NMR system and detection method for detecting large-scale geological density

Abstract

This invention discloses an NMR system for large-scale geodensity detection, comprising a computer that controls a radio frequency transmitter to emit radio frequency signals. These signals are amplified by a radio frequency power amplifier and then distributed and transmitted to the main probe via a power divider. The main probe and secondary probes receive NMR relaxation signals in parallel and transmit them to the computer. The permanent magnets of each probe in the probe group can rotate along their own axes. This invention also discloses an NMR detection method for large-scale geodensity detection, which involves determining the number n of probes in the probe group and their positions; identifying and classifying target detection areas based on their positions; measuring the geodensity of each target detection area and summing the results to obtain the total geodensity of the detection area. This invention achieves large-scale density detection by cooperating with multiple probes to measure multiple target detection areas.

Description

Technical Field

[0001] This invention belongs to the field of nuclear magnetic resonance (NMR) instrument technology, specifically relating to an NMR system for large-scale geological density detection, and also to an NMR detection method for large-scale geological density detection. Background Technology

[0002] Before constructing large-scale buildings (dams, transportation hubs, tunnels, etc.), geological investigation of the foundation is essential. Among the various methods, the density of the foundation is particularly important, directly affecting its mechanical properties, bearing capacity, and the long-term stability of the large-scale building. Currently used methods for geological density testing (mercury intrusion testing, gas adsorption, ultrasonic testing, etc.) involve taking foundation samples and testing them on the ground. These methods cannot perform in-situ density testing of the foundation geology, thus failing to obtain the most original density information. In contrast, NMR-based methods can perform non-destructive in-situ density testing of the foundation geology, offering higher accuracy and providing more specific porosity information, such as the proportion of pores of different sizes and the presence of large, destructive pores that severely affect the foundation's bearing capacity.

[0003] The working principle of an NMR system used for geological density detection is as follows: CPMG (Carr-Purcell-Meiboom-Gill) sequences are used to collect data from geological pores. 1 The echo signal of H was then subjected to an inverse Laplace transform to obtain the pore. 1 The T2 information of H is used to obtain the pore size and the proportion of various pores by measuring the T2 length and the peak area. Through calculation, the porosity and density information of the geology can be obtained, and then it can be determined whether the tested foundation meets the requirements for subsequent construction.

[0004] For geological testing of large-scale buildings and facilities, a larger testing area represents a larger sample size, which can more reliably reflect the general density information of the geological area being tested. Currently, NMR systems used for geological density testing are typically equipped with a single probe. A single probe can only generate a static magnetic field that meets the detection requirements in a fixed area, and cannot flexibly adjust the testing area over a large range as needed, thus failing to universally reflect the geological density over a wide area. Patent document CN102360703 B discloses a "magnet structure for a downhole nuclear magnetic resonance logging-while-drilling tool in oil wells." This patent uses a combination of permanent magnet blocks and ferromagnetic material blocks to increase the horizontal detection range while ensuring sufficient depth in the detection area. Patent document CN 111472766 A discloses a "magnet for a downhole nuclear magnetic resonance logging tool," which uses several small magnets bonded together to form a larger magnet, with the N pole at the top set in an arc shape, increasing the vertical detection depth of the probe while saving costs.

[0005] While the aforementioned patents expand the probe's detection range and optimize the magnet's structure to some extent, they all focus on optimizing the structure of a single magnet within a single probe. The magnetic field strength is also lower in regions far from the probe, resulting in a low signal-to-noise ratio (SNR) for the detected magnetic resonance signal (SNR ~ B0 under low field conditions). 2 Since ~ is proportional to B0 (where B0 is the static magnetic field strength of the detection area), the location of the detection area is often fixed (the sensitive area of ​​the probe attachment), making it impossible to perform large-scale measurements. Summary of the Invention

[0006] The purpose of this invention is to address the problems of limited detection range, low static and radio frequency field strength, and low signal transmission and reception efficiency in existing single-probe, single-magnet NMR systems for detecting geological density. This invention proposes an NMR system and method for large-area geological density detection. The NMR system includes a probe group. The system distributes radio frequency signals to the probe in main probe mode for transmission, while probes in main and secondary probe modes receive NMR relaxation signals in parallel. The probe group in this invention can achieve dynamic multi-target detection over a larger total detection area and generates stronger static and radio frequency fields. Regarding signal transmission and reception, the system improves efficiency through a "single-channel transmission, multi-channel parallel reception" approach. Therefore, this NMR system can acquire NMR relaxation signals with a stronger signal-to-noise ratio over a larger detection area, while simultaneously improving the accuracy of geological density detection.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: An NMR system for large-scale geological density detection includes a computer and a probe group comprising multiple probes. The probes operate in two modes: a main probe mode and a secondary probe mode. The computer controls a radio frequency transmitter to emit radio frequency signals. These signals are amplified by a radio frequency power amplifier (single-channel transmission) and then distributed to the probes in main probe mode via a power divider for transmission. Both main and secondary probes receive NMR relaxation signals. The signal output of each probe is connected to an independent preamplifier. The NMR relaxation signals are amplified by the preamplifiers corresponding to each probe and then transmitted in parallel to a multi-channel radio frequency receiver (multi-channel parallel reception). The multi-channel radio frequency receiver transmits the parallel-received NMR relaxation signals to the computer.

[0008] The probe includes a permanent magnet, a magnet fixing shell, an RF coil, a probe housing, a connecting frame, a gear, and a limiting screw. The permanent magnet is embedded in the magnet fixing shell, which is rotatably disposed inside the probe housing. Limiting rings are provided at both the upper and lower ends of the magnet fixing shell, and the limiting rings are clearance-fitted with the probe housing. The probe housing has a threaded hole. When the bottom of the magnet fixing shell contacts the bottom inner wall of the probe housing, the limiting screw passes through the threaded hole on the probe housing, and the threaded head of the limiting screw is located above the limiting ring at the upper end of the magnet fixing shell. A gap is provided between the threaded head of the limiting screw and the limiting ring at the upper end of the magnet fixing shell. A connecting frame is provided on the top end face of the magnet fixing shell, and the connecting frame has an integral rotating shaft. The gear is fixedly sleeved on the rotating shaft of the connecting frame. The RF coil is fixedly installed on the inner wall of the probe housing.

[0009] The radio frequency coil is a transceiver coil. When the probe is in main probe mode, the radio frequency coil is in transceiver mode. When the probe is in secondary probe mode, the radio frequency coil is in signal receiving mode.

[0010] The radio frequency coil is a solenoid radio frequency coil, and the permanent magnet is a regular hexagonal prism permanent magnet. The static magnetic field B0 generated by the permanent magnet is orthogonal to the radio frequency field B1 generated by the radio frequency coil.

[0011] An NMR detection method for large-scale geological density detection, utilizing the aforementioned NMR system, includes the following steps: Step 1: Determine the number n of probes in the probe group and the position of the probes; Step 2: Based on the probe location, confirm the target detection area according to the target detection area classification rules; Step 3: Measure the geological density of each target detection area; Step 4: Summarize the geological density of each target detection area to obtain the total geological density of the detection area.

[0012] The number of probes n≥3, the probes are distributed on a probe distribution circle with the center point of the total detection area as the center and the detection radius of the probe as the radius, and the probe distribution circle is evenly divided into n equal parts.

[0013] The target detection area classification rules are as follows: Type 1 target detection area: The target detection area is located between adjacent probes, and the center point of the target detection area coincides with the midpoint of the line connecting the center positions of the adjacent probes. Second type of target detection area: The center point of the target detection area coincides with the center point of the total detection area; The third type of target detection area: The center point of the target detection area is located on the line connecting the center point of the total detection area and the center position of the probe, and is located at the midpoint of the line; The fourth type of target detection area: The center point of the target detection area is located on the extension of the line connecting the center point of the total detection area and the center position of the probe, and the distance from the center point of the target detection area to the center position of the probe is less than the detection radius of the probe.

[0014] The measurement of the geological density of each target detection area is based on the following steps: Step 3.1: Select a target detection area and determine the main probe and sub-probes for the target detection area during this measurement. Based on the classification of the target detection area, rotate the main probe. When the number of main probes is 1, make the static magnetic field generated by the permanent magnet of the main probe in the target detection area point to the center of the target detection area. When the number of main probes is greater than 1, make the static magnetic fields generated by the permanent magnets of each main probe in the target detection area superimpose each other to obtain a magnetic field enhancement effect. Step 3.2: Set the RF coil in the main probe to transceiver mode and set the RF coil in the secondary probe to receive signal mode; Step 3.3: Obtain the NMR relaxation signal from this measurement, perform inverse Laplace transform on the obtained NMR relaxation signal to obtain the transverse relaxation time T2, analyze the transverse relaxation time T2 and process the value and distribution of the transverse relaxation time T2 to obtain the geological density of the target detection area in this detection. Step 3.4: Select the next target detection area, and repeat steps 3.1 to 3.3 until the geological density of all target detection areas is obtained.

[0015] The main and secondary probes for determining the target detection area are based on the following rules: The m probes located within a region centered on the center point of the target detection area and with the probe's detection radius as the radius are the main probes, where m is greater than or equal to 1 and less than or equal to n, and the remaining nm probes are the auxiliary probes.

[0016] The classification rotating master probe based on the target detection area includes the following steps: Type 1 target detection area: Rotate the main probe so that the static magnetic field generated by the permanent magnet in one main probe points to the center of the target detection area, while the static magnetic field generated by the permanent magnet in the other main probe moves away from the center of the target detection area. The third type of target detection area: rotate the main probe so that the static magnetic field generated by the permanent magnet inside the main probe points to the center of the target detection area; Fourth type of target detection area: Rotate the main probe so that the static magnetic field generated by the permanent magnet inside the main probe points to the center of the target detection area; The second type of target detection area: When the number of probes n is odd, rotate the main probe so that the static magnetic field generated by the permanent magnet inside each main probe points to the center of the target detection area; when the number of probes n is even, divide all the main probes into groups of two main probes on the same diameter of the probe distribution circle, rotate the main probe so that the static magnetic field generated by the permanent magnet inside one of the main probes in the group points to the center of the target detection area, and the static magnetic field generated by the permanent magnet inside the other main probe in the group moves away from the center of the target detection area.

[0017] Compared with the prior art, the present invention has the following advantages: 1. In this invention, the permanent magnet of each probe can rotate along its own axis under the action of external drive, thereby changing the direction of the static magnetic field generated by the permanent magnet in space, and thus focusing the static magnetic field of multiple probes on the target detection area. Through the cooperation of multiple probes, the measurement of multiple target detection areas can be realized, thereby realizing large-scale density detection.

[0018] 2. This invention increases the radio frequency field intensity in the target detection area by cooperating with the probes, thereby reducing the excitation power of the radio frequency coils of each probe, and thus improving the efficiency of radio frequency coil transmission and reception. Attached Figure Description

[0019] Figure 1 A schematic diagram of a probe group consisting of four probes and the target detection area. Figure 2 A schematic diagram of the structure of a single probe and the static magnetic field and radio frequency field it generates; Figure 3 A cross-sectional view of a single probe structure; Figure 4 Taking the detection of the fifth detection area 35 as an example, the schematic diagram of the main and auxiliary probe allocation scheme is shown. Figure 5 A schematic diagram of the NMR system structure is shown, taking the detection of the fifth detection region 35 as an example. Among them, 1-probe, 2-total detection area, 4-permanent magnet, 5-magnet fixing shell, 6-RF coil, 7-probe shell, 8-connecting bracket, 9-gear, 10-limiting screw, 11-first probe, 12-second probe, 13-third probe, 14-fourth probe, 31-first detection area, 32-second detection area, 33-third detection area, 34-fourth detection area, 35-fifth detection area, 36-sixth detection area, 37-seventh detection area, 38-eighth detection area, 39-ninth detection area, 310-tenth detection area, 311-eleventh detection area, 312-twelfth detection area, 313-thirteenth detection area, 41-first permanent magnet, 42-second permanent magnet, 43-third permanent magnet, 44-fourth permanent magnet, 51-limiting ring, B0-static magnetic field, B1-RF field, B 01 -The static magnetic field generated by the first permanent magnet, B 02 -The static magnetic field generated by the second permanent magnet, SS pole, NN pole. Detailed Implementation

[0020] To facilitate understanding and implementation of the present invention by those skilled in the art, the present invention will be further described in detail below with reference to embodiments. It should be understood that the embodiments described herein are for illustration and explanation only and are not intended to limit the present invention. Example 1:

[0021] like Figure 5 As shown, an NMR system for large-scale geological density detection includes a computer and a probe array.

[0022] The above probe group includes several probes 1, and the total number of probes 1 can be set to n.

[0023] like Figure 2 (The probe housing 7 is not shown to facilitate understanding of the probe's internal structure.) Figure 3As shown, each of the probes 1 includes a permanent magnet 4, a magnet fixing shell 5, an RF coil 6, a probe housing 7, a connecting frame 8, a gear 9, and a limiting screw 10. The permanent magnet 4 is embedded in the magnet fixing shell 5, which is located inside the probe housing 7 and can rotate relative to the probe housing 7 about its own axis. Limiting rings 51 are provided at both the upper and lower ends of the magnet fixing shell 5. The limiting rings 51 are clearance-fitted with the probe housing 7, and serve as lateral limiting rings to prevent radial displacement between the magnet fixing shell 5 and the probe housing 7. The probe housing 7 has threaded holes. When the bottom of the magnet fixing shell 5 contacts the bottom inner wall of the probe housing 7, the limiting screw 10 passes through the threaded hole on the probe housing 7, and the threaded head of the limiting screw 10 is positioned above the limiting ring 51 at the upper end of the magnet fixing shell 5. A gap is provided between the threaded head of the limiting screw 10 and the limiting ring 51 at the upper end of the magnet fixing shell 5, ensuring that the limiting screw 10 does not affect the rotation of the magnet fixing shell 5 and simultaneously prevents axial displacement between the magnet fixing shell 5 and the probe housing 7. The connecting frame 8 is fixed to the top end face of the magnet fixing shell 5. The connecting frame 8 has an integral rotating shaft. The gear 9 is fixedly sleeved on the rotating shaft of the connecting frame 8. The rotating shafts of the gear 9, connecting frame 8, magnet fixing shell 5, and permanent magnet 4 are coaxial and perpendicular to the horizontal plane. A gear drive device is arranged outside the probe to drive the gear 9 to rotate. The gear drive device can drive the gear 9 to rotate, thereby driving the connecting frame 8 and magnet fixing shell 5 to rotate, and finally driving the permanent magnet 4 to rotate.

[0024] In this embodiment, the permanent magnet 4 is a regular hexagonal prism permanent magnet. Compared with cylindrical permanent magnets, regular hexagonal prism permanent magnets are easier to magnetize laterally, have lower manufacturing costs, and are easier to fit into the magnet fixing shell 5, rotating along the rotation axis with the magnet fixing shell 5. Therefore, the permanent magnet 4 adopts a regular hexagonal prism structure. The magnet fixing shell 5 is a cylinder with a regular hexagonal prism groove inside. The regular hexagonal prism permanent magnet is embedded in the regular hexagonal prism groove of the magnet fixing shell 5 (the bottom of the regular hexagonal prism groove is closed, and the top of the groove is open, so the regular hexagonal prism permanent magnet will not slip off the bottom of the magnet fixing shell). When the magnet fixing shell 5 rotates along the rotation axis, the regular hexagonal prism permanent magnet rotates synchronously with the magnet fixing shell 5.

[0025] Each probe 1 has the same structure, such as Figure 2 and Figure 3As shown, the magnetization direction of the regular hexagonal prism permanent magnet inside probe 1 is perpendicular to the rotation axis, and the magnetization direction is also perpendicular to one side of the regular hexagonal prism permanent magnet (inside the regular hexagonal prism permanent magnet, the magnetic field direction is from the S pole to the N pole). The regular hexagonal prism permanent magnet is encased in a magnet fixing shell 5, which is made of non-magnetic, hydrogen-free plastic material. The shell 5 has regular hexagonal prism slots inside for embedding the regular hexagonal prism permanent magnet and for limiting its position. The radio frequency coil 6 is a solenoid radio frequency coil, fixed to the inner wall of the probe housing 7. The static magnetic field B0 generated by the permanent magnet 4 in the detection area is horizontal, while the radio frequency field B1 generated by the radio frequency coil 6 in the detection area is vertical. The static magnetic field B0 and the radio frequency field B1 are orthogonal. The outermost part of probe 1 is the probe housing 7, which protects the internal structure of probe 1.

[0026] By laterally magnetizing the prism permanent magnet along one of its sides perpendicular to the prism permanent magnet, a static magnetic field in the horizontal direction can be generated in the detection area. When the magnet fixing shell 5 rotates along its own axis, the direction of the static magnetic field generated by the prism permanent magnet in space also changes, but it is still in the horizontal plane. Therefore, by adjusting the rotation angle of each probe 1 (rotation angle of the permanent magnet 4), the direction of the static magnetic field generated by the permanent magnet 4 inside each probe 1 in space can be changed, thereby focusing the static magnetic field generated by the permanent magnet 4 inside each probe onto the target detection area. Therefore, the rotation angle of each probe can be adjusted in real time according to the current position of the detection area to achieve dynamic detection of the detection area.

[0027] The aforementioned radio frequency coil 6 is installed on the inner wall of the probe housing 7. When the permanent magnet 4 rotates with the magnet fixing shell 5, the radio frequency coil 6 and the probe housing 7 remain stationary. The radio frequency coil 6 adopts a solenoid coil structure. The radio frequency coil 6 can generate a vertical radio frequency field B1 in the detection area. The radio frequency field B1 is orthogonal to the static magnetic field B0 generated by the permanent magnet 4.

[0028] The working modes of probe 1 include main probe mode and auxiliary probe mode. When probe 1 is in main probe mode, it is the main probe, and when probe 1 is in auxiliary probe mode, it is the auxiliary probe.

[0029] The RF coil 6 is a transceiver coil. When the probe 1 is in the main probe mode, the RF coil 6 receives the RF signal transmitted by the power divider, and the RF coil 6 generates an RF field B1 in the detection area. The working mode of the RF coil 6 is the transceiver mode. When the probe 1 is in the auxiliary probe mode, the RF coil 6 acts as a receiving coil, and the working mode of the RF coil 6 is the receiving signal mode.

[0030] During the transmission phase, the computer controls the radio frequency transmitter to emit radio frequency signals. These signals are amplified by a radio frequency power amplifier and then transmitted to the radio frequency coils 6 in the m main probes via a power divider. During the reception phase, the radio frequency coils 6 corresponding to the m main probes and the nm secondary probes all receive NMR relaxation signals (in this embodiment: 1 The NMR relaxation signal of H is amplified by the corresponding preamplifiers of m main probes and nm sub-probes, and then transmitted in parallel to a multi-channel RF receiver. The RF receiver then transmits the parallel-received NMR relaxation signal to a computer. Example 2:

[0031] It is worth noting that the number of probes 1 involved in this invention, and the spatial relationship between probes 1, the total detection area 2, and the target detection area are not limited to this embodiment. They can be set according to specific actual needs. This embodiment is only for reference. Figure 1 The positional relationships shown are explained in detail to enable those skilled in the art to understand the invention.

[0032] A method for large-scale geological density detection using the NMR system for large-scale geological density detection described in Example 1 includes the following steps: Step 1: Determine the number n of probe 1 in the probe group and the position of probe 1; The number of probes 1, n, in the probe group is determined based on the size of the total detection area 2. The larger the total detection area 2, the larger the number of probes 1, n. Generally, n ≥ 3. In this embodiment, n = 4 (an even number) is taken.

[0033] like Figure 1 As shown, the probe group comprises four probes 1: probe 11, probe 12, probe 13, and probe 14, with corresponding permanent magnets 4: permanent magnet 41, permanent magnet 42, permanent magnet 43, and permanent magnet 44. The total detection area 2 is square, with the four probes 1 distributed on a probe distribution circle centered at the center of the total detection area and having a detection radius equal to the probe 1's radius, thus dividing the probe distribution circle into four equal parts.

[0034] It should be noted that the larger the total detection area 2 is, the larger the probe distribution circle where probe 1 is located will be. In this case, increasing the size of the permanent magnet in probe 1 can increase the detection radius of probe 1.

[0035] Step 2: Based on the position of probe 1, confirm the target detection area according to the target detection area classification rules; the target detection area classification rules are as follows: The first type of target detection area is located between adjacent probes 1, and the center point of the first type of target detection area coincides with the midpoint of the line connecting the center positions of the adjacent probes 1. In this embodiment, the first type of target detection area includes the fifth detection area 35, the sixth detection area 36, ​​the seventh detection area 37 and the eighth detection area 38.

[0036] Second type of target detection area: The center point of the second type of target detection area coincides with the center point of the total detection area 2. In this embodiment, the second type of target detection area includes the thirteenth detection area 313.

[0037] The third type of target detection area: The center point of the third type of target detection area is located on the line connecting the center point of the total detection area and the center of probe 1, and is located at the midpoint of the line. In this embodiment, the third type of target detection area includes the first detection area 31, the second detection area 32, the third detection area 33, and the fourth detection area 34.

[0038] The fourth type of target detection area: The center point of the fourth type of target detection area is located on the extension line of the line connecting the center point of the total detection area and the center position of probe 1, and the distance from the center point of the fourth type of target detection area to the center position of probe 1 is less than the detection radius of probe 1. In this embodiment, the fourth type of target detection area includes the ninth detection area 39, the tenth detection area 310, the eleventh detection area 311 and the twelfth detection area 312.

[0039] Step 3: Measure the geological density of each target detection area.

[0040] When conducting geological density testing on a large-scale foundation, it is necessary to test the density of each target detection area within the total detection area 2, and finally summarize the density of each target detection area to obtain the density of the entire total detection area 2. The steps for measuring each target detection area are as follows: Step 3.1: Select a target detection area and determine the main probe and auxiliary probe of the target detection area during this measurement process; rotate each main probe according to the classification of the target detection area so that the static magnetic field generated by the permanent magnet 4 of each main probe in the target detection area meets the preset conditions.

[0041] Based on the location of the target detection area within the total detection area, m probes are distributed within a region centered on the center point of the target detection area and with the detection radius of probe 1 as the radius. These m probes are the main probes, where m is greater than or equal to 1 and less than or equal to n. The remaining nm probes are the secondary probes. Since the magnetic field generated by the permanent magnet 4 decays rapidly with distance (generally, it can be approximated as the magnetic field strength at the field point decays with the cube of the distance from the field point to the permanent magnet), the contribution of the permanent magnet 4 in the secondary probes to the static magnetic field of the target detection area is very small. Therefore, only the static magnetic field generated by the permanent magnet 4 in the main probes in the target detection area is considered. After determining the main probes and secondary probes, the main probes are rotated so that the static magnetic fields generated by the permanent magnets 4 in the target detection area of ​​each main probe are superimposed and enhanced.

[0042] It should be noted that, based on the target detection area classification and main probe selection method, each first-class target detection area has only two main probes; each third-class target detection area and each fourth-class target detection area has only one main probe; and all probes in the second-class target detection area are main probes.

[0043] When detecting targets belonging to the first type of target detection area (the fifth detection area 35, the sixth detection area 36, ​​the seventh detection area 37 and the eighth detection area 38), there are two main probes for the first type of target detection area; Let's take the fifth detection area 35 of the first type of target detection area as an example: Based on the location of the fifth detection area 35, the distance from the center point of the fifth detection area 35 to the first probe 11 is equal to the distance from the center point of the fifth detection area 35 to the second probe 12, and is less than the detection radius of probe 1. The first probe 11 and the second probe 12 are the main probes. The distance from the center point of the fifth detection area 35 to the third probe 13 is equal to the distance from the center point of the fifth detection area 35 to the fourth probe 14, and is greater than the detection radius of probe 1. The third probe 13 and the fourth probe 14 are the auxiliary probes.

[0044] like Figure 4 As shown, rotating the first probe 11 and the second probe 12 causes the static magnetic field B generated by the first permanent magnet 41 in the first probe 11 to... 01 The static magnetic field B generated by the second permanent magnet 42 in the second probe 12 points to the center of the fifth detection area 35. 02 Since the two hexagonal permanent magnets 4 are located away from the center of the fifth detection area 35, the static magnetic fields generated by them in the fifth detection area 35 are in the same direction. After vector superposition, the static magnetic field strength is twice that of a single hexagonal permanent magnet 4, thus improving the signal-to-noise ratio of the detection signal in the fifth detection area 35. The third probe 13 and the fourth probe 14 are auxiliary probes, and the static magnetic fields generated by the third permanent magnet 43 and the fourth permanent magnet 44 inside them in the fifth detection area 35 are negligible.

[0045] When detecting the second type of target detection area (the thirteenth detection area 313), all probes 1 are the main probes; The following example uses the thirteenth detection area 313 of the second type of target detection area for illustration: When detecting the thirteenth detection area 313, the distance from the center point of the thirteenth detection area 313 to each probe 1 (first probe 11, second probe 12, third probe 13, and fourth probe 14) is equal to the detection radius of probe 1. Each probe 1 is a main probe. The rotation angle of each probe 1 is adjusted so that the static magnetic field generated by the permanent magnet in the first probe 11 and the second probe 12 points to the center of the thirteenth detection area 313, while the static magnetic field generated by the permanent magnet in the third probe 13 and the fourth probe 14 moves away from the center of the thirteenth detection area 313. This results in a superimposed and enhanced effect of the static magnetic fields generated by the first probe 11, the second probe 12, the third probe 13, and the fourth probe 14 in the first detection area 313. That is, all the main probes are divided into groups of two main probes on the same diameter of the probe distribution circle. The static magnetic field generated by the permanent magnet 4 in one main probe in the group points to the center of the thirteenth detection area 313, while the static magnetic field generated by the permanent magnet 4 in the other main probe in the group moves away from the center of the thirteenth detection area 313.

[0046] When detecting targets belonging to the third category (first detection area 31, second detection area 32, third detection area 33 and fourth detection area 34), there is one main probe for the third category target detection area; The following example uses the first detection area 31 of the third type of target detection area for illustration: When detecting the first detection area 31, the distance from the center point of the first detection area 31 to the first probe 11 is less than the detection radius of the probe 1, and the first probe 11 is the main probe; the distances from the center point of the first detection area 31 to the second probe 12, the distances from the center point of the first detection area 31 to the third probe 13, and the distances from the center point of the first detection area 31 to the fourth probe 14 are equal and greater than the detection radius of the probe 1, and the second probe 12, the third probe 13, and the fourth probe 14 are the auxiliary probes. By rotating the main probe, the static magnetic field generated by the permanent magnet 4 inside the first probe 11 is directed to the center position of the first detection area 31.

[0047] When detecting targets belonging to the fourth category (the ninth detection area 39, the tenth detection area 310, the eleventh detection area 311 and the twelfth detection area 312), there is one main probe for the fourth category target detection area.

[0048] The following example uses the ninth detection area 39 of the fourth type of target detection area as an illustration: When detecting the ninth detection area 39, the distance from the center point of the ninth detection area 39 to the second probe 12 is less than the detection radius of probe 1, and the second probe 12 is the main probe; the distances from the center point of the ninth detection area 39 to the first probe 11, the distances from the center point of the ninth detection area 39 to the third probe 13, and the distances from the center point of the ninth detection area 39 to the fourth probe 14 are equal and greater than the detection radius of probe 1, and the first probe 11, the third probe 13, and the fourth probe 14 are the auxiliary probes; by rotating the main probe, the static magnetic field generated by the permanent magnet 4 inside the main probe (second probe 12) is directed to the center position of the ninth detection area 39.

[0049] Step 3.2: Determine the operating mode of the radio frequency coil 6 in each probe being measured. The RF coil 6 inside the main probe is configured as a transceiver, meaning it transmits RF signals during the transmission phase and receives NMR relaxation signals during the reception phase. The RF coil 6 inside the secondary probe is configured as a signal receiver. During the transmission phase, the control power divider distributes all the power of the RF power amplifier to the corresponding RF signals of each main probe. Each RF signal is then transmitted to the corresponding RF coil 6 inside the main probe. The RF coil 6 inside the secondary probe does not transmit RF pulses during the transmission phase; it only serves to receive NMR relaxation signals.

[0050] Let's take the fifth detection area 35 of the first type of target detection area as an example: The RF coils 6 in the main probes (first probe 11 and second main probe 12) are all set to transceiver mode, and the RF coils 6 in the auxiliary probes (third probe 13 and fourth auxiliary probe 14) are all set to signal receiving mode.

[0051] like Figure 5 As shown, during the transmission phase, the computer controls the radio frequency transmitter to output radio frequency signals. The radio frequency power amplifier amplifies the radio frequency signals output by the radio frequency transmitter, and the power divider splits the amplified radio frequency signals into two paths, which are transmitted to the radio frequency coils 6 in the first probe 11 and the second probe 12 respectively. During the reception phase, the radio frequency coils 6 in the main probe (first probe 11 and second probe 12) and the auxiliary probe (third probe 13 and fourth probe 14) receive NMR relaxation signals. The weak NMR relaxation signals received are amplified by the preamplifier and then received by the multi-channel radio frequency receiver, and then transmitted to the computer.

[0052] The thirteenth detection area 313 of the second type of target detection area, the first detection area 31 of the third type of target detection area, and the ninth detection area 39 of the fourth type of target detection area operate in a similar manner, and will not be described in detail here.

[0053] Step 3.3: Obtain the NMR relaxation signal measured at the moment, and process the NMR relaxation signal to obtain the geological density of the target detection area for this detection.

[0054] The NMR relaxation signal within the target detection area was measured using CPMG sequences. The transverse relaxation time T2 was obtained by performing an inverse Laplace transform on the measured NMR relaxation signal. The transverse relaxation time T2 was analyzed, and its numerical value and distribution were processed to finally obtain the geological compactness of the target detection area. Step 3.4: Select the next target detection area and repeat steps 3.1 to 3.3 until the geological density of all target detection areas is obtained.

[0055] Step 4: Summarize the geological density of each target detection area to obtain the geological density of the total detection area 2. Example 3:

[0056] In this embodiment, the number n of probes 1 is odd. When detecting the detection area of ​​the second type of target, all probes 1 are main probes. The static magnetic field generated by the permanent magnet 4 inside each main probe is adjusted to point to the center position of the thirteenth detection area 313, so that the static magnetic fields generated by all main probes in the thirteenth detection area 313 are superimposed and enhanced. Everything else is the same as in embodiment 2.

[0057] This invention reflects the density information of the total detection area 2 by detecting the geological density of each target detection area.

[0058] It should be noted that the embodiments described in this invention are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains can make various modifications or additions to the described embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. An NMR system for large-scale geological density detection, comprising a computer, characterized in that, It also includes a probe group, which includes multiple probes (1). The working modes of the probes (1) include main probe mode and sub-probe mode. The computer controls the radio frequency transmitter to emit radio frequency signals. After the radio frequency signals are amplified by the radio frequency power amplifier, they are distributed and transmitted to the probes (1) in the main probe mode for transmission by the power divider. The probes (1) in both the main probe mode and the sub-probe mode receive NMR relaxation signals. The signal output terminal of each probe (1) is connected to an independent preamplifier. After the NMR relaxation signals are amplified by the preamplifiers corresponding to each probe (1), they are transmitted in parallel to the multi-channel radio frequency receiver. The multi-channel radio frequency receiver transmits the parallel-received NMR relaxation signals to the computer.

2. The NMR system for large-scale geological density detection according to claim 1, characterized in that, The probe (1) includes a permanent magnet (4), a magnet fixing shell (5), an RF coil (6), a probe housing (7), a connecting frame (8), a gear (9), and a limiting screw (10). The permanent magnet (4) is embedded in the magnet fixing shell (5), which is rotatably disposed inside the probe housing (7). Limiting rings (51) are provided at both the upper and lower ends of the magnet fixing shell (5), and the limiting rings (51) are clearance-fitted with the probe housing (7). The probe housing (7) is provided with threaded holes. When the bottom of the magnet fixing shell (5) and the bottom of the probe housing (7) are... When the wall contacts, the limiting screw (10) passes through the threaded hole on the probe housing (7), and the threaded head of the limiting screw (10) is located above the limiting ring (51) at the upper end of the magnet fixing shell (5). A gap is provided between the threaded head of the limiting screw (10) and the limiting ring (51) at the upper end of the magnet fixing shell (5). A connecting frame (8) is provided on the top end face of the magnet fixing shell (5). The connecting frame (8) has an integral rotating shaft. The gear (9) is fixedly sleeved on the rotating shaft of the connecting frame (8). The radio frequency coil (6) is fixedly installed on the inner wall of the probe housing (7).

3. The NMR system for large-scale geological density detection according to claim 2, characterized in that, The radio frequency coil (6) is a transceiver coil. When the probe (1) is in the main probe mode, the radio frequency coil (6) is in the transceiver mode. When the probe (1) is in the secondary probe mode, the radio frequency coil (6) is in the signal receiving mode.

4. The NMR system for large-scale geological density detection according to claim 3, characterized in that, The radio frequency coil (6) is a solenoid radio frequency coil, and the permanent magnet (4) is a regular hexagonal prism permanent magnet. The static magnetic field B0 generated by the permanent magnet (4) is orthogonal to the radio frequency field B1 generated by the radio frequency coil (6).

5. An NMR detection method for large-scale geological density detection, utilizing the NMR system described in claim 4, characterized in that, Includes the following steps: Step 1: Determine the number n of probes (1) in the probe group and the position of probes (1); Step 2: Based on the position of the probe (1), confirm the target detection area according to the target detection area classification rules; Step 3: Measure the geological density of each target detection area; Step 4: Summarize the geological density of each target detection area to obtain the geological density of the total detection area (2).

6. The NMR detection method for large-scale geological density detection according to claim 5, characterized in that, The number of probes (1) is n≥3. The probes (1) are distributed on a probe distribution circle with the center point of the total detection area as the center and the detection radius of the probe (1) as the radius, and the probe distribution circle is evenly divided into n equal parts.

7. The NMR detection method for large-scale geological density detection according to claim 6, characterized in that, The target detection area classification rules are as follows: The first type of target detection area: The target detection area is located between adjacent probes (1), and the center point of the target detection area coincides with the midpoint of the line connecting the center position of the adjacent probe (1); Second type of target detection area: The center point of the target detection area coincides with the center point of the total detection area (2); The third type of target detection area: The center point of the target detection area is located on the line connecting the center point of the total detection area and the center position of the probe (1), and is located at the midpoint of the line; The fourth type of target detection area: The center point of the target detection area is located on the extension line of the line connecting the center point of the total detection area and the center position of the probe (1), and the distance from the center point of the target detection area to the center position of the probe (1) is less than the detection radius of the probe (1).

8. The NMR detection method for large-scale geological density detection according to claim 7, characterized in that, The measurement of the geological density of each target detection area is based on the following steps: Step 3.1: Select a target detection area and determine the main probe and sub-probe of the target detection area during this measurement process; rotate the main probe based on the classification of the target detection area. When the number of main probes is 1, the static magnetic field generated by the permanent magnet (4) of the main probe in the target detection area is directed to the center of the target detection area. When the number of main probes is greater than 1, the static magnetic fields generated by the permanent magnets (4) of each main probe in the target detection area are superimposed to obtain the magnetic field enhancement effect. Step 3.2: Set the RF coil (6) in the main probe to transceiver mode and set the RF coil (6) in the secondary probe to receive signal mode; Step 3.3: Obtain the NMR relaxation signal from this measurement, perform inverse Laplace transform on the obtained NMR relaxation signal to obtain the transverse relaxation time T2, analyze the transverse relaxation time T2 and process the value and distribution of the transverse relaxation time T2 to obtain the geological density of the target detection area in this detection. Step 3.4: Select the next target detection area, and repeat steps 3.1 to 3.3 until the geological density of all target detection areas is obtained.

9. The NMR detection method for large-scale geological density detection according to claim 8, characterized in that, The main and secondary probes for determining the target detection area are based on the following rules: The m probes located in the area centered on the center point of the target detection area and with the detection radius of the probe (1) as the radius are the main probes, where m is greater than or equal to 1 and less than or equal to n, and the remaining nm probes are the auxiliary probes.

10. The NMR detection method for large-scale geological density detection according to claim 9, characterized in that, The classification rotating master probe based on the target detection area includes the following steps: The first type of target detection area: rotate the main probe so that the static magnetic field generated by the permanent magnet (4) in one main probe points to the center of the target detection area, and the static magnetic field generated by the permanent magnet (4) in the other main probe moves away from the center of the target detection area. The third type of target detection area: rotate the main probe so that the static magnetic field generated by the permanent magnet (4) inside the main probe points to the center of the target detection area; Fourth type of target detection area: Rotate the main probe so that the static magnetic field generated by the permanent magnet (4) inside the main probe points to the center of the target detection area; Second type of target detection area: When the number of probes (1) n is odd, rotate the main probe so that the static magnetic field generated by the permanent magnet (4) inside each main probe points to the center of the target detection area; when the number of probes (1) n is even, divide all the main probes into groups of two main probes on the same diameter of the probe distribution circle, rotate the main probe so that the static magnetic field generated by the permanent magnet (4) inside one of the main probes in the group points to the center of the target detection area, and the static magnetic field generated by the permanent magnet (4) inside the other main probe in the group moves away from the center of the target detection area.

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