Penetration rate / depth monitor for boreholes formed by millimeter wave beams.

Abstract

An apparatus and method are described for drilling deep boreholes into earthen materials with millimeter wave radiation to access deep resources such as geothermal. The borehole depth and temperature at the bottom of the borehole can be monitored by probe signals and / or radiation emissions from the bottom of the borehole.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. patent application Ser. No. 63 / 291,731, entitled “Rate of Penetration / Depth Monitor for a Millimeter-Wave Beam Made Hole,” filed December 20, 2021, which is incorporated by reference in its entirety herein.

[0002] government support This invention was made with Government support under Grant No. DE-AR0001051 awarded by the Department of Energy. The Government has certain rights in this invention. [Background technology]

[0003] The high power millimeter wave beam produced by a gyrotron can create a borehole in the rock by melting and / or vaporizing the rock. This borehole opening process operates at temperatures above the melting and vaporization temperatures of rock, which are 1000 °C and 3000 °C, respectively. Traditional sensors used to monitor mechanically drilled boreholes require physical contact with the bottom of the borehole, which may not exist at these temperatures. Summary of the Invention

[0004] This technique can be used to drill deep boreholes into rock with high power millimeter wave beams and monitor the bottom of the borehole, and the monitoring equipment can remain on the earth's surface at ambient temperature and pressure, regardless of the borehole depth or temperature.

[0005] The present technique includes a method of measuring the depth and / or penetration rate of a borehole drilled by a millimeter wave drilling beam directed by a transmission line to the bottom of the borehole. The method includes coupling a probe signal into the transmission line. The transmission directs the probe signal to the bottom of the borehole, and at least a portion of the probe signal is reflected and / or scattered from the bottom of the borehole as a return beam. The transmission line directs the return beam from the bottom of the borehole. The return beam is coupled out of the transmission line and mixed with a local oscillator to generate an intermediate frequency beam whose amplitude and / or frequency is used to determine the depth and / or penetration rate of the borehole.

[0006] In some cases, the amplitude of the probe signal may be modulated and the borehole depth and / or penetration rate may be based on the amplitude of the intermediate frequency beam. In other cases, the frequency of the probe signal may be modulated and the borehole depth and / or penetration rate may be based on the frequency of the intermediate frequency beam. In still other cases, the probe signal includes pulses and the borehole depth and / or penetration rate is based on the time of flight of the pulses.

[0007] The probe signal may be generated at a frequency different from that of the mm-wave borehole beam or may be taken from the mm-wave borehole beam.

[0008] If desired, a temperature signal at a temperature signal frequency different from the frequency of the mmWave borehole beam (and different from the probe frequency) can be coupled into the transmission line with the probe signal to determine the temperature at the bottom of the borehole.

[0009] Another embodiment of the present technology includes a system for drilling a borehole with a source, a transmission line, a depth / penetration rate monitor, and a beam combiner. In operation, the source generates a millimeter wave borehole beam. The transmission line coupled to the source directs the millimeter wave borehole beam to the bottom of the borehole. The depth / penetration rate monitor, also coupled to the transmission line, monitors the depth / penetration rate of the borehole. And, the beam combiner coupled to the transmission line and the depth / penetration rate monitor couples a probe signal to the transmission line for transmission to the bottom of the borehole and couples a return beam generated by reflection and / or scattering of the probe signal from the bottom of the borehole from the transmission line to the depth / penetration rate monitor.

[0010] The depth / penetration rate monitor may be configured to operate as a reflectometer, a frequency modulated radar, or a pulse modulated time-of-flight radar. The depth / penetration rate monitor may generate a probe signal at a frequency different than the frequency of the mmWave borehole beam. In other cases, the beam combiner directs a portion of the mmWave borehole beam returning from the bottom of the borehole as a return beam to the depth / penetration rate monitor.

[0011] The beam combiner may include a miter mirror to reflect the mm-wave drilling beam around a bend in the transmission line. The miter mirror may have a hole in it to pass radiation at the probe frequency and reject radiation at the frequency of the mm-wave drilling beam.

[0012] The system may also include a temperature monitor to receive a temperature signal for monitoring the temperature of the borehole at a temperature signal frequency different from the frequency of the mmWave borehole beam and different from the probe frequency, and the system may include a small signal beam combiner coupled to the depth / penetration rate monitor, the temperature monitor, and the beam combiner to combine the temperature signal with the probe signal.

[0013] All combinations of the above concepts and additional concepts discussed in more detail below (provided that such concepts are not mutually inconsistent) are contemplated as part of the inventive subject matter disclosed herein. In particular, all combinations of the subject matter described in the claims at the end of this disclosure are contemplated as part of the inventive subject matter disclosed herein. Terms explicitly used in any disclosure incorporated herein by reference should be given the meaning most consistent with the specific concepts disclosed herein. [Brief description of the drawings]

[0014] The drawings are primarily for illustrative purposes and are not intended to limit the scope of the inventive subject matter. The drawings are not necessarily to scale, and in some instances, various aspects of the inventive subject matter disclosed herein may be shown exaggerated or enlarged in the drawings to facilitate understanding of different features. In the drawings, like reference characters generally refer to like features (e.g., functionally similar and / or structurally similar elements).

[0015] [Figure 1A] FIG. 1A shows a directed energy millimeter wave (MMW) drilling system that uses a high power borehole beam to drill a borehole, and at least one collinear small signal probe signal at a different frequency than the high power borehole beam to monitor the rate of penetration (ROP) and / or depth of the borehole.

[0016] [Figure 1B] FIG. 1B shows an implementation of an MMW drilling system that uses a high powered borehole beam to drill a borehole and to monitor the rate of penetration (ROP) and / or depth of the borehole.

[0017] [Figure 1C] FIG. 1C shows details of an ROP / depth monitor that may be implemented with an isolator and detector and may use MMW radiation used to drill a borehole.

[0018] [Figure 2A] FIG. 2A shows an implementation of a beam combiner for combining a high power heating beam into one or more higher frequency probe signals in the directed energy MMW drilling system of FIG. 1A.

[0019] [Figure 2B] FIG. 2B shows another implementation of a beam combiner for combining a high-power heating beam into one or more lower frequency probe signals in a directed energy MMW drilling system similar to that shown in FIG. 1A.

[0020] [Diagram 3] FIG. 3 is a photograph of a portion of the MMW borehole system of FIG. 1A including a small signal combiner that combines a 94 GHz beam for monitoring borehole ROP / depth and a 137 GHz beam for monitoring borehole temperature with a 28 GHz high power heating beam propagating through a high power miter bend on a 28 GHz overmodulated waveguide (e.g., as shown diagrammatically in FIG. 2A ).

[0021] [Figure 4] Figure 4 shows a circuit for generating a 94 GHz probe signal for monitoring the ROP / depth of a borehole using either reflectometry or frequency modulation (FM) radar techniques. The circuit is shown with an optional small signal combiner for combining in a 137 GHz temperature signal.

[0022] [Diagram 5] Figure 5 shows the connection of the borehole ROP / depth monitor with other electronics operating as a reflectometer.

[0023] [Figure 6] Figure 6 shows the connection of a borehole ROP / depth monitor with other electronics operating as an FM radar.

[0024] [Figure 7]FIG. 7 is a plot of the signal generated by a 94 GHz ROP / depth monitor beam probing a flat lead target moving uniformly away from the monitor at a starting distance of approximately 18 cm and a rate of 0.5 mm / min.

[0025] [Figure 8] Figure 8 shows plots of the signals generated by the 94 GHz ROP / depth monitor beam (right axis) and the 28 GHz borehole beam (left axis), both used to simultaneously probe the formation of melt craters on the basalt surface while the borehole beam removes rock.

[0026] [Figure 9] Figure 9 is a photograph of the broken open cross section of a melt crater of an MMW beam in basalt. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] FIG. 1A shows a high power directed energy millimeter wave (MMW) drilling system 100 with at least one monitor 160 for sensing the rate of penetration (ROP) and / or depth of a borehole 110 created by the system of material 150, such as a geological material. The system 100 may also include one or more auxiliary monitors 170 to monitor one or more conditions at the bottom of the borehole 110, such as temperature, surface emissivity ε, and melt turbulence, among other features. The drilling system 100 uses a high power MMW drilling beam 105 to "drill" the borehole 110 into the material 150. Drilling may include melting, vaporizing, and / or removing material as the borehole deepens.

[0028] High power MMW radiation 103 from the high power source 120 may be coupled into a high power transmission line 130 (which may be implemented as a waveguide) and delivered to the bottom of the borehole 110. At the bottom of the borehole, the high power MMW radiation 103 may exit a distal end 138 of the transmission line 130 to form a borehole beam 105 that interacts with material in its path. According to some embodiments, the borehole beam 105 has sufficient power to melt granite, for example. A high power source 120, such as a gyrotron, may generate MMW radiation 103 at frequencies in the range of about or just 30 GHz to about or just 300 GHz, with average powers in the range of about or just 100,000 Watts to about or just 2,000,000 Watts. The MMW radiation 103 may be continuous wave at these power levels, or may be pulsed to produce momentary higher power levels.

[0029] The power and frequency of the MMW radiation 103 from the high-power source 120 may be constant over time as the system 100 is drilling the borehole 110. For example, the power and frequency may be held constant over seconds, minutes, tens of minutes, or even hours as the borehole is drilled. In some cases, one or both of the power and frequency of the MMW radiation 103 from the high-power source 120 may vary within the frequency and power ranges discussed above as the system 100 is drilling the borehole 110. For example, the frequency and / or power may vary when engaging different materials to improve energy coupling to and heating of the different materials.

[0030] The probe signals 108, 109 from the monitors 160, 170 may be radially coupled to and from the high power transmission line 130 to one or more combiners 140, 142. The probe signals 108, 109 may be at one or more frequencies different than the frequency of the high power borehole beam 105 to sense conditions at the bottom of the borehole 110. Additionally, the probe signals 108 from the ROP / depth monitor 160 may be at a different frequency than the frequency used for the auxiliary monitor 170. For example, the frequency used for the ROP / depth monitor may be in the range of 10 GHz to 300 GHz. In some cases, the frequency used for the ROP / depth monitor may be in the range of 10 GHz to 1 THz. The frequency used for the auxiliary monitor may also be in the range of 10 GHz to 300 GHz. In some cases, the frequency used for the auxiliary monitor may be in the range of 10 GHz to 1 THz. The power level of the probe signal may be in the range of 0.1 watts to 100 watts, or in some cases even higher power levels. When detecting thermal radiation from the bottom of the borehole 110, the power level of the received probe signal 109 may be less than 0.1 watts. The term "probe signal" is used herein to refer to a signal that is emitted into the borehole by the ROP / depth monitor 160 or the auxiliary monitor 170 and used to measure properties of the borehole 110. In some cases, the auxiliary monitor (such as a temperature radiometer) may not emit a probe signal, but instead receive and monitor radiative emissions (e.g., blackbody radiation) from the borehole. Conditions that may be sensed by the ROP / depth monitor 160 and the auxiliary monitors include, but are not limited to, the rate of penetration of the borehole 110 as the borehole is drilled, the depth of the borehole 110, the surface emissivity ε of the earth material, and the temperature of the earth material 150 as the borehole 110 is drilled. The depth of the borehole 110 may or may not be measured while the borehole is actively drilled.

[0031] The high power transmission line 130 directs the high power MMW radiation 103 from the high power source 120 to the bottom of the borehole where the borehole beam 105 may vaporize rock and other materials. The high power transmission line 130 is typically hollow and sized to be two or more wavelengths in diameter (oversized) that can handle the high power. The high power transmission line is configured to reduce or minimize mode conversion so that the most efficient fundamental mode can propagate with minimal loss. There may be some intentional mode conversion for special applications when launching from the distal end 138 or to traverse bends more efficiently, but generally the MMW radiation 103 propagates through the high power transmission line 130 in a single mode for efficient long distance transmission. Linearly polarized HE 11 The mode is most efficient in circular high power transmission lines 130 that are internally corrugated with good conductor metal surfaces such as copper. Other suitable modes include azimuthally polarized TE in smooth wall metal waveguides. 01 This mode is the HE mode in an internally corrugated circular waveguide. 11 The efficiency of the HE mode is 69%. 11 Modes may also be guided by the borehole 110, which acts like a dielectric waveguide, such as a hollow fiber optic cable. For example, the borehole beam 105 may vitrify the walls of the borehole 110 as the beam advances, forming a hollow circular dielectric waveguide.

[0032] The MMW drilling system 100 may include a ROP / depth monitor 160 radially coupled to the high power transmission line 130. The ROP / depth monitor 160 generates a small signal probe signal, also referred to as a probe signal 108 or probe beam, which may be at a different frequency than the borehole beam 105 for sensing the ROP and / or depth of the borehole. For example, the probe signal 108 may be centered at a probe frequency in the 30 GHz to 1 THz frequency range, with a bandwidth of up to 5 GHz. In some cases, the frequency of the probe signal 108 may vary across frequencies within this frequency range when making measurements. The probe signal 108 may preferably be induced by the same high power transmission line 130 that induces the borehole beam 105. The average probe signal power may be in the range of 0.01 Watts to 10 Watts, and may be constant or vary across a range of power levels within this range when making measurements.

[0033] One or more auxiliary monitors 170 may generate and / or receive one or more other small signal probe signals 109 at other frequencies to probe or monitor the borehole 110 and other parameters associated with the MMW drilling system 100. In some cases, the auxiliary monitor 170 receives radiative emissions from the borehole for analysis. For example, the auxiliary monitor 170 may monitor the temperature of the borehole by radiometry and / or the waveguide / borehole fill composition by mmWave or Terahertz spectroscopy, respectively. In some implementations, the temperature may be monitored by mmWave thermal emissions from the bottom of the borehole 110 that couple into the transmission line 130 and propagate back to the auxiliary monitor 170. The radiometer antenna pattern defined by the high power transmission lines 130 and 138 selects the apparent spot size that forms the return temperature signal. (Thermal radiation occurs in all modes, and the receiver antenna pattern selects which mode propagates along the high power transmission line 130 and is detected by the auxiliary monitor 170. Thus, the return temperature signal 109 has characteristics that are defined, at least in part, by the field of view of the radiometer.) In some cases, the fill composition may be monitored by millimeter wave or terahertz spectroscopic emission or absorption. The spectroscopy performed by the auxiliary monitor 170 may be passive, using thermal blackbody radiation background or local plasma excited radiation, or active with a frequency swept probe signal. For passive spectroscopy, the receiving antenna (essentially, the high power transmission line 130) essentially defines the return signal, as in the case of the return temperature signal. For active probes, the high power waveguide defines and guides the probe signal. These auxiliary probe signals may have average power levels in the range of 0.1 Watts to 100 Watts and may operate at one or more frequencies in the range of 30 GHz to 1 THz.

[0034] In some implementations, the temperature monitoring may include emitting a temperature probe signal into the borehole 110 by the auxiliary monitor 170. The temperature probe signal may be used to determine the surface emissivity ε of the bottom of the borehole. Generally, the radiative emission from the bottom of the borehole (and detected by the temperature radiometer) is the product of the surface emissivity and temperature (εT). The emissivity may have a value ranging from 0 to 1. Once the surface emissivity is known, the temperature of the earthen material 150 at the bottom of the borehole 110 may be more accurately determined. The emissivity may be measured by measuring the reflectivity of the surface with the temperature probe signal. For opaque surfaces, the emissivity may be calculated from the equation ε=1-r, where r is the surface reflectivity.

[0035] Temperature monitoring can be useful for two reasons. First, the temperature of the earthen material can be monitored during borehole to improve removal efficiency. For example, a particular temperature can be maintained during borehole (e.g., using a feedback loop that receives a signal indicative of the temperature from the borehole area and adjusts the delivered power accordingly). The maintained temperature can be a temperature that will vaporize the earthen material or remove it into predominantly microparticles that can be expelled from the borehole using high pressure gas. The pressurized gas can be pumped along the transmission line 130 and pressurized by energy from the borehole beam 105, which increases the temperature of the material within the confined borehole volume as described by the ideal gas and real gas laws while vapors and / or particulates are expelled upwardly outside the transmission line.

[0036] Second, for geothermal access, temperature monitoring is also useful when drilling stops and allows the earthen material to cool to a lower temperature or its steady state temperature. In this case, temperature monitoring can determine the time to reach a sufficient depth to access the geothermal. For example, the temperature can drop to its steady state temperature, which can be measured by a temperature monitor. Alternatively, at least one lower temperature can be measured as the temperature drops to its steady state temperature. The steady state temperature (the temperature at which geothermal energy can be utilized and no further drilling is required) can range from 50°C to 500°C. An exponential or function fit to the drop temperature can be used to determine the final steady state temperature at the bottom of the borehole.

[0037] The auxiliary probe signal 109 from the auxiliary monitor 170 may be combined with the probe signal 108 from the ROP / depth monitor 160 by a small signal combiner 142 onto a common small signal transmission line 133 that carries the probe signals 108, 109 to and from the power combiner 140, as shown in FIG. 1A. The power combiner 140 couples the combined small signal probe signal 108, 109 from the small signal transmission line 133 to a high power transmission line 130 that also directs the high power MMW radiation 103 to the bottom of the borehole 110. At the bottom of the borehole 110, the probe signals 108, 109 may exit and / or enter a distal end 138 of the transmission line 130 to sense physical properties (e.g., borehole penetration rate, temperature, depth, material composition, melt turbulence, etc.). Blackbody radiation emissions from the heated soil material 150 may also enter the distal end 138 of the transmission line 130 for transmission to the auxiliary monitor 170 .

[0038] The small signal probe signals 108, 109 may return from the bottom of the borehole 110 and may be directed by high power transmission lines 130 back to the power combiner 140 which couples the return small signal probe signals 108, 109 via small signal transmission lines 133 to the small signal combiner 142. The small signal combiner 142 directs the different small signal probe signals 108, 109 and / or radiated emissions to the respective monitors 160, 170. The return signals may be split using frequency, polarization, or time and sent to the respective monitors which demultiplex the signals at the small signal combiner 142. The monitors 160, 170 measure the amplitude, frequency, and / or phase of their corresponding received probe signals to derive some information regarding the conditions at the bottom of the borehole 110, such as the drilling beam penetration rate, borehole depth, surface emissivity, melt turbulence, and / or temperature, among other features. Because the monitors 160, 170 derive information from return probe signals 108, 109 and / or radiative emissions, they can remain on the earth's surface, far away from the bottom of the borehole 110, which may be heated to extreme temperatures. As a result, the monitors do not need to be as robust as downhole monitors used to monitor, for example, mechanical drilling. The monitors may also monitor conditions at the bottom of the borehole 110 that are more extreme (e.g., hotter) than conditions at the bottom of a borehole created by mechanical drilling.

[0039] An assessment of melt turbulence can be beneficial for drilling deep boreholes. Melt turbulence can indicate the level of viscosity of the molten earth material 150. In some borehole applications, once the appropriate viscosity is reached, the molten earth material 150 can be mobilized to the borehole wall and cooled to form a solid casing over the borehole 110. This self-casing can stabilize the borehole 110 and in some cases can be strong enough to prevent the borehole from collapsing.

[0040] 1B and 1C show an alternative high power directed energy MMW borehole system 102. The system 102 may monitor the borehole penetration rate and / or depth using a portion of high power MMW radiation instead of a probe signal 108 at a different frequency. As in FIG. 1A, a gyrotron or other high power MMW source 120 generates high power MMW radiation 103 that is guided to the bottom of the borehole by a high power waveguide or transmission line 130. However, in this case, the penetration rate / depth monitor does not generate and launch the probe signal at a different frequency than the frequency of the high power MMW radiation 103 that forms the borehole beam 105. Instead, beam sampling by an isolator 180 couples a small portion of the forward high power MMW radiation 103 through beam dumps 181, 182 to a gyrotron frequency detector 184 at one of the beam dumps. Additionally, the reflected power isolator 180 simultaneously couples a portion of the high power MMW radiation 103 returned from the bottom of the borehole 110 to a detector 184. The returned MMW radiation 103 may be coupled from the reflected power beam dump 182 through a small signal transmission line 186.

[0041] 1C shows further details of the isolator 180 and detector 184, which may be part of another implementation of the ROP / depth monitor 162. More specifically, the reflective power isolator 180 may include a copper wire polarizer grille 183 at a 45 degree angle to the axis of the high power transmission line 130. There may be two beam dumps 181, 182, facing each other on each side of the polarizer grille 183. The MMW radiation 103 passes through the polarizer grille of the power isolator 180. 11The MMW radiation 103 is primarily linearly polarized in the forward mode, but there is typically a small unwanted power component in the wrong polarization that does not pass through the polarizer. The power isolator 180 filters and / or reflects the other polarization components of the MMW radiation 103 to the side beam dumps 181, 182 depending on the direction of travel of the MMW radiation 103. One forward beam dump 181 may absorb the filtered forward power and the second reflected power beam dump 182 may absorb the return reflected power. In some cases, the beam dumps 181, 182 may not be complete and cross-scatter the power to each other. In such cases, the small signal waveguide 186 may not be needed. The polarizer grille 183 reflects the power returned from the target at the bottom of the borehole 110 to the reflected power beam dump 182. This return and reflected power has its polarization flipped by 90 degrees by a circular polarizing mirror in a miter bend, which may be part of the small signal and high power beam combiner 140 as shown in FIG. 2A. The circular polarizing mirror has grooves on its surface that circularly polarize the incident linearly polarized beam and repolarize the return reflection to an orthogonal linear polarization to the incident beam. A detector 184 (e.g., a 28 GHz diode) located away from the center of the forward beam dump 81 picks up those signals rejected by the polarizer grill. The forward and return signal components coherently interfere in the diode detector to produce a detected signal amplitude that is then dependent on the relative phase of the forward and reflected signals, which is dependent on the distance to the rock melt surface. The isolator 180 and detector 184 can be used to implement a second ROP / depth monitor 162, which can be used alone or in addition to the ROP / depth monitor 160 (which may operate at a different frequency) to improve the reliability and accuracy of the distance measurements.

[0042] In the second ROP / depth monitor 162, the reflected signal from the polarizing grille coherently mixes with a portion of the forward moving high power MMW radiation 103 coupled to the ROP / depth monitor 162 for use as a local oscillator. The ROP / depth monitor 160 may have its own frequency source that provides a local oscillator signal for that monitor, which may be at a different frequency than the frequency of the MMW radiation 103. The ROP / depth monitors 160, 162 detect their beat or intermediate frequencies caused by mixing their respective local oscillator signals with the reflected signal received from the borehole 110, and the monitors process their respective detected beat signals to determine the ROP or depth of the borehole 110. When the MMW radiation 103 and the downhole beam 105 are at a constant frequency, the rate of penetration / depth monitor 162 acts as a reflectometer, and the number of amplitude maxima and minima represent the depth of the borehole, as described below. If the frequency of the MMW radiation 103 and the downhole beam 105 is chirped or swept, the rate of penetration / depth monitor 162 acts as a frequency modulated (FM) radar, with the phase or beat frequency representing the borehole depth, as described below. Similarly, the frequency source of the ROP / depth monitor 160 may be fixed or swept to operate the ROP / depth monitor as a reflectometer or FM radar, respectively.

[0043] 1. Combining frequency-multiplexed probe signals with guided high-power MMW radiation In some embodiments, the high power MMW radiation 103, the small signal probe signals 108, 109, and the radiation emission of interest from the bottom of the borehole 110 are at different frequencies so that they can be frequency multiplexed and demultiplexed using the small signal combiner 142 and the power combiner 140. The small signal monitors 160, 170 can be radially coupled to the high power transmission line 130 by using fundamental mode microwave / mm wave waveguide components such as signal splitters, directional couplers, or frequency multiplexers. (A waveguide that supports only the fundamental mode has a cross section of less than one wavelength, or as little as half the wavelength of the radiation used by the small signal monitors 160 or 170.) The monitor probe signals 108, 109 can be coupled to the high power transmission line 130 by using the configurations described in connection with Figures 2A and 2B. The high power transmission line 130 may be oversized relative to the probe signal (eg, the diameter of the waveguide used for the transmission line 130 may be much larger than the wavelength of the radiation used for the probe signals 108, 109).

[0044] 2A and 2B show examples of power combiners 140a, 140b that may radially combine small signal probe signals 108, 109 propagating on and from a high power transmission line 130 carrying high power MMW radiation 103 to the bottom of a borehole 110. The configuration of the power combiners depends on the relative frequencies of the small signal probe signals 108, 109, the radiated emissions of interest, and the high power MMW radiation 103. The power combiner 140a of FIG. 2A may combine a higher frequency small signal probe signal with a lower frequency high power MMW radiation 103 traveling along the transmission line 130. The power combiner 140b of FIG. 2B may combine a lower frequency small signal probe signal 108 and / or probe signal 109 with a higher frequency high power borehole beam.

[0045] In the beam combiner of Figure 2A, a small coupling hole 205 in a miter mirror 210 attached to a bend in the high power transmission line 130 can be used to combine the higher frequency probe signals 108, 109 with the lower frequency MMW radiation 103 without perturbing the borehole beam 105. The high power MMW radiation 103 reflects off the miter mirror 210, for example, downwards towards the bottom of the borehole 110, as shown in Figure 1A. At the same time, the small signal probe signals 108, 109 from the monitors 160, 170 propagate downwards through the coupling hole 205 in the miter mirror 210 to the bottom of the borehole. The return small signal probe signals 108, 109 and / or radiation emissions pass upwards through the hole 205 in the miter mirror 210 to the monitors 160, 170. The diameter of the coupling hole 205 and / or the inner diameter of the small signal transmission line 133 is less than half the wavelength of the high power MMW radiation 103 to prevent the MMW radiation 103 from propagating towards the monitor 160, 170. In other words, the coupling hole 205 and / or the small signal transmission line 133 in the miter mirror 210 allows only ephemeral propagation of the MMW radiation 103 towards the monitor 160, 170, effectively acting as a low cutoff frequency or high pass filter. The mode conversion loss in the coupling between the small signal transmission line 133 and the high power transmission line 130 can be calibrated.

[0046] In the power combiner 140b of FIG. 2B, a dichroic or polarization dependent filter 220 in the power transmission line 130 passes the high frequency high power MMW radiation 103 and reflects the low frequency small signal probe signals 108, 109 and / or radiation emission from the bottom of the borehole 110. More specifically, the polarization dependent filter 220 may be implemented as a wire grill or mesh with a low density of wires. For example, a wire grill (comprising straight parallel conductive wires or conductive traces) may transmit a first linearly polarized wave and reflect a second linearly polarized wave with its polarization oriented orthogonal to the first polarized wave. As another example, a dielectric window mounted at an appropriate angle or thickness may be used as a frequency selective filter 220 to separate the two different frequencies of radiation. The filter 220 is configured to transmit the high power MMW radiation 103 downwards towards the bottom of the borehole 110. The dichroic or polarization dependent filter 220 also reflects the probe signals 108, 109 downwards from the monitors 160, 170 towards the bottom of the borehole 110 via the high power transmission line 130 and reflects the return probe signals 108, 109 and / or the radiated emissions of the subject back towards the monitors 160, 170 via the small signal transmission line 133.

[0047] A transition region comprising a section of tapered waveguide 230 may be located between the small signal transmission line 133 and the high power transmission line 130 and may be located near the dichroic or polarization dependent filter 220 (e.g., within 10 cm of the mirror). The tapered waveguide 230 may convert transverse modes from the small signal transmission line 133 to better match the modes supported by the high power transmission line 130 and vice versa, thereby reducing mode coupling losses between the two transmission lines. In some cases, the diameter of the small signal transmission line 133 may be much smaller than the diameter of the high power transmission line 130, even though the wavelength of the probe signal may be longer than the wavelength of the high power MMW radiation 103. The tapered waveguide 230 serves to couple the probe signal 108, 109 propagating in the small signal transmission line 133 to and from the larger high power transmission line 130. The taper of the tapered waveguide 230 may be linear or parabolic and should be long enough to reduce or minimize mode conversion losses (e.g., to less than 10 dB). Parabolic tapers are generally shorter than linear tapers. The inner surface of the tapered waveguide 230 preferably matches the inner surface of the transmission line it connects. For example, the tapered waveguide 230 may be a HE waveguide when the transmission lines 130, 133 it connects are implemented as waveguides with corrugated inner surfaces. 11 It may have a corrugated inner surface to efficiently transmit the mode. The end width (diameter) of the tapered waveguide 230 is sized to match the width (diameter) of the transmission line that the end connects to. A high power gyrotron frequency notch filter may also be added to the small signal monitor waveguide to further reject any stray or scattered high power gyrotron electromagnetic radiation.

[0048] One example of a high-power dielectric component that can be used in the power combiner 140b is a diamond plate oriented at Brewster's angle, which is 67 degrees for diamond in air at 1 atmosphere pressure. The diamond plate combines and separates the beams with orthogonal linear polarizations. At Brewster's angle, the polarization in the plane of incidence (the plane containing the incident and reflected beam vectors and perpendicular to the plate) is transmitted without loss (except for very small plate absorption), and the beam with polarization perpendicular to the plane of incidence is highly reflective. The transmitted beam may be the high-power MMW radiation 103 propagating along the high-power transmission line 130, and the reflected beam may be the probe signal 108 and / or the probe signal 109.

[0049] FIG. 3 is a photograph of a portion of a directed energy MMW borehole system, showing a small signal beam combiner 142 coupled to a power combiner 140a, such as the power combiner shown in FIG. 2A. The small signal beam combiner 142 decouples the 135-139 GHz radiated emission or temperature signal 109 received from the borehole 110 for temperature radiation measurement, and the 94 GHz ROP / depth return probe signal 108 from a small signal transmission line 133 (implemented as a rectangular waveguide, circular to rectangular transition, and circular waveguide) that makes a 90 degree turn toward a high power miter mirror 210. A temperature radiometer is used as an auxiliary monitor 170. The hole in the miter mirror 210 radially couples the collinear combined return probe signal 108 and temperature signal from a larger diameter high power transmission line (not visible in the photograph) below the miter mirror 210 to the monitors 160, 170, such that the probe signal 108 and temperature signal propagate along the high power transmission line with the 28 GHz high power radiation used to form the borehole beam 105. The top of the copper miter mirror 210 mounted on the 3 inch (76 mm) inner diameter 28 GHz high power transmission line is shown in the bottom right of Figure 3. The small signal transmission line 133 is attached to the miter mirror 210 and comprises a 0.097 inch (2.5 mm) inner diameter copper circular waveguide with a vertical portion aligned with the same diameter hole in the center of the miter mirror, which introduces the probe signal 108 into the high power transmission line to propagate collinearly with the high power MMW radiation.

[0050] Continuing upward from the miter mirror 210 of the exemplary system of FIG. 3, a circular to rectangular transition of the waveguide to the wr-8 band (90-140 GHz) is attached, followed by a wr-8 E-plane bend. The E-plane bend is attached to a horizontally oriented 3 dB wr-8 directional coupler, which splits the combined return probe signal 108 and temperature signal between the two monitors 160, 170. The return probe signal 108 to the 94 GHz ROP / depth monitor 160, and the temperature signal 109 to the GHz temperature radiometer are separated by the directional coupler in combination with the wr-8 to wr-6 (110-170 GHz) transition of the waveguide to the auxiliary monitor 170, and the wr-8 to wr-10 (75-110 GHz) transition to the ROP / depth monitor 160 (which is partially visible). The WR-6 to WR-8 transition in the waveguide prevents the ROP / depth probe signal from interfering with the radiometer's received temperature signal since 94 GHz cannot propagate in the WR-6 waveguide. Similarly, the 28 GHz high power borehole beam cannot propagate in the 0.097" diameter waveguide, shielding the monitor while allowing full access for monitoring the borehole target surface with the higher frequency beam.

[0051] 2. Penetration rate / depth monitoring equipment The ROP / depth monitor 160 may operate as either a reflectometer (reflective interferometer), a frequency modulated (FM) radar, or a pulsed time-of-flight radar. In a reflectometer configuration, the frequency of the small signal ROP / depth probe signal 108, also referred to as the probe frequency, is fixed. The return probe signal 108 is mixed with an untransmitted portion of itself to generate a DC signal whose amplitude depends on the round-trip return phase of the return probe signal 108 relative to the one before transmission. For a change in depth equal to one-quarter of a wavelength at the probe frequency, the detected signal amplitude should change from a maximum value to a minimum value, or vice versa. In other words, in a reflectometer configuration, the ROP / depth monitor 160 has a depth resolution Δz, which may be described as:

number

[0052] In an FM radar configuration, the probe frequency is some modulation frequency rate f m The phase shift of the round trip reflection as detected at the mixer by an untransmitted copy of the ROP / depth probe signal 108 has an intermediate beat frequency f that is proportional to the depth Z. B The sound is generated with a depth given by:

number

number

[0053] The relative merits of reflectometer and FM radar configurations can be understood by considering example frequencies. At 94 GHz (λ=3.19 mm) and 1 GHz tuning bandwidths typically available for commercial Gunn oscillators, the depth resolution of the reflectometer would be 0.8 mm (Eq. 1) and 150 mm (Eq. 3) for FM radar. The reflectometer is more suitable for small shallow boreholes (e.g., laboratory boreholes) that are less than a few times deeper than the FM radar resolution, while the FM radar is more suitable for deep boreholes in the field. Also, frequency measurements are more reliable for deep boreholes than amplitude changes that may vary for reasons other than phase changes.

[0054] In the time-of-flight configuration, a short electromagnetic pulse (with a pulse duration of full width at half maximum of τ) is transmitted towards the bottom of the borehole. The round trip time delay for the pulse to return to the surface electronics can be used to determine the distance to the bottom of the hole. The relationship is given by: Z = cΔτ / 2(4) where c is the velocity of the transmitted pulse and Δt is the round trip delay time. The spatial resolution depends on the pulse length, τ, and the velocity of the transmitted pulse. Δz=cτ(5) In air at atmospheric pressure, the propagation speed is the speed of light. For a 1 ns pulse, corresponding to available 1 GHz electronics, the resolution would be 300 mm.

[0055] Peak pulse power levels can be as high as 100 kW in pulsed operation. The high power and low spatial resolution with time-of-flight configuration will be suitable for the deepest boreholes ever drilled.

[0056] FIG. 4 shows the circuit details of the 94 GHz ROP / depth monitor 160, which can operate as a reflectometer or FM radar. The circuit is constructed with wr-10 waveguide components for use in the 75-110 GHz band. A voltage regulated 94 GHz ±0.5 GHz Gunn oscillator 410 is driven by an 8V, 800 mA power supply 405 and connected to a 10 dB directional coupler 415 through an isolator 412 that protects the Gunn oscillator from back reflections. The directional coupler directs 10% of the Gunn oscillator output to a bias mixer 418, powered by a 12V, 20 mA power supply 420, as a local oscillator for determining depth. The directional coupler directs the remaining 90% of the Gunn oscillator output to a three-port circulator 425 as a small signal ROP / depth probe signal. A three-port circulator 425 directs the probe signal from port 1 to port 2, which is coupled to a solid-state single-pole double-throw (SPDT) PIN switch 430 powered by a ±5V, 20mA power supply 440 and controlled by a transistor-transistor-transistor logic (TTL) signal provided on a control input 435. The PIN switch 430 has one output connected to a load 432 and another output connected via a waveguide component to a small single combiner 142, as shown in FIG. 3 and described above. The small signal combiner 142 combines the 94GHz ROP / depth probe signal with the high power transmission line 130 that leads to the borehole. The small signal combiner 142 may further direct the radiometric signal from the borehole and the high power transmission line 130 to a temperature radiometer for temperature monitoring. The combiner 142 also directs the return probe signal from the borehole to port 2 of a three-port circulator 425 which outputs the return probe signal via port 3 to the bias mixer 418 for detection. The return signal from the bias mixer can then go to a reflectometer or FM radar electronics for processing and determining the borehole depth.

[0057] The circuit of FIG. 4 can be operated as an FM radar if a frequency sweep voltage is applied to the Gunn oscillator and the PIN switch is set to transmit and receive signals continuously. The PIN switch 430 can also be removed from the circuit to reduce signal loss due to transmission and absorption in the switch. FIG. 5 shows the ROP / depth monitor 160 connected to a lock-in amplifier 510, a TTL signal generator 520, and data acquisition electronics 530 for operation as a reflectometer. The TTL generator typically provides a 5V square wave at a frequency above 100Hz to the control input 435 (FIG. 4) of the PIN switch, which alternately directs the probe signal to the target or load 432, which modulates the probe signal from the ROP / depth monitor 160 sent down the borehole. A feedback amplitude modulated (AM, on / off) reflectometer signal is acquired by the lock-in amplifier 510 using the signal from the TTL generator 520 as a reference. The use of the lock-in amplifier 510 allows for the detection of very weak signals. The output of the lock-in amplifier 510 is directed to a data acquisition system that may process, store, and / or display the signal from the lock-in amplifier.

[0058] FIG. 6 shows the ROP / depth monitor 160 connected to a voltage sweep generator 610, a frequency-to-voltage converter 620, and data acquisition electronics 530 for operation as an FM radar. The output from the sweep voltage generator 610 is applied to the Gunn oscillator 410 (FIG. 4) to modulate the probe frequency (e.g., linearly sweep or chirp the frequency of the probe signal). The bias mixer 418 mixes the resulting return probe signal with a local oscillator copy of the untransmitted swept frequency probe signal to generate a tone at the intermediate frequency (IF) port of the mixer whose beat frequency is proportional to the distance to the target (bottom of the borehole). This beat frequency may be picked up directly by the data acquisition electronics 530 for further processing or may be converted to a voltage by the frequency-to-voltage converter 620 and provided to the data acquisition electronics 530. The received data may be processed, displayed, and / or stored by the data acquisition electronics 530. When the ROP / depth monitor is configured for FM radar operation, the PIN switch 430 is maintained in a position that continuously transmits and receives signals (e.g., no switching signal to the load 432). In some implementations, the PIN switch 430 can be removed from the circuit to increase signal strength.

[0059] FIG. 7 shows an example of a signal detected by the ROP / depth monitor 160 when operated as a reflectometer. The target in this case is a flat lead brick on a motorized translation stage located about 18 cm from the launch horn connected to the reflectometer's wr-10 waveguide output (e.g., output from PIN switch 430), which may typically be coupled to a small signal combiner 142 as shown in FIG. 4. The target was translated in depth at a uniform rate of 0.5 mm / hr. The signal moves through a peak (max-min-max) every half a wavelength of 1.6 mm. FIG. 7 shows a total of six such fringes passing a total distance of 9.6 mm. In an actual borehole application on a melt surface target, the fringes may not be as uniform due to uneven, varying surfaces and / or non-uniform penetration rates. For example, if a portion of the surface at the bottom of a borehole changes height by more than about ¼ of a wavelength at the probe frequency, it may reflect a portion of the probe signal out of phase with the remainder of the probe signal, reducing the fringe peak signal strength (and fringe contrast).

[0060] FIG. 8 shows the reflectometer signal from a crater melted on a basalt surface by a 28 GHz borehole beam with a power of about 4.5 kW, about 40 mm in diameter, incident on the basalt surface. The system uses two reflectometer beams of different frequencies, one at the 28 GHz borehole beam frequency sampled from the return borehole beam radiation, and a separate 94 GHz monitor beam (e.g., both FIG. 1A and FIG. 1B together). The top plot is the reflectometer signal from the 94 GHz monitor 160, and the bottom plot is the reflectometer signal from a 28 GHz Schottky diode detector / mixer 184 coupled to a reflected power isolator 180 (FIG. 1B), which also samples the forward high power MMW radiation. The fringe peaks are not as ideal as shown in laboratory tests from flat solid surfaces. Both plots show about six 94 GHz peaks to about two 28 GHz peaks in the direct to probe signal wavelength ratio for the formation of a crater about 10 mm deep.

[0061] FIG. 9 shows a cross-section of a basalt crater 910 created by exposing solid basalt 920 to a borehole beam as described in connection with FIG. 8. The crater deposit is about 10 mm. Molten rock has solidified and created a reservoir in the crater filling a substantial portion of the borehole area. Having different reflectometer probe signals at different frequencies can reduce the uncertainty of monitoring non-ideal reflectometer signals. For example, the certainty of the depth measurement can be increased by detecting peaks according to the probe wavelength ratio of the different probe signals and cross-correlating the number of detected peaks.

[0062] Apparatus for measuring the depth or rate of penetration of a borehole drilled by a mmWave directed energy borehole beam may be implemented and / or included in a drilling system in a variety of configurations. Exemplary configurations are listed below. Corresponding methods for measuring the depth or rate of penetration may also be implemented. (1) A system for drilling a borehole comprising: a source for generating millimeter wave radiation; a transmission line coupled to the source for directing millimeter wave radiation to a bottom of the borehole and forming a millimeter wave borehole beam in a region at a distal end of the transmission line; a rate of penetration / depth monitor coupled to the transmission line for monitoring the depth and / or rate of penetration of the borehole; and a beam combiner coupled to the transmission line and to the rate of penetration / depth monitor for coupling a probe signal to the transmission line for transmission to the bottom of the borehole and for combining a return probe signal generated by reflection and / or scattering of the probe signal from the bottom of the borehole from the transmission line to the rate of penetration / depth monitor. (2) The system of (1), wherein the penetration rate / depth monitor is configured to operate as a reflectometer. (3) The system of (1), wherein the penetration rate / depth monitor is configured to operate as a frequency modulated radar. (4) The system of any one of (1) to (3), wherein the penetration rate / depth monitor is configured to operate as a pulse modulated time-of-flight radar. (5) The system of any one of (1) to (4), wherein the penetration rate / depth monitor is configured to generate a probe signal at a frequency different from the frequency of the millimeter wave radiation. (6) The system of any one of (1) to (5), wherein the beam combiner is configured to direct a portion of the millimeter wave radiation returning from the bottom of the borehole to the penetration rate / depth monitor as a return probe signal. (7) The system of any one of configurations (1) to (6), wherein the beam combiner comprises a miter mirror having a hole therein for reflecting the millimeter wave radiation around a bend in the transmission line and passing the probe signal. (8) The system of any one of configurations (1) to (7), further comprising: a temperature monitor for receiving radiation indicative of a temperature in the borehole; and a small signal beam combiner coupled to the transmission line for combining radiation from the transmission line, the radiation propagating along the transmission line with a return probe signal. (9) A method of measuring depth and / or rate of penetration of a borehole drilled by millimeter wave radiation directed to a bottom of the borehole by a transmission line and formed into a millimeter wave drilling beam, the method comprising: coupling a probe signal into the transmission line; directing the probe signal by the transmission line to a bottom of the borehole where at least a portion of the probe signal is reflected and / or scattered from the bottom of the borehole as a return probe signal; directing the return probe signal from the bottom of the borehole by the transmission line; coupling the return probe signal external to the transmission line; mixing the return probe signal with a local oscillator to generate an intermediate frequency signal; and determining the depth and / or rate of penetration of the borehole from the amplitude and / or frequency of the intermediate frequency signal. (10) The method of (9), further comprising modulating an amplitude of the probe signal and determining a borehole depth and / or penetration rate based on the amplitude of the intermediate frequency signal. (11) The method of (9), further comprising modulating a frequency of the probe signal and determining a borehole depth and / or penetration rate based on the frequency of the intermediate frequency signal. (12) The method of (9), further comprising forming the probe signal into at least one pulse and determining a borehole depth and / or rate of penetration based on a time of flight of the at least one pulse. (13) A method according to any one of (9) to (12), further comprising generating a probe signal at a frequency different from the frequency of the millimeter wave radiation. (14) A method according to any one of (9) to (12), further comprising forming the probe signal from a portion of millimeter wave radiation. (15) A method according to any one of (9) to (14), further comprising receiving radiation indicative of a temperature at the bottom of the borehole via the transmission line. (16) A method of forming a borehole with a millimeter wave drilling beam and determining a depth and / or rate of borehole penetration comprising: coupling millimeter wave radiation into a transmission line; coupling a probe signal into the transmission line; directing the millimeter wave radiation and the probe signal to a bottom of the borehole with the transmission line; forming a millimeter wave drilling beam at a distal end of the transmission line; increasing the depth of the borehole with the millimeter wave drilling beam; directing a return probe signal from the bottom of the borehole with the transmission line, the return probe signal being at least a portion of the probe signal that is reflected and / or scattered from the bottom of the borehole; coupling the return probe signal external to the transmission line; mixing the return probe signal with a local oscillator to generate an intermediate frequency signal; and determining the depth and / or rate of borehole penetration from the amplitude and / or frequency of the intermediate frequency signal. (17) The method of (16), further comprising modulating an amplitude or frequency of the probe signal and determining a borehole depth and / or penetration rate based on the amplitude of the intermediate frequency signal. (18) The method of (16), further comprising forming the probe signal into at least one pulse and determining a borehole depth and / or rate of penetration based on a time of flight of the at least one pulse. (19) The method of any one of (16)-(18), further comprising: while drilling with the millimeter wave drilling beam, coupling a first temperature signal radiating from a bottom of the borehole into a transmission line; coupling the first temperature signal from the transmission line into a temperature monitor; determining a first temperature with the temperature monitor; adjusting an amount of power in the millimeter wave radiation based on the first temperature; ceasing to direct the millimeter wave radiation to the bottom of the borehole, allowing the bottom of the borehole to reach a lower temperature; coupling at least a second temperature signal radiating from the bottom of the borehole into the transmission line; coupling the at least a second temperature signal from the transmission line into the temperature monitor; determining at least a second temperature with the temperature monitor; and determining whether the borehole has reached a sufficient depth to access geothermal heat based on the at least the second temperature.

[0063] 4. Conclusion All parameters, dimensions, materials, and configurations described herein are meant to be exemplary, and the actual parameters, dimensions, materials, and / or configurations will depend on the particular application in which the teachings of the present invention are used. It is to be understood that the foregoing embodiments are presented primarily by way of example, and that within the scope of the appended claims and equivalents thereof, the embodiments of the present invention may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and / or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and / or methods is included within the inventive scope of the present disclosure, if such features, systems, articles, materials, kits, and / or methods are not mutually inconsistent.

[0064] Also, various inventive concepts may be embodied as one or more methods, of which at least one example is provided. The acts performed as part of a method may, in some cases, be ordered in a different manner. Thus, in some inventive implementations, the acts of a given method may be performed in an order different than that specifically illustrated, which may include performing some acts simultaneously (even though such acts are shown as sequential acts in an illustrative embodiment).

[0065] All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.

[0066] All definitions and uses herein should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and / or ordinary meanings of the defined terms.

[0067] As used herein, in the specification and the claims, the indefinite articles "a" and "an" should be understood to mean "at least one," unless clearly indicated to the contrary.

[0068] The term "and / or" as used in the specification and claims should be understood to mean "either or both" of the elements so conjoined, i.e., elements that are conjunctive in some cases and disjunctive in other cases. Multiple elements listed with "and / or" should be construed in the same manner, i.e., "one or more" of the elements so conjoined. Other elements, whether related or unrelated to the elements specifically identified, other than the elements specifically identified may optionally be present. Thus, as a non-limiting example, a reference to "A and / or B," when used in combination with open-ended language such as "comprising," can refer in one embodiment to only A (optionally including elements other than B), in another embodiment to only B (optionally including elements other than A), in yet another embodiment to both A and B (optionally including other elements), etc.

[0069] "Or" as used herein and in the claims should be understood to have the same meaning as "and / or" as defined above. For example, when separating items in a list, "or" or "and / or" shall be interpreted as being inclusive, i.e., including at least one of, but also including two or more of, some or the listed elements, and optionally, other unlisted items. Terms clearly indicated to the contrary, such as "only one of" or "exactly one of," or "consisting of," when used in the claims, only refer to the inclusion of exactly one element of some or the listed elements. In general, the term "or" as used herein shall only be interpreted as indicating exclusive alternatives (i.e., "one or the other but not both") when preceded by a term of exclusivity, such as "either," "one of," "only one of," or "exactly one of." "Consisting essentially of," as used in the claims, shall have its ordinary meaning as used in the field of patent law.

[0070] As used herein and in the claims, the term "at least one" in connection with a list of one or more elements should be understood to mean at least one element selected from one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed in the list of elements, and not excluding any combination of elements in the list of elements. This definition also allows for the optional presence of elements other than those specifically identified in the list of elements to which the phrase "at least one" refers, whether related or unrelated to the specifically identified element. Thus, as a non-limiting example, "at least one of A and B" (or, equivalently, "at least one of A or B" or, equivalently, "at least one of A and / or B") can refer in one embodiment to at least one, optionally two or more, A, and no B (and optionally including elements other than B); in another embodiment to at least one, optionally two or more, B, and no A (and optionally including elements other than A); in yet another embodiment to at least one, optionally two or more, A, and at least one, optionally two or more, B (and optionally including other elements); and so forth.

[0071] In the claims, as well as in the above specification, all transitional phrases, such as "comprising," "including," "holding," "having," "including," "involving," "holding," "comprising," and the like, are to be understood to be open-ended, i.e., meaning including but not limited to. Only the transitional phrases "consisting of" and "consisting essentially of" shall be closed or semi-closed transitional phrases, respectively, as defined in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.

Claims

1. 1. A system for drilling a borehole, comprising: a source for generating millimeter wave radiation; a transmission line coupled to the source for directing the millimeter wave radiation to the bottom of the borehole and forming a millimeter wave borehole beam in a region of a distal end of the transmission line; a rate of penetration / depth monitor coupled to said transmission line for monitoring the depth and / or rate of penetration of said borehole; a beam combiner coupled to said transmission line and to said rate / depth monitor for directing a probe signal into said transmission line for transmission to the bottom of the borehole and for directing a return probe signal generated by reflection and / or scattering of the probe signal from the bottom of the borehole from said transmission line to said rate / depth monitor.

2. The system of claim 1 , wherein the penetration rate / depth monitor is configured to operate as a reflectometer.

3. The system of claim 1 , wherein the penetration rate / depth monitor is configured to operate as a frequency modulated radar.

4. The system of claim 1 , wherein the penetration rate / depth monitor is configured to operate as a pulse-modulated time-of-flight radar.

5. The system of claim 1 , wherein the penetration rate / depth monitor is configured to generate the probe signal at a frequency different from a frequency of the millimeter wave radiation.

6. 10. The system of claim 1, wherein said beam combiner is configured to direct a portion of said millimeter wave radiation returned from said bottom of said borehole to said rate of penetration / depth monitor as said return probe signal.

7. 10. The system of claim 1, wherein the beam combiner comprises a miter mirror having a hole therein to reflect the millimeter-wave radiation around a bend in the transmission line and to pass the probe signal.

8. a temperature monitor for receiving radiation indicative of a temperature in the borehole; 10. The system of claim 1, further comprising: a small signal beam combiner coupled to the transmission line for guiding the radiation from the transmission line, the radiation propagating along the transmission line due to the return probe signal.

9. 1. A method of measuring depth and / or rate of penetration of a borehole drilled by millimeter wave radiation directed by a transmission line to the bottom of said borehole and formed into a millimeter wave drilling beam, comprising: inducing a probe signal onto the transmission line; directing the probe signal by the transmission line to the bottom of the borehole, wherein at least a portion of the probe signal is reflected and / or scattered from the bottom of the borehole as a return probe signal; directing said return probe signal from said bottom of said borehole by said transmission line; directing the return probe signal external to the transmission line; mixing the returned probe signal with a local oscillator signal to generate an intermediate frequency signal; and determining the depth and / or the rate of penetration of the borehole from the amplitude and / or frequency of the intermediate frequency signal.

10. The method of claim 9 , further comprising modulating the amplitude of the probe signal.

11. The method of claim 9 further comprising modulating the frequency of the probe signal.

12. forming the probe signal into at least one pulse; and 10. The method of claim 9, further comprising determining the depth and / or the rate of penetration of the borehole based on a time of flight of the at least one pulse.

13. 10. The method of claim 9, further comprising generating the probe signal at a frequency different from a frequency of the millimeter-wave radiation.

14. The method of claim 9 further comprising forming the probe signal from a portion of the millimeter-wave radiation.

15. 10. The method of claim 9, further comprising receiving radiation indicative of a temperature at the bottom of the borehole via the transmission line.

16. 1. A method of forming a borehole with a millimeter wave drilling beam and determining the depth and / or rate of penetration of said borehole, comprising: Guiding millimeter wave radiation into a transmission line; inducing a probe signal onto the transmission line; directing said millimeter wave radiation and said probe signal to the bottom of said borehole by said transmission line; forming the millimeter-wave borehole beam at a distal end of the transmission line; increasing the depth of the borehole with the millimeter wave drilling beam; directing a return probe signal from the bottom of the borehole by said transmission line, said return probe signal being at least a portion of said probe signal that is reflected and / or scattered from the bottom of the borehole; directing the return probe signal external to the transmission line; mixing the returned probe signal with a local oscillator signal to generate an intermediate frequency signal; and determining the depth and / or the rate of penetration of the borehole from the amplitude and / or frequency of the intermediate frequency signal.

17. The method of claim 16 , further comprising modulating the amplitude or frequency of the probe signal.

18. forming the probe signal into at least one pulse; and 17. The method of claim 16, further comprising determining the depth and / or the rate of penetration of the borehole based on a time of flight of the at least one pulse.

19. directing a first temperature signal emanating from the bottom of the borehole onto the transmission line while drilling with the millimeter wave drilling beam; directing the first temperature signal from the transmission line to a temperature monitor; determining a first temperature with said temperature monitor; adjusting the amount of power in the millimeter wave radiation based on the first temperature; ceasing the directing of the millimeter wave radiation to the bottom of the borehole; allowing the bottom of the borehole to reach lower temperatures; directing at least a second temperature signal emanating from the bottom of the borehole into the transmission line; directing at least the second temperature signal from the transmission line to a temperature monitor; determining at least a second temperature with said temperature monitor; and 17. The method of claim 16, further comprising determining whether the borehole has reached a sufficient depth to access geothermal heat based on at least the second temperature.

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