Underground water flow velocity testing method

Abstract

The invention discloses a method for testing the flow velocity of underground water, which is used for testing the flow velocity of the underground water by using equipment containing an internal heating optical cable, and comprises the following steps of: implementing ATRT, namely placing the internal heating optical cable in a drill hole, backfilling filler in the drill hole, carrying out DTS temperature measurement calibration and carrying out subsequent heating operation, recording the temperature change of the heating optical cables at different depths within the heating duration t1 as a real value temperature rise response curve; data of the heating starting duration t0 are removed, a predicted value temperature rise response curve containing temperature changes is fitted within the heating duration t1-t0 according to the MILS theory, the RMSE between a predicted value and a true value is calculated, and when RMSEt is greater than RMSEt, the RMSEt is greater than RMSEt; fitting data are recorded at 0.1 K; calculating the flow velocity q based on the MILS theory, namely the underground water flow velocity; the method provided by the invention overcomes the influence of an optical cable sheath, a backfill material and the like on the underground water flow velocity measurement error, so that the result is more accurate.

Description

Technical Field

[0001] The present invention relates to the field of groundwater testing, and in particular, provides a method for testing the groundwater flow velocity. Background Art

[0002] Seepage refers to the flow phenomenon of fluids in porous media (such as rock and soil masses). In rock and soil masses, seepage may directly change their mechanical properties, reduce the stability of rock and soil masses, and trigger geological disasters such as landslides or ground collapses. In engineering construction, seepage can cause soil particles to be carried away by water flow, forming piping or quicksand, thereby destroying the stability of engineering structures. At the same time, seepage may also carry pollutants to migrate and pollute groundwater.

[0003] Traditional methods for obtaining the groundwater seepage velocity of rock and soil masses include pumping tests, direct monitoring methods using piezometers, current meters, etc., and indirect inversion methods represented by isotope tracer methods, temperature tracer methods, ground penetrating radar, and potentiometric methods. The pumping test is carried out in a closed borehole within a certain depth range. By continuously pumping water through a pumping well and monitoring the water level changes in nearby observation wells, the permeability coefficient (K) of the aquifer within this depth range is calculated in combination with a mathematical model. According to the monitored hydraulic head (J), the groundwater seepage velocity can be calculated. This method is suitable for the evaluation of the characteristics of large-scale aquifers, and has a simple principle and convenient calculation. However, because it requires simultaneous pumping and monitoring of the surrounding water levels, multiple boreholes are needed for pumping tests, which are costly and time-consuming. The measured groundwater seepage velocity is the average equivalent value within a certain depth range of the borehole, and the test results have multiple solutions and uncertainties, and cannot accurately reflect the groundwater seepage situation and the spatial heterogeneity of rock and soil masses. The isotope tracer method is to inject a tracer, such as fluorescent dye, isotope, etc., into the groundwater, and directly calculate the seepage velocity by measuring the arrival time and concentration change of the isotope tracer in the observation well. The seepage obtained by this method is the actual groundwater flow velocity (pore velocity), and can be converted into Darcy velocity according to the effective porosity of the rock and soil mass as needed. The results of this method are intuitive and suitable for complex geological conditions, but appropriate tracers and injection methods need to be selected to reduce the impact of environmental pollution, tracer adsorption and diffusion. The seepage meter method is a type of point sensor. By directly burying the sensor within the monitoring area, the groundwater flow velocity is monitored. The principle of the seepage meter often obtains the seepage velocity by measuring the flow rate of groundwater flowing into / out of the seepage meter. There are also seepage meters based on electrical methods for measuring the seepage velocity. Limited by the complexity of the underground environment, seepage meters are often difficult to deploy and have a limited monitoring range. The sensors buried underground are difficult to maintain and recover, and the monitoring cost is high. The isotope tracer method has the risk of polluting the environment. The potentiometric method and ground penetrating radar are easily affected by strong interference such as electromagnetic signal interference and high salinity areas during the monitoring process, and the measurement accuracy needs to be improved.

[0004] The temperature tracing method has the characteristics of simple theory, environmental friendliness, and convenient data collection, and is an important method for understanding the thermal conductivity of rock and soil masses and the underground water flow velocity. Among them, the in-situ thermal response test technology (TRT) is currently the main means to obtain the in-situ thermal conductivity of rock and soil masses. By circulating water with a constant power in the transducer pipe, based on the infinite line source (ILS) theory according to the water temperatures at the inlet and outlet, the average thermal conductivity of the rock and soil mass at the borehole is calculated. However, this method only obtains the average thermal conductivity of the rock and soil mass within the depth range of the transducer pipe.

[0005] To improve the test efficiency and obtain the thermal conductivity of rock and soil masses at different depths, the existing technology has used the thermal response test technology with actively heated optical fibers (ATRT). By integrating a temperature measurement optical cable based on distributed temperature sensing (DTS) as a temperature sensor and a heating cable and arranging them in the formation at the same time, heating the underground environment with a constant power in the form of an infinite line source hypothesis, and obtaining the thermal conductivity at different depths according to the temperature rise curves at different depths. Distributed measurement can accurately describe the variation of the thermal conductivity with depth caused by the spatial heterogeneity of the rock and soil mass. When groundwater movement exists, the groundwater flow will further accelerate the heat dissipation process in the rock and soil mass, thereby reducing the temperature rise observed at the heat source location. Based on this principle, it can be used to estimate the groundwater flow velocity.

[0006] However, there are still some technical problems in the existing technology when using the moving infinite line source (MILS) theory to calculate the groundwater seepage velocity: First, although the direct push method conforms to the MILS theory hypothesis, it is difficult to monitor the groundwater seepage at deeper depths, which limits its application scope; Second, the thermal resistance generated by the borehole backfill will affect the heat conduction process, making the groundwater seepage monitoring and calculation complex and reducing the measurement accuracy; In addition, different forms of heat sources and optical cable sheaths have a significant impact on the heat transfer process, making the thermal response test process not fully conform to the theoretical hypothesis of calculating the groundwater seepage velocity by the moving infinite line source theory, resulting in errors in the measurement results. These problems seriously restrict the accuracy and applicability of the groundwater flow velocity measurement method based on the MILS theory. Summary of the Invention

[0007] In order to overcome the defects of the existing technology, the purpose of the present invention is to provide at least one novel method for testing the groundwater flow velocity.

[0008] To solve the above problems, the present invention provides a method for testing the groundwater flow velocity. This method uses a device containing an internally heated optical cable to test the groundwater flow velocity. The method for testing the groundwater flow velocity includes the following steps:

[0009] S2. Implement ATRT, including placing the internal heating optical cable in the borehole, backfilling the borehole with filler, calibrating the DTS temperature measurement, and subsequent heating operations, and recording the temperature change of the heating optical cable at different depths within the heating duration t1 as the true value temperature rise response curve;

[0010] S3. According to the MILS theory, fit the predicted temperature rise response curve including temperature changes within the heating duration t1, and calculate the RMSE between the predicted value and the true value. When RMSE < 0.1K, record the fitting data and execute step S4; otherwise, stop executing step S4;

[0011] S4. Calculate the flow velocity q based on the MILS theory, which is the groundwater flow velocity.

[0012] In some embodiments, within the heating duration t1, the true value temperature rise response curve gradually increases, and the predicted value temperature rise response curve gradually increases.

[0013] In some embodiments, before executing step S2, the method further includes step S1, performing numerical analysis based on data information and the MILS theory to determine the finite time range corresponding to different flow velocity values and the corresponding temperature rise response curve conforming to the line heat source theory assumption;

[0014] Step S4 further includes: recording the assumed flow velocity q0 corresponding to the assumed temperature rise response curve corresponding to the predicted value temperature rise response curve. If the flow velocity q is consistent with the assumed flow velocity q0, the flow velocity q is recorded as the groundwater flow velocity q; otherwise, discard the flow velocity q.

[0015] In some embodiments, in step S4, calculating the flow velocity q based on the MILS theory includes the following steps:

[0016] When considering convective heat transfer, the one-dimensional heat diffusion equation satisfying the homogeneous assumption in the heat response process is:

[0017]

[0018] T is the temperature, K; t is the heating time, s; λ is the thermal conductivity of the rock and soil mass, W / (m·K); ρ is the density of the rock and soil mass, kg / m 3 ; c is the specific heat capacity of the rock and soil mass, J / (kg·K); q is the groundwater flow velocity, m / s; ρ w 、c w are respectively the density of water (kg / m 3 ) and the specific heat capacity (J / (kg·K));

[0019] When the water flow participates in heat transfer and reaches a steady state, there is:

[0020]

[0021] U is the equivalent velocity used to replace the moving velocity of the heat source, m / s; α is the thermal diffusivity of the rock and soil mass, m 2 / s;

[0022] Under the ideal condition of a constant heat input p (W / m) per unit length in the borehole, the instantaneous relative temperature change of the line heat source is:

[0023]

[0024] ΔT = T - T0, where T0 is the initial temperature, K; D is the thermal diffusivity of the homogeneous medium, m 2 / s; r is the radius of the heat source (m), W is the Hantush well function;

[0025] When the time t approaches infinity, the corresponding steady-state final temperature ΔT(t ∞ ) can be expressed as:

[0026]

[0027] K0 is the second-kind modified Bessel function of the zero order, and ΔT(t ∞ ) can be approximately reached after heating for a sufficient long time in the thermal response test, and its value is only related to the thermal conductivity and the groundwater flow velocity in the aquifer; then the temperature rise of the line heat source can be expressed by the relationship with ΔT(t ∞ ):

[0028]

[0029] The parameter ΔT(t ∞ ), A, r / B converge to a unique value. By combining Equation (0-25) and Equation (0-26), the calculation formula for the groundwater flow velocity can be obtained:

[0030]

[0031] In some embodiments, in step S1, the data information includes one or more of the heating power of the heating optical cable, the borehole diameter, and the filler.

[0032] In some embodiments, the heating power is 20 W / m, the borehole diameter is 0.065 m, and the filler is quartz sand.

[0033] In some embodiments, t0 is 3 h and t1 is 36 h.

[0034] In some embodiments, t0 is 6 h and t1 is 36 h.

[0035] The present invention also provides a method for testing the groundwater flow velocity. This method uses a device containing an internal heating optical cable to test the groundwater flow velocity. The method for testing the groundwater flow velocity includes the following steps:

[0036] S1. Perform numerical analysis based on data information and the MILS theory to determine the finite time range corresponding to different flow velocity values and the corresponding temperature rise response curve that conforms to the assumptions of the line heat source theory.

[0037] S2. Implement ATRT, including placing the internal heating optical cable in the borehole, backfilling the borehole with filler, calibrating the DTS temperature measurement, and subsequent heating operations, and record the temperature change of the heating optical cable at different depths within the heating duration t1 as the true value temperature rise response curve.

[0038] S3. Fit the predicted value temperature rise response curve including temperature changes within the heating duration t1 according to the MILS theory, and calculate the RMSE between the predicted value and the true value. When RMSE < 0.1K, record the fitting data and execute step S4; otherwise, stop executing step S4.

[0039] S4. Calculate the flow velocity q based on the MILS theory, record the assumed flow velocity q0 corresponding to the assumed temperature rise response curve corresponding to the predicted value temperature rise response curve. If the flow velocity q is consistent with the assumed flow velocity q0, then the flow velocity q is recorded as the groundwater flow velocity q; otherwise, discard the flow velocity q.

[0040] In some embodiments, in step S1, the data information includes one or more of the heating power of the heating optical cable, the borehole diameter, and the filler; and / or, the heating power is 20 W / m, the borehole diameter is 0.065 m, and the filler is quartz sand.

[0041] In some embodiments, in step S3, the heating duration t1 is continuous and uninterrupted, and the initial value is greater than 1 h, and the temperature rise does not reach the maximum value at the end point.

[0042] The beneficial effects of the present invention are as follows:

[0043] Compared with the prior art, the groundwater flow velocity testing method provided by the present invention separates the heat transfer stages of the thermal response test of actively heating the optical fiber in the borehole, and comprehensively considers the effects of the cable sheath, backfill, and surrounding rock and soil on the heat source diffusion in sequence. It separates the heat conduction stage that can ignore the effects of the sheath and backfill and conforms to the moving line heat source theory (MILS) to calculate the groundwater flow velocity. It proposes to use numerical simulation methods to determine the finite time range corresponding to different flow velocity values, and ensures the credibility of the calculation results by verifying the consistency between the calculated results and the preset groundwater flow velocity values and ranges. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1a It is a schematic diagram of the TRT device;

[0045] Figure 1b It is a schematic diagram of the ATRT device;

[0046] Figure 2a , Figure 2b is a schematic diagram of the temperature distribution of the ATRT device in a porous medium in the first embodiment of the present invention;

[0047] Figure 3 It is a schematic diagram for dividing different groundwater seepage temperature rise stages in the first embodiment of the present invention;

[0048] Figure 4 It is a flowchart of the groundwater flow velocity measurement method in the first embodiment of the present invention;

[0049] Figure 5 It is a schematic diagram of the results measured by the groundwater flow velocity measurement method provided by the present invention in the first embodiment of the present invention.

[0050] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. Specific Embodiments

[0051] The present invention can be further clearly understood through the following specific embodiments given, but they do not limit the present invention.

[0052] This embodiment provides a groundwater flow velocity measurement method. Field tests are carried out in an investigation borehole near a certain Yangtze River bank collapse section in Zhenjiang to measure the groundwater flow velocity in rock and soil masses at different depths. Copper mesh inner heating optical cables are vertically U-shaped arranged in the borehole and backfilled with low-permeability quartz sand for ATRT testing. When there is no groundwater movement, as the heating time increases, the temperature gradually accumulates in the rock and soil mass and the influence of the borehole can be ignored. The temperature response curve shows a linear trend in the latter part on a semi-logarithmic scale, and the temperature rise rate gradually reaches a constant value.

[0053] When there is groundwater movement, the temperature response curve becomes non-linear on a semi-logarithmic scale. As time increases, the temperature rise rate gradually decreases to zero to reach the maximum temperature rise. Under the action of groundwater flow, the heat generated by the heat source and the heat diffused into the rock and soil mass reach a dynamic equilibrium, and this equilibrium will be reached earlier as the flow velocity increases. At this time, heat can be injected into the surrounding rock and soil mass earlier and faster, and the heat source can meet the linear heat source hypothesis conditions earlier relative to the rock and soil mass. At this time, it is necessary to confirm the finite time range corresponding to the aforementioned dynamic equilibrium for different flow velocity values.

[0054] As shown in Figure 2, during the test, the ATRT in the backfilled borehole can be regarded as a transient heat transfer process of multi-layer media. The porous difference in temperature in the direction parallel to the line heat source is caused by the vertical thermal conductivity of the medium. The isotherms on the cable sheath near the heat source can be regarded as straight lines parallel to the line heat source, and the difference gradually becomes prominent outward. The temperature difference in the direction perpendicular to the line heat source is controlled by different materials. Under a constant heating power, the heat generated by the internally heated optical cable gradually diffuses outward due to the temperature gradient, accumulates in and around the borehole, and gradually decreases with the radial distance. The non-linear temperature gradient reflects the different thermophysical properties of the sheath, borehole backfill, and porous medium around the heat source. The presence of groundwater movement will further accelerate the heat dissipation process in the rock and soil mass, reducing the temperature rise observed at the heat source location.

[0055] During the thermal response test, heat gradually diffuses outward and accumulates among multi-layer media. According to the proportion of heat in each layer of media in the total heat, the heat transfer process can be divided into four stages. When only considering heat conduction, the four stages correspond respectively to:

[0056] ① Initial stage (Sc), this stage is mainly affected by the material of the internally heated optical cable, with a short duration and a rapid temperature rise. The heat source and the sheath are heated, reflecting the low thermal conductivity of the sheath.

[0057] ② Borehole-dominated heat transfer stage (Sb), at this time, the borehole dominates the heat transfer, and the heat propagates radially along the backfill.

[0058] ③ Stage of joint influence of borehole and rock and soil mass on heat transfer (Sj), as a transition stage, the proportion of heat absorbed by the backfill becomes less and less, and the influence of the rock and soil mass on heat transfer becomes greater and greater. The duration of this stage.

[0059] ④ Rock and soil mass-dominated heat transfer stage (Sg), in this stage, the rock and soil mass plays a dominant role in heat transfer, and the heat absorbed by the backfill can be ignored. Based on the temperature data of this stage, the heat source reaches the line heat source assumption condition relative to the rock and soil mass (under the ideal borehole condition, the line heat source is in direct contact with the porous medium without extra heat transfer medium), and the corresponding hydrothermal parameters can be solved based on the line heat source theory.

[0060] S1, in this embodiment, the device with a heating power of 20 W / m has a borehole diameter of 150 mm, and the filler is quartz sand with a permeability coefficient less than 0.0001 m / s. Based on the foregoing data and the MILS theory, numerical analysis is carried out to determine the finite time range corresponding to different flow velocity values and the corresponding temperature rise response curve that conforms to the line heat source theory assumption. This only needs to be done once for each different ATRT device, so a separate and comprehensive analysis of common ATRT devices will effectively simplify practical applications.

[0061] S2. Implement ATRT, including placing the internal heating optical cable in the borehole, backfilling the borehole with filler, DTS temperature measurement calibration, and subsequent heating operations. Record the temperature changes of the heating optical cable at different depths within 72 hours of heating, and create a two-dimensional temperature rise-time diagram.

[0062] S3. According to the Moving Infinite Line Sources (MILS) theory, fit the temperature change curve (i.e., the two-dimensional temperature rise-time diagram) within the previously defined preset time range, and simultaneously calculate the RMSE (RMSE, i.e., Root Mean Square Error, which is used to quantify the deviation between the predicted value and the true value, and is the square root of the mean of the squared prediction errors). At the start of the heating period, the RMSE will be ≥0.1 K because this short time is in the Sc - Sj stage, resulting in a deviation between the fitted curve and the measured temperature. When the threshold RMSE < 0.1 K, it indicates that the Sg stage has started, and the corresponding hydrothermal parameters can be solved. Record the fitting data and execute step S4; otherwise, stop executing step S4.

[0063] When considering convective heat transfer, the one-dimensional heat diffusion equation that satisfies the homogeneous assumption in the heat response process is:

[0064]

[0065] In the formula: T is the temperature, K; t is the heating time, s; λ is the thermal conductivity of the rock and soil mass, W / (m·K); ρ is the density of the rock and soil mass, kg / m 3 ; c is the specific heat capacity of the rock and soil mass, J / (kg·K); q is the groundwater flow velocity, m / s; ρ w , c w are the density of water (kg / m 3 ) and specific heat capacity (J / (kg·K)), respectively.

[0066] When heat transfer is jointly participated by water flow and reaches a steady state:

[0067]

[0068] In the formula: U is the equivalent velocity used to replace the moving velocity of the heat source, m / s; α is the thermal diffusivity of the rock and soil mass, m 2 / s.

[0069] According to the MILS theory assumptions: ① Under ideal drilling conditions, the line heat source is in direct contact with the porous medium, without any extra heat transfer medium; ② The porous medium is homogeneous and infinite; ③ The groundwater flow velocity in the porous medium is uniform and stable, conforming to Darcy flow; ④ The thermophysical properties of groundwater do not change with temperature. By solving the analytical solution of Equation (0-22), under the ideal condition of a constant heat input p (W / m) per unit length in the borehole, the instantaneous relative temperature change of the line heat source is as follows:

[0070]

[0071] In the formula: ΔT = T - T0, where T0 is the initial temperature, in K; D is the thermal diffusivity of the homogeneous medium, in m 2 / s; r is the radius of the heat source (m), and W is the Hantush well function.

[0072] When the time t approaches infinity, the corresponding steady-state final temperature ΔT(t ∞ ) can be expressed as:

[0073]

[0074] In the formula: K0 is the modified Bessel function of the second kind of order zero, and ΔT(t ∞ ) can be approximately reached after heating for a long enough time in the thermal response test, and its value is only related to the thermal conductivity in the aquifer and the groundwater flow velocity. By combining Equation (0-24) and Equation (0-28), the temperature rise of the line heat source can be expressed through the relationship with ΔT(t ∞ ) as:

[0075]

[0076] If the temperature rise in the thermal response test is fitted with Equation (0-29), the parameters ΔT(t ∞ ), A, and r / B converge to unique values. By combining Equation (0-25) and Equation (0-26), the calculation formula for the groundwater flow velocity can be obtained:

[0077]

[0078] Step S4: Calculate the flow velocity q based on Equation (0-31). If the flow velocity q is consistent with the corresponding flow velocity range in Step S1, then the flow velocity q is recorded as the effective flow velocity q; otherwise, discard the flow velocity q.

[0079] According to the test method proposed in this patent, the test device and test results are as Figure 5 shown. The maximum and minimum temperature rises recorded at 25.5 m in the silt layer and 75.5 m in the gravel layer correspond to the low-flow velocity and high-flow velocity sections respectively.

[0080] The depth of the borehole test is 82 m. The bottom highly permeable aquifer is mainly composed of loose sediments such as gravel and sand. According to the steady pumping test of a single incomplete well and the water level of the Yangtze River, the groundwater flow velocity in the aquifer within the depth range of 12.8 - 45.6 m is about 9×10 -7 -5×10 -6 m / s.

[0081] Within the depth range of 5 - 78 m, the average groundwater flow rate calculated in this patent is 3.6×10 -6 m / s. The groundwater flow velocity is usually relatively high in gravel sand, and the average flow rate can reach 4.0×10 -6 m / s, showing good consistency with the lithology of the formation. The measured groundwater flow velocity is relatively stable within the range of 5 - 48.8 m, with an average value of 3.3×10 -6 m / s, which is consistent with the results independently obtained from the pumping test within the aquifer range of 12.8 - 45.6 m (9×10 -7 m / s - 5×10 -6 m / s).

[0082] Compared with the prior art, the test method provided in this embodiment separates each heat transfer stage of the thermal response test of the actively heated optical fiber in the borehole, and comprehensively considers the effects of the cable sheath, backfill, and surrounding rock and soil on the heat source diffusion in sequence. It separates the heat conduction stage that can neglect the effects of the sheath and backfill and conforms to the moving line heat source theory (MILS) to calculate the groundwater flow velocity. It proposes to use the numerical simulation method to determine the finite time range corresponding to different flow velocity values, and ensures the credibility of the calculation results by verifying the consistency between the calculated results and the preset groundwater flow velocity values and ranges.

[0083] The above-disclosed are only the preferred embodiments of the present invention. Of course, the scope of the rights of the present invention cannot be limited thereby. Therefore, equivalent changes made according to the scope of the patent application of the present invention still fall within the scope covered by the present invention.

Claims

1. A method for testing groundwater flow velocity, the method using a device containing an internally heated optical cable to test groundwater flow velocity, characterized in that: The groundwater flow rate testing method comprises the following steps: S2, implement ATRT, including placing the internal heating cable in the borehole, backfilling the borehole with fillers, DTS temperature measurement calibration and subsequent heating operations, and record the temperature changes of the heating cables at different depths within the heating time t1 as the true value temperature rise response curve; S3, calculate the RMSE between the predicted value and the true value, and if the RMSE is ≥ 0.1K in the time period of the start heating time t0, remove the data of the start heating time t0, fit the predicted value temperature rise response curve containing temperature changes in the heating time t1-t0 according to the MILS theory, and calculate the RMSE between the predicted value and the true value. When the RMSE is < 0.1K, record the fitting data and execute step S4; otherwise, stop executing step S4; S4, the flow velocity q calculated based on MILS theory is the groundwater flow velocity.

2. The groundwater flow velocity testing method according to claim 1, characterized in that: During the heating time t1, the real value temperature rise response curve gradually increases, and the predicted value temperature rise response curve gradually increases.

3. The groundwater flow velocity testing method according to claim 1, characterized in that: Before executing step S2, the method further includes step S1, performing numerical analysis based on data information and MILS theory to determine a finite time range corresponding to different flow rate values ​​and a corresponding temperature rise response curve that conforms to the linear heat source theory assumption; The step S4 also includes: recording the assumed flow velocity q0 corresponding to the assumed temperature rise response curve corresponding to the predicted value temperature rise response curve; if the flow velocity q is consistent with the assumed flow velocity q0, the flow velocity q is recorded as the groundwater flow velocity q; otherwise, the flow velocity q is discarded.

4. The groundwater flow velocity testing method according to claim 1, characterized in that: In step S4, the flow velocity q is calculated based on the MILS theory, including the following steps: When considering convective heat transfer, the one-dimensional heat diffusion equation that satisfies the homogeneous assumption during the thermal response process is: T is temperature, K; t is heating time, s; λ is the thermal conductivity of rock and soil, W / (m·K); ρ is the density of rock and soil, kg / m 3 ; c is the specific heat capacity of rock and soil, J / (kg·K); q is the groundwater velocity, m / s; ρ w 、c w are the densities of water (kg / m 3 ) and specific heat capacity (J / (kg·K); When water flow participates in heat transfer and reaches a steady state: U is the equivalent velocity used to replace the moving velocity of the heat source, m / s; α is the thermal diffusion coefficient of the rock and soil, m 2 / s; Under ideal conditions of constant heat input p (W / m) per unit length in the borehole, the instantaneous relative temperature change of the line heat source is: ΔT=T-T0, T0 is the initial temperature, K; D is the thermal diffusion coefficient of the homogeneous medium, m 2 / s; r is the heat source radius (m), W is the Hantush well function; When time t tends to infinity, the corresponding steady-state final temperature ΔT(t ∞ ) can be expressed as: K0 is the second kind of zero-order modified Bessel function, ΔT(t ∞ ) can be approximately reached after a sufficiently long heating time in the thermal response test, and its value is only related to the thermal conductivity of the aquifer and the groundwater flow rate; then the temperature rise of the line heat source can be obtained by comparing ΔT(t ∞ ) is expressed as: Parameter ΔT(t ∞ ), A, r / B converge to a unique value, and the formula (0-25) and (0-26) can be used to calculate the groundwater velocity:

5. The groundwater flow velocity testing method according to claim 3, characterized in that: In step S1, the data information includes one or more of the heating power of the heating optical cable, the drilling hole diameter and the filler permeability coefficient.

6. The groundwater flow velocity testing method according to claim 5, characterized in that: The heating power is 10-50 W / m, the borehole diameter is 150 mm, the filler is selected from one or more of medium sand, fine sand, silt sand, clay balls, and cement, and the permeability coefficient of the filler is less than 0.0001 m / s.

7. The groundwater flow velocity testing method according to claim 5, characterized in that: The t0 is 3 hours, the t1 is 36 hours; and / or, The t0 is 6 hours, and the t1 is 36 hours.

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