Drilling radar antenna with directionality and implementation method thereof

Abstract

The invention relates to a directional drilling radar antenna and an implementation method thereof. The directional drilling radar antenna comprises a metal radiation sheet, a dielectric substrate, a loading resistor, a loading medium, a metal reflecting plate and a loading capacitor. Carrying out distributed resistance loading on the antenna arm, defining the resistance value of a loading resistor at the tail end, and optimizing other resistors; an exponential gradual change structure is arranged at a feeding point to suppress the input impedance change of the antenna, and a semi-elliptical arc structure is arranged at the tail end; a loading medium and a reflecting plate are introduced, the distance between the metal reflecting plate and the antenna is adjusted along with the change of the dielectric constant of the dielectric substrate, and the beam width of the antenna is adjusted; the drill hole radar is placed in a drill hole to trigger pulses at specified time, the antenna transmits electromagnetic waves to the periphery of the drill hole and receives the electromagnetic waves, and the drill hole radar moves along the drill hole to successively obtain all stratum information of the whole drill hole. Electromagnetic wave directional emission and receiving or directional accurate detection of target objects in a narrow frequency band of a single frequency and around a drill hole is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of antenna design of ground penetrating radar, and particularly relates to a directional borehole radar antenna and a method for realizing the same. Background Art

[0002] As a special detection means of ground penetrating radar, a borehole radar places a ground penetrating radar antenna in a borehole to approach a detection target for close-range detection; electromagnetic waves enter the formation from the borehole and propagate, and formation information near the borehole is obtained, thereby interpreting the underground structure around the borehole; the advantage is that it has a relatively high spatial resolution and a relatively large detection depth. Borehole radar detection is divided into single-hole detection and cross-hole detection. The transmitting and receiving antennas of the borehole radar in single-hole detection receive echo signals in the same borehole to achieve target detection, while in cross-hole detection, the transmitting and receiving antennas are separated, and the amplitude and time difference of the signals of the transmitting and receiving antennas are measured and compared to interpret the formation characteristics between the two holes. Single-hole measurement has a lower operation cost than cross-hole measurement, but in single-hole measurement, using an omnidirectional antenna can only obtain the distance information of the target and cannot obtain the azimuth information.

[0003] Ground penetrating radar is one of the geophysical exploration tools, with characteristics such as high resolution and large detection depth. The working principle of ground penetrating radar is to transmit electromagnetic waves to the ground and receive echo signals, and by analyzing the propagation characteristics of the echo signals in the formation, the formation information under the ground surface is obtained. A borehole radar is a special form of ground penetrating radar, which is usually placed in a borehole and measures along the borehole wall to obtain the formation information around the borehole.

[0004] Currently, there are mainly two measurement methods for borehole radar, namely single-hole measurement and cross-hole measurement. In the former, the borehole radar is placed in a borehole, and through a time-triggered pulse, the antenna of the borehole radar emits and receives electromagnetic waves around the borehole. The borehole radar moves along the borehole to successively obtain all the formation information of the entire borehole. In the latter, two borehole radars are respectively placed in two boreholes, where one borehole radar is responsible for emitting electromagnetic waves and the other borehole radar is responsible for receiving electromagnetic waves. Different from single-hole measurement, during operation, the borehole radar responsible for emitting electromagnetic waves is fixed at a certain depth, and the other borehole radar responsible for receiving electromagnetic waves is successively moved along the borehole to obtain multiple formation information at this depth. After the borehole radar responsible for receiving electromagnetic waves moves to the borehole mouth, it is reinserted into the borehole, the depth of the borehole radar responsible for emitting electromagnetic waves is changed, and the borehole radar responsible for receiving electromagnetic waves is moved again for measurement. The above process is repeated until the borehole radar responsible for emitting electromagnetic waves reaches the borehole mouth and the measurement is completed. It can be seen that single-hole measurement is simple to implement and easy to operate, so single-hole measurement is the primary choice in the fields of advanced geological prediction, geological exploration, etc. at present.

[0005] The dipole antenna is the most commonly used antenna for borehole radar. The conventional dipole antenna radiates electromagnetic waves omnidirectionally and cannot provide precise positioning for target detection. Therefore, it has become a difficult problem to achieve borehole directional detection.

[0006] To achieve the directivity of borehole radar, the existing methods mainly include: achieving directional detection by analyzing the phase difference of the array receiving antennas in the hole, and designing a directional antenna for directional emission and reception of electromagnetic waves in the hole to determine the target.

[0007] The positioning of the in-hole array antenna is to place multiple receiving antennas in the same borehole at a certain interval. Since the time difference of each antenna receiving the same target signal is different, the direction from which the target signal comes can be inferred by analyzing the phase difference of the signals, thus realizing positioning detection. However, usually when detecting a far target, the wavelength adopted by the borehole radar is much larger than the interval of the array antennas in the borehole, making it difficult to extract the phase difference. Therefore, this method is not applicable to the application environment of long-distance detection by borehole radar.

[0008] However, although current borehole radars all support the single-hole measurement mode, most borehole radars have no directivity for the detected objects. Specifically, since current borehole radars usually adopt omnidirectional transmitting and receiving dipole antennas, the electromagnetic waves they transmit and receive have no azimuth information. Therefore, a borehole radar using single-hole measurement can judge the existence of an object at a certain depth from the borehole and a certain depth from the borehole wall, but cannot further judge the azimuth angle information of the object around the axis of the borehole. Therefore, when the borehole radar is in the single-hole measurement working mode, the obtained information is limited. Therefore, how to improve the directivity of the antenna of the borehole radar in the single-hole measurement mode, realize the directional emission and reception of electromagnetic waves or the directivity only in a narrow frequency band of a single frequency, and accurately detect the target objects around the borehole is a technical problem that urgently needs to be solved at present. Summary of the Invention

[0009] The purpose of the present invention is to provide a borehole radar antenna with directivity and its implementation method, so as to improve the directivity of the antenna of the borehole radar in the single-hole measurement mode, realize the directional emission and reception of electromagnetic waves or the directivity in a narrow frequency band of a single frequency, and accurately detect the target objects around the borehole.

[0010] To solve the above 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, which includes a metal radiation sheet, 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 the rectangular surface of the loading dielectric is connected to the dielectric substrate, and the outside is wrapped by a metal reflector that matches its semi-cylindrical arc surface. The metal radiation sheet is attached to the surface of the dielectric substrate, and both ends of the metal radiation sheet are electrically connected to the metal reflector through a series-connected loading capacitor.

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

[0013] Preferably, the metal radiation sheet is composed of two single sheets. Each single sheet is composed of a front-end exponential gradient structure and a terminal semi-elliptical structure. The two single sheets are connected by feeding.

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

[0015] Preferably, each single sheet is provided with a group of loading resistors. Each group of loading resistors has at least 5, and the two groups of resistors are symmetrically distributed with the feeding as the center.

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

[0017] In a second aspect, a method for realizing a directional borehole radar antenna is provided, which is realized based on the above-mentioned directional borehole radar antenna, and includes the following steps:

[0018] S1: Perform distributed resistance loading on the antenna arms of the directional borehole radar antenna. The formula for performing distributed resistance loading is as follows:

[0019]

[0020] Where, R i is the resistance value of the i-th resistor, ψ is the resistance value of the terminal loading resistor, Lf is the length of a single antenna arm, p i is the distance between the i-th resistor and the feeding point, and l is the distance between adjacent resistors;

[0021] S2: Define the resistance value of the end-loaded resistor, and the resistance values of other resistors change with the change of ψ. Optimize other resistors through optimizing other resistors;

[0022] S3: Set an exponential gradient structure at the feeding point to suppress the change of the antenna input impedance, reduce the reflection of signals on the two arms of the antenna, and set a semi-elliptical arc structure at the end;

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

[0024] S5: Place the borehole radar of the adjusted directional borehole radar antenna into the borehole and trigger a pulse at a specified time. The directional borehole radar antenna emits and receives electromagnetic waves around the borehole, and the borehole radar moves along the borehole to successively obtain all the formation information of the entire borehole.

[0025] The beneficial effects of the present invention include:

[0026] The directional borehole radar antenna and its implementation method provided by the present invention include a metal radiation sheet, a dielectric substrate, a loading resistor, a loading medium, a metal reflector and a loading capacitor. Perform distributed resistance loading on the antenna arms, define the resistance value of the end-loaded resistor, and optimize other resistors; set an exponential gradient structure at the feeding point to suppress the change of the antenna input impedance, and set a semi-elliptical arc structure at the end; introduce a loading medium and a reflector, and adjust the distance between the metal reflector and the antenna with the change of the dielectric constant of the dielectric substrate and adjust the antenna beam width; place the borehole radar into the borehole and trigger a pulse at a specified time, the antenna emits and receives electromagnetic waves around the borehole, and the borehole radar moves along the borehole to successively obtain all the formation information of the entire borehole. Realize the directional emission and reception of electromagnetic waves or the directivity within a narrow frequency band of a single frequency and the precise detection of target objects around the borehole.

[0027] First, through distributed resistance loading on each antenna arm, based on the Wu-King loading theory, first define the resistance value of the end-loaded resistor, and the resistance values of other resistors will change with the change of ψ. Optimize this resistance value. The distributed resistance loading weakens the frequency selection characteristic of the antenna, thereby increasing the overall bandwidth and enabling the antenna to meet the broadband requirements.

[0028] Secondly, suppress the change of the antenna input impedance by using an exponential gradient structure at the feeding point, reduce the reflection of signals on the two arms of the antenna, and the value of the gradient index g needs to be determined by optimization. The end adopts a semi-elliptical arc structure, which is beneficial to reducing the deterioration of the voltage standing wave ratio caused by current cutoff, and the semi-minor axis b is the main parameter affecting the voltage standing wave ratio.

[0029] Finally, by introducing a loading medium and a reflector into the antenna structure, where the dielectric constant of the loading medium affects the wavelength of electromagnetic waves between the antenna and the reflector, so 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, and the central angle corresponding to the arc length of the reflector affects the beam width of the finally designed antenna, thereby effectively improving the directional performance of the antenna. Description of the Drawings

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

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

[0032] Figure 3 It is a schematic diagram of the parameter curve of the directional borehole radar antenna of the present invention.

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

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

[0035] The following further elaborates on the present invention in conjunction with the attached Figures 1 to 5 for a more detailed description:

[0036] Embodiment 1

[0037] Referring to the attached Figure 1 and Figure 2 as shown, a directional borehole radar antenna includes a metal radiation sheet, a dielectric substrate, a loading resistor, a loading medium, a metal reflector, and a loading capacitor. The loading medium is a semi-cylinder, one side of the rectangular surface of the loading medium is connected to the dielectric substrate, and the outside is wrapped by a metal reflector that matches its semi-cylindrical arc surface. The metal radiation sheet is attached to the surface of the dielectric substrate, and both ends of the metal radiation sheet are electrically connected to the metal reflector through a series-connected loading capacitor.

[0038] In this embodiment, the length of the antenna is L = 200 mm, the width is W = 60 mm, and the thickness is 32 mm. The metal radiation sheet is composed of two single sheets, and each single sheet is composed of a front-end exponential gradient structure and a terminal semi-elliptical structure. The two single sheets are connected by feeding. The front-end exponential gradient structure is represented by the formula y = c·e gxIt is determined that the gradient index g = 0.15, the length of the exponential 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 feeding gap in the middle of the metal radiation sheet is w_gap = 1 mm, the substrate material of the radiation sheet is selected as FR-4, the dielectric constant of the substrate is 4.6, and the tangent of the loss angle 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. To fit the hole environment, the shape is set as a semi-cylinder and is connected to the metal reflector on the outside. The thickness of the metal reflector is 0.5 mm, the fan-shaped angle of the metal reflector is 150°, and both ends of the metal radiation sheet are connected to the metal reflector through series-loaded capacitors with a capacitance value of 12 pF.

[0040] A distributed loading resistor method is used to load 5 resistors on a single radiation sheet. The resistance values of the 5 loading resistors are 3.39 Ω, 4.18 Ω, 5.45 Ω, 7.82 Ω, and 18 Ω respectively.

[0041] Embodiment 2

[0042] On the basis of Embodiment 1, each single piece is provided with a group of loading resistors. Each group of loading resistors has at least 5 resistors, and the two groups of resistors are symmetrically distributed with the feeding as the center. The distance between the loading resistor closest to the feeding point in each group of loading resistors and the feeding is 20 mm, and the distance between adjacent loading resistors is 15 mm.

[0043] Embodiment 3

[0044] On the basis of Embodiment 1 or Embodiment 2, a method for realizing a directional borehole radar antenna is based on the described directional borehole radar antenna and includes the following steps:

[0045] S1: Perform distributed resistance loading on the antenna arms of the directional borehole radar antenna. The formula for distributed resistance loading is as follows:

[0046]

[0047] where R i 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, p i is the distance between the i-th resistor and the feeding point, and l is the distance between adjacent resistors;

[0048] S2: Define the resistance value of the end-loaded resistor, and the resistance values of other resistors change with the change of ψ. Optimize other resistors through

[0049] S3: Set an exponential gradient structure at the feeding point to suppress the change of the antenna input impedance, reduce the reflection of signals on both arms of the antenna, and set a semi-elliptical arc structure 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 of the dielectric constant of the dielectric substrate, and the antenna beam width is adjusted according to the central angle corresponding to the metal reflector;

[0051] S5: Place the borehole radar of the adjusted directional borehole radar antenna into the borehole and trigger a pulse at a specified time. The directional borehole radar antenna emits and receives electromagnetic waves around the borehole, and the borehole radar moves along the borehole to successively obtain all the formation information of the entire borehole.

[0052] In this embodiment, distributed resistance loading is performed on each antenna arm. First, the resistance value of the end-loaded resistor is defined, and the resistance values of other resistors will change with the change of ψ. Optimize this resistance value. The distributed resistance loading weakens the frequency selection characteristic of the antenna, thereby improving the overall bandwidth and enabling the antenna to meet the wideband requirements. The feeding point adopts an exponential gradient structure to suppress the change of the antenna input impedance and reduce the reflection of signals on both arms of the antenna. The value of the gradient index g needs to be determined through optimization. The end adopts a semi-elliptical arc structure, which is beneficial to reducing the deterioration of the standing wave ratio caused by current cutoff. The semi-minor axis b is the main parameter affecting the standing wave ratio.

[0053] Introduce 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 directivity performance. In addition, the distance between the reflector and the antenna needs to be adjusted according to the change of the dielectric constant of the dielectric plate, and the central angle corresponding to the arc length of the reflector affects the finally designed antenna beam width. These parameters need to be optimized during the design process.

[0054] See Figure 3 , simulate the above model through an electromagnetic simulation software to obtain the S11 parameter as Figure 3 shown. The working bandwidth range of the antenna is 118 MHz - 403 MHz (-8 dB), and the relative bandwidth reaches 109%, belonging to a broadband antenna.

[0055] To verify the directivity of the antenna, set far-field monitors at frequency points of 120 MHz, 200 MHz, 260 MHz (center frequency), 280 MHz, 360 MHz, and 400 MHz respectively, and obtain the far-field E-plane and H-plane patterns of the antenna as Figure 4 and Figure 5 shown. Figure 4E-plane radiation pattern: (a) 120 MHz; (b) 200 MHz; (c) 260 MHz; (d) 280 MHz; (e) 360 MHz; (f) 400 MHz. Figure 5 H-plane radiation pattern: (a) 120 MHz; (b) 200 MHz; (c) 260 MHz; (d) 280 MHz; (e) 360 MHz; (f) 400 MHz.

[0056] According to Figure 4 and Figure 5 it can be seen that the 3dB beamwidth of the antenna in the E-plane within a certain frequency range is less than 150°, showing good directivity. Especially at the center frequency of 260 MHz, the 3dB beamwidth of the antenna is 93.6°. However, the directivity of the antenna in the H-plane is weaker than that in the E-plane. It can be known that the present invention is based on a butterfly antenna and is designed under the constraint that the width of the antenna is less than 60 mm, having the following effective effects: the width of the entire antenna does not exceed the diameter of the drill hole, and it is semi-cylindrical and can be conformal with the inner wall of the drill hole, enabling the entire system to be smoothly placed into the drill hole. By changing the current in the way of distributed loading resistors on the basis of Wu-king loading, the antenna frequency band is broadened, enabling the antenna to operate in a wide frequency range of 118 MHz - 403 MHz (109%); by using the loaded medium to reduce the wavelength of the electromagnetic wave and using a reflector to reflect the backward electromagnetic wave, the directivity in the far field is achieved, so as to complete the underground target directional detection in combination with the RF system.

[0057] In summary, the directional borehole radar antenna and its implementation method provided by the present invention include a metal radiation sheet, a dielectric substrate, a loading resistor, a loaded medium, a metal reflector, and a loading capacitor. Conduct distributed resistance loading on the antenna arms, define the resistance value of the end loading resistor, and optimize other resistors; set an exponential gradient structure at the feeding point to suppress the change of the antenna input impedance, and set a semi-elliptical arc structure at the end; introduce a loaded medium and a reflector, adjust the distance between the metal reflector and the antenna according to the change of the dielectric constant of the dielectric substrate, and adjust the antenna beamwidth; place the borehole radar in the borehole and trigger a pulse at a specified time, the antenna emits and receives electromagnetic waves around the borehole, and the borehole radar moves along the borehole to successively obtain all formation information of the entire borehole. Realize the directional emission and reception of electromagnetic waves or the directivity in a narrow frequency band of a single frequency and the precise detection of target objects around the borehole.

[0058] By performing distributed resistance loading on each antenna arm, based on the Wu-King loading theory, first define the resistance value of the end-loaded resistor, and the resistance values of other resistors will change with the change of ψ. Optimize this resistance value. The distributed resistance loading weakens the frequency selection characteristic of the antenna, thereby improving the overall bandwidth and enabling the antenna to meet the broadband requirements. By adopting an exponentially tapered structure at the feeding point, the change of the antenna input impedance is suppressed, and the reflection of the signal on the two arms of the antenna is reduced. The value of the taper index g needs to be determined by optimization. The end adopts a semi-elliptical arc structure, which is beneficial to reducing the deterioration of the voltage standing wave ratio caused by current cutoff. The semi-minor axis b is the main parameter affecting the voltage standing wave ratio. By introducing a loading medium and a reflector into the antenna structure, the permittivity 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 with the change of the permittivity of the dielectric plate, and the central angle corresponding to the arc length of the reflector affects the beam width of the finally designed antenna, thereby effectively improving the directional performance of the antenna.

Claims

1. A directional borehole radar antenna, characterized in that, It includes a metal radiation sheet, 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 the rectangular surface of the loading dielectric is connected to the dielectric substrate, and the outside is wrapped by a metal reflector that matches its semi-cylindrical arc surface. The metal radiation sheet is attached to the surface of the dielectric substrate, and both ends of the metal radiation sheet are electrically connected to the metal reflector through a series-connected loading capacitor.

2. The directional borehole radar antenna according to claim 1, wherein, The length of the antenna is L = 200 mm, the width is W = 60 mm, and the thickness is 32 mm.

3. The directional borehole radar antenna according to claim 1, characterized in that, The metal radiation sheet consists of two single pieces. Each single piece consists of a front-end exponential gradient structure and a terminal semi-elliptical structure. The two single pieces are connected by feeding.

4. The directional borehole radar antenna according to claim 3, characterized in that, The front-end exponential gradient structure is determined by the formula y = c·e gx + d, where the gradient exponent g = 0.15, the length of the exponential 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 radiation patch is w_gap = 1 mm, the substrate material of the radiation patch is selected as FR-4, the dielectric constant of the substrate is 4.6, and the tangent of the loss angle of the substrate is 0.

019.

5. The directional borehole radar antenna according to claim 3, wherein, Each single piece is provided with a group of loading resistors. Each group of loading resistors has at least 5, and the two groups of resistors are symmetrically distributed with the feeding as the center.

6. The directional borehole radar antenna according to claim 5, characterized in that, The distance between the loading resistor closest to the feeding point in each group of loading resistors and the feeding is 20 mm, and the distance between adjacent loading resistors is 15 mm.

7. A method for implementing a directional borehole radar antenna, which is implemented based on the directional borehole radar antenna according to any one of claims 1-6, characterized in that It includes the following steps: S1: Perform distributed resistance loading on the antenna arm of the directional borehole radar antenna. The formula for distributed resistance loading is as follows: Among them, R i 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, p i is the distance between the i-th resistor and the feeding point, and l is the spacing between adjacent resistors; S2: Define the resistance value of the end-loaded resistor, and the resistance values of other resistors change with the change of ψ. Optimize other resistors by performing optimization on other resistors; S3: Set an exponential gradient structure at the feeding point to suppress the change of the antenna input impedance, reduce the reflection of signals on the two arms of the antenna, and set a semi-elliptical arc structure at the end. S4: Introduce a loading dielectric and a reflector. The distance between the metal reflector and the antenna is adjusted according to the change of the dielectric constant of the dielectric substrate, and the antenna beam width is adjusted according to the central angle corresponding to the metal reflector. S5: Place the borehole radar of the adjusted directional borehole radar antenna into the borehole and trigger a pulse at a specified time. The directional borehole radar antenna emits and receives electromagnetic waves around the borehole, and the borehole radar moves along the borehole to successively obtain all the formation information of the entire borehole.

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