Underground cavity detection method, system and readable medium

Abstract

The present invention relates to the field of underground detection and signal analysis technology, and in particular to an underground cavity detection method, system and readable medium. The underground cavity detection method proposed in the present invention detects the underground state of a road based on an electromagnetic detection method. The present invention sets two parallel insulated wires in the area to be detected, and loads an electric pulse signal at one end of the wire. If there is a cavity in the stratum between the parallel wires, there will be a difference in their permittivity. When the electric pulse signal passes through the capacitance change area, a pulse reflection signal will be generated, thereby judging whether there is a cavity in the stratum and other problems. The present invention has low detection cost, has a unique solution, and achieves extremely high detection accuracy.

Description

Technical Field

[0001] The present invention relates to the technical field of underground detection and signal analysis, and in particular to an underground cavity detection method, system and readable medium. Background Art

[0002] The issue of underground road safety has now become one of the major public safety issues in cities. The main problem with underground road safety is that various reasons lead to the loss of support for road structural stability, resulting in road collapse, which further affects public safety in ground transportation. The main reasons for the loss of support for underground road structural stability are: 1. The underground rainwater and sewage pipes in the road rupture, and water seeps into the soil layer below the roadbed. When the amount of water increases, it forms runoff, which slowly carries away the soil particles below the roadbed, causing the soil layer below the roadbed to gradually become loose. Because the surface of modern roads is a relatively rigid structure containing concrete, when the soil layer below gradually becomes loose, and the relatively rigid structure above temporarily supports the surface, the soil layer below will form cavities as it develops further. 1. When the relatively rigid structure above is not sufficient to support the ground, or when a heavy-loaded vehicle on the ground exceeds the relatively rigid structure above, the road will collapse, causing casualties and property losses; 2. The local groundwater level is high, and the foundation treatment is not carried out for some reason during road construction. The groundwater activity will carry away the soil particles under the road, gradually develop into cavities, and form collapse conditions; 3. Due to insufficient survey work during the construction of underground facilities such as subways, the stability of the soil layer above the road is disturbed, gradually becomes loose, and finally develops into cavities.

[0003] Currently, the primary method for detecting underground road safety issues is ground-penetrating radar (GPR). This method transmits electromagnetic waves into the ground. When dielectric differences exist in the subsurface structure, the electromagnetic waves are partially reflected. The radar antenna receives the reflected signals, stores them, and generates radar spectra. Further analysis of the spectra reveals changes in the subsurface structure. However, using GPR for underground road detection still presents the following challenges: 1. GPR uses electromagnetic wave reflections to detect underground cavities. Due to the complex underground structure, various media reflect electromagnetic waves, resulting in multiple interpretations of the electromagnetic reflection signals, which can interfere with the determination of cavities and lead to misjudgments. 2. GPR is a one-time method for road inspections. The measured data only represents the subsurface conditions at that moment and cannot capture the spatial and temporal evolution of the subsurface conditions. Even when retesting the same route over time, inconsistencies in equipment, personnel, and survey lines can hinder temporal and spatial comparison of data, making it difficult to predict the future health of the subsurface based on temporal and spatial relationships.

[0004] Another road safety detection method is the road curvature method. The road curvature method determines the underground conditions of a section of road through surface settlement caused by underground cavities or underground subsidence. This method cannot detect potential cavities that have not yet settled, and the detection effect of this type of problem is inadequate. Summary of the Invention

[0005] To overcome the above-mentioned shortcomings of the prior art in underground cavity detection, such as multiple solutions and low detection accuracy, the present invention proposes an underground cavity detection method that has low detection cost, a unique solution, and extremely high detection accuracy.

[0006] The present invention proposes a method for detecting underground cavities, comprising the following steps:

[0007] S1. Arrange two parallel wires on both sides of the area to be detected; select a signal receiver and a signal generator;

[0008] S2. Connect a signal receiver and a signal generator in parallel at the same end of two wires. The signal generator sends a periodic pulse signal to the wires, and the signal receiver receives the echo signal as the measured waveform. The measured waveform collected during the detection is obtained as the full-wave signal, and the measured waveform collected when the area to be detected is intact is obtained as the incident wave.

[0009] S3. Decompose the full wave into the incident wave and the echo signal; calculate the reflection starting point of the full wave relative to the incident wave; the reflection starting point is the point within one signal cycle where the echo signal has the maximum floating trend in the opposite phase relative to the incident first wave;

[0010] S4. Calculate the distance L between the hole and the starting point of the parallel wire based on the reflection starting point p ;

[0011]

[0012] t is the time corresponding to the starting point of reflection, the unit of t is nanosecond, d L is the set correction value.

[0013] Preferably, the periodic pulse signal emitted by the signal generator satisfies the following conditions: the signal duty cycle is less than or equal to 1:10, and the signal frequency f k Meet L / (c / f k )>0.1; where L is the length of the parallel section of the conductor and c is the speed of the electromagnetic wave.

[0014] Preferably, obtaining the reflection starting point includes the following steps:

[0015] S31, construct a time point sequence, sample the incident wave and the full wave, record the incident wave sampling signal as P0, and record the full wave sampling signal as P;

[0016] S32, calculating the reflected wave sequence P1=P-P0 in the full wave;

[0017] S33, perform Fourier transform on the reflected wave sequence P1 to obtain the reflected wave frequency domain

[0018] S34, read frequency domain Frequency peak sequence before attenuation

[0019] S35, frequency peak sequence Perform spline fitting, establish the fitting curve G, and calculate the frequency envelope of the fitting curve G (U,f U ), represents the amplitude of the kth sampling point on the fitting curve G, Represents the frequency of the kth sampling point on the fitting curve G;

[0020] S36. Set the amplitude parameter T min and T max And the frequency position parameters PosMin and PosMax, initialize PosMax=PosMin=1;

[0021] S37, determine whether it is satisfied The initial value of k is 1;

[0022] If yes, let T min Updated to PosMin is updated to k; then it is determined whether the envelope frequency sequence f is traversed U If no, then update k to k+1 and execute step S37 again; if yes, execute step S39;

[0023] Otherwise, T min Updated to Update PosMin to k and set T max Updated to T min , PosMax is updated to PosMin; then determine whether the envelope frequency sequence f is traversed U If yes, then update k to k+1 and execute step S38; if yes, execute step S39;

[0024] S38, determine whether it is satisfied

[0025] If yes, let T max Updated to PosMax is updated to k, and then it is determined whether the envelope frequency sequence f has been traversed. UIf yes, then update k to k+1 and execute step S38 again; if yes, execute step S39;

[0026] If not, update PosMax to k-1, and then execute step S39;

[0027] S39, select envelope frequency sequence f U The PosMaxth frequency point As a reference point, calculate the time t of the reflection start:

[0028]

[0029] Preferably, in S34, let:

[0030]

[0031] Indicates the first frequency peak, f1 indicates The corresponding frequency; Indicates the second peak frequency, f2 indicates The corresponding frequency; Indicates the sth frequency peak, f s express The corresponding frequency; f s / f1 is greater than the set threshold, which is set in the interval [10,100].

[0032] Preferably, in S2, when there is no measured waveform collected when the area to be detected is intact, the echo signal received by the signal receiver when the signal generator is directly connected to the signal receiver is used as the incident wave.

[0033] Preferred:

[0034] d L =p1·t 3 +p2·t 2 +p3·t+p4

[0035] Among them, t is the time corresponding to the starting point of reflection; p1, p2, p3, p 4为 A fixed value determined by the soil structure and properties.

[0036] Preferably, when the area to be inspected is located above a road pipe, the wires are arranged as follows: one wire is attached to an airbag, which is then placed into the pipe and inflated, so that the airbag carries the wire and extends along the pipe, and the wire is attached to the inner wall of the pipe; another wire is laid on the ground parallel to the pipe, so that the two wires are parallel to each other.

[0037] Preferably, the wire arrangement method is: vertically drill holes on both sides of the area to be detected, and then insert two wires into the holes respectively, so that the two wires are arranged vertically in parallel; or, during the road construction stage, parallel small PVC pipes are arranged above the soil cushion layer, with wires passed through the inside; after the road construction is completed, the wire reserved interface is retained for connecting the signal receiver and the signal generator, and the wires parallel to each other on both sides of the soil layer to be detected are connected to the signal generator and the signal receiver, and then the measured waveform is obtained.

[0038] The present invention proposes an underground cavity detection system, which includes a memory and a processor. The memory stores a computer program. The processor is connected to the memory. The processor obtains a full-wave signal and an incident wave, and executes the computer program to implement the underground cavity detection method.

[0039] The present invention provides a readable medium storing a computer program, which is used to implement the underground cavity detection method when executed.

[0040] The advantages of the present invention are:

[0041] (1) The underground cavity detection method proposed in the present invention detects the underground status of a road based on electromagnetic detection. The present invention sets two parallel insulated wires in the area to be detected, and applies an electric pulse signal to one end of the wires. If there is a cavity in the stratum between the parallel wires, there will be a difference in their permittivity. When the electric pulse signal passes through the capacitance change area, a pulse reflection signal is generated, thereby determining whether there is a cavity in the stratum.

[0042] (2) The present invention detects soil cavities in the area where the conductors pass through by changing the permittivity of the soil between the conductors. The signal is transmitted through the conductors, thus avoiding the multi-solution problem of electromagnetic wave reflection in radar detection.

[0043] (3) The present invention can be used to detect cavities in the soil above road pipelines, filling the current lack of effective long-term monitoring methods for underground road safety. The present invention can arrange conductors in key areas of concern, collect data regularly, and understand the dynamic changes in underground health through the changes in data. In this way, the underground cavity detection method proposed by the present invention, combined with the reserved conductors, can achieve long-term monitoring of underground cavities.

[0044] (4) The detection cost of the present invention only requires an electromagnetic pulse transceiver and a conductor, which has a significant cost advantage over expensive ground penetrating radar. Even if the present invention is used for long-term monitoring, it only requires the cost of drilling construction. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 This is a flow chart of the underground cavity detection method;

[0046] Figure 2Flowchart of the reflection starting point search method;

[0047] Figure 3 This is a schematic diagram of the test model of Example 1;

[0048] Figure 4 These are the echo waveforms of the six working conditions in Example 1;

[0049] Figure 5(a) shows the reflected wave waveform of working condition 3;

[0050] Figure 5(b) shows the frequency domain waveform of working condition 3;

[0051] Figure 5(c) shows the fitting curve G for working condition 3;

[0052] Figure 6(a) shows the reflected wave waveform of working condition 4;

[0053] Figure 6(b) shows the frequency domain waveform of working condition 4;

[0054] Figure 6(c) shows the fitting curve G for working condition 4;

[0055] Figure 7(a) shows the reflected wave waveform of working condition 5;

[0056] Figure 7(b) shows the frequency domain waveform of working condition 5;

[0057] Figure 7(c) is the fitting curve G for working condition 5. DETAILED DESCRIPTION

[0058] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0059] Example 1

[0060] Reference Figure 1 , an underground cavity detection method proposed in this embodiment includes the following steps:

[0061] S1. Arrange two parallel wires on both sides of the area to be inspected on the road pipeline; select an oscilloscope and a signal generator as the detection signal receiver and detection signal transmitter respectively;

[0062] For the area to be inspected above the road pipeline, when setting up the wires, one wire can be attached to an airbag, which is then placed into the pipeline and inflated, so that the airbag carries the wire and extends along the pipeline, and the wire is attached to the inner wall of the pipeline; the other wire is laid on the ground parallel to the pipeline, thus forming two wires parallel to each other.

[0063] The signal generator is used to generate and transmit periodic pulse signals. The signal duty cycle is not greater than 1:10, and the frequency selection meets the following conditions: L / (c / f k )>0.1; L is the length of the conductor to be tested, that is, the length of the parallel section of the conductor; c is the speed of the electromagnetic wave 3e8m / s; f k is the preselected detection frequency, i.e. the pulse frequency of the signal generator; if the ratio L / (c / f k ) is greater than 0.1, the detection frequency is appropriate, otherwise reselect the frequency f k , until the ratio L / (c / f k ) is greater than 0.1.

[0064] S2. Obtain the full-wave signal and incident wave corresponding to the area to be inspected; connect the two wiring points of the signal generator and the two wiring points of the oscilloscope at the beginning of the two wires to form a structure in which the signal generator and the oscilloscope are connected in parallel at the beginning of the wires; the signal generator sends a periodic pulse signal to the wires, the oscilloscope receives and displays the echo signal, and the oscilloscope display signal is obtained as the measured waveform; the full-wave signal is the measured waveform obtained during the inspection;

[0065] When the signal generator is directly connected to an oscilloscope, the waveform measured by the oscilloscope is used as the original waveform;

[0066] The periodic pulse signal sent by the signal generator to the conductor adopts the parameters set above, that is, the signal duty cycle is not greater than 1:10 and the frequency is f k .

[0067] There are two types of incident waves: the first is the measured waveform collected when the area to be inspected is intact (i.e., there are no holes); the second is the original waveform. In specific implementations, the first type of incident wave is preferred.

[0068] S3. Decompose the full wave into incident wave and echo signal; calculate the reflection starting point of the full wave relative to the incident wave;

[0069] The reflection starting point is the point with the maximum floating trend of the echo signal relative to the incident first wave within one signal cycle;

[0070] The incident first wave is the first waveform of the incident wave.

[0071] S4. Calculate the distance L between the hole and the starting point of the parallel wire based on the reflection starting point p ;

[0072]

[0073] t is the time corresponding to the starting point of reflection, the unit of t is nanosecond, d L is the set correction value.

[0074] Obviously, if there is no reflection starting point, it means that there is no hole, or the hole is located at the starting point or end point of the parallel wire.

[0075] Correction value d L It is related to the structural characteristics of the soil layer. Considering that the soil layer structure and characteristics also affect the signal propagation, the correction value d is constructed. L The mapping relationship of the time t corresponding to the starting point of reflection is as follows:

[0076] d L =p1·t 3 +p2·t 2 +p3·t+p4

[0077] p1, p2, p3, p 4为 The fixed value is determined by the soil structure and characteristics, and its influence changes are only affected by the road structure design. For common strata, p1 is 1.1807e+21, p2 is -7.2901e+13, p3 is 4.1529e+06, and p4 is 2.276. The selection of p1, p2, p3, and p4 coefficients for more sites only needs to be calibrated according to local measured data.

[0078] In this embodiment, the inflection point of the curve in a single cycle on the echo waveform is obtained, and compared with the incident first wave waveform, the point where the peak absolute value of the inflection point of the echo waveform with the opposite trend to the incident first wave waveform is the minimum is taken as the reflection starting point; Figure 2 ,The method for finding the reflection starting point specifically includes the following steps.

[0079] S31, construct a time point sequence to sample the incident wave and the full wave;

[0080] The incident wave sampling signal is denoted as Represents the sampling value at the i-th time point on the incident wave waveform; n is the set number of sampling points;

[0081] The full-wave sampling signal is denoted as Represents the sampling value at the i-th time point on the full-wave waveform;

[0082] S32, let the inverse phase of the incident wave P0 be recorded as P0', P0'=-P0; combine the full wave P and the incident wave P0 to calculate the reflected wave sequence P1 in the full wave;

[0083]

[0084] S33, perform Fourier transform on the reflected wave sequence P1 to obtain the reflected wave frequency domain fft means Fourier transform;

[0085] S34, read frequency domain Frequency peak sequence before attenuation

[0086]

[0087] Indicates the first peak frequency, choose The first frequency peak point after 0, f1 represents The corresponding frequency; Indicates the second peak frequency, f2 indicates The corresponding frequency; Indicates the sth frequency peak, f s express The corresponding frequency;

[0088] The denser the frequency peak sampling is, the higher the calculation accuracy is, but the calculation cost is also higher. The sampling interval can be set as needed, that is, f s / f1 is greater than the set threshold, and the set threshold can be taken in the interval [10,100].

[0089] S35, frequency peak sequence Perform spline fitting to establish a fitting curve G; extract H sampling points from the fitting curve, and let the kth sampling point on the fitting curve be Indicates frequency, Indicates the amplitude; combined with all sampling points Forming the frequency envelope (U,f U ); U is the envelope amplitude sequence, f U is the envelope frequency sequence;

[0090]

[0091] S36. Set the amplitude parameter T min and T max And the frequency position parameters PosMin and PosMax, initialize PosMax=PosMin=1;

[0092] S37, determine whether it is satisfied The initial value of k is 1;

[0093] If yes, let T min Updated to PosMin is updated to k; then it is determined whether the envelope frequency sequence f is traversed U If no, then update k to k+1 and execute step S37 again; if yes, execute step S39;

[0094] Otherwise, Tmin Updated to Update PosMin to k and set T max Updated to T min , PosMax is updated to PosMin; then determine whether the envelope frequency sequence f is traversed U If yes, then update k to k+1 and execute step S38; if yes, execute step S39;

[0095] S38, determine whether it is satisfied

[0096] If yes, let T max Updated to PosMax is updated to k, and then it is determined whether the envelope frequency sequence f has been traversed. U If yes, then update k to k+1 and execute step S38 again; if yes, execute step S39;

[0097] If not, update PosMax to k-1, and then execute step S39;

[0098] S39, select envelope frequency sequence f U The PosMaxth frequency point As a reference point, calculate the time t of the reflection start:

[0099]

[0100] In this embodiment, the test model parameters are as follows: the length between the two observation wells of the pipeline is 20m, where m represents the unit meter; that is, the parallel section length of the conductor L = 20m, the depth of the pipe top from the ground is 1m, a conductor is placed above the pipeline using an airbag, and holes are drilled at 5m, 10m, and 15m respectively to place 100uF capacitors simulating defects. The simplified model is shown as follows Figure 3 As shown; the signal generator transmits a periodic pulse signal with a frequency of f k =4 MHz, duty cycle is 10%. The test model has:

[0101] L / (c / f k )=20 / [3×10 8 / (4×10 6 )]=0.2667>0.1, meeting the test conditions.

[0102] In this embodiment, six working conditions are set:

[0103] Working condition 1: The signal generator is directly connected to the oscilloscope, and the original waveform is measured as the incident wave;

[0104] Working condition 2: No simulated defect capacitor is placed in the model, and the signal generator and oscilloscope are connected to measure the actual waveform without defects as the full-wave signal;

[0105] Condition 3: Place a defective capacitor at 5m, connect the signal generator and oscilloscope, and measure the actual waveform as the full-wave signal;

[0106] Condition 4: Place a defective capacitor at a distance of 10 m, connect a signal generator and an oscilloscope, and measure the actual waveform as the full-wave signal;

[0107] Condition 5: Place a defective capacitor at a distance of 15 m, connect the signal generator and oscilloscope, and measure the actual waveform as the full-wave signal;

[0108] Working condition 6: Place a defective capacitor at a distance of 20 m, connect the signal generator and oscilloscope, and measure the actual waveform as the full-wave signal.

[0109] In this embodiment, the oscilloscope displays signals under various working conditions as follows: Figure 4 shown.

[0110] In this embodiment, Figure 4 The full-wave signals collected in the middle working conditions 2-6 are analyzed using the above steps S31-S39.

[0111] During the full-wave signal analysis of working condition 3, the waveform of the reflected wave sequence P1 is shown in Figure 5(a), the reflected wave frequency domain is shown in Figure 5(b), and the frequency domain envelope is shown in Figure 5(c).

[0112] During the full-wave signal analysis of working condition 4, the waveform of the reflected wave sequence P1 is shown in Figure 6(a), the reflected wave frequency domain is shown in Figure 6(b), and the frequency domain envelope is shown in Figure 6(c).

[0113] During the full-wave signal analysis of working condition 5, the waveform of the reflected wave sequence P1 is shown in Figure 7(a), the reflected wave frequency domain is shown in Figure 7(b), and the frequency domain envelope is shown in Figure 7(c).

[0114] In this embodiment, the correction value calculation formula p1, p2, p3, and p4 are based on common formation parameters in combination with experimental conditions, i.e., p1 = 1.1807e+21, p2 = -7.2901e+13, p3 = 4.1529e+06, p4 = 2.276, d L =p1·t 3 +p2·t 2 +p3·t+p4.

[0115] In this embodiment, the calculation results of each working condition are summarized as follows:

[0116] The calculation result of the reflection starting point of working condition 5 is t = 1.2750e-07 seconds. Substituting it into the correction value calculation formula, the correction value d is obtained.L =4.0675; the calculation result of the defect is: tc / 2-d L =1.2750e-07×3×10 8 / 2-4.0675=15.05 meters, which is consistent with the defect capacitor being set at 15 meters;

[0117] The calculation result of the reflection starting point of working condition 4 is t = 8.4672e-08 seconds. Substituting it into the correction value calculation formula, the correction value d is obtained. L =2.82, the calculation result of the defect is: tc / 2-d L =8.4672e-08×3×10 8 / 2-2.82=9.88 meters, which is consistent with the defect capacitor being set at 10 meters;

[0118] The calculation result of the reflection starting point of working condition 3 is t = 5.0335e-08 seconds. Substituting it into the correction value calculation formula, the correction value d is obtained. L =2.45, the calculation result of the defect is: tc / 2-d L =5.0335e-08×3×10 8 / 2-2.45=5.09 meters, which is consistent with the defective capacitor being set at 5 meters.

[0119] It can be seen that in this embodiment, the calculation results under various working conditions are very close to the true values, which proves the effectiveness of the detection method.

[0120] During specific implementation, correction values ​​can be set based on experience and actual measurement results to further improve calculation accuracy.

[0121] Example 2

[0122] Compared with Example 1, this embodiment drills vertical holes on both sides of the area to be detected, and then inserts two wires into the holes respectively, so that the two wires are arranged vertically and parallel. Obviously, the wire arrangement method of this embodiment is applicable to any scenario.

[0123] Example 3

[0124] In this embodiment, during the road construction phase, small PVC pipes with parallel distribution are arranged above the soil cushion layer, and wires are passed through the inside; after the road construction is completed, a reserved interface for the wires is retained for connecting the oscilloscope and the signal generator, and the wires parallel to each other on both sides of the soil layer to be tested are connected to the signal generator and the oscilloscope, and then the measured waveform is obtained.

[0125] In this embodiment, the wires are reserved during the construction phase, and there is no need to set up the wires during the subsequent detection process. It is only necessary to connect the wires to the oscilloscope and the signal generator, which is simple and quick.

[0126] Of course, it will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, but also encompasses the same or similar structures that can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims, not the foregoing description, and it is intended that all variations that fall within the meaning and range of equivalents of the claims be encompassed within the present invention. Any reference signs in the claims should not be construed as limiting the claim to which they relate.

[0127] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

[0128] The technology, shape, and structure not described in detail in the present invention are all well-known technologies.

Claims

1. A method for detecting underground cavities, characterized in that: The following steps are involved: S1. Arrange two parallel wires on both sides of the area to be detected; select a signal receiver and a signal generator; S2. Connect a signal receiver and a signal generator in parallel at the same end of two wires. The signal generator sends a periodic pulse signal to the wires, and the signal receiver receives the echo signal as the measured waveform. The measured waveform collected during the detection is obtained as the full-wave signal, and the measured waveform collected when the area to be detected is intact is obtained as the incident wave. S3. Decompose the full wave into the incident wave and the echo signal; calculate the reflection starting point of the full wave relative to the incident wave; the reflection starting point is the point within one signal cycle where the echo signal has the maximum floating trend in the opposite phase relative to the incident first wave; S4. Calculate the distance L between the hole and the starting point of the parallel wire based on the reflection starting point p ; t is the time corresponding to the starting point of reflection, the unit of t is nanosecond, d L is the set correction value.

2. The underground cavity detection method according to claim 1, wherein: The periodic pulse signal emitted by the signal generator meets the following conditions: the signal duty cycle is less than or equal to 1:10, and the signal frequency f k Meet L / (c / f k )>0.1; where L is the length of the parallel section of the conductor and c is the speed of the electromagnetic wave.

3. The underground cavity detection method according to claim 1, wherein: Obtaining the reflection starting point includes the following steps: S31, construct a time point sequence, sample the incident wave and the full wave, record the incident wave sampling signal as P0, and record the full wave sampling signal as P; S32, calculating the reflected wave sequence P1=P-P0 in the full wave; S33, perform Fourier transform on the reflected wave sequence P1 to obtain the reflected wave frequency domain S34, read frequency domain Frequency peak sequence before attenuation S35, frequency peak sequence Perform spline fitting, establish the fitting curve G, and calculate the frequency envelope of the fitting curve G (U,f U ), represents the amplitude of the kth sampling point on the fitting curve G, Represents the frequency of the kth sampling point on the fitting curve G; S36. Set the amplitude parameter T min and T max And the frequency position parameters PosMin and PosMax, initialize PosMax=PosMin=1; S37, determine whether it is satisfied The initial value of k is 1; If yes, let T min Updated to PosMin is updated to k; then it is determined whether the envelope frequency sequence f is traversed U ; Otherwise, update k to k+1 and execute step S37 again; If yes, proceed to step S39; Otherwise, T min Updated to Update PosMin to k and set T max Updated to T min , PosMax is updated to PosMin; then determine whether the envelope frequency sequence f is traversed U If yes, then update k to k+1 and execute step S38; if yes, execute step S39; S38, determine whether it is satisfied If yes, let T max Updated to PosMax is updated to k, and then it is determined whether the envelope frequency sequence f has been traversed. U If yes, then update k to k+1 and execute step S38 again; if yes, execute step S39; If not, update PosMax to k-1, and then execute step S39; S39, select envelope frequency sequence f U The PosMaxth frequency point As a reference point, calculate the time t of the reflection start:

4. The underground cavity detection method according to claim 3, wherein: In S34, let: Indicates the first frequency peak, f1 indicates The corresponding frequency; Indicates the second peak frequency, f2 indicates The corresponding frequency; Indicates the sth frequency peak, f s express The corresponding frequency; f s / f1 is greater than the set threshold, which is set in the interval [10,100].

5. The underground cavity detection method according to claim 1, wherein: In S2, when there is no measured waveform collected when the to-be-detected area is intact, the echo signal received by the signal receiver when the signal generator is directly connected to the signal receiver is used as the incident wave.

6. The underground cavity detection method according to claim 1, wherein: d L =p1·t 3 +p2·t 2 +p3·t+p4 Among them, t is the time corresponding to the starting point of reflection; p1, p2, p3, p 4为 A fixed value determined by the soil structure and properties.

7. The underground cavity detection method according to claim 1, wherein: When the area to be inspected is located above a road pipeline, the wires are arranged as follows: one wire is attached to an airbag, which is then placed in the pipeline and inflated, allowing the airbag to carry the wire and extend along the pipeline, attaching the wire to the inner wall of the pipeline; the other wire is laid on the ground parallel to the pipeline, so that the two wires are parallel to each other.

8. The underground cavity detection method according to claim 1, wherein: The wire arrangement method is as follows: vertical holes are drilled on both sides of the area to be detected, and then two wires are inserted into the holes respectively, so that the two wires are arranged vertically and parallel; or, during the road construction stage, parallel small PVC pipes are arranged above the soil cushion layer, with wires passed through the inside; after the road construction is completed, the wire reserved interface is retained for connecting the signal receiver and the signal generator, and the parallel wires on both sides of the soil layer to be detected are connected to the signal generator and the signal receiver, and then the measured waveform is obtained.

9. An underground cavity detection system, characterized in that: It includes a memory and a processor, the memory stores a computer program, the processor is connected to the memory, the processor obtains the full-wave signal and the incident wave, and executes the computer program to implement the underground cavity detection method as described in any one of claims 1-8.

10. A readable medium, characterized in that A computer program is stored, and when the computer program is executed, it is used to implement the underground cavity detection method according to any one of claims 1 to 8.

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