Method for analyzing acoustic properties of frozen soil by using low-field nuclear magnetic resonance and device thereof
By setting probe buffer blocks of NMR-compatible material at both ends of the frozen soil sample, and combining acoustic wave and low-field NMR testing, the error and interference problems in the measurement of the acoustic properties of frozen soil were solved, the accurate measurement of the unfrozen water content of frozen soil was realized, and the reliability of the data and the ability to support activities in the frozen soil area were improved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- QINGDAO INST OF MARINE GEOLOGY
- Filing Date
- 2023-11-28
- Publication Date
- 2026-06-09
Smart Images
Figure CN117571835B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to a method and apparatus for jointly testing and analyzing the acoustic properties of frozen soil using both acoustic wave and low-field nuclear magnetic resonance, and belongs to the field of frozen soil testing. Background Technology
[0002] Currently, approximately 24% of the Earth's Northern Hemisphere is covered by permafrost. The characteristics of permafrost play a crucial role in human activities such as infrastructure construction, major engineering projects, and resource exploration and development in these regions. Existing research indicates that permafrost warming and degradation can trigger severe geological disasters such as rock slope collapses, roadbed settlement, and tunnel collapses. At the same time, permafrost regions contain abundant fossil fuels, such as oil and natural gas hydrates. Therefore, studying the physical properties of different types of permafrost; determining the freeze-thaw zone, the upper limit of permafrost, and permafrost thickness; and understanding permafrost structure and the distribution of surface ice are important prerequisites for human activities in permafrost regions.
[0003] The characteristics of permafrost include physical properties (such as unfrozen water, ice, and porosity) and mechanical properties (such as bulk modulus and shear modulus, or compression and shear wave velocities). Acoustic testing is one of the effective methods for studying the physical and mechanical properties of permafrost, offering advantages such as speed, simplicity, and non-destructive testing. Information on the propagation velocity and attenuation characteristics of sound waves in permafrost can be used to determine parameters such as the dynamic elastic modulus of permafrost, and is also significant for seismic and acoustic logging exploration in permafrost regions. The pores of permafrost contain ice, unfrozen water, and small amounts of gas (including water vapor), which have only a minor impact on the properties of permafrost. The phase equilibrium of pore water changes under the influence of climatic temperature: ice content increases as temperature decreases, while the amount of unfrozen water increases as temperature increases. Changes in the percentage of solid (ice) and liquid (water) components of pore water affect the acoustic properties of permafrost.
[0004] Current technologies for testing the acoustic properties of permafrost typically rely on established empirical relationships between sound wave velocity and unfrozen water content, leading to significant errors in the test results. Therefore, it is necessary to accurately determine the specific unfrozen water content. Existing testing methods for measuring unfrozen water content include time domain reflectance (TDR), differential scanning calorimetry, and nuclear magnetic resonance (NMR).
[0005] The time-domain reflectometry method requires determining the soil's dielectric constant and establishing different calibration curves to determine the soil's water content. The differential scanning calorimetry method performs a series of calculations based on the measured heat input value to obtain the unfrozen water content. Both methods are relatively cumbersome and subject to numerous limitations. The low-field nuclear magnetic resonance method measures the free induction decay of hydrogen nuclei in a magnetic field and calculates the unfrozen water content based on the ratio of signal intensity to liquid water. This method is widely used by research institutions due to its high accuracy and fast testing speed.
[0006] Although there are separate cases and related studies on acoustic property testing and low-field NMR testing of permafrost, joint acquisition is quite difficult. The main reason is that in-situ testing acoustic probes can interfere with NMR signals, and non-in-situ testing poses a challenge to the reliability of test data.
[0007] In view of the above, this patent application is hereby filed. Summary of the Invention
[0008] The method and apparatus for combined low-field nuclear magnetic resonance analysis of the acoustic properties of permafrost described in this invention aim to solve the problems existing in the prior art by combining two different types of testing methods, acoustic wave and low-field nuclear magnetic resonance, to obtain quantitative analytical indicators between the acoustic properties of permafrost and the content of unfrozen water, so as to effectively improve the analysis capability and data accuracy of the acoustic properties of permafrost.
[0009] To achieve the above design objectives, the method of applying low-field nuclear magnetic resonance combined analysis of the acoustic properties of frozen soil adopts solid buffer method acoustic wave testing, and probe buffer blocks made of nuclear magnetic resonance compatible material are set at both ends of the frozen soil sample.
[0010] The testing method includes the following steps.
[0011] Step A: Calibration of the acoustic travel time at both ends of the frozen soil sample;
[0012] In step A, a pair of acoustic probes and probe buffer blocks are set at both ends of the frozen soil sample; the acoustic probes and probe buffer blocks are then clamped together.
[0013] The arrival time of the first wave after emitting a sound pulse is denoted as t, and the natural time of the ultrasonic probe system is denoted as t0. Then, the sound wave travel time of the probe buffer blocks at both ends of the frozen soil sample is denoted as t1=t-t0.
[0014] Step B: Sound wave test;
[0015] In step B, the water-containing frozen soil sample is loaded into the testing device, and the pore pressure is introduced to measure the transverse relaxation spectrum curve of the frozen soil sample to obtain the initial free induction decay (FID) signal.
[0016] Adjust the acoustic probe and test the acoustic signal of the frozen soil sample to calculate the longitudinal and transverse wave velocities at this time. The formula for calculating the longitudinal and transverse wave velocities is: where L is the distance between the upper and lower acoustic probes at both ends of the frozen soil sample, t is the arrival time of the first wave read from the waveform signal of the acoustic test, t0 is the inherent time of the ultrasonic probe system, and t1 is the time required for the acoustic wave to penetrate the probe buffer blocks at both ends of the frozen soil sample.
[0017] Step C: Calculate the unfrozen water content and simulate the low-temperature freezing and thawing processes for the frozen soil samples.
[0018] in accordance with Calculate the unfrozen water content under frozen or warmed conditions, where It represents the percentage of unfrozen water at the freezing or warming temperature T. It is the FID value of the frozen sample at the freezing or heating temperature T; It is the FID value of a fully saturated sample at room temperature;
[0019] Step D: Test and analyze the changes in sound wave velocity. During the freezing and thawing processes of the frozen soil sample at low temperature, the transverse relaxation spectrum curve is measured using a nuclear magnetic resonance spectrometer to obtain the free induction decay (FID) signal after freezing or heating. The sound wave signal of the frozen soil sample is tested using ultrasound to calculate the longitudinal and transverse wave velocities.
[0020] Step E: Quantifying acoustic properties. The acoustic properties of the temperature, unfrozen water content, and corresponding sound wave test data obtained during the freezing and thawing processes in the above test steps are expressed by the following formulas related to the unfrozen water content: ;
[0021] Among them, W u V is the unfrozen water content, and V is the measured velocity of sound. f The ultrasonic velocity was measured under completely frozen conditions.
[0022] V d =V f -V u This represents the difference in ultrasonic velocity between the point of complete freezing and the point of freezing, or between the point of heating and the point of heating.
[0023] The nuclear magnetic resonance compatible materials include non-magnetic, biocompatible, and / or electronically inactive materials.
[0024] Based on the above-mentioned method of using low-field nuclear magnetic resonance (NMR) combined analysis of the acoustic properties of frozen soil, this application proposes a novel testing device, including a reaction vessel connected to an external low-field NMR analyzer, and a pore pressure control module, a confining pressure control module, and a data measurement module of the testing system connected to both ends of the frozen soil sample, respectively. The frozen soil sample is placed in a fluorinated oil ring-filled cavity inside the reaction vessel, and an RF coil is wrapped around the outer periphery of the reaction vessel body. An upper end cover and a lower end cover are respectively provided at both ends of the reaction vessel. An upper probe buffer block and an upper acoustic probe, and a lower probe buffer block and a lower acoustic probe are respectively connected in pairs at both ends of the frozen soil sample. An axial pore pressure outlet, a fluorinated oil outlet, and a thermocouple connected to the frozen soil sample are provided in the upper end cover of the reaction vessel, and an axial pore pressure inlet and a fluorinated oil inlet are provided in the lower end cover of the reaction vessel.
[0025] The upper and lower end covers of the reactor are both the same as the outer diameter of the reactor body.
[0026] The inner diameters of the upper and lower end covers of the reactor are respectively sealed to the upper acoustic probe, the upper probe buffer block, the lower acoustic probe, and the lower probe buffer block through several sealing rings.
[0027] The confining pressure control module includes a confining pressure circulation pipeline consisting of a normal temperature and normal pressure fluorinated oil storage tank, a confining pressure loading pump, a low temperature water bath, a low temperature and high pressure fluorinated oil storage tank, and a high pressure fluorinated oil circulation pump connected in sequence.
[0028] The pore pressure control module includes a high-pressure gas cylinder and a pore pressure pipeline equipped with a pore pressure sensor. High-pressure gas enters the frozen soil sample through the pore pressure pipeline.
[0029] The data measurement module includes a data acquisition unit that connects and collects data from the lower acoustic probe, upper acoustic probe, confining pressure sensor, pore pressure sensor, thermocouple, and nuclear magnetic resonance signal acquisition system.
[0030] In summary, the method and apparatus for combined low-field NMR analysis of the acoustic properties of permafrost have the following advantages: This application combines low-field NMR testing with the acoustic analysis of permafrost samples using the solid buffer method, thereby accurately measuring and analyzing the quantitative relationship between the unfrozen water content and the acoustic properties of permafrost. This overcomes the shortcomings of existing technologies that use a single measurement technique, and improves the reliability and accuracy of acoustic measurement results through in-situ data, thus providing solid support for infrastructure construction and resource exploration in permafrost areas. Attached Figure Description
[0031] Figure 1 This is a cross-sectional structural diagram of the testing device for analyzing the acoustic properties of frozen soil using low-field nuclear magnetic resonance as described in this application;
[0032] Figure 2 For applications such as Figure 1 The structure and flowchart of the acoustic combined low-field NMR analysis system of the test device shown;
[0033] Figure 3 This is a comparison of the intensity of free-inducible decay (FID) signals measured by low-field NMR; among them, Figure 3 'a' represents the signal graph before freezing. Figure 3 b represents the signal graph during freezing. Figure 3 c represents the signal diagram after freezing;
[0034] Figure 4 This is a comparison diagram of waveforms obtained from acoustic wave testing; among them, Figure 4 'a' represents the waveform before freezing. Figure 4 b represents the waveform during freezing. Figure 4 c represents the waveform after freezing;
[0035] Figure 5 This is a comparison graph showing the changes in unfrozen water content and acoustic signal with temperature during freezing and thawing; among them, Figure 5 'a' is a schematic diagram of the changes during the freezing process. Figure 5 b is a schematic diagram of the changes during the thawing process;
[0036] In the above figures, 1. Frozen soil sample; 2. Radio frequency coil; 3. Fluorinated oil ring filling cavity; 4. Lower acoustic probe; 5. O-ring seal; 6. Thermocouple; 7. Pore pressure inlet; 8. Fluorinated oil inlet; 9. Fluorinated oil outlet; 10. Probe pressure block; 11. Upper end cover of reactor; 12. Lower end cover of reactor; 13. Foam insulation layer; 14. Upper acoustic probe; 15. Pore pressure outlet; 16. Lower probe buffer block; 17. Upper probe buffer block; 18. Frozen soil acoustic property testing device; 19. High-pressure gas cylinder; 20. Normal temperature and pressure fluorinated oil storage tank; 21. Confining pressure loading pump; 22. Low temperature water bath; 23. Low temperature and high pressure fluorinated oil storage tank; 24. Confining pressure sensor; 25. Pore pressure sensor; 26. High pressure fluorinated oil circulation pump; 27. Data acquisition device; Detailed Implementation
[0037] The present invention will be further described below with reference to the accompanying drawings and embodiments.
[0038] Example 1, such as Figures 1 to 5 As shown, this application proposes a method for analyzing the acoustic properties of permafrost using low-field nuclear magnetic resonance (NMR). This method addresses the deficiency in existing technologies where acoustic wave testing for evaluating the important parameter of unfrozen water content cannot establish a quantitative relationship between acoustic wave parameters and unfrozen water content. To address this, this application proposes combining low-field NMR techniques with acoustic wave testing to jointly analyze and derive the quantitative relationship between the acoustic properties of permafrost and the unfrozen water content.
[0039] Specifically, the acoustic wave testing method adopted in this application is the solid buffer method, in order to avoid the interference of the hydrogen-containing components on the nuclear magnetic signal of the frozen soil sample within a certain range caused by the metal probe tightly coupled to the frozen soil sample in the existing acoustic wave testing.
[0040] At both ends of the frozen soil sample 1, probe buffer blocks (including lower probe buffer block 16 and upper probe buffer block 17) made of nuclear magnetic resonance compatible material are set.
[0041] The nuclear magnetic resonance compatible materials include, but are not limited to, non-magnetic materials (such as polymers, ceramics, and glass), biocompatible materials (such as polylactic acid and titanium alloys), and electronically inactive materials (such as plastics and glass fibers). In this embodiment, a lightweight material (such as polyetheretherketone) is preferred to adapt to the high-pressure internal testing environment and will not interfere with the nuclear magnetic resonance signal.
[0042] The testing method includes the following steps:
[0043] Step A: Calibration of acoustic travel time at both ends of the frozen soil sample
[0044] A pair of acoustic probes and probe buffer blocks were set at both ends of the frozen soil sample.
[0045] Clamp the acoustic probe and the probe buffer block together;
[0046] The arrival time of the first wave after emitting a sound pulse is denoted as t, and the natural time of the ultrasonic probe system is denoted as t0. Then, the sound travel time of the probe buffer blocks at both ends of the frozen soil sample is denoted as t1=t-t0.
[0047] Step B, Sound Wave Test
[0048] A water-containing frozen soil sample is placed into a testing device, and after pore pressure is introduced, the transverse relaxation spectrum curve of the frozen soil sample is measured to obtain the initial free induction decay (FID) signal.
[0049] Adjust the acoustic probe and test the acoustic signal of the frozen soil sample to calculate the longitudinal and transverse wave velocities. The formulas for calculating the longitudinal and transverse wave velocities are as follows: Where L is the distance between the upper and lower acoustic probes at both ends of the frozen soil sample, and t is the arrival time of the first wave read from the waveform signal of the acoustic test (e.g., Figure 4 (As shown in the figure), t0 is the inherent time of the ultrasonic probe system, and t1 is the time required for the sound wave to penetrate the probe buffer blocks at both ends of the frozen soil sample.
[0050] Step C: Calculate the unfrozen water content
[0051] The freezing and thawing processes of frozen soil samples were simulated separately.
[0052] in accordance with Calculate the unfrozen water content under frozen or warmed conditions, where The percentage of unfrozen water at freezing or warming temperature T; FID value of frozen sample at freezing or heating temperature T; It is the FID value of a fully saturated sample at room temperature;
[0053] Step D: Test and analyze the changes in sound wave velocity
[0054] During the freezing and thawing processes of frozen soil samples, the transverse relaxation spectrum curves were measured using a nuclear magnetic resonance spectrometer to obtain the free induction decay (FID) signal after freezing or thawing; the acoustic signal of the frozen soil samples was tested using ultrasound to calculate the longitudinal and transverse wave velocities.
[0055] Analysis of the above test and analysis process of sound wave velocity change shows that:
[0056] D1. The free induction decay (FID) signal intensity continuously decreases during the freezing process of the frozen soil sample, indicating that the unfrozen water content gradually decreases (e.g., Figure 3 (as shown)
[0057] D2. During the freezing process of the frozen soil sample, the arrival time of the first wave of the acoustic test signal gradually decreased and the acoustic velocity gradually increased, indicating that most of the pore water condensed into ice (e.g., Figure 4 (as shown)
[0058] D3. During the thawing process of the frozen soil sample, the intensity of the free induction decay (FID) signal gradually increased, indicating that the unfrozen water content gradually increased.
[0059] D4. During the thawing process of the frozen soil sample, the arrival time of the first wave in the acoustic test signal gradually increased and the acoustic velocity gradually decreased, indicating that the unfrozen water content gradually increased.
[0060] Step E: Quantifying acoustic properties
[0061] The data obtained from the above test steps, including temperature and unfrozen water content during freezing and thawing, as well as the corresponding acoustic wave test data, can be expressed using the following formulas related to acoustic properties and unfrozen water content: ;
[0062] Among them, W u V is the unfrozen water content, and V is the measured velocity of sound. f The ultrasonic velocity was measured under completely frozen conditions.
[0063] V d =V f -V u The difference in ultrasonic velocity between the time of complete freezing and the time of freezing, or between the time of heating and the time of heating.
[0064] To implement the aforementioned method for analyzing the acoustic properties of permafrost using low-field nuclear magnetic resonance, this application proposes the following... Figure 1 The apparatus for testing the acoustic properties of permafrost shown, and as shown Figure 2 The test system shown.
[0065] The aforementioned frozen soil acoustic property testing device 18 is a measuring device that combines acoustic wave testing and low-field nuclear magnetic resonance analysis of acoustic properties. The measuring device includes a reaction vessel connected to an external low-field nuclear magnetic resonance analyzer, and the frozen soil sample 1 is connected to the pore pressure control module, confining pressure control module and data measurement module of the testing system at both ends.
[0066] Specifically, a frozen soil sample 1 is placed in the fluorinated oil ring cavity 3 inside the reactor, a radio frequency coil 2 is wrapped around the outer periphery of the reactor body, and an upper end cover 11 and a lower end cover 12 are respectively placed at both ends of the reactor.
[0067] At both ends of the frozen soil sample 1, the upper probe buffer block 17 and the upper acoustic probe 14, and the lower probe buffer block 16 and the lower acoustic probe 4 are connected in pairs.
[0068] The upper end cover 11 of the reactor is provided with an axial pore pressure outlet 15 and a fluorinated oil outlet 9, as well as a thermocouple 6 for connecting the frozen soil sample 1. The lower end cover 12 of the reactor is provided with an axial pore pressure inlet 7 and a fluorinated oil inlet 8.
[0069] Furthermore, the upper end cover 11 and the lower end cover 12 of the reactor are both the same as the outer diameter of the reactor body;
[0070] Furthermore, the inner diameters of the upper end cover 11 and the lower end cover 12 of the reactor are respectively sealed to the upper acoustic probe 14 and the upper probe buffer block 17, and the lower acoustic probe 4 and the lower probe buffer block 16 through several O-rings 5; wherein, the upper probe buffer block 17 and the lower probe buffer block 16 are respectively used to avoid interference of the metal ultrasonic probe to the low-field nuclear magnetic resonance signal;
[0071] Furthermore, the outer ends of the upper end cover 11 and the lower end cover 12 of the reactor are respectively connected to the probe pressure block 10 via internal threads. The probe pressure block 10 can adjust the distance between the upper acoustic probe 14 and the lower acoustic probe 4 according to the size of the frozen soil sample 1, so as to tightly couple the probe buffer blocks at both ends with the frozen soil sample 1.
[0072] Furthermore, the outer periphery of the upper end cover 11 and the lower end cover 12 of the reactor is covered with a heat insulation layer 13 (such as a foam insulation layer), that is, the area on the outer periphery of the reactor body that is not covered by the radio frequency coil 2 is covered with the heat insulation layer 13.
[0073] The confining pressure control module is used to achieve dual control of confining pressure (i.e., fluorinated oil) pressure and temperature. It includes a confining pressure circulation pipeline consisting of a normal temperature and normal pressure fluorinated oil storage tank 20, a confining pressure loading pump 21, a low temperature water bath 22, a low temperature and high pressure fluorinated oil storage tank 23, and a high pressure fluorinated oil circulation pump 26 connected in sequence.
[0074] The confining pressure circulating fluid is fluorinated oil without nuclear magnetic resonance signal. The confining pressure loading pump 21 pumps the fluorinated oil in the normal temperature and pressure fluorinated oil storage tank 20 to the confining pressure circulation pipeline. The confining pressure circulation pipeline is cooled inside the low temperature water bath 22. The cooled fluorinated oil is circulated in the confining pressure circulation pipeline by the high pressure fluorinated oil circulation pump 26.
[0075] When the valve is opened, the confining pressure loading pump 21 pumps the fluorinated oil from the ambient temperature and pressure fluorinated oil storage tank 20 to the confining pressure circulation pipeline and pressurizes it to the set confining pressure value. The confining pressure circulation pipeline is cooled inside the low-temperature water bath 22, and the cooled confining pressure liquid circulates in the confining pressure circulation pipeline through the high-pressure fluorinated oil circulation pump 26, creating a low-temperature environment suitable for water freezing within the fluorinated oil confining pressure chamber inside the reactor. Specifically, to ensure smooth circulation by the high-pressure fluorinated oil circulation pump 26, a low-temperature high-pressure fluorinated oil storage tank 23 is installed at the inlet of the high-pressure fluorinated oil circulation pump. This eliminates the need for an additional cooling circulation system, reducing system weight and preventing the additional cooling layer outside the reactor from affecting NMR penetration.
[0076] The pore pressure control module is used to connect the high-pressure gas cylinder and the reaction vessel to provide the required pore pressure. It includes a high-pressure gas cylinder 19 and a pore pressure pipeline equipped with a pore pressure sensor 25. High-pressure gas enters the frozen soil sample 1 through the pore pressure pipeline.
[0077] The data measurement module includes a data acquisition unit 27 that connects and collects data from a lower acoustic probe 4, an upper acoustic probe 14, a confining pressure sensor 24, a pore pressure sensor 25, a thermocouple 6, and an NMR signal acquisition system.
[0078] Using the above-mentioned permafrost acoustic property testing device and system, acoustic property measurement and analysis can be carried out according to the following steps:
[0079] (1) Connect the lower acoustic probe 4, the lower probe buffer block 16, the probe pressure block and the lower end cover 12 of the reactor in sequence and install them on the reactor body;
[0080] (2) Install frozen soil sample 1, connect the upper acoustic probe 14, probe pressure block and upper end cover 11 of the reactor and install them in the reactor;
[0081] (3) Connect the reactor to the low-field nuclear magnetic resonance analyzer;
[0082] (4) Connect the lower acoustic probe 4 and the upper acoustic probe 14, the confining pressure sensor 24, the pore pressure sensor 25, the thermocouple and 6 to the data acquisition unit 27 respectively.
[0083] (5) Connect the ambient temperature and pressure fluorinated oil storage tank 20, the confining pressure loading pump 21, the low temperature water bath 22, the low temperature and high pressure fluorinated oil storage tank 23, the high pressure fluorinated oil circulation pump 26 and valves, and introduce the confining pressure.
[0084] (6) Connect the high-pressure gas cylinder 19 and its valve, and close the inlet when the pressure reaches the set value;
[0085] (7) Turn on the air bath temperature control box and the circulating water bath chiller to control the temperature of the high-pressure reaction chamber 1;
[0086] (8) Perform low-field nuclear magnetic resonance data acquisition and ensure the normal operation of the 27 data acquisition unit;
[0087] (9) As the temperature of the frozen soil sample decreases, the acoustic characteristics of the frozen soil during the freezing process and the change in the unfrozen water content can be detected. Temperature and pressure conditions can be controlled to raise the temperature of the frozen soil sample and obtain the acoustic characteristics of the frozen soil during the thawing process.
[0088] Through the above testing method, this embodiment can achieve the joint collection of acoustic parameters of frozen soil and the content of unfrozen water in frozen soil samples during the freezing or thawing process. The measurement data is more accurate, the operation is simple, and the test can be repeated.
[0089] As described above, similar technical solutions can be derived from the solutions presented in conjunction with the accompanying drawings and description, and all of them still fall within the scope of the claims of the present invention.
Claims
1. A method for analyzing the acoustic properties of frozen soil using low-field nuclear magnetic resonance spectroscopy, characterized in that: The solid buffer method was used for acoustic wave testing, with probe buffer blocks made of nuclear magnetic resonance compatible material placed at both ends of the frozen soil sample; The testing method includes the following steps. Step A: Calibration of the acoustic travel time at both ends of the frozen soil sample; In step A, a pair of acoustic probes and probe buffer blocks are set at both ends of the frozen soil sample; the acoustic probes and probe buffer blocks are then clamped together. The arrival time of the first wave after emitting a sound pulse is denoted as t, and the natural time of the ultrasonic probe system is denoted as t0. Then, the sound wave travel time of the probe buffer blocks at both ends of the frozen soil sample is denoted as t1=t-t0. Step B: Sound wave test; In step B, the water-containing frozen soil sample is loaded into the testing device, and the pore pressure is introduced to measure the transverse relaxation spectrum curve of the frozen soil sample to obtain the initial free induction decay (FID) signal. Adjust the acoustic probe and test the acoustic signal of the frozen soil sample to calculate the longitudinal and transverse wave velocities at this time. The formula for calculating the longitudinal and transverse wave velocities is: where L is the distance between the upper and lower acoustic probes at both ends of the frozen soil sample, t is the arrival time of the first wave read from the waveform signal of the acoustic test, t0 is the inherent time of the ultrasonic probe system, and t1 is the time required for the acoustic wave to penetrate the probe buffer blocks at both ends of the frozen soil sample. Step C: Calculate the unfrozen water content and simulate the low-temperature freezing and thawing processes for the frozen soil samples. in accordance with Calculate the unfrozen water content under frozen or warmed conditions, where It represents the percentage of unfrozen water at the freezing or warming temperature T. It is the FID value of the frozen sample at the freezing or heating temperature T; It is the FID value of a fully saturated sample at room temperature; Step D: Test and analyze the changes in sound wave velocity. During the freezing and thawing processes of the frozen soil sample at low temperature, the transverse relaxation spectrum curve is measured using a nuclear magnetic resonance spectrometer to obtain the free induction decay (FID) signal after freezing or heating. The sound wave signal of the frozen soil sample is tested using ultrasound to calculate the longitudinal and transverse wave velocities. Step E: Quantifying acoustic properties. The acoustic properties of the temperature, unfrozen water content, and corresponding sound wave test data obtained during the freezing and thawing processes in the above test steps are expressed by the following formulas related to the unfrozen water content: ; Among them, W u V is the unfrozen water content, and V is the measured velocity of sound. f The ultrasonic velocity was measured under completely frozen conditions. V d =V f -V u This represents the difference in ultrasonic velocity between the point of complete freezing and the point of freezing, or between the point of heating and the point of heating.
2. The method for analyzing the acoustic properties of frozen soil using low-field nuclear magnetic resonance as described in claim 1, characterized in that: The nuclear magnetic resonance compatible materials include non-magnetic, biocompatible, and / or electronically inactive materials.
3. The testing apparatus for the method of low-field nuclear magnetic resonance combined analysis of the acoustic properties of permafrost as described in claim 1 or 2, characterized in that: The system includes a reaction vessel connected to an external low-field nuclear magnetic resonance analyzer, and the frozen soil sample is connected to the pore pressure control module, confining pressure control module, and data measurement module of the testing system at both ends. A frozen soil sample was placed in the fluorinated oil ring cavity inside the reactor. A radio frequency coil was wrapped around the outer periphery of the reactor body. An upper end cover and a lower end cover were respectively installed at both ends of the reactor. Connect the upper probe buffer block and upper acoustic probe, and the lower probe buffer block and lower acoustic probe to the two ends of the frozen soil sample respectively. The upper end cover of the reactor is equipped with an axial pore pressure outlet, a fluorinated oil outlet, and a thermocouple for connecting the frozen soil sample. The lower end cover of the reactor is equipped with an axial pore pressure inlet and a fluorinated oil inlet.
4. The testing apparatus according to claim 3, characterized in that: The upper and lower end covers of the reactor are both the same as the outer diameter of the reactor body.
5. The testing apparatus according to claim 3, characterized in that: The inner diameters of the upper and lower end covers of the reactor are respectively sealed to the upper acoustic probe, the upper probe buffer block, the lower acoustic probe, and the lower probe buffer block through several sealing rings.
6. The testing apparatus according to claim 3, characterized in that: The confining pressure control module includes a confining pressure circulation pipeline consisting of a normal temperature and normal pressure fluorinated oil storage tank, a confining pressure loading pump, a low temperature water bath, a low temperature and high pressure fluorinated oil storage tank, and a high pressure fluorinated oil circulation pump connected in sequence.
7. The testing apparatus according to claim 3, characterized in that: The pore pressure control module includes a high-pressure gas cylinder and a pore pressure pipeline equipped with a pore pressure sensor. High-pressure gas enters the frozen soil sample through the pore pressure pipeline.
8. The testing apparatus according to claim 3, characterized in that: The data measurement module includes a data acquisition unit that connects and collects data from the lower acoustic probe, upper acoustic probe, confining pressure sensor, pore pressure sensor, thermocouple, and nuclear magnetic resonance signal acquisition system.