US2590874A - Multiple element radioactive ray recording - Google Patents

Description

April 1, 1952 s. KRASNOW ET AL MULTIPLE ELEMENT RADIOACTIVE RAY RECORDING Filed July 24, 1942 8 Sheets-Sheet 1 Apnl 1, 1952 s. KRASNOW ET'AL 2,590,874

MULTIPLE ELEMENT RADIOACTIVE RAY RECORDING Filed July 24. 1942 s Sheets-Sheet 2 W 60 I s 0 6 r 58 W A i o #0 m 3 M 7 2 M M 2 2 2 l 2 2 April 1,1952 5. KRASNOW ETAL MULTIPLE ELEMENT RADIOACTIVE RAY RECORDING 8 Sheets-Sheet 3 Filed July 24, 1942 April 1952 s. KRASNOW ET AL MULTIPLE ELEMENT RADIOACTIVE RAY RECORDING 8 Sheets-Sheet 4 Filed July 24, 1942 r/ z z z 1 z z z z April 1952 I s. KRASNOW ETAL 2,590,874

MULTIPLE ELEMENT RADIOACTIVE RAY RECORDING Filed July 24, 1942 8 Sheets-Sheet 5 /22 i l- /2/ -l27 4*105' 4402 Z fi-JO! A TTOR/V April 1, 1952 s. KRASNOW ET AL 2,590,874

MULTIPLE ELEMENT RADIOACTIVE RAY RECORDING Filed July 24, 1942 8 Sheets-Sheet 6 L I42 5+ L Am g L36 S. KRASNOW ETAL MULTIPLE ELEMENT RADIOACTIVE RAY RECORDING April 1, 1952 8 Sheets-Sheet 7 Filed July 24, 1942 y R0 5 W5 w 55 wA w a Ap M N w A u... N 2 w 1V6 MA M w 2 u A M 87 "r y 1 F: F 4 7V 8 rm m April 1, 1952 s. KRASNOW ET AL 2,590,874

MULTIPLE ELEMENT RADIOACTIVE RAY RECORDING Filed July 24, 1942 8 aneets-Sheet 8 Shelia Krasnow 3 Leon F curilss Patented Apr. 1, 1952 UNITED STATES PATENT OFFICE MULTIPLE ELEMENT RADIOACTIVE RAY RECORDING Application July 24, 1942, Serial No. 452,228

Claims.

This invention relates to an improved method and apparatus for measuring radioactivity, and has particular reference to a method and apparatus for measuring radioactivity in inaccessible locations, such as in boreholes or at considerable depths in bodies of water.

One object of the invention is to provide a method and apparatus useful for locating deposits of minerals having radioactive properties. Another object of the invention is to provide an apparatus by which one may measure radioactive properties continuously from the top to the bottom of a borehole, and have both an immediate indication, and a permanent record, of the radioactivity at various depths.

In locating deposits of radioactive minerals it is often the custom to drill a number of boreholes in localities where such deposits might exist. It is further the practice to bring samples or cores of the drilled material to the surface of the earth, and there examine them for radioactivity by well known methods and apparatus. This method has several drawbacks. First, a deposit of ore may exist close to the borehole, but not be traversed by it, by which the deposit will be missed. Second, it is possible to make an error in ascertaining the exact depth from which a core or sample has been taken. Finally, it is rarely possible to bring all of the core to the surface, a certain percentage always being lost in the drilling or handling.

It is further known that deposits of petroleum are often markedly radioactive as compared with the surrounding rock material. This is believed to be due to the superior absorptive property of petroleum for radium emanation. Natural gas and ground water are also known to be somewhat more radioactive than their surrounding rock material. In drilling for either petroleum or natural gas, or ground water, it is desirable to know the exact level at which the strata having these are traversed by the drilled 'hole. This is often difiicult to determine, particularly when drilling has been done by the rotary method, in which the use of mud under pressure tends to wall off the strata. .Often too, the drilled hole will be lined with a metallic casing. which casing by accident or intention may seal on strata having the desired fluid.

It is the intention in the present invention to provide an apparatusso sensitive, and a method appropriate to its use, that the relatively taint radioactivity of oil and ground water may be detected in place in a borehole. An apparatus sensitive enough to serve this function will by its nature differentiate between the different though faint radioactivities of the rock material. Rock materials, dependent upon their origin and dependent upon the minerals contained in them, have different radioaetivities. Thus, it has been found that granite, shales having organic materials embodied therein, sedimentary rocks containing zircon, and rock materials having mica associated with them, are all slightly more radioactive than for example limestone or chalk deposits. Sandstones will differ in their natural radioactivity, depending upon the minerals contaminating them. Organic deposits, such as coal, oil and natural gas, as mentioned above, petrified vegetable matter, etc., will show higher radioactivities than for instance limestone and chalk. Thus, with an apparatus as sensitive as that described herein it will be possible to differentiate between different layers of rock by the differences in their radioactivities. Each layer in an area will have a characteristic radioactivity, just as it has a characteristic chemical compostion, and for the same reason. Thus, the radioactivity of a layer will serve as a variety of marker, serving to identify the layer wherever it might be in an area.

It thus becomes possible to identify rock layers in different bore-holes drilled in an area and thus correlate the strata.

Further objects of the invention described are to obviate the difiiculties mentioned and secure the advantages mentioned above.

Reference is had to the accompanying drawings in which:

Figure 1 shows a more convenient form of apparatus for measuring radioactivity at various depths in a borehole.

Figure 2 shows a cross sectional view of the element l shown in Figure 1, taken across the plane 22.

Figure 3 shows the sensitive element employed in the apparatus shown in Figure 2.

Figure 4 shows a cross section of the element in Figure 3 taken across the plane 33.

Figure 5 shows the circuit diagram of the apparatus shown in Figure 1.

Figure 6 shows a vertical cross sectional view of still a further type of apparatus which may be used for the purpose described.

Figure 7 shows essentially the same apparatus as is shown in Figure 6 with the addition of another electroscope to serve as a control.

Figure 8 shows a modification of the apparatus designed to permit perforation of the casing in a well.

Figure 9 shows another type of apparatus for measuring radioactivity at various depths and giving an immediate indication at the surface of the ground of the value of the radioactive intensity. I

Figure 10 shows a detail of the lower portion of the apparatus shown in Figures 14 and 15.

Figure 11 shows the electrical circuit suitable for use in the modification shown in Figure 9.

Figure 12 shows an apparatus for measuring radioactive gradient.

Figure 13 shows a modification of the apparatus indicated in Figure 12 to render measure ments of radioactivity more dependable.

Figure 14 shows an apparatus for measuring radioactive intensity with the interposition of a filter.

Figure 15 shows an apparatus with plural measuring elements, one of the elements being provided with a filter.

Figure 16 shows an arrangement of plural elements, in which both a Geiger-Muller counter and an ionization chamber are employed.

Figure 17 shows an apparatus to be lowered into a borehole, consisting of two Geiger-Muller counters of different size.

Figure 18 shows an apparatus adapted to be lowered into a borehole, having two Geiger- Miiller counters, each of different characteristics.

Figure 19 shows an apparatus employing plural elements, with recording provided at the surface of the earth for the composite response of the two elements.

Figure 20 shows a plural radioactive measuring element employed in conjunction with a perforator.

A more convenient form of the apparatus shown in Figure 5 employs a cartridge ll suspended in the borehole by a conducting cable 12. The cable i2 passes over a measuring wheel 13 and thence onto a reel 14 operated by a crank l5. A pair of slip-rings Ma and lib fastened to the shaft of the reel M .have bearing upon them the brushes I6 and IT. These brushes are connected through the medium of wires 18, I8, to a recording element is. Referring now to Figures 2 and 3, the cartridge ll consists essentially of a radioactive-sensitive member 23 mounted at the bottom of a pressure-tight cartridge 20. A rack 21 holds the element 25 and serves further to hold batteries 24, vacuum tube 25, and relay 21. Springs 22 serve to prevent violent contact of the frame 2! with cartridge 20. A cap 3! is fastened by means of a threaded or other connection onto one end of cartridge 20. A fluid tight seal is had by the use of gasket 33. The wires necessary to convey the sig nals from the cartridge I! pass through insulating bushing 3 and are looped onto ring 32 and thence pass to the surface. In this way the wire serves also for raising and lowering member H.

The sensitive element 23 consists essentially of a sealed glass vessel 35 which has within it a' E. be used for long periods of time without further attention. In operation the members 36 and 38 are kept at a high potential relative to each other by means of batteries 56 and it operating through high resistance leak 'i'l, as shown in Figure 5. A suitable value for the voltage of battery 46 is volts; of battery it, 380 volts. The positive end of battery M is connected to one side of the filament 15 of vacuum tube 25. The member 36 is connected to a blocking condenser 52 and thence to the grid $3 of the same vacuum tube. The plate 58 of this, tube is connected through relay 2'6 to the positive end of battery Q6. The relay 2?, when tie-energized, serves to close contacts 28 and 2%, thus allowing a current to flow through wires l2, slip-rings Ma and Nb, brushes l5 and H, electro-magnet 5t and battery it. The electro-magnet 5i? serves to attract armature 5! which further serves to move pen 52 across the tape 53 kept in constant uniform motion by means of drum 5% operated by driving means 55.

The operation of the apparatus is as follows: The members 36 and 38 are charged'at a controlled rate to a high potential relative to each other by means of the batteries 45 and M operating through leak ll. In the presence of radioactive material the gas in the container 35 will be partially ionized and will thus change the potential of the member 36.. This will result in a change of potential of the grid 53 which will reduce the current normally flowing between filament 1 5 and plate 48 of vacuum tube 25. This will in turn reduce the current in relay 21 sufficiently to allow its armature 28 to be retracted, closing the circuit between member 28 and contact 29. The closing of this circuit will cause a current to flow through slip-ring Mb, brush 1 ll, battery '59, electro-magnet 5%, brush I55, and slipring Mia. The energizing of electro-magnet 50 will cause armature 55 to move pen 52, causing a break in the line traced on tape 53.

Upon the operation of the circuit in this fashion, the potential of member as will be restored to its original value, increasing the filament-toplate current in tube 25, energizing relay 2'! and thereby causing the circuit made by members 28 and 29 to open.

Upon the further ionization of the gas in container 35 the operation above described will be repeated. Thus the frequency of the pulses finally received by pen 52 will be a measure of the radioactivity of the material in the vicinity of member 23. It will be noted that the rays given off by radioactive substances have considerable penetrating power and can therefore easily penetrate the shield at even if the latter be made of metal. To reduce the absorption of these rays by the metal, however, that portion of the cartridge 20 which houses the member 23 is provided with thinner walls than the remainder; a construction made possible by the smaller diameter of the said portion. it will be noted further that even if a metallic casing such as 56 exists in the borehole the presence of a radioactive layer such as B may be noted because of the easy penetration of the rays through the thickness of metal ordinarily employed for casing.

Another type of apparatus which may be employed is shown in vertical cross section in Figure 6. In this, cartridge 656 is provided with a threaded cap 1'2 sealed by gasket i i and raised and lowered by ring 73, Within thecartridge 86 is a frame file which holds an ionization from radiation, to serve as a control.

chamber 68 connected to an electroscope 13, which may be of any suitable type, but which is shown schematically as a gold leaf eleetroscope employing leaves 1 I. Springs 69 serve to cushion the shocks imparted to the frame 31a. A circumferential thin portion 61 of the cartridge is provided to reduce the absorption of the rays in passing to the chamber 58. In use the electroscopeis charged to a high potential relative to the outer walls of the ionization chamber 63. It is then mounted on frame 61a, inserted in cartridge 66, the latter sealed with cap 72, and the whole lowered into the well. The apparatus is allowed to remain at a depth where presence of radioactive material is suspected, for a suitable length of time. It is then raised and the alteration in charge on the electroscope noted. A second electroscope similar to electroscope I9 may be mounted in the cartridge 66 and shielded Thus, the cartridge 66 may be made longer and an electroscope and ionization chamber, entirely shielded with lead, mounted above the chamber 38. This arrangement is shown in Figure 7, the control electroscope and ionization chamber being designated respectively as 39 and 33.

In the type of apparatus shown in Figure 2 the chronograph and entire recording system may be clock operated and mounted in the cartridge so that no conducting wires need pass to the surface. As a further alternative the motion of the tape may be made not a function of time, but rather of the position of a measuring wheel suchsas I3.

Another apparatus which may be used for the same purpose is shown in Figure 9. This consists of a cartridge IBI, which is provided with a gas-tight partition I05 and a gas-filled space I 52. Located preferably centrally within the space I02 is an electrode I03, carefully insulated by means of amber or other low leakage insulating material IM. In the partition I85 is mounted a valve I2I by which gas may be introduced to attain any desired pressure within the enclosure I62, after which the valve I2I may be shut and the said pressure maintained. The wall IGI is made of strong material, as thin as possible to reduce the absorption of rays of radioactive material passing into the space I92. A material which will combine strength and transparency to rays from radioactive substances is utilized. Suitable materials are: magnesium alloys, aluminum alloys such as duralumin, beryllium, or beryllium alloys. A very thin steel housing may be used, the greater strength allowing the material to be so thin that absorption is not serious. Ihe space H22 may be filled with any one of a number of gases. A suitable gas for this purpose is nitrogen, although other gases may be used with almost equally good results.

It is of advantage to rib or corrugate the surface of the insulation as shown, to increase the leakage path. Although element IEI is shown as a valve, in practice it may be advantageous to use a standard type of sealed-off glass joint, as employed in the glass blowing art.

The pressure in the ionization chamber is preferably higher than atmospheric so as to give a greater ionization current, as will be familiar to those versed in the art. A pressure of several hundred pounds per square inch will be found suitable. The voltage across the chamber is made as high as possible so as to obtain an increased ionization for a given change in intensity of ionizing rays. The voltage is limited, however,

by the fact that if it is made too high, ionization by collision will result and the chamber will support a steady discharge regardless of the intensity of ionizing rays in its vicinity. The value of the resistance is such as to cause an easily measurable voltage drop across its terminals for the usual intensity of ionizing rays. Its value will be chosen with regard to this and with regard to the requirements of the voltage indicating device. Good results may be obtained with a resistance having a value comparable and preferably approximately equal to the effective resistance of the ionization chamber. Suitable values are: a battery voltage of 130, and a resistance value of 10 megohms.

In the modification shown in Figure 9, the in-' formation is transmitted to the surface through wires I23, allowing immediate observation at the surface, as well as recording. The central electrode I03 of the ionization chamber is connected to element I22 which represents schematic-ally the electrical apparatus more fully shown hereinafter. A lead I2! is connected to metallic casing IBI serving to ground certain of the elements employed in the apparatus I22. Leads I23 extend to the surface of the ground, where they may pass over a wheel such as I3, onto a reel such as I4 provided with slip rings such as Ma, and Nb. Connecting wires such as I6 and I8 serve to connect to frequency measuring apparatus, substituted for element I9 shown in Figure 1.

Referring now to Figure 11, IOI and IE3 represent the elements of the ionization chamber. The side IBI is grounded, while the electrode I03 is connected to one terminal of a high resistance I 23. The other terminal of resistance I28 is connected to the positive end of a high voltage battery I23, the negative end of this battery being grounded. A potentiometer I30 is connected across the terminals of battery I29, with its movable contact 153 connected to the cathode of a triode vacuum tube I3I. A lead joins electrode H33 and the grid of a pentode vacuum tube I32. While a pentode is shown in the specific embodiment disclosed, any multi-element tube having three or more elements, and having the proper characteristics, may be used.

A conventional battery I33 is shown to provide the heater current for the tubes. A B" battery i3 2 is shown connected to battery I33 and leading to choke I35, the other terminal of the choke being connected to the plate of tube I32. A tap is taken off battery I34 to provide the screen grid voltage for tube I32. The plate of tube I32 is connected through condenser I36 to inductance I31. Inductance I37 terminates at terminal point I38 to which is connected one end of resistor I39. The other end of resistor I39 is connected to contact I53 as shown.

Across inductance I31 and resistance I39 is placed a condenser I40. The common terminal point I38 is connected to a grid of vacuum tube I32. While connection to a specific grid has been indicated, it is also possible to connect a lead from terminal I38 to the cathode of tube I32 or to any other element except the plate of the same tube, with satisfactory results. It will be understood that proper biasing means will be utilized for the specific type of tube chosen,

The cathode of tube I32 is connect-ed through a conventional self-biasing arrangement I10 to the contact I53. The grid of tube I3I is connected through a conventional self-biasing arrangement I ll to inductance I3'I. The plate of tube I3l is connected to the primary of a coupling transformer I42, the other end of the primary being connected to the B battery as shown; The secondary of coupling transformer I42 may be connected to leads which are brought directly to the surface of the borehole and which are connected to a suitable frequency or other measuring device such as I9. A suitable frequency can be chosen, high enough to be easily measurable, and low enough to avoiddifiiculties due to capacity and inductance effects along the transmitting cable.

The coupling transformer may be connected to an amplifier such as i 1, the amplifier feeding into the external cable as shown. The amplifier will be found particularly valuable in preventing external load variations from reacting on the principal circuit and thus causing a disturbing change of frequency.

While Figure 11 discloses an apparatus for generating a frequency in proportion to the intensity of radioactivity, it will be understood that an apparatus utilizing phase shift or amplitude variation as a function of radioactive intensity, may be used instead of one employing frequency variation.

An amplifying stage I62 may be inserted in the lead from element I63 of the ionization chamber to the grid of tube I32. This will be a direct current amplifier, and will serve to increase the voltage change on the grid for a given change in potential on element I63. Where the change in potential on element I03 is sufficient to cause a proper voltage change of the grid, the amplifier I62 may be omitted, and a direct connection made between the lead I03 and the grid element.

The operation of the circuit may be described as follows: The elements I31, I39, and I40, together with tube I3I, biasing arrangement MI and the proper A and B voltage supplies, constitute an oscillatory circuit whose natural frequency is dependent on the values of inductance I31, condenser I lll, and resistance I39.

There will be an alternating voltage across the terminals of resistance I39, which voltage will be in phase with the current flowing through the resistor. This voltage will be impressed between the cathode and one grid of tube I32, and will cause in general an alternating voltage of the same frequency between the grid and plate of tube I32. The voltage across resistor I39 is 90 out of phase with that across I31. Consequently, the voltage impressed by vacuum tube I32 across inductance I31 will also be 90 out of phase with the voltage in inductance I3 1. The magnitude of this voltage will be dependent upon the amplification due to tube I32. Any out-of-phase voltage across inductance I31 will have the effect of changing the apparent value of the inductance and will thereby cause a change in the frequency generated by the oscillatory circuit.

i Any increase in radioactive intensity will alter the effective resistance between the electrodes IilI and'lllfi. Through the agency of battery I29, an, increased current will flow through the circuit-composed of battery I29, resistance I28, and electrodes IDI and. H33. This increased current will cause a greater voltage drop between terminals of resistance I28, which increased voltage drop, after amplification by amplifier I52, will be impressed across the cathode and a grid of tube I32. If the screen grid voltage has been properly adjusted, any change in the potential of the grid connected to amplifier I62, will cause a change in the effective amplification factor of tube I32. This change, as described previously will cause a change in the out-of-phase voltage impressed acros inductance I31 and will thereby cause a change in the natural frequency of the oscillatory circuit described herein. The alternating current flowing through inductance I31 will induce voltages of equal frequency in transformer I 22 and consequently in amplifier I54.

It is therefore seen that an alteration of radioactive intensity will cause a related and functionally connected change in frequency in the output of amplifier It2.

The voltage of battery I29 should be so chosen as to obtain the maximum ionizing effect without actual breakdown, and the value of resistance I23 should be of such value as to cause a significant voltage change across the cathode and grid of tube I32. The values of the constants in the remainder of the circuit should be chosen so that with the voltage changes normally obtained across the cathode and grid of tube 132, a sufii cient change in frequency will be obtained in the output.

While a variety of vacuum tubes may be used for elements I3! and E32, a suitable set will be had by using an RCA type #957 tube as element I3I and an RCA type #959 tube as element I32. suitable values for the other elements are as follows:

I29--l35 volts I34-135 volts I28-10 megohms fill-4800 ohms and .01 mf. i36.002 mf.

I31-700 microhenries Nil-50,000 ohms and .0005 mi. I39100 ohms As described herein, the frequency may vary over a wide range, depending upon the particular mode of transmission of information to the surface. A suitable frequency is one megacycle.

Figure 12 shows an apparatus which may be utilized to measure what may be termed as the radioactive gradient along the length of a borehole. This apparatus is comprised of elements such as shown in Figure 10 in duplicate, and mounted at a substantial axial distance from each other. Each unit is connected to its associated. measuring circuit I22, the outputs of the two circuits being each connected to separate frequency measuring systems at the surface of the ground. The two frequency measuring systems may be interconnected so as to superpose one frequency on the other and give the difference of the two frequencies as a result. In this way a measure will be obtained of the relative radioactivity of the rock materials at the respective levels of each of the ionization chambers. Thus the gradient or rate of change of radioactivity may be detected.

This modification will permit further distinguishing the actual radioactivities of the strata from the possible individual erratic behavior of each of the measuring elements.

If the latter feature is sought rather than the actual measurement of gradient of radioactivity, the measuring elements may be placed close together and their combined effect noted.

This modification is shown in Figure 13.

Figure 14 show a modification of the apparatus shown in Figure 9, suitable for measurement of radioactive intensity through a filter. It will be understood that in certain areas little contrast will be noted in radioactive intensity throughout the lengthy of the borehole. Advantage may be taken of the fact that different radioactive materials emit rays having different distributions of intensity in the radioactive spectrum. Thus, if a layer in the borehole is contaminated with thorium, the total intensity recorded on the apparatus may be the same as that for a layer contaminated with radium. However, if measurements be taken with a filter, the intensity due to radium will appear greater than that due to thoriand the difference may be noted. The filter may also be found particularly valuable in cases where radioactive material is used as an indicator, as will be hereinafter described. Different materials may be introduced in the borehole, each having different radioactive properties. They may later be identified by measurements taken with a filter.

The apparatus shown in Figure it resembles that shown in Figure 14, except that an outer cylindrical shield or filter I64 has been placed completely surrounding the cartridge IfiI. A latch I35 is held by a spring in an indentation I66 in the filter. This latch may be operated by a solenoid IE'I actuated by wires I 58 which pass to the surface of the borehole. On passing an energizing current through wires I68, solenoid I8! will cause latch I65 to be withdrawn from indentation IE6, allowing filter I64 to drop till it strikes the circumferential stop I69.

With filter I8 3 in the raised position, the apparatus will operate as previously described. The only filtering action will bethat of the car-.

tridge, and which is intrinsic in the material used. If it is desired to make the measurement with a filter, the energizing current can be applied while the apparatus is in the borehole, which will cause the filter to assume an operative position, after which a further measurement can be taken. With the filter I54 against stop I 69 all rays entering the ionization chamber radially will have to pass through the filter. Since most of the rays enter the chamber in this way, the equivalent of a nearly complete filter will be obtained.

The filter may be made or" any metal or substance having the desired absorbing properties. Examples of suitable materials are copper, lead aluminum, etc. It is understood that the filter may be incorporated with the cartridge IEII, and be made permanent, in which case only the filtered rays will impinge on the instrument.

There will be a special advantage in the utilization of the filter about one of the units shown in Figure 12 or 13. Here a differential result will be obtained, giving to the observer the difierence between the filtered and unfiltered rays. Alternatively, filters may be used about both of the elements shown in Figures 12 and 13, a different filter being used about each element. By successive runs, with different pairs of filters, the data collected will be of greater value than that obtained by the mere measurement of unfiltered radiation.

Figure 14.- shows an apparatus similar to that shown in Figure 13 with the addition of a filter H34 about the upper ionization chamber I 03. Duplicate leads I23, 23, conduct the amplified responses from elements I22, I22 to apparatus 169 at the surface of the earth. Element I69 constitutes means for comparing the two frequencies from elements I22, I22. It will be noted from the description of the function of element [22 given herein, particularly as to its operationas shown in Figure 11, that each element I22 will give an output frequency related to the intensity of radioactivity as measured by its respective member I03.

The apparatus shown in Figure 2 particularly,

may be made extremely sensitive to the rays emitted by radioactive substances and so the sometimes faint radioactivity of petroleum, natural gas, and ground water detected. As has been pointed out previously, this may be done in spite of anycovering of mud or of metallic casing intervening between the walls of the borehole and the cartridge II. It is in fact, possible to run the cartridge II inside of the standard drill pipe used in rotary drilling and 15 thus make measurements with a minimum of disturbance to drilling. Because of the limited absorptive power of the metals customarily used for drilling, it will be possible to detect radioactive rays through the thickness of metal in the drill pipe, or even through the several inch thickness of the drilling tools.

While, from what has been disclosed above, it is evident that strata may be differentiated from each other by means of the quantitative difference in the amount of associated radioactive material, it will be appreciated that strata need not necessarily be widely different in their associated radioactivity to enable one to differentiate them from one another. In cases where the associated radioactivities are not conspicuously different in conducting measurements from one end of the bore hole to the other,

valuable information may still be obtained by.

considering the manner in which the radioactivity varies, or phrased differently, the function by which radioactive intensity changes as the depth is altered. This will be found particularly valuable in searching for oil deposits. It will be recalled that petroleum deposits in the natural state have water associated with them. In many cases the water underlies the petroleum, and will have a radioactivity markedly different from that of the petroleum itself. Thus if an apparatus as described above, were lowered past a formation, a sudden change would be observed in passing from rock to petroleum, another sudden change in passing from petroleum to water, and still another sudden change in passing from water to rock. The

layers might thus be easily identifiable despite the fact that their radioactivity may be no greater or less than that of most of the rock lining the borehole.

It is obvious that any other means than those shown or described may be used to convey the frequency of the impulses produced by the apparatus in Figure 2, to the surface.

The elements employed in the member II may be combined with a perforating tool as ordinarily used for perforating casing in oil, gas or water wells. With this it will be possible to lower the apparatus slowly until an indication of radioactivity is received. The apparatus may then be stopped and the perforating procedure carried on as usual. This will have the advantage of eliminating the inaccuracy usually made in measurement. Heretofore, it has been the custom to measure the depth to the level in question, then run the perforator to that depth.

This involved two measurements, the combined error of which was at times suificient to cause perforation to be performed at the wrong level. The method described above can have none of these errors, since it is not dependent in any way on a measurement of depth.

A schematic showing of this appears in Figure 8. Here an apparatus capable of measuring radioactivity is shown schematically asBl. This is connected to a perforating element 99. This latter arrangement may be of any of the well known types. -I-Iowever, the type known as a "gun perforator, which perforates the casing by firing bullets through the casing, will be found particularly suitable. The assemblage made up of radioactive apparatus 81, connector 98 and perforating element 39 may be raised and lowered by means of cable Ifiii. This cable will serve both for raising and lowering and 31 01 operating the apparatus and making observations. The cable passes over wheel l3, onto reel 96. Connection is made between the reel and apparatus 95. Apparatus 95 serves to make observations and to control the elements that are being raised and lowered.

It will be noted that one of the fundamental concepts here involved is that of utilizing plural elements which may be lowered as a unit, and which will furnish simultaneous responses. A number of useful modifications may be constructed utilizing this principle. A number of representative modifications are given below, it being understood that these are not limiting but rather exemplary. Thus, advantage may be taken of the different behavior of different types of radioactive measuring elements. For example, one element may be a Geiger-Muller counter with the proper associated circuit, while the other element may be an ionization chamber with the proper associated circuit. The responses from each of the members may be transmitted separately to the surface of the earth and there re corded in correlation with depth. Although both pieces of apparatus mentioned are subject to extraneous disturbances, the disturbances will in general not occur at the same time nor will they be of the same nature or pattern. Thus, only those responses which are common to both mem bers need be recognized.

This is shown schematically in Figure 16. Here, a cartridge ll, adapted to be lowered within the borehole has within it a Geiger-Muller counter 23, together with its associated circuit IN. This associated circuit is the equivalent of that shown for instance in Figure 5. It will give at its output a response proportional to the radioactivity acting upon the Geiger-Muller counter. Similarly, an ionization chamber l'H, similar to that shown in Figures 9 and 10, is placed within the same cartridge H. This has its associated circuit I22, described in detail previously, together with leads going to the surface of the earth.

Similarly, a Geiger-Muller counter and ionization chamber may be utilized, one apparatus utilizing high voltages fed from the surface of the earth, the other utilizing high voltage supplied by batteries lowered with the sensitive member. Or two Geiger-Muller counters or two ionization chambers may be utilized, one of the two operating from voltages supplied from the surface of the earth. Still further, plural elements may be utilized, one type of circuit being utilized for one element, another type of circuit utilized for the other. Thus, one circuit may be responsive, for instance, to temperature andmay have temperature enter as a factor in the results. The other circuit may be responsive, for instance, to vibration and have the effects thereof enter as a factor into the results. The use of both circuits simultaneously allows one to correct or compensate for that portion of a record obtained under adverse conditions for a given circuit, having at all times a usable record from at least one circuit or the other. It will be obvious that the probability of the same disturbance arising in both circuits to the same extent due to different causes is exceedingly small, so small that it may in practice be neglected.

Thus, the arrangement as shown in Figures l2 and 13 may be utilized, the two elements I22,

l22, having been indicated above.

Still another use which may be made of plural elements would involve the conjoint use of short and of long elements sensitive to radioactivity mounted adjacent each other, and provided with separate amplifying and transmitting apparatus. A short element (either a Geiger- Miiller counter or an ionization chamber) will distinguish thin layers readily, but is not very eiiicient for the rapid logging of relatively weak formations. On the other hand, a long counter is less capable of discriminating and resolving thin layers while acting efiiciently for rapid logging of weak formations. The use of the two elements operating and recording simultaneously allows one to operate rapidly for certain portions of a borehole, and to slow down operations for other portions. In the first case, the responses from the large measuring element would be given chief attention, while in the latter case the small element would enable one to indicate position of a layer with greater accuracy. This modification is also highly useful for the location of radioactive markers, whether natural or artificial.

The large measuring element may be utilized while lowering the apparatus rapidly, to find the approximate position of the radioactive markers. Such an element, being larger, will in general be more sensitive and can detect the marker at a greater distance and with a greater speed of lowering. Once the approximate position has been obtained, the small element can be utilized to locate the position of the marker with certainty. It is understood that when large and small ele ments are referred to, they may be different as to diameter or different as to length or both, or they may have differences in their physical structure which will cause them to act respectively as large or small elements. Or the two elements may be different, one having a greater sensitivity of response than the other, due to treatment of the active surfaces, the circuit involved, etc. This is shown schematically in Figure 17, ll being the enclosing cartridge lowered into the earth as before. A large Geiger-Muller counter is shown together with its associated circuit I10, with leads going to the surface of the earth. Similarly, there is provided a small Geiger-Muller counter and associated circuit I10, with leads going to the surface of the earth.

Similarly, plural elements may be utilized one responsive chiefly to hard gamma rays, the other chiefly to soft gamma rays. Or one element may be made sensitive particularly to neutrons, while the other element is sensitive chiefly to gamma rays. As for example, a Geiger-Muller tube with a cylinder of rhodium, silver, or metal associated or lined with boron compounds will respond readily to neutrons, while a tube with a copper cylinder will respond primarily to gamma rays and will be little affected by neutrons. Where both neutrons and gamma rays are expected, such plural elements allow the simultaneous detection. Since neutrons are often given off by sources which also emit gamma rays, a log of neutron intensity is made far more certain by a concomitant log of gamma ray intensity. This is shown schematically in Figure 18, in which the enclosing cartridge is H, a Geiger-Muller counter of one type, I14, connected to its associated circuit I70, with leads going to the surface of the earth, and another Geiger-Muller counter I'Ha placed within the same enclosure has its associated circuit I with its leads going to the surface of the earth. Thus, as before, separate indications will be transmitted upward separately, and may be received and detected separately.

It will be understood that any one of the measuring systems shown herein may be utilized for measuring with one element, while any of the other systems shown herein may be utilized for the other element, or that both elements may have the same sensitive members and transmitting systems.

Another use of the plural elements will be where the response from one of the plural elements will actuate some mechanism such as a gun perforator. The method of doing this has been fully described in our co-pending application, Serial No. 438,475. Since such an element need be responsive only to strong sources, and might have an auxiliary actuating system which affects the.

accuracy of results obtained thereby, it will be found desirable to utilize this element for actuating purposes only, while utilizing a separate element to transmit responses to the surface. Both elements may, however, be actuated from the same power source, so that one will act as a pilot element indicating that the system is functioning, while the other will serve only for automatic actuation. The basic arrangement is shown in Figure 8, and the details thereof in Figure 20. Here two elements responsive to radioactivity are provided, identified as I18 and I79. Element I18 serves to actuate apparatus I80, which in turn operates the gun perforator. Element I19 is the pilot element, indicating that the system is functioning.

It will be understood that each measuring element may furnish a separate trace on a chart, produced either photographically or with a pen, this chart being if desired a single one whose movement is proportional to the position of a measuring sheave. The responses from both members may, however, be fed into a single recording galvanometer or pen recorder, which will then give the sum of the responses from the members. Thus, one obtains a record which is automatically made proportional to the average integrated response from the members. This is shown schematically in Figure '19, pairs of leads I and H6 leading upward from cartridge ll. Both of these leads I15 and H6 feed into a single recording galvanometer l 11, which serves to record the combined response.

This case continues and extends the teachings of our co-pending applications Serial No. 137,380 and 301,078.

The scope of the invention is defined by the appended claims.

We claim:

1. In a method of investigating radioactivity in a bore hole drilled into the earth, the steps of simultaneously lowering into the bore hole a plurality of radiation responsive devices having different radiation response characteristics, exposing said devices to radioactivity in the bore hole 14 so as to cause at least one of said devices to respond significantly thereto, and obtaining indications of the responses of said devices whereby information about radioactive conditions in the bore hole may be obtained by comparing the indications of the responses of said devices.

2. In a method of investigating radioactivity in a bore hole drilled into the earth, the steps of simultaneously lowering into the bore hole a plurality of radiation responsive devices, shielding at least one of said devices from radioactivity originating externally thereof, exposing said devices to radioactivity in the bore hole so as to cause at least one of said devices to respond significantly thereto, and obtaining indications of the responses of said devices, whereby information about radioactive conditions in the bore hole may be obtained by comparing the indications of the responses of said devices.

3. In a method of investigating radioactivity in a bore hole drilled into the earth, the steps of lowering into the bore hole a radiation responsive device of given radiation response characteristics, exposing said device to radioactivity in the bore hole so as to cause it to respond significantly thereto, obtaining indications of the response of said device, lowering into the bore hole another radiation responsive device of different radiation response characteristics, exposing said other device to radioactivity in the bore hole in the same manner as said first device, and obtaining indications of the response of said other device, whereby information about radioactive conditions in the bore hole may be obtained by comparing the indications of the responses of said devices.

4. In apparatus for investigating radioactivity in a bore hole, the combination of a plurality of radiation responsive devices having different radiation response characteristics mounted for movement through the bore hole in fixed relation to one another, and means for providing indications of the responses of said devices.

5. In apparatus for investigating radioactivity in a bore hole, the combination of a plurality of radiation responsive devices mounted for movement through the bore hole in fixed relation to one another, means for shielding at least one of said devices from radioactivity originating externally thereof, and means for providing indications of the responses of said devices.

SHELLEY KRASNOW. LEON F. CURTISS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,097,760 Failla' Nov. 2, 1937 2,133,776 Bender Oct. 18, 1938 2,197,453 Hassler Apr. 16, 1940 2,275,456 Neufeld Mar. '10, 1942 2,285,840 Scherbatskoy June 9, 1942 2,296,176 Neufeld Sept. 15, 1942 2,303,638 Fearon Dec. 1, 1942 OTHER REFERENCES Geophysics, vol. IV. pp. 106 to 114, March 1939.

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