A directional borehole radar antenna and its implementation method

Abstract

This invention relates to a directional borehole radar antenna and its implementation method, comprising a metal radiating plate, a dielectric substrate, a loading resistor, a loading dielectric, a metal reflector, and a loading capacitor. Distributed resistance loading is applied to the antenna arm, defining the resistance value of the end loading resistor and optimizing other resistors; an exponentially tapered structure is set at the feed point to suppress changes in the antenna input impedance, and a semi-elliptical arc structure is set at the end; a loading dielectric and a reflector are introduced, the distance between the metal reflector and the antenna is adjusted according to the dielectric constant of the dielectric substrate, and the antenna beamwidth is also adjusted; the borehole radar is placed inside the borehole and a trigger pulse is applied at a specified time, causing the antenna to transmit and receive electromagnetic waves around the borehole. The borehole radar moves along the borehole, successively acquiring all stratigraphic information of the entire borehole. This achieves directional transmission and reception of electromagnetic waves, or directional and precise detection of target objects within a narrow frequency band and around the borehole.

Description

Technical Field

[0001] This invention belongs to the field of antenna design technology for ground-penetrating radar, and particularly relates to a directional borehole radar antenna and its implementation method. Background Technology

[0002] Borehole radar, a special type of ground-penetrating radar, uses a ground-penetrating radar antenna placed inside a borehole to conduct close-range detection of the target. Electromagnetic waves propagate through the strata from the borehole, acquiring information about the surrounding geological formations and thus interpreting the underground structure. Its advantages lie in its high spatial resolution and large detection depth. Borehole radar detection is divided into single-hole detection and cross-hole detection. In single-hole detection, the transmitting and receiving antennas receive echo signals within the same borehole to detect the target. In cross-hole detection, the transmitting and receiving antennas are separated, and the stratigraphic characteristics between the two boreholes are interpreted by measuring and comparing the amplitude and time difference of the transmitted and receiving antenna signals. Single-hole measurement is less expensive than cross-hole measurement, but in single-hole measurement, using an omnidirectional antenna only provides distance information to the target, not azimuth information.

[0003] Ground-penetrating radar (GPR) is a geophysical exploration tool characterized by high resolution and deep penetration. GPR works by emitting electromagnetic waves into the ground and receiving the echo signals. By analyzing the propagation characteristics of the echo signals through the strata, it obtains information about the subsurface strata. Borehole radar is a special type of GPR, typically placed inside a borehole to measure along the borehole wall and acquire information about the surrounding strata.

[0004] Currently, borehole radar mainly employs two measurement methods: single-hole measurement and cross-hole measurement. The former places the borehole radar within a single borehole. Time-triggered pulses activate the radar's antenna, which then transmits and receives electromagnetic waves around the borehole. The radar moves along the borehole, sequentially acquiring information about all formations within the borehole. The latter places two borehole radars, one transmitting and the other receiving. Unlike single-hole measurement, in this method, the transmitting radar remains stationary at a certain depth. The receiving radar moves sequentially along the borehole to acquire information about multiple formations at that depth. Once the receiving radar reaches the borehole opening, it is repositioned, its transmission depth changed, and the receiving radar is moved again for measurement. This process is repeated until the transmitting radar reaches the borehole opening, completing the measurement. It is evident that single-hole measurement is simple to implement and easy to operate, making it the primary choice for advanced geological prediction, geological exploration, and other fields.

[0005] Dipole antennas are the most commonly used antennas in borehole radar. Conventional dipole antennas radiate electromagnetic waves in all directions, which cannot provide accurate positioning for target detection, making it difficult to achieve directional borehole detection.

[0006] To achieve the directionality of borehole radar, existing methods mainly include: analyzing the phase difference of the receiving antenna array inside the borehole to achieve directional detection, and designing directional antennas to determine the target by directional transmission and reception of electromagnetic waves inside the borehole.

[0007] Hole-hole array antenna localization involves placing multiple receiving antennas at specific intervals within the same borehole. Since each antenna receives the same target signal at different time differences, the direction from which the target signal originates can be inferred by analyzing the phase difference, thus achieving localization detection. However, for detecting distant targets, the wavelength used in borehole radar is typically much larger than the spacing between the array antennas within the borehole, making phase difference extraction quite difficult. Therefore, this method is not suitable for long-range detection applications using borehole radar.

[0008] However, while current borehole radars all support single-hole measurement mode, most lack directionality in detecting objects. Specifically, because current borehole radars typically use omnidirectional dipole antennas for transmission and reception, the transmitted and received electromagnetic waves lack azimuth information. Therefore, while a single-hole measurement borehole radar can determine the presence of an object at a certain distance from the borehole depth and from the borehole wall, it cannot further determine the object's azimuth angle with respect to the borehole axis. Thus, the information acquired by borehole radars in single-hole measurement mode is limited. Therefore, improving the directionality of the antennas in single-hole measurement mode to achieve directional transmission and reception of electromagnetic waves, or directionality limited to a narrow frequency band, and accurate detection of targets around the borehole, is a pressing technical problem that needs to be solved. Summary of the Invention

[0009] The purpose of this invention is to provide a directional borehole radar antenna and its implementation method, which improves the directionality of the antenna of a borehole radar in single-hole measurement mode, enabling directional transmission and reception of electromagnetic waves or directional transmission within a narrow frequency band of a single frequency, as well as accurate detection of target objects around the borehole.

[0010] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0011] In a first aspect, a directional borehole radar antenna is provided, comprising a metal radiating plate, a dielectric substrate, a loading resistor, a loading dielectric, a metal reflector, and a loading capacitor. The loading dielectric is a semi-cylinder, one side of its rectangular surface is connected to the dielectric substrate, and the outside is wrapped by a metal reflector that matches the arc surface of the semi-cylinder. The metal radiating plate is attached to the surface of the dielectric substrate, and the two ends of the metal radiating plate are electrically connected to the metal reflector through a series loading capacitor.

[0012] Preferably, the antenna has a length of L = 200 mm, a width of W = 60 mm, and a thickness of 32 mm.

[0013] Preferably, the metal radiating sheet consists of two individual sheets, each sheet having an exponentially gradient structure at the front end and a semi-elliptical structure at the rear end, and the two individual sheets are connected by a power supply.

[0014] Preferably, the front-end exponential gradient structure is derived from the formula y = c·e gx +d is determined, where the gradient index g = 0.15, the length of the index segment is 90mm, the semi-major axis of the semi-elliptical structure at the end is a = 30mm, the semi-minor axis is b = 9.5mm, the feed gap in the middle of the metal radiating sheet is w_gap = 1mm, the substrate material of the radiating sheet is FR-4, the dielectric constant of the substrate is 4.6, and the loss tangent of the substrate is 0.019.

[0015] Preferably, each of the single chips is provided with a set of loading resistors, each set of loading resistors has at least 5 resistors, and the two sets of resistors are symmetrically distributed with the power supply as the center.

[0016] Preferably, the distance between the loading resistor closest to the feed point and the feed point in each group of loading resistors is 20mm, and the distance between adjacent loading resistors is 15mm.

[0017] Secondly, a method for implementing a directional borehole radar antenna, based on the aforementioned directional borehole radar antenna, includes the following steps:

[0018] S1: Distributed resistance loading is applied to the antenna arm of the directional borehole radar antenna. The formula for distributed resistance loading is as follows:

[0019]

[0020] Among them, R i ψ is the resistance value of the i-th resistor, ψ is the resistance value of the end-loaded resistor, Lf is the length of a single antenna arm, and p i Let be the distance between the i-th resistor and the feed point, and l be the spacing between adjacent resistors;

[0021] S2: Defines the resistance value of the end-loaded resistor; the resistance values ​​of other resistors change with ψ. Optimize other resistors;

[0022] S3: An exponentially tapered structure is set at the feed point to suppress changes in the antenna input impedance, reduce signal reflection at both arms of the antenna, and a semi-elliptical arc structure is set at the end.

[0023] S4: Introduce a loading medium and a reflector. The distance between the metal reflector and the antenna is adjusted according to the change in the dielectric constant of the substrate, and the antenna beamwidth is adjusted according to the central angle corresponding to the metal reflector.

[0024] S5: The adjusted directional borehole radar antenna is placed inside the borehole and a trigger pulse is applied at a specified time. The directional borehole radar antenna emits and receives electromagnetic waves around the borehole. The borehole radar moves along the borehole and obtains all the strata information of the entire borehole one by one.

[0025] The beneficial effects of this invention include:

[0026] This invention provides a directional borehole radar antenna and its implementation method, comprising a metal radiating plate, a dielectric substrate, a loading resistor, a loading dielectric, a metal reflector, and a loading capacitor. Distributed resistance loading is applied to the antenna arm, defining the resistance value of the end loading resistor and optimizing other resistors. An exponentially tapered structure is set at the feed point to suppress changes in the antenna input impedance, and a semi-elliptical arc structure is set at the end. A loading dielectric and a reflector are introduced; the distance between the metal reflector and the antenna is adjusted according to the change in the dielectric constant of the dielectric substrate, and the antenna beamwidth is also adjusted. The borehole radar is placed inside the borehole and a trigger pulse is applied at a specified time. The antenna transmits and receives electromagnetic waves around the borehole, and the borehole radar moves along the borehole, successively obtaining all strata information of the entire borehole. This achieves directional transmission and reception of electromagnetic waves, or directional and precise detection of target objects within a narrow frequency band and around the borehole.

[0027] First, by applying distributed resistance loading to each antenna arm, based on the Wu-King loading theory, the resistance value of the end loading resistor is first defined. The resistance values ​​of other resistors will change with ψ. This resistance value is optimized. Distributed resistance loading weakens the frequency selectivity of the antenna, thereby improving the overall bandwidth and enabling the antenna to meet the wideband requirements.

[0028] Secondly, by employing an exponentially tapered structure at the feed point, the variation in antenna input impedance is suppressed, reducing signal reflection at both arms of the antenna. The value of the tapering exponent g needs to be determined through optimization. The semi-elliptical arc structure at the end helps to reduce the deterioration of the VSWR caused by current cutoff, with the semi-minor axis b being the main parameter affecting the VSWR.

[0029] Finally, by introducing a loading medium and a reflector into the antenna structure, the dielectric constant of the loading medium affects the wavelength of the electromagnetic wave between the antenna and the reflector. Therefore, the material selection affects the overall directional performance. The distance between the reflector and the antenna is adjusted as the dielectric constant of the dielectric plate changes. The central angle corresponding to the arc length of the reflector affects the final designed antenna beamwidth, thereby effectively improving the directional performance of the antenna. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the structural composition of the directional borehole radar antenna of the present invention.

[0031] Figure 2 This is a schematic diagram of the disassembled structure of the directional borehole radar antenna of the present invention.

[0032] Figure 3 This is a schematic diagram of the parameter curves for the directional borehole radar antenna of the present invention.

[0033] Figure 4 This is the far-field E-plane radiation pattern of the directional drilling radar antenna of the present invention.

[0034] Figure 5 This is the far-field H-plane radiation pattern of the directional borehole radar antenna of the present invention. Detailed Implementation

[0035] The following is in conjunction with the appendix Figures 1-5 The present invention will be further described in detail below:

[0036] Example 1

[0037] See appendix Figure 1 and Figure 2 As shown, a directional borehole radar antenna includes a metal radiating plate, a dielectric substrate, a loading resistor, a loading dielectric, a metal reflector, and a loading capacitor. The loading dielectric is a semi-cylinder, and one side of its rectangular surface is connected to the dielectric substrate. The outside is wrapped by a metal reflector that matches the curved surface of the semi-cylinder. The metal radiating plate is attached to the surface of the dielectric substrate, and the two ends of the metal radiating plate are electrically connected to the metal reflector through a series loading capacitor.

[0038] In this embodiment, the antenna has a length L = 200 mm, a width W = 60 mm, and a thickness of 32 mm. The metal radiating plate consists of two monoliths, each monolith comprising a front-end exponentially tapered structure and a rear-end semi-elliptical structure, connected by a feed. The front-end exponentially tapered structure is defined by the formula y = c·e. gx+d is determined, where the gradient index g = 0.15, the length of the index segment is 90mm, the semi-major axis of the semi-elliptical structure at the end is a = 30mm, the semi-minor axis is b = 9.5mm, the feed gap in the middle of the metal radiating sheet is w_gap = 1mm, the substrate material of the radiating sheet is FR-4, the dielectric constant of the substrate is 4.6, and the loss tangent of the substrate is 0.019.

[0039] The loading medium 1-2 is connected to the dielectric substrate 1-3. The dielectric substrate is made of marble with a dielectric constant of 9. In order to suit the environment inside the hole, the shape is set as a semi-cylinder. The outside is connected to the metal reflector. The thickness of the metal reflector is 0.5mm and the fan angle of the metal reflector is 150°. The two ends of the metal radiating sheet are connected to the metal reflector through series loading capacitors with a capacitance value of 12pF.

[0040] Five resistors were applied to a single radiating sheet using a distributed resistor loading method. The resistance values ​​of the five resistors were 3.39Ω, 4.18Ω, 5.45Ω, 7.82Ω, and 18Ω, respectively.

[0041] Example 2

[0042] Based on Embodiment 1, each of the single-chip chips is provided with a set of loading resistors, and each set of loading resistors has at least 5 resistors. The two sets of resistors are symmetrically distributed with the power supply as the center. In each set of loading resistors, the distance between the loading resistor closest to the power supply and the power supply is 20mm, and the distance between adjacent loading resistors is 15mm.

[0043] Example 3

[0044] Based on Embodiment 1 or Embodiment 2, a method for implementing a directional borehole radar antenna, comprising the following steps:

[0045] S1: Distributed resistance loading is applied to the antenna arm of the directional borehole radar antenna. The formula for distributed resistance loading is as follows:

[0046]

[0047] Among them, R i ψ is the resistance value of the i-th resistor, ψ is the resistance value of the end-loaded resistor, Lf is the length of a single antenna arm, and p i Let be the distance between the i-th resistor and the feed point, and l be the spacing between adjacent resistors;

[0048] S2: Defines the resistance value of the end-loaded resistor; the resistance values ​​of other resistors change with ψ. Optimize other resistors;

[0049] S3: An exponentially tapered structure is set at the feed point to suppress changes in the antenna input impedance, reduce signal reflection at both arms of the antenna, and a semi-elliptical arc structure is set at the end.

[0050] S4: Introduce a loading medium and a reflector. The distance between the metal reflector and the antenna is adjusted according to the change in the dielectric constant of the substrate, and the antenna beamwidth is adjusted according to the central angle corresponding to the metal reflector.

[0051] S5: The adjusted directional borehole radar antenna is placed inside the borehole and a trigger pulse is applied at a specified time. The directional borehole radar antenna emits and receives electromagnetic waves around the borehole. The borehole radar moves along the borehole and obtains all the strata information of the entire borehole one by one.

[0052] In this embodiment, distributed resistance loading is applied to each antenna arm. The resistance value of the end-loading resistor is first defined, and the resistance values ​​of other resistors change with ψ. This resistance value is optimized. Distributed resistance loading weakens the antenna's frequency selectivity, thereby improving the overall bandwidth and enabling the antenna to meet wideband requirements. The feed point adopts an exponentially gradient structure to suppress changes in the antenna input impedance and reduce signal reflection at the antenna arms. The value of the gradient exponent g needs to be determined through optimization. The end adopts a semi-elliptical arc structure, which helps reduce the deterioration of the VSWR due to current cutoff. The semi-minor axis b is a key parameter affecting the VSWR.

[0053] In the antenna structure, a loading medium and a reflector are introduced. The dielectric constant of the loading medium affects the wavelength of the electromagnetic wave between the antenna and the reflector, so the material selection affects the overall directional performance. In addition, the distance between the reflector and the antenna needs to be adjusted as the dielectric constant of the medium changes. The central angle corresponding to the arc length of the reflector affects the final antenna beamwidth. These parameters need to be optimized during the design process.

[0054] See Figure 3 The above model was simulated using electromagnetic simulation software, and the S11 parameters were obtained as follows: Figure 3 As shown, the antenna's operating bandwidth range is 118MHz-403MHz (-8dB), with a relative bandwidth of 109%, classifying it as a broadband antenna.

[0055] To verify the antenna's directivity, far-field monitors were set up at frequencies of 120MHz, 200MHz, 260MHz (center frequency), 280MHz, 360MHz, and 400MHz. The far-field E-plane and H-plane radiation patterns of the antenna were obtained as follows: Figure 4 and Figure 5 As shown. Figure 4The radiation patterns of the E-plane are: (a) 120MHz; (b) 200MHz; (c) 260MHz; (d) 280MHz; (e) 360MHz; (f) 400MHz. Figure 5 H-plane radiation patterns: (a) 120MHz; (b) 200MHz; (c) 260MHz; (d) 280MHz; (e) 360MHz; (f) 400MHz.

[0056] according to Figure 4 and Figure 5 It is known that the antenna has a 3dB beamwidth of less than 150° in the E-plane within a certain frequency range, exhibiting good directivity, especially at a center frequency of 260MHz where the 3dB beamwidth is 93.6°. However, the antenna's directivity in the H-plane is weaker than in the E-plane. This invention uses a butterfly antenna as the basic antenna, designed within the constraint of an antenna width of less than 60mm, achieving the following effective effects: the entire antenna width does not exceed the borehole diameter, forming a semi-cylindrical shape that conforms to the borehole wall, allowing the entire system to be smoothly placed into the borehole. Based on Wu-king loading, the current is changed by using distributed loading resistors to broaden the antenna bandwidth, enabling the antenna to operate in a wide frequency range of 118MHz-403MHz (109%). The loading medium reduces the wavelength of electromagnetic waves, and a reflector is used to reflect back electromagnetic waves, achieving far-field directivity, thus enabling directional detection of underground targets in conjunction with a radio frequency system.

[0057] In summary, the directional borehole radar antenna and its implementation method provided by this invention include a metal radiating plate, a dielectric substrate, a loading resistor, a loading dielectric, a metal reflector, and a loading capacitor. Distributed resistance loading is applied to the antenna arm, defining the resistance value of the end loading resistor and optimizing other resistors. An exponentially tapered structure is set at the feed point to suppress changes in the antenna input impedance, and a semi-elliptical arc structure is set at the end. A loading dielectric and a reflector are introduced; the distance between the metal reflector and the antenna is adjusted according to the change in the dielectric constant of the dielectric substrate, and the antenna beamwidth is also adjusted. The borehole radar is placed inside the borehole and a trigger pulse is applied at a specified time. The antenna transmits and receives electromagnetic waves around the borehole, and the borehole radar moves along the borehole, successively obtaining all the strata information of the entire borehole. This achieves directional transmission and reception of electromagnetic waves or directional, narrow-band detection of targets around the borehole.

[0058] Based on the Wu-King loading theory, distributed resistance loading is applied to each antenna arm. First, the resistance value of the end-loading resistor is defined. The resistance values ​​of other resistors change with ψ. Optimizing this resistance value weakens the antenna's frequency selectivity, thereby improving the overall bandwidth and enabling the antenna to meet wideband requirements. An exponentially gradient structure at the feed point suppresses changes in the antenna's input impedance, reducing signal reflection at both arms. The value of the gradient exponent g needs to be determined through optimization. A semi-elliptical arc structure at the end helps reduce the deterioration of the VSWR due to current cutoff. The semi-minor axis b is a key parameter affecting the VSWR. By introducing a loading medium and a reflector into the antenna structure, the dielectric constant of the loading medium affects the wavelength of electromagnetic waves between the antenna and the reflector. Therefore, the material selection affects the overall directional performance. The distance between the reflector and the antenna is adjusted according to the dielectric constant of the dielectric material. The central angle corresponding to the reflector arc length affects the final designed antenna beamwidth, thus effectively improving the antenna's directional performance.

Claims

1. A directional borehole radar antenna, characterized in that, The device includes a metal radiating sheet, a dielectric substrate, a loading resistor, a loading dielectric, a metal reflector, and a loading capacitor. The loading dielectric is a semi-cylinder, and one side of its rectangular surface is connected to the dielectric substrate. It is wrapped by a metal reflector that matches the curved surface of the semi-cylinder. The metal radiating sheet is attached to the surface of the dielectric substrate, and the two ends of the metal radiating sheet are electrically connected to the metal reflector through a series loading capacitor. The metal radiating sheet consists of two single pieces, each piece consisting of an exponentially gradient structure at the front end and a semi-elliptical structure at the rear end, and the two single pieces are connected by a power supply. The front-end exponential gradient structure is derived from the formula. It is determined that the gradient index g = 0.15, the length of the index segment is 90 mm, the semi-major axis of the semi-elliptical structure at the end is a = 30 mm, the semi-minor axis is b = 9.5 mm, the feed gap in the middle of the metal radiating sheet is w_gap = 1 mm, the substrate material of the radiating sheet is FR-4, the dielectric constant of the substrate is 4.6, and the loss tangent of the substrate is 0.

019.

2. A directional borehole radar antenna according to claim 1, characterized in that, The antenna has a length of L=200 mm, a width of W=60 mm, and a thickness of 32 mm.

3. A directional borehole radar antenna according to claim 1, characterized in that, Each of the aforementioned single chips is provided with a set of loading resistors, and each set of loading resistors has at least 5 resistors. The two sets of resistors are symmetrically distributed with the power supply as the center.

4. A directional borehole radar antenna according to claim 3, characterized in that, In each group of loading resistors, the distance between the loading resistor closest to the feed point and the feed point is 20mm, and the distance between adjacent loading resistors is 15mm.

5. A method for implementing a directional borehole radar antenna, based on the directional borehole radar antenna described in any one of claims 1-4, characterized in that, Includes the following steps: S1: Distributed resistance loading is applied to the antenna arm of the directional borehole radar antenna. The formula for distributed resistance loading is as follows: ； in, R i It is the resistance value of the i-th resistor. ψ The resistance value of the resistor applied at the end. Lf The length of a single antenna arm. p i For the first i The distance between the resistor and the feed point l The spacing between adjacent resistors; S2: Defines the resistance value of the end-loaded resistor; the resistance values ​​of other resistors follow the same logic. ψ Change for the sake of change, through Optimize other resistors; S3: An exponentially tapered structure is set at the feed point to suppress changes in the antenna input impedance, reduce signal reflection at both arms of the antenna, and a semi-elliptical arc structure is set at the end. S4: Introduce a loading medium and a metal reflector. The distance between the metal reflector and the antenna is adjusted according to the change in the dielectric constant of the substrate, and the antenna beamwidth is adjusted according to the central angle corresponding to the metal reflector. S5: The adjusted directional borehole radar antenna is placed inside the borehole and a trigger pulse is applied at a specified time. The directional borehole radar antenna emits and receives electromagnetic waves around the borehole. The borehole radar moves along the borehole and obtains all the strata information of the entire borehole one by one.

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