Superconducting induction loops for mineral exploration

Abstract

Electromagnetic induction systems that include superconducting wire are disclosed. The electromagnetic induction systems are suitable for mineral prospecting.

Description

[0001] SUPERCONDUCTING INDUCTION LOOPS FOR MINERAL EXPLORATION

[0002] FIELD OF THE INVENTION

[0003] The invention relates to systems and methods using superconducting induction loops for mineral exploration.

[0004] BACKGROUND

[0005] Electromagnetic induction has been used for identifying ore bodies and electromagnetic anomalies in the subsurface. For example, electromagnetic induction has been used to locate conductive massive sulfide deposits, or structures with high magnetic susceptibility. Both airborne and ground current loops can be used. An example of airborne survey and equipment: https: / / gq.mines.gouv.qc.ca / documents / examine / GM69149 / GM69149.pdf

[0006] Current loops today are typically rigid or semi-rigid airborne copper or aluminum conductors with ~10m radius, or wires spooled out on the ground to form square loops ~lkm on a side. Currents applied are typically limited by power dissipation in the current loops. Typical currents in airborne conductors are a few kA, in ground conductors are a few hundred amps. There may be one or many turns in these current loops.

[0007] SUMMARY

[0008] Recently, superconducting wires have been developed for carrying large amounts of current with zero resistance that produce large magnetic fields. See, e.g., superconducting wires described at https: / / www.advancedconductor.com / about-us / . Such wires typically need to be kept cold as the critical temperature - the temperature at which they cease to act as superconductors - is generally well below room temperature. Accordingly, cryogens, such as liquid nitrogen, are often used to cool these wires to their operational temperatures. Superconducting wires can be incorporated into electromagnetic induction systems for mineral exploration. Such systems can include both ground-based and air-based operation.

[0009] Among other advantages, implementations of the technologies described herein can enable electromagnetic induction prospecting with low (e.g., zero) power dissipation during on-time (superconductivity). Accordingly, on-times for inductors can be longer than for similar inductors based on conventional ohmic conductors. Longer on-times can provide greater sensitivity to subsurface deposits, particularly in minerals in which eddy current excitations are slow to respond to excitation fields.

[0010] In some examples, induction systems offering higher magnetic moment per unit mass compared to systems using conventional conductors is possible. Such systems may be particularly useful in air-based applications, where the system is carried by an aircraft.

[0011] Other features and advantages will be apparent from the description below, the drawings, and the claims.

[0012] BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 A is a schematic diagram of an example system for mineral exploration using a superconducting induction loop.

[0014] FIG. IB is a schematic diagram of an example induction loop from the system shown in FIG. 1A.

[0015] FIG. 1C is a schematic diagram of an example source coil for the induction loop from the system shown in FIG. 1 A.

[0016] FIG. 2A is a plot of current v. time for an example implementation of a superconducting induction loop.

[0017] FIG. 2B is a plot of current v. time for another example implementation of a superconducting induction loop.

[0018] FIG. 2C is a plot of current v. time for a further example implementation of a superconducting induction loop.

[0019] FIG. 3 A is a schematic diagram of another example system using a superconducting induction loop.

[0020] FIG. 3B is a schematic diagram showing an example of a length of a superconducting cable useful in a superconducting induction loop.

[0021] In the figures, like symbols indicate like elements.

[0022] DETAILED DESCRIPTION

[0023] Referring to FIG. 1 A and IB, an induction detection system 100 includes a pair of coils 110 connected to a control module 101. Coils 110 include a source coil 111 and a receive coil 112 that share a common axis. Control module 101 includes a controller 120 (e.g., a computer), a detection module 130, a current source 140, energy storage system 150 (e.g., capacitor bank, battery), and an energy source 160 (e.g., a generator, battery, fuel cell, renewable source, mains). Controller 120 controls and synchronizes operation of the detection module 130, current source 140, energy storage 150, and energy source 160. Generally, the coil 110 has an area (in a plane orthogonal to the coil axis) suitable for use in subterranean mineral exploration. For example, the loop can have an area of 10 m2or more (e.g., 100 m2or more, 1,000 m2or more).

[0024] Coil 110 is at least partially composed of superconducting wire. For example, one or more portions of either or both the source coil 111 and receive coil 112 can be superconducting wire. Example coils composed of segments of superconducting wire are described in more detail below. Generally, each of the coils are composed of one more loops of wire (e.g., two or more loops, three or more loops, five or more loops, 10 or more loops). In some cases, Low-temperature superconductor (LI'S) wires can be used. Such wires can be made from superconductors with low critical temperature, such as NbiSn (niobium-tin) and NbTi (niobium-titanium). Often the superconductor is in filament form in a copper or aluminum matrix which carries the current should the superconductor quench for any reason. The superconductor filaments can form a third of the total volume of the wire. In certain cases, High-temperature superconductor (HTS) wires can be used. Such wires are made from superconductors with high critical temperature (high-temperature superconductivity), such as YBCO and BSCCO. Commercially-available superconducting wire can be used.

[0025] During use, system 100 pulses current through source coil 110 and then uses receive coil 112 to pick up the change in magnetic field during or after the current pulse through the loop. When oriented with the coil axis perpendicular (e.g., substantially perpendicular, precisely perpendicular, within about 20% of perpendicular) to the earth’s surface, the strength of the induced current in the subsurface, which generates a signal in the detection module 130 via the receive coil 112, is proportional to the magnetic field produced by the source coil 111. The signal to noise ratio is typically proportional to the current in the loop forming source coil 111. Increased current and loop magnetic field strength can lead to improved signal to noise in the detection module 130 and deeper or more accurate identification of geological structures of interest.

[0026] The magnetic field strength generated by coil 110 can vary as desired and is generally sufficient to produce a detectable response from a subterranean deposit of interest. In some embodiments, the magnetic field strength can be 10 Gauss or more (e.g., 15 Gauss or more, 20 Gauss or more).

[0027] Generally, the current pulses through coil 110 can include on times (when current is flowing) that are relatively long. For example, current pulses in a range from 0.1 seconds or more can be used (e.g., 0.2 seconds or more, 0.3 seconds or more, 0.5 seconds or more, 0.8 seconds or more, 1 second or more, 2 seconds or more, 3 seconds or more, 5 seconds or more, 8 seconds or more, 10 seconds or more, such as up to about 50 seconds). The current pulses can be followed by off-times during which no current flows or by reverse pulses where the current is reversed in the loop. The off-time pulses or reverse current pulses can be the same or different as the on-time pulses. The duty cycle of the current pulses can be 1% or more (i.e., the on-time is 1% or more of the period), e.g., 2% or more, 5% or more, 10% or more, 20% or more, 30% or more, 50% or more, such as up to 90%. Typical ramp up and down times to full current can be on the order of a few milliseconds, for example.

[0028] It is believed that use of superconducting wire can facilitate long pulses using wires that are relatively small compared to ohmic conductors because the superconductors do not generally generate as much heat as ohmic conductors, e.g., at high current. It is further believed that long on-time pulses can be beneficial for mineral exploration because the eddy currents generated by an external magnetic field in subterranean deposits respond on relatively long time scales, e.g., 10 ms or more, 20 ms or more, 50 ms or more, 100 ms or more, 200 ms or more, 500 ms or more, 500 ms or more, or even 1 second or more, such as several seconds.

[0029] In some implementations, coil 110 can dump energy into energy storage system 150 during the periods when the current through the coil is turned off.

[0030] Referring to FIG. 2A, an example pulse sequence for current in a source coil features a 1 second on time and a 10 second period. In other words, the off time is 9 seconds and the duty cycle is 10%. The current direction is reversed for each sequential pulse.

[0031] FIG. 2B shows another example pulse sequence in which the period is 20 seconds and the on-time is 10 seconds (i.e., a 50% duty cycle, and a 10 second off-time).

[0032] Referring to FIG. 1C and 2C, in some examples, source coil 111’ is composed of two loops configured to carry current in opposing directions, as depicted. In particular, here source coil I l l is composed of a first loop 114a carrying current in a clockwise direction and a second loop 114b carrying counterclockwise current. Both loops are at least partially composed of superconducting wire (e.g., one or both are entirely composed of superconducting wire). The loops can be coplanar or displaced relative to each along the coil axis.

[0033] Referring specifically to FIG. 2C, a current pulse sequence for loop 114a is shown by trace 210 and the current pulse sequence for loop 114b is shown by trace 220. Both sequences are varied in 10 second intervals. During the first interval, the current in loop 114a is at its peak value, while the current in loop 114b is zero. The resulting magnetic field in this time will have magnetic flux lines along a first direction and have a peak field strength.

[0034] From 10 seconds to 20 seconds, loops 114a and 1 lb carry the same current but in opposite directions. The result in that the net magnetic field from coil 111’ is zero (e.g., approximately zero, exactly zero) because the fields from each coil are canceled by the other.

[0035] From 20 seconds to 30 seconds, loop 114a carries no current while loop 114b is at peak current. The result is a magnetic field having flux lines along the opposite direction from the first 10 seconds.

[0036] In some examples containing superconducting loops configured to carry current in opposing directions, the current can be reversed by directing current from one loop to the other with little to no loss. Accordingly, the magnetic field direction from the source coil can be reversed with little or no energy loss.

[0037] The time intervals presented in FIGS. 2A-C are examples only and other time intervals may be used. Specific time intervals can be established based on the details of the system and the subterranean deposits being sought.

[0038] In general, the conductors of either or both of the source coil and pick up coil can be wholly or partially composed of superconducting wire. In some embodiments, at least one of the coils is composed of two or more segments of superconducting wire connected by segments of conventional wire. For example, referring to FIG. 3A, a system 300 includes four lengths of superconducting wire 310, 312, 314, and 316, connected by lengths 320, 322, and 324 of ohmic conducting wire (e.g., copper or aluminum) to form a loop of a source coil. In some examples, the superconducting wire can compose at least 50% of the length of the loop (e.g., 60% or more, 70% or more, 80% or more, 90% or more, such as 99% or less). The lengths of superconducting wire can be rigid or semi-rigid (e.g., more rigid than the lengths 321, 322, 323 of ohmic wire, which can be flexible. Accordingly, the loop in FIG. 3 A is substantially rectangular in shape, with curved corners. A rigid frame 350 maintains the shape of the loop during transport and deployment. More generally, any appropriate number of superconducting lengths can be used to form the loop, resulting in polygonal loops, e.g., having more than four sides (e.g., five, six, second, eight, or more), where each apex of the polygon is composed of an ohmic wire section. Generally, the superconducting lengths can all have the same length, or lengths or two or more can differ.

[0039] A current source and controller 370 is connected to the coil via ohmic wires 371.

[0040] Operation can be implemented as described above. The lengths of superconducting wire 310, 312, 314, and 316 are each housed in a corresponding sleeve 315, 316, 317, 318 that holds a cryogen (e.g., liquid nitrogen) that cools the superconducting wire to its operational temperature. Cryogenic fluid connectors 331, 332, and 333 link sleeves and allow the entire coil to be filled with cryogen from a single port. A cryogen pumping and storage station 360 supplies the cryogen to the loop via tubes 361. Connectors 331, 332, and 333 allow pumping of the cryogen through the entire loop and back to the pumping station 360, bypassing the ohmic sections 321, 322, 323. In some implementations, cryogen is supplied to the loop prior to operation of the system, during which time the pumping station 360 is disconnected from the loop.

[0041] In systems in which the coil is composed of multiple loops, more than one loop can be housed with sleeves 315, 316, 317, 318. Alternatively, or additionally, a coil can include loops that don’t share at least one sleeve (e.g., they have no shared sleeves).

[0042] In some examples, a detection antenna associated with system 300 also contains one or more loops contained within sleeves 315, 316, 317, and 318.

[0043] Referring to FIG. 3B, a superconducting segment 400 suitable for system 300 includes a superconducting wire 410 housed within a sleeve 420. Both ends of the superconducting wire 410 are connected to a corresponding section 440 of ohmic wire (e.g., copper or aluminum wire) by electrical connectors 430. A port 450 for accessing a cryogen chamber within sleeve 420 is attached at the end of sleeve 420. Port 450 can include a valve allowing cryogen into sleeve 420 but sealing the cryogen within the sleeve. The port can be disconnected from the cryogen source prior to deployment.

[0044] The length segment 400 can vary depending on the application. For example, segment 400 can have a length in a range from 1 m to 100 m or more (e.g., 1 m to 10 m, 10 m to 20 m, 20 m to 50 m).

[0045] In some embodiments, segment 400 is flexible and can be rolled up when not in use and then unrolled again before deployment. The segment can be rolled up while sleeve 420 contains the cryogen, or when empty. Segments can be disconnected from each other, facilitating storage and transportation when not in use.

[0046] The superconducting wire can be surrounded or spaced at intervals away from any other conductor by an insulating material (e.g., polyethylene, polyimide or other insulating polymers). In some examples, the insulator can shield the system from effects of voltage spikes, e.g., as the current in the wire ramps up or down.

[0047] In general, the systems described above can be deployed as either land-based or airbased systems. In either case, the system can fill the sleeves for the loops forming the coils with the cryogen before operation of the system. The pumping station can be disconnected from the loop prior to deployment. The pumping station can be continuously connected to the sleeves to continue to move cryogen during operation to achieve uniform temperature distribution. In air-based deployment this can reduce the overall weight and volume of the system, this can allow for larger loops and / or longer deployments of the system between servicing / refueling.

[0048] While the foregoing examples feature a receive coil for detecting changes in a magnetic field generated by subterranean substances, other detection technologies can be used for example, in some cases, a SQUID detector can be used.

[0049] A number of embodiments have been described. Additional embodiments are described below and in the claims that follow.

[0050] In one embodiment, an electromagnetic induction system includes an source coil with a loop of an electrically conducting wire, at least a portion of the loop including a superconducting wire; a receive coil; and a control module configured to vary a current in the source coil to periodically apply an excitation magnetic field to a volume of earth proximate to the source coil and to detect, using the detection antenna, changes in a magnetic field generated by a substance in the volume responsive to the excitation magnetic field.

[0051] In some embodiments, the source coil includes multiple loops of electrically conducting wire, a portion of each loop comprising superconducting wire.

[0052] In any of the preceding embodiments, the loop can include at least a portion of conventionally conducting wire.

[0053] In any of the preceding embodiments, the source coil can include a sleeve around the portion of the loop including the superconducting wire, the sleeve providing a chamber for housing a cryogen during operation of the system.

[0054] In any of the preceding embodiments, the source coil can include multiple loops, each loop comprising at least a portion of a superconducting wire.

[0055] In any of the preceding embodiments, the superconducting wire include an insulating sheath or spacers.

[0056] In any of the preceding embodiments, wherein the receive coil comprises a second superconducting material (e.g., the same as the first superconducting material). At least a portion of the receive coil and the source coil can share a common cryogenic sleeve.

[0057] In any of the preceding embodiments, the receive coil includes multiple loops. In any of the preceding embodiments, an on time for the current is 0.1 seconds or more (e.g., 0.5 seconds or more, 1 second or more, 2 seconds or more 5 seconds or more, 10 seconds or more, 20 seconds or more, such as up to 60 seconds).

[0058] In any of the preceding embodiments, the loop encompasses an area of 10 m2or more (e.g., 100 m2or more, 1,000 m2or more).

[0059] In any of the preceding embodiments, the control module can be further configured to determine information about the substance in the volume based on the detected responsive magnetic field.

[0060] In certain implementations, a method includes using the system of any of the preceding embodiments to identify subterranean mineral deposits. Using the system can include moving the system over an area of land using an aircraft (e.g., a drone, a helicopter, and airplane).

[0061] Using the system can include installing the system over an area of land.

[0062] In some implementations, a method includes: providing a superconducting cable including a length of a superconducting wire within a sleeve housing a cryogen sufficient to cool the superconducting wire to an operational temperature, the length of superconducting cable being would in a roll; unrolling the roll of superconducting cable; forming a coil including the unrolled superconducting cable; and conducting mineral exploration using the coil.

Claims

What is claimed is:

1. A electromagnetic induction system, comprising: an source coil comprising a loop of an electrically conducting wire, at least a portion of the loop comprising a superconducting wire; a receive coil; and a control module configured to vary a current in the source coil to periodically apply an excitation magnetic field to a volume of earth proximate to the source coil and to detect, using the detection antenna, changes in a magnetic field generated by a substance in the volume responsive to the excitation magnetic field.

2. The electromagnetic induction system of claim 1, wherein the source coil comprises multiple loops of electrically conducting wire, a portion of each loop comprising superconducting wire.

3. The electromagnetic induction system of claim 1, wherein the loop comprises at least a portion of conventionally conducting wire.

4. The electromagnetic induction system of claim 1, wherein the source coil comprises a sleeve around the portion of the loop comprising superconducting wire, the sleeve providing a chamber for housing a cryogen during operation of the system.

5. The electromagnetic induction system of claim 1, wherein the source coil comprises multiple loops, each loop comprising at least a portion of a superconducting wire.

6. The electromagnetic induction system of claim 1, wherein the superconducting wire comprises an insulating sheath.

7. The electromagnetic induction system of claim 1, wherein the receive coil comprises a second superconducting material.

8. The electromagnetic induction system of claim 7, wherein at least a portion of the receive coil and the source coil share a common cryogenic sleeve.

9. The electromagnetic system of claim 7, wherein the receive coil comprises multiple loops.

10. The electromagnetic induction system of claim 1, wherein an on time for the current is 0.1 seconds or more.

11. The electromagnetic induction system of claim 1, wherein the loop encompasses an area of 10 m2or more.

12. The electromagnetic induction system of claim 1, wherein the control module is further configured to determine information about the substance in the volume based on the detected responsive magnetic field.

13. A method, comprising using the system of claim 1 to identify subterranean mineral deposits.

14. The method of claim 13, wherein using the system comprises moving the system over an area of land using an aircraft.

15. The method of 13, wherein using the system comprises installing the system over an area of land.

16. A method, comprising: providing a superconducting cable comprising a length of a superconducting wire within a sleeve housing a cryogen sufficient to cool the superconducting wire to an operational temperature, the length of superconducting cable being would in a roll; unrolling the roll of superconducting cable; forming a coil comprising the unrolled superconducting cable; and conducting mineral exploration using the coil.

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