effective to displace the bore-hole liquids or to shield the detector against capture gamma rays originating there.

Also, in neutron logging the neutron sources most commonly used also emit substantial quantities of gamma rays,- which may pass up the bore hole and be received and recorded along with the gamma rays of neutron capture in the formations, which rays it is desired to observe. The flexible shielding devices disclosed herein are effective in minimizing this source of error and .thus improve the accuracy of the neutron log for determining porosity.

While I have thus described my invention in terms of the foregoing specific embodiments and details, it will be apparent that further modifications and combinations of these elements and details can be made and will be apparent to those skilled in the art. The invention, therefore, should not be considered as limited to the exact details described, but its scope is preferably to be ascertained by reference -to the appended claims.

.I claim:

1. Apparatus for logging wells comprising an elongated housing adapted to be moved through a well bore, a source of penetrating nuclear radiations in said housing, a detector of nuclear radiations in said housing at a fixed spacing from said source, spring means for holding said source and detector and one side of said housing against the wall on one side of a well bore, a substantial thickness of fixed nuclear-radiation shielding at least partially surrounding each of said source and said detector on the side Opposite that held against a well wall, flexible nuclearradiation shielding formed of a plurality of overlapping radiation-absorbing elements at least partially encircling said housing at a position intermediate between said source and said detector, said elements being urged outwardly to extend across a space between said housing .8 and .a well wall, the outer periphery of said flexible shielding being movable to maintain contact with a well wall of changing shape, and means for recording an indication of the output of said detector as a function of depth in a well.

2. Apparatus for logging wells comprising an elongated housing adapted to be moved through a well bore, a concentrated source 'of gamma rays in said housing, a gamma-ray detector in said housing at a fixed spacing from said source, spring means for holding said source and detector and one side of said housing against-the wall on one side of a well bore, a substantial thickness .of fixed gamma-ray shielding at least partially surrounding each of said source and said detector on the side op posite to that held against a well wall, flexible gamma-ray shielding formed of a plurality of overlapping, gammaray-absorbing elements at least partially encircling said housing at a position intermediate between said source and said detector, said elements being urged outwardly to extend across a space between said housing and a well wall, the outer periphery of said flexible shielding being movable to maintain contact with a Well wall of changing shape, and means for recording an indication of the output of said detector as a function of depth in a well.

3., Apparatus according to claim 2 in which said gamma-ray-absorbing elements completely surround said housing at a position intermediate between said source and detector and extend substantially entirely across the well bore at said position.

4. Apparatus according to claim 2 in which said gamma-ray-absorbing elements are wires extending outwardly from a mounting hub.

5. Apparatus according to claim 2 in which said flexible gamma-ray shielding comprises a flexible dense-fluidcontaining sleeve encircling said housing between said source and detector, said sleeve being urged outwardly to maintain contact with a well wall of changing shape.

6. Apparatus according to claim 2 in which said gamma-ray-absorbing elements comprise a plurality of pivoted plates of gamma-ray-absorbing material, each of said plates substantially blocking off a segment of a wellbore cross section and each of plates pivoting to adapt to different well diameters.

7. Apparatus according to claim 2 in which said gamma-rayaabsorbing elements comprise a plurality of resilient, radially projecting fingers containing a gamma ray-absorbing material.

'8. Apparatus according to claim 2 in which said gamma-ray-absorbing elements comprise a plurality of disks of gamma-ray-absorbing material, each of said disks having flexible, gamma-ray-aosorbing means projecting outwardly from its rim and adapted to maintain contact with wall of a bore hole.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE oERTIFICATE 0F CORRECTION Non 8 Daniel Silverman It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 19, for "substantialy" read substantial line 20, for "formation" read M formations line 69, strike out "Well column 2 line 3, strike out "contact between the irradiation source and the formations" and insert instead tions in the detector due to hole diame'ter change-so line 5 for "formation" read formations w Signed and sealed this 16th day of September 1958 (SEAL) Attest:

KARL El, AXLINE Attesting Officer ROBERT C. WATSON Commissioner of Patents

Claims

1. APPARATUS FOR LOGGING WELLS COMPRISING AN ELONGATED HOUSING ADAPTED TO BE MOVED THROUGH A WELL BORE, A SOURCE OF PENETRATING NUCLEAR RADIATIONS IN SAID HOUSING, A DETECTOR OF NUCLEAR RADIATIONS IN SAID HOUSING AT A FIXED SPACING FROM SAID SOURCE, SPRING MEANS FOR HOLDING SAID SOURCE AND DETECTOR AND ONE SIDE OF SAID HOUSING AGAINST THE WALL ON ONE SIDE OF A WELL BORE, A SUBSTANTIAL THICKNESS OF FIXED NUCLEAR-RADIATION SHIELDING AT LEAST PARTIALLY SURROUNDING EACH OF SAID SOURCE AND SAID DETECTOR ON THE SIDE OPPOSITE THAT HELD AGAINST A WELL WALL, FLEXIBLE NUCLEARRADIATION SHIELDING FORMED OF A PLURALITY OF OVERLAPPING RADIATION-ABSORBING ELEMENTS AT LEAST PARTIALLY ENCIRCLING SAID HOUSING AT A POSITION INTERMEDIATE BETWEEN SAID SOURCE AND SAID DETECTOR, SAID ELEMENTS BEING URGED OUTWARDLY TO EXTEND ACROSS A SPACE BETWEEN SAID HOUSING AND A WELL WALL, THE OUTER PERIPHERY OF SAID FLEXIBLE SHIELDING BEING MOVABLE TO MAINTAIN CONTACT WITH A WELL WALL OF CHANGING SHAPE, AND MEANS FOR RECORDING AN INDICATION OF THE OUTPUT OF SAID DETECTOR AS A FUNCTION OF DEPTH IN A WELL.