US3468387A

US3468387A - Thermal coring method and device

Info

Publication number: US3468387A
Application number: US631233A
Authority: US
Inventors: Glendon M Benson
Original assignee: NEW PROCESS IND Inc
Priority date: 1967-04-17
Filing date: 1967-04-17
Publication date: 1969-09-23
Anticipated expiration: 1986-09-23

Description

Sept. 23, 1969 THERMAL CORING METHOD AND DEVICE Filed April 17, 1967 G. M. BENSON 3 Sheets-Sheet 1 2 GENERATOR F SOURCE 4 27 j 1 j g I 2 I I g 20 M 1 4 if:

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vllll'lf l'l r'la INVENTOR. GLENDON M. BENSON Filed April 17, 1967 FlG 2 r$rllurff 1 I wh am ATTORNEYS 76 r J TIMER Sept. 23, 1969 s. M. BENSON 3,468,337

THERMAL CORING METHOD AND DEVICE Filed April 17, 1967 s Sheets-Sheet 3 CURRENT SENSOR a CONTROLLER SEQUENCER '7 J 66 J4 A E 3 E 70 70f 70 70d 70 2b 70b 720 72g 72f 72e 72d 566 m-u, 56c

56d i so 56f F 56 2 7 g INVENTOR. GLENDON M. BENSON F I G 8 BY 7 ATTORNEYS United States Patent 3,468,387 THERMAL CORING METHOD AND DEVICE Glendon M. Benson, Danville, Calif., assignor to New Process Industries, Inc., Minneapolis, Minn., a corporation of Minnesota Filed Apr. 17, 1967, Ser. No. 631,233 Int. Cl. E21b 7/18, 49/02 US. Cl. 175-16 14 Claims ABSTRACT OF THE DISCLOSURE A thermal drilling device for isolating and recovering an elongated core segment from within the earths crust. The device includes a pair of concentric, tubular members that define an annular fluid path therebetween for directing molten lava against the earths crust for thermal erosion thereof. Heat-generating means are included along said fluid path to raise the temperature of the molten lava.

This inventon is directed to apparatus for the thermal penetration of the earths crust in an annular configuration so as to isolate and recover a central core therefrom.

When drilling for mineral deposits, it is advantageous to drill for and recover a central core suitable for visual examination of the earths crust as well as for physical and chemical testing purposes. In addition, core drilling is more economical than other drilling operations such as anger drilling because the amount of energy expended is minimized, i.e., it is only necessary to form the annular groove about the core rather than to drill into the entire core itself. In conventional mechanical coring, this is accomplished through the use of a rotatable, annular bit which cuts a narrow kerf in the earths crust. Thereafter, this undisturbed central core is raised and recovered.

More recent developments in mineral drilling have included fusion piercing techniques wherein a borehole is formed by melting the earths crust with a suitable heat transfer medium. One of the earliest forms of fusion piercing devices employed an oxygen-acetylene blow pipe to apply intense heat to the floor of the borehole. The molten lava produced was either removed or dispersed into the pervious portions of the substrata and then allowed to solidify to form, in essence, a continuous casing around the borehole. Still other thermal drilling devices have been developed; see, for example, U.S. Patent 1,- 898,926, issued February 21, 1933, wherein borehole formation is accomplished by providing an advancing electric are which melts the earth in its path to form a hole. Using this method, the resistance of the earth to the passage of the electric arc generates the requisite heat. When sufficient heat is produced, the earth becomes molten and can be displaced to form the desired borehole. Nuclear sources have also been employed to fuse the earths crust; see, for example, US. Patent 3,115,194, issued December 24, 1963. However, prior devices that penetrate the earths crust primarily by the application of heat do not operate to form and recover the desired central core, but produce a continuous borehole. As a result, when fusion piercing is employed, it is extremely difiicult to determine with accuracy the natural composition of the particular formation through which the earth penetration has been directed.

Broadly stated, the present invention is directed to a device for isolating and recovering an unaltered core sample by the controlled application of heat to mineral deposits within the earths crust. The device of this invention includes a pair of coaxially aligned, concentric tubular members that provide heat transfer surfaces that define an annular path therebetween through which molten lava can be directed as the molten lava moves downwardly and into intimate contact (and thereby heat transfer relationship) with the mineral deposit at the bottom of the borehole. The tubular members are coupled to a source of heat to maintain the aforesaid heat transfer surfaces at the desired elevated temperature to create the molten lava. The molten lava is forcibly expelled at the bottom end of the tubular member and melts the earth. The tubular members are advanced downwardly in surrounding relationship to a core at a rate correlated with the thermal erosion of the earth, i.e., the transforamtion of the earth strata into a molten state. Through movement of heat transfer surfaces of the tubular members relative to one another, the molten lava flowing therebetween follows a spiral path for greater contact with the heat transfer surfaces. As a result, this greater contact between the heat transfer surfaces and the molten lava produces an attendant enhancement of the rate of heat transfer from the heat transfer surfaces to the molten lava.

In a preferred aspect of this invention, the heat transfer surfaces are formed so that the annular path therebetween is of continually diminishing cross section so that the molten lava being emitted from the lower end of the device will be subjected to an increase in velocity along the annular path. In this manner, heat transfer from the molten lava to the solid substrata (and rate of fusion thereof) will be further enhanced by the forcible impingement of the molten lava on the substrata.

In another aspect of the invention, the rate of heat transfer from the molten lava to the solid substrata is further accelerated by providing the lower end of the coring device with abrasive rollers or drag bits that initially break down the solid substrata. The broken particles create greater surface area for the heat transfer from the molten lava to the solid strata. Therefore, more efficient heat transfer is accomplished.

The invention will be more fully understood when reference is made to the following detailed disclosure, especially in view of the attached drawings wherein:

In the drawings:

FIG. 1 is a schematic view illustrating one embodiment of this invention;

FIG. 2 is a fragmentary vertical section of the embodiment of FIG. 1;

FIG. 3 is a fragmentary vertical section of the device as shown in FIG. 2 and disposed for severing an elongated central core portion;

FIG. 3a is an enlarged fragmentary view of that portion of the device shown in FIG. 3 which is located within line 3a;

FIG. 4 is a fragmentary vertical section of an alternate embodiment of this invention;

FIG. 5 is an enlarged fragmentary view of an alternate embodiment of this invention;

FIG. 6 is an enlarged fragmentary view illustrating one embodiment for producing heat transfer;

FIG. 7 is an end view of an alternate embodiment of this invention; and

FIG. 8 is a schematic view illustrating the wiring diagram required for operation of the embodiment shown in FIG. 7.

Referring now to the drawings, wherein similar characters of reference represent corresponding parts in each of the several views, there is shown a core-forming head A attached to elongated conduit B in some conventional manner such as by screw threads 9. As shown most clearly in FIG. 2, head A consists of concentric outer and inner

tubular members

10 and 12, respectively, formed from a material having a high melting temperature and a relatively large coeflicient of thermal conductivity, such as tungsten, molybdenum, hafnium carbide and the like.

Inner member 12 is mounted on

circular bearings

14 and 14 for rotational movement within outer member 10. It will be apparent to one skilled in this art that, alternately, outer member may be rotatable or that both

members

10 and 12 may be rotatable in the same direction at different speeds or in opposite directions. Variable rotational movement of member 12 with respect to member 10 is produced by an electric motor (not shown) or some other conventional driving means. For example, inner tubular member 12 can be rotated by the upper portion of inner member 12 acting as the rotor of a squirrel cage induction motor and the upper portion of outer member 10 acting as the stator thereof.

For purposes of description,

members

10 and 12 will hereinafter be characterized as including lower region A, upper region A" and central region A. Lower region A is designed for heating and accelerating the downward movement of molten lava. Upper region A" is designed for providing the circulation of drilling mud from the exterior of conduit B through

members

10 and 12 to the interior of conduit E. Central region A' provides a seal for separating lower region A from upper region A".

Referring more particularly to region A, inner member 12 is modified by machining or casting to define, in combination with inner surface 15 of member 10, cylindrical flow path 16. Inner member 12 further defines a descending helical convoluted surface 18 spaced from inner surface 15 of outer member 10. As shown in more detail with reference to FIG. 6,

cylindrical members

10 and 12 are equipped with heat-generating means 42 and 42 along region A. Outer member 10 includes a series of perforations 20 spaced around the perimeter thereof to allow for introduction of molten lava (not shown) into flow path 16.

Upper region A includes concentric channel 22 commencing in the upper end of outer member 10. The upper perimeter of member 10 is further provided with flared flange 23 for guiding drilling mud into channel 22 as hereinafter described. The outer surface of inner cylindrical member 12 is formed to define concentric channel 22' in fluid communication with channel 22. The upper portion of inner cylindrical member 12 is also formed with a series of perforations 26 in fluid communication with channels 22 and 22'. Member 12 includes a series of low-pitched forming threads 27 around its inner circumference and extending downwardly from about perforations 26 in upper region A" towards central region A.

As shown most clearly in FIGS. 3 and 3a, inner cylindrical member 12 includes an annular sleeve 12. Member 12 and sleeve 12 include mating threaded

surfaces

28 and 28, respectively, for raising and lowering inner cylindrical member 12 with respect to outer cylindrical member 10. Key 29 is normally disposed in slot 30 in outer member 10 and above slot 31 in annular sleeve 12. When it is desired to raise inner member 12 relative to outer member 10, key 29 is lowered into slot 31, thereby locking sleeve 12' to outer member 10. Therefore, as member 12 continues to rotate it will be drawn upward a distance corresponding to the length of threads on sleeve 12'. When it is desired to lower member 12, the rotational movement is reversed. The movement of key 2? is controlled by conventional switching means not shown. Of course, it will be apparent to those of skill in this art that other conventional locking devices can be employed in place of key 29. Furthermore, threaded surfaces 28 and 28 can also be replaced with conventional rack and pinion devices and the like.

Region A includes means such as labyrinth seal on the outer surface of member 12 and in close tolerance with member 10 for isolating region A from region A.

In general, the core drilling apparatus of this inven tion functions in the following manner: Head A and conduit B are disposed at a geographic location where penetration of the earths crust is desired. It will be apparent to one skilled in this art that sufiicient molten lava should be present during initiation of the operation to submerge at least lower region A of core-forming head A. This can be accomplished by conventional drilling to a depth suflicient to allow for insertion of region A of head A into the earths crust. Then, sufficient synethetic lava, i.e., molten sodium or the like, is introduced to submerge region A. Alternately, a fluid-impervious dam for enclosing region A of head A can be constructed at the earths surface. Again, sufficient synthetic lava is introduced to submerge region A.

Rotation of inner member 12 is initiated and the synthetic lava is circulated through perforations 20 into flow path 16. As the rotation of inner member 12 continues, descending helical convoluted surface 18 draws the synthetic lava along inner surface 15 of outer cylindrical member 10. In this manner, heat transfer is produced across the surface of inner member 12 and outer member 10 to provide the desired temperature elevation of the synthetic lava.

As a result of the rotation of the descending helical convoluted surface 18, the synthetic lava is ejected from path 16 and impinges upon the earth formation immediately below core-forming head A. The lava is deflected by the formation into radial flow in the space separating head A from the formation.

Members

10 and 12 act as a thrust bearing with respect to the earth formation at the end of the borehole. The layer of molten lava being radially deflected around the ends of

members

10 and 12 acts, in a sense, like a lubricant. The downward pressure on head A minimizes the flow area between the ends of

members

10 and 12 and the earth formation. Therefore, rapid radial flow is produced with the attendant high rate of heat transfer. Minimizing the flow area (and thereby creating a high flow velocity that generates a turbulent boundary layer) also minimizes the thermal impedance of the formation. The resulting intimate contact between molten lava and the solid portion of the formation produces heat transfer to the formation. When heat transfer is sufficient, the exposed earth formation is converted to fused earth (hereinafter referred to as molten lava). By maintaining the lower end of the core-forming head A in close proximity to the solid formation at the bottom of the borehole, the thickness of the molten lava layer flowing between the lower circular end of core-forming head A and the bottom of the borehole is minimized. As a result, a high rate of heat transfer is produced between the heat-generating surface and the solid strata, and a significant rate of thermal erosion is obtained.

As the lowering of the core-forming head A into the earths crust is continued, the molten lava is displaced upwardly along the inside of member 12 and the outside of member 10. A portion of the molten lava being displaced along the outside of member 10 is recycled through perforations 20 into path 16 for further heating and acceleration. The remainder of the molten lava either permeates the side walls of the borehole or is displaced upwardly around head A and conduit B. As illustrated schematically in FIG. 1, that portion of the molten lava that permeates the side walls of the borehole produces a hardened ceramic-type casting 34. Annular deflector collar 39, secured to outer member 10 in some conventional manner such as welding, is provided to guide this portion of displaced molten lava out against the side walls of the borehole to produce a smooth hardened casing thereon. That portion of molten lava which is displaced along the inside of member 12 isolates a core 38, having a semi-molten plastic exterior 38a. The interior of core 38 is retained in its original non-fused form.

As earth penetration continues, drilling mud is introduced from drilling mud source 36 through a surface pump (not shown). The drilling mud is forced downward along the outer surface of elongated conduit B where it cools the mechanical components. The drilling mud is then guided by flange 23 on outer member through channel 22, channel 22 and perforations 26 into the interior of inner cylindrical member 12. As the drilling mud is forced into the interior of inner member 12 through perforations 26, it contacts hot exterior 38a causing controlled cooling of exterior 38a. Furthermore, the molten lava which is displaced upwardly within tubular member 12 is also quenched into fine particulate matter as it encounters the cool drilling mud. If core 38 contains sand or other particulate earthen matter, it is encased by exterior 38a and recovered.

When it is desired to remove a section of core 38, the advancement of core-forming head A is terminated. Key 29 is positioned within slot 31 in annular sleeve 12 and rotation on inner member 12 is continued. As a result, member 12 is raised the length of threaded surface 28. As shown most clearly in FIG. 3, the molten lava is then directed radially inwardly producing thermal erosion of core 38 and forming a necked-down portion at an area adjacent the outer end of core-forming head A. The necked-down portion can be completely severed by the thermal erosion of the molten lava or it can be subsequently fractured when contacted by the cool drilling mud as penetration continues.

When the section of core 38 has been severed, it is drawn up through inner tubular member 12 by the continued rotation of forming threads 27. As the lower end of the core section is forced above the upper end of the forming threads, the force exerted by the drilling mud raises the core segment through conduit B to the surface. Alternately, continued downward movement of head A causes relative upward displacement of the section of core 38 and its eventual release from threads 27.

The advancement of core-forming head A into the borehole can be accomplished by any of the procedures well-known to those skilled in the drilling art. For example, core-forming head A may be forced downward by the weight of up-standing conduit B. In addition, weight can be added to the device at the earths surface. Furthermore, the positive application of a driving force by a fluid or electric drive means can also be utilized. When it is desired to form shallow or short horizontal boreholes, compressed air can be used in place of drilling mud.

It will be recognized that the thickness of hardened casing 34 as well as the final diameter of core 38 can generally be controlled by regulating the quantity of molten lava flowing radially inwardly or outwardly around the ends of

members

10 and 12. The quantity of molten lava is, in turn, controlled by the rate of descent of head A as well as by increasing the temperature of the molten lava or its rate of flow. Variations in both the thickness of casing 34 and the diameter of core 38 can also be provided by changing the position of the outer end of

members

10 and 12 with respect to each other. When the outer ends are in alignment, radial flow will normally be approximately evenly divided between inward and outward flow. However, by raising or lowering the outer end of inner member 12, the proportion of lava fiow directed inwardly can be substantially varied.

Referring now to an alternate configuration for coreforming head A as shown in FIG. 4, inner rotatable member 12 is formed with an outer taper so that the cross-sectional area of flow path 16 becomes more restricted near the outer ends of

members

10 and 12. In this manner, molten lava which enters flow path 16 through perforations is subjected to both heat and pressure as it advances along downwardly-extending flow path 16. In this manner, the molten lava is ejected from the end of core-forming head A at substantially increased velocities. It will be apparent to one of skill in this art that other modifications may be employed for increasing the velocity of the molten lava being ejected from the lower end of flow path 16.

The apparatus for pumping molten lava described above is only exemplary of many suitable types of pumping devices that may be employed to pump the molten lava through annular fluid path 16. For example, a low speed eccentric screw or auger pump having either a continuous or interrupted convoluted surface can also be em ployed. In addition, conventional electrostatic and electromagnetic devices may be utilized for conveying the molten lava into annular contact with the earth formation.

The rate of thermal erosion of the formation directly below the lower end of core-forming head A can also be increased by supplying the lower end of head A with abrasive rollers or drag bits 40 (see FIG. 5). As inner member 12 rotates with its outer end in close proximity to the solid portion of the earths crust at the bottom of the borehole, rollers 40 will produce abrasion, thereby breaking off small segments in the mineral formation. In this manner, the surface area of the formation is increased and the rate of heat transfer from the :molten lava will also be increased.

Referring now to FIG. 6, there is shown an extended area heater such as an electrical resistance heater having coils 42 and 42' disposed within the confronting heat transfer surfaces of

members

16 and 12, respectively. Heater coils 42 and 42' are electrically connected to energy source 44 through electrical cables 46, shown schematically in FIG. 1. The particular configuration of the coils 42 and 42' are not critical although maximum heat transfer to the molten lava is of course desired.

Here again, other means for providing heat transfer from a heat transfer surface to the solid earthen formation can be employed. For example, vibrating means for reciprocating the heat transfer surfaces at high frequency will increase the heat transfer rates and correspondingly, the rate of thermal erosion.

Various methods can be employed to transmit electric power to core-forming head A. For example, electrical cables 46 can be located within conduit B, intergal with conduit B or conduit B itself can serve as the conductor. Various guidance systems commonly employed to control the direction of penetration of mechanical drilling devices may also be combined with the operation of core-forming head A.

Referring now to FIG. 7, an inner tubular member 50, substantially identical to member 12 described above in connection with FIG. 4, is rotatably driven with respect to an outer tubular member 52 by previously described mechanisms. Outer tubular member 52 is circumferentially segmented into a plurality of insulative segments 54 alternating with plural conductive segments or electrodes 56. A voltage is imposed between one or more of the electrodes 56 and inner tubular member 50 to cause a current flow through the earthen material 58 within the annular space between the inner and outer members. The voltage produced is of such magnitude that the current flow through material 58 is sufficient to heat the material to a molten state. Thus, the material 58 is heated by the deposition of electrical energy directly within the material rather than through the use of a heat transfer surface.

Reference numeral 60 in FIG. 7 indicates a deposit or limited body of material having a much lower electrical resistance than the remainder of earthen material 58 within the annular space. Without appropriate switching apparatus, the magnitude of current flow from an electrode 56a in contact with deposite 60 would exceed the capacity of electrode 56a as well as that of the power supply since essentially all of the electrical energy would be through this single electrode and low resistance material element. As a result of this action this material body would be vaporized and an arc would be formed between the electrode 56:: and tube 50. This are would have sufiiciently low resistance as to destroy both the electrode and power supply. In order to prevent such condition, the present invention provides a system for sequentially switching voltage pulses to the various electrodes 56 around the annular body of earthen material 58. For producing the necessary voltage pulses to be applied across the annular body of material 58, a transformer 62 having a primary winding 63 and a secondary winding 64 is provided. One side of the secondarry winding is grounded at 66 (thereby connecting it electrically to inner tubular member 50) and the other side of the secondary winding is connected to a bus 68. Bus 68 is connected to individual electrodes 56 through

respective switches

70a, 70b, 700, etc. Switches 74 are here exemplified as siliconcontrolled rectifiers, each of which includes a control terminal 72a, 72b, etc. The control terminals are connected to a timing and sequencing circuit 74. Timing and sequencing circuit 74 is a conventional element that applies a trigger pulse to the various control terminals 72 so that a high voltage and current pulse from transformer 62 is applied to the appropriate electrodes 56 as the respective switches 70 are pulsed on and off. It is preferred that the rate of sequential switching effected by circuit 74 is equal to the frequency of an AC source 76 used to drive the apparatus, the source being connected to primary winding 63 of transformer 62. The timing and sequencing circuit is arranged so that adjacent electrodes 56 are never sequentially pulsed; circuit 76 is arranged so that the Voltage pulse through the material 58 skips about. If an arc is temporarily formed at any electrode, the arc is quenched by the surrounding material before a voltage is applied at an immediately adjacent electrode. Pulsing spaced apart or remote electrodes (rather than pulsing immediately adjacent electrodes) prevents an arc once formed at an electrode from traveling around the annular region as would occur if the electrodes were sequentially pulsed.

Thus, the switches 70 limit the energy deposited per pulse by controlling the duration of the voltage pulse applied to the conductive deposit 60 and thereby prevent the destruction of the power supply 76 or electrode 56 or tube 50. The sequential pulsing of non-adjacent electrodes prevents are tracking. Additionally, a circuit 78 for sensing and controlling the output current is operatively connected with the power supply so as to prevent temporary over-currents as well as to regulate the voltage applied to the electrodes as various earth strata are encountered by the corer. For example, if a lower electrical resistance strata is encountered, the current overshoots for a given voltage. The current sensor then reduces the turns ratio of the transformer 62 and a lower secondary voltage is applied to the electrodes. It will thus be seen that the material 58 within the annular space is heated to a molten state by the in-situ deposition of electrical energy Within the material and when the molten material is ejected through the annular opening as described above, the coring device progresses through the earth formation.

The circuit shown in FIG. 8 and described next above is only exemplary of any suitable known switching circuit. For example, the switching functions can be performed at the low voltage side of the circuit, in which case an individual variable step-up pulse transformer is provided for each electrode segment 56 and is individually regulated by Zener diode or like element. Moreover, both of the above electrical heating circuits can be employed to heat the molten lava. The resistance heaters can be used to heat the molten lava to a particular state and then the electrodes can be used to heat the lava to a temperature higher than the melting temperature of the heaters.

Thus it will be seen that the present invention provides an efficient coring device that is capable of high speed advancement through an earth formation. Moreover, the core formed by apparatus according to this invention is encased and isolated in a hard ceramic shell, and a continuous ceramic casing that lines the borehole is formed. The invention affords variation of porosity of the bore casing by permitting injection of volatile material into B the lava used for the casing in a manner similar to that used in making foamed metal and foamed ceramic material.

It will be appreciated by those skilled in this art that the core forming device of this invention can be used for other purposes in addition to thermal penetration of the earths crust. For example, the device may be employed to penetrate concrete and other artificially disposed deposits such as stock piles of sulphur, taconite and the like, in addition to its obvious application to naturally occurring deposits.

Although several embodiments of the invention have been shown and described, it will be obvious that other modifications and adaptations can be made without departing from the true spirit and scope of the invention.

What is claimed is:

1. In a core-forming device of the class having means for generating heat energy at a temperature above the melting temperature of the earth strata, means for applying heat from said generating means to the earth strata to produce molten lava, and means, for circulating the molten lava between the generating means and the earth strata, the improvement comprising means for directing said molten lava against said solid earth strata, said di recting means having first and second tubular surfaces defining an annular fiuid path, said surfaces adapted for simultaneous advancement into said solid earth strata, at least one of said first and second surfaces being rotatable about the central axis of said fluid path, and means secured to said rotatable surface for advancing said molten lava along said path for impingement upon said solid earth strata so that said solid earth strata will be melted in an annular configuration corresponding to the cross section of said fluid path leaving a central core.

2. A core-forming device in accordance with claim 1 wherein said advancing means comprises a descending helical convoluted surface.

3. A core-forming device in accordance with claim 1 and further characterized by means spaced from said directing means for providing drilling mud to cool spent molten lava displaced from said directing means.

4. A core-forming device in accordance with claim 1 wherein said rotatable surface is disposed within said other surface, and wherein is included means coupled with said rotatable surface for raising the same relative to said other surface to define an open region disposed to permit said molten lava to be directed inwardly and against said central core to sever the latter by melting.

5. A core-forming device comprising a housing having an inner cylindric surface terminating in a first circular edge, a hollow member having an inverted conical outer surface that diverges outwardly to a second circular edge having a diameter less than said first circular edge, means for supporting said hollow member within said cylinder for relative rotation and with said circular edges in coplanar relationship to define an annular opening, the confronting surfaces of said housing and member defining a downwardly converging path toward said annular opening, means for generating heat to fuse material in said path, and means for forcing material through said path and out said annular opening.

6. A core-forming device in accordance with claim 5 wherein said material forcing means includes a helical convoluted surface on the outer surface of said hollow member.

7. A core-forming device in accordance with claim 5 wherein said heat generating means comprises an electrical resistance heater.

8. The invention of claim 5 wherein said heat generating means comprises a voltage source, and means for connecting said source between said housing and said hollow member, thereby to produce current flow through the material in said path.

9. The invention of claim 8 wherein said connecting means comprises a plurality of electrodes mounted on said housing or said hollow member and spaced circumferentially therearound, means for switching said voltage source to said electrodes in a time sequence such that immediately adjacent electrodes are not sequentially connected to said voltage source, and means for regulating the magnitude of the current fiow through said electrodes.

10. A method for the thermal isolation of a core from the earths crust, comprising the steps of: providing a stream of molten lava at a temperature sufiicient to fuse the earths crust; establishing an annular flow path for said stream of molten lava; discharging said stream along said annular path to cause fusion of the earths crust at a corresponding annular cavity; continuing the discharge of said lava for a time sufiicient to fuse the earths crust in said annular cavity; and advancing the discharge of said lava along said annular cavity as said crust is fused to isolate a non-fused core.

11. A method in accordance with claim 10 and further characterized by periodically discharging said stream of molten lava in .a direction normal to said annular flow path to sever said core.

12. In a core drilling apparatus: a head having first and second spaced, tubular members, said members defining spaced heat transfer surfaces defining a fluid passage therebetween, said head adapted to be inserted into an annular kerf in the crust of the earth; means associated with said head for heating said heat transfer surfaces to a temperature sufiicient to generate molten material; means in said fluid passage for causing flow of said molten material therethrough in heat-transfer relationship to said heat-transfer surfaces, when said head is inserted in said annular kerf whereby the molten material will melt the earth below said head; and means for moving said head downwardly as the kerf is formed in advance thereof by said molten material.

13. A core drilling apparatus in accordance with claim 12 wherein said first tubular member is rotatable, said flow causing means including a helical convoluted surface on said first tubular member and confronting on said second tubular member.

14. A core-forming device in accordance with claim 12 wherein said first member includes mechanical drilling means extending outwardly from the lower end thereof.

References Cited UNITED STATES PATENTS 914,636 3/1909 'Case -15 X 1,348,815 8/1920 Lewis 175-323 1,456,983 5/1923 Hansen 175-323 X 1,989,926 2/1933 Aarts et a1. 175-16 2,313,806 3/1943 Crites 175-93 2,341,572 2/1944 Reed 175-17 2,623,149 12/1952 Amar 175-16 3,357,505 12/1967 Armstrong et a1 175-16 CHARLES E. OCONNELL, Primary Examiner JAN A. CALVERT, Assistant Examiner US. Cl. X.R. 175-58, 249, 323