A non-contact detection system and method for soil moisture content based on antenna sensor

Abstract

The present invention relates to a non-contact soil moisture content detection system and method based on an antenna sensor, belonging to the technical field of soil property measurement. The system comprises five steps: Step S1, selecting an appropriate antenna sensor frequency and ground-penetrating radar center frequency based on the detected soil moisture content range and depth; Step S2, using the ground-penetrating radar to detect the spectrum response of the antenna sensor in the air and obtain the minimum value of the spectrum signal. f a Step S3, using ground penetrating radar to detect the spectrum response of the antenna sensor buried in the ground to obtain the minimum value of the spectrum signal f d ; Step S4, according to the formula e r =( f a / f d ) 2 Calculate the dielectric constant of the measured stratum; Step S5, calculate the water content of the measured stratum using the Topp formula. The present invention has the advantage of non-contact water content detection, and reduces the difficulty of signal processing based on the traditional water content detection method based on ground penetrating radar.

Description

Technical Field

[0001] The present invention belongs to the technical field of soil property measurement, and in particular relates to a non-contact detection system and method for soil moisture content based on an antenna sensor. Background Art

[0002] Moisture content has a significant impact on the mechanical properties of soil. Changes in soil moisture content directly affect the safety of underground engineering structures (such as pile foundations of buildings and bridges, slopes, and tunnels).

[0003] In the prior art, the determination of soil moisture content usually adopts the drying method, tensiometer method, X-ray method, acoustic wave method, heat capacity method, spectroscopy method and dielectric method. Among them, the dielectric method is based on the dielectric properties of soil, and has produced a variety of methods for measuring soil moisture content, mainly including microwave absorption method, frequency domain decomposition method, high-frequency capacitance method, time domain reflectometry and standing wave ratio method. Among these methods, time domain reflectometry (TDR), frequency domain decomposition method (FDR), high-frequency capacitance method and standing wave ratio method are relatively mature, but they all belong to contact detection. The probe or detector of the contact detection method needs to be inserted into the soil to be tested. The contact state between the probe and the soil has a great influence on the test results, and the depth of contact measurement is limited, which makes it difficult to apply on a large scale.

[0004] Ground-penetrating radar (GPR) has been rapidly adopted in nondestructive testing in recent years due to its advantages, including wide-area continuous detection, rapid speed, non-destructiveness, and low cost. The GPR hardware system primarily consists of a host computer, transmitting antenna, receiving antenna, display unit, and control unit. The host computer generates high-frequency electromagnetic waves within a certain range (1 MHz to 2 GHz). These waves are emitted by the transmitting antenna into the dielectric layer. As they propagate underground, they encounter targets or interfaces with different electrical properties, causing reflection, refraction, and transmission. The receiving antenna then receives the echoes.

[0005] Patent CN117008125A discloses a method for predicting the moisture content of loose aggregates using non-contact ground-penetrating radar (GPR). This method uses an artificial neural network to establish a predictive model between the moisture content of the loose aggregate and electromagnetic signals. The GPR's electromagnetic signals are then used to predict the moisture content. This method requires a training dataset for the neural network prediction model, which is costly and time-consuming, and the model's accuracy requires further verification.

[0006] Invention CN101915771A discloses a method for measuring the moisture content of sandy soils using ground-penetrating radar (GPR). Ground-penetrating radar collects radar wave information from a soil profile, and the soil's dielectric constant is measured based on changes in electromagnetic wave velocity, thereby calculating the moisture content. The dielectric constant measured by this method is an average value across the soil profile. The test results and accuracy are susceptible to uneven moisture content, and strict testing procedures are required (the common midpoint method and the fixed spacing method).

[0007] Invention CN113740354A discloses a method for measuring soil moisture content in coal gangue reclamation sites. This method pre-embeds aluminum ingots at different depths along a radar survey line, measures the electromagnetic wave velocity when reflected by the ingots, and establishes a relationship model between electromagnetic wave velocity and moisture content. This method enables soil moisture measurement using ground-penetrating radar. While this invention uses aluminum ingots to enhance the reflected signal, it still essentially uses electromagnetic wave velocity to predict soil moisture. Summary of the Invention

[0008] The purpose of the present invention is to solve the problems existing in the above-mentioned background technology and provide a non-contact detection system and method for soil moisture content based on antenna sensors. This method has the advantages of non-contact detection of moisture content and reduces the difficulty of signal processing on the basis of traditional moisture content detection methods based on ground penetrating radar.

[0009] To achieve the above-mentioned purpose, the technical solution of the present invention is: a non-contact soil moisture content detection system based on an antenna sensor, comprising: an antenna sensor and a ground penetrating radar; the antenna sensor and the ground penetrating radar are connected via wireless electromagnetic waves; wherein,

[0010] The antenna sensor is a passive device whose sensing body is a receiving antenna with a fixed resonant frequency, which can convert spatial electromagnetic wave signals into alternating current on the transmission line;

[0011] The ground-penetrating radar transmits an electromagnetic wave signal of a predetermined frequency spectrum from a transmitting antenna. After the electromagnetic wave attenuates and diffuses, it reaches an antenna sensor pre-buried in the measured stratum. The antenna sensor absorbs electromagnetic waves in the predetermined frequency band and reflects electromagnetic waves in the remaining frequency bands, which are then received by the ground-penetrating radar's receiving antenna. The ground-penetrating radar analyzes the frequency spectrum changes of the reflected wave to obtain the dielectric constant of the stratum around the antenna sensor, and then calculates the water content of the stratum.

[0012] In one embodiment of the present invention, the antenna sensor is in the form of a dipole antenna or a patch antenna.

[0013] In one embodiment of the present invention, the ground penetrating radar further includes a computer console connected to the transmitting antenna and the receiving antenna. The computer console includes a host and a display unit and a control unit connected to the host.

[0014] The present invention also provides a non-contact detection method for soil moisture content based on an antenna sensor, which uses the system described above and includes the following steps:

[0015] Step S1: Select the resonant frequency f of the antenna sensor according to the detected soil moisture content range and soil depth h. a0 and the GPR center frequency f D0 ;

[0016] Step S2: Use ground penetrating radar to detect the spectrum response of the antenna sensor in the air and obtain the minimum value of the spectrum signal f a That is, a ground penetrating radar is used on the ground to detect the antenna sensor exposed to the air, and the echo spectrum reflected by the antenna sensor is measured. The echo spectrum is analyzed to obtain the minimum value f of the spectrum signal. a ;

[0017] Step S3: Use ground penetrating radar to detect the spectrum response of the antenna sensor buried in the ground to obtain the minimum value of the spectrum signal f d That is, the antenna sensor is buried at a depth h in the stratum to be measured, and the ground penetrating radar is used to transmit an electromagnetic wave signal of a specific spectrum, and the echo signal reflected by the antenna sensor in the stratum is received, and the minimum value f of the spectrum signal is obtained by analysis. d ;

[0018] Step S4: According to formula ε r =(f a / f d ) 2 Calculate the dielectric constant of the measured formation; that is, according to the minimum value f of the spectrum signal of the antenna sensor in the air and the formation a and f d , using the formula ε r =(f a / f d ) 2 Calculate the dielectric constant ε at the measured formation h r ;

[0019] Step S5: Calculate the water content of the measured stratum using the Topp formula; that is, according to the dielectric constant ε of the soil r , use the Topp formula to calculate the soil moisture content of the measured stratum

[0020] In one embodiment of the present invention, in step S1, a resonant frequency f a0 The antenna sensor can receive electromagnetic wave signals with a frequency of f a0 The electromagnetic waves nearby, the reflected signal frequency is far away from f a0 The electromagnetic wave, so the response function of the antenna sensor in the air to the electromagnetic wave spectrum signal of the ground penetrating radar is T a (f) is expressed as,

[0021]

[0022] Where S a0 is the antenna sensor at the frequency resonance point f a0 Absorption coefficient, 1-S a0 The antenna sensor is at the frequency resonance point f a0 The reflection coefficient, ia0 It is a bandwidth parameter related to the antenna sensor structure.

[0023] In one embodiment of the present invention, in step S2, the ground penetrating radar transmits a Gaussian pulse signal, whose spectrum function is S(f), and the frequency response functions of the ground penetrating radar transmitting antenna and receiving antenna are At(f) and Ar(f) respectively, then the received signal spectrum R a (f) is,

[0024] R a (f) = S(f)gAt(f)gAr(f)gT a (f) (2)

[0025] Find the spectrum function R of the received signal a The minimum point of (f) is recorded as f a .

[0026] In one embodiment of the present invention, in step S2, the spectrum function S(f) of the Gaussian pulse signal emitted by the ground penetrating radar is,

[0027]

[0028] p is a parameter determined by the time width of the pulse signal. The transmitting and receiving antennas of the ground penetrating radar can be regarded as filtering the electromagnetic wave signal. Since the transmitting and receiving antennas are symmetrical, the frequency response function A is t (f)A r (f) is expressed as,

[0029]

[0030] Among them, f D0 is the center frequency of the ground penetrating radar, S D is the reflection coefficient of the frequency resonance point, i D is a parameter determined by the bandwidth of the GPR transceiver antenna; the received signal spectrum R a (f) is,

[0031]

[0032] When S a0 ≈1, R a The minimum point f of (f) a satisfy,

[0033] f a ≈f a0 (6).

[0034] In one embodiment of the present invention, after the antenna sensor is buried at a depth h in the soil layer to be measured, the ground penetrating radar also uses a Gaussian pulse signal to detect the antenna sensor, as follows:

[0035] Compared with the air, the response function T of the antenna sensor to the electromagnetic spectrum signal of the ground penetrating radar d (f) is expressed as,

[0036]

[0037] Where S d0 is the frequency resonance point f of the antenna sensor in the formation d0 Absorption coefficient, 1-S d0 is the frequency resonance point f of the antenna sensor in the formation d0 The reflection coefficient, i d0 is the bandwidth parameter of the antenna sensor in the stratum; the electromagnetic wave signal of the ground penetrating radar experiences attenuation and diffusion in the soil layer, and the corresponding frequency response function P(f) is,

[0038] P(f)=f B exp(Af+C) (8)

[0039] Among them, the parameters A, B, and C are related to the electromagnetic characteristic parameters of the soil layer and the propagation distance d, so the spectrum of the ground penetrating radar receiving signal R d (f) is,

[0040] R d (f)=S(f)gAt(f)gAr(f)gT(d)gP(f) (9)

[0041] Substituting formulas (7) and (8) into formula (9), we get

[0042]

[0043] When S d0 ≈1, parameter A<150, the received spectrum signal R d The minimum point f of (f) d satisfy,

[0044] f d ≈f d0 (10).

[0045] Compared to existing technologies, the present invention offers the following advantages: The proposed system and method for non-contact soil moisture detection based on antenna sensors, combined with ground-penetrating radar, enables remote, non-contact detection of the dielectric constant of the soil surrounding the embedded antenna sensor. The dielectric constant is then used to convert the moisture content at the depth of the measured soil layer. The embedded sensor significantly reduces the difficulty of processing the ground-penetrating radar's reflected signal and improves the accuracy of moisture detection. The antenna sensors employed in the present invention are inexpensive and readily available for large-scale deployment. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 Schematic diagram of a non-contact soil moisture detection system according to an embodiment of the present invention;

[0047] Figure 2 1 is a schematic diagram of an antenna sensor in one embodiment of the present invention;

[0048] Figure 3 is a flow chart of an embodiment of the present invention;

[0049] Figure 4 The mathematical principle of the ground penetrating radar detection antenna sensor in one embodiment of the present invention;

[0050] Figure 5 1 is a time domain and frequency domain diagram of a ground penetrating radar pulse signal in one embodiment of the present invention;

[0051] Figure 6 is a spectrum diagram of the ground penetrating radar system itself in one embodiment of the present invention;

[0052] Figure 7 The artificially generated echo signal spectrum of the antenna sensor in the air detected by the ground penetrating radar in one embodiment of the present invention;

[0053] Figure 8 : is a frequency spectrum diagram of the frequency response function P(f) of a ground penetrating radar signal due to electromagnetic wave attenuation and diffusion in one embodiment of the present invention;

[0054] Figure 9 This is an artificially generated spectrum diagram of the echo signal of the antenna sensor in the ground penetrating radar detecting the ground layer in one embodiment of the present invention. DETAILED DESCRIPTION

[0055] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings.

[0056] like Figure 1 As shown, the system required for implementing the non-contact soil moisture detection method of the present invention includes an antenna sensor and a ground-penetrating radar. The detection system works as follows: The ground-penetrating radar transmits electromagnetic wave signals with a specific frequency spectrum from its transmitting antenna. After attenuation and diffusion, the electromagnetic waves reach the antenna sensor pre-buried in the measured stratum. The antenna sensor absorbs electromagnetic waves in a specific frequency band and reflects electromagnetic waves in other frequency bands, which are then received by the ground-penetrating radar. The ground-penetrating radar analyzes the spectral changes of the reflected waves to determine the dielectric constant of the stratum surrounding the antenna sensor, thereby calculating the stratum moisture content.

[0057] The working principle and equivalent circuit of the antenna sensor are as follows: Figure 2 As shown, including receiving antenna, input impedance Z in, receiving load Z1. The receiving antenna can receive incident electromagnetic waves in a specific frequency band and reflect electromagnetic waves in other frequency bands. Receiving antennas come in various forms and types, such as dipole antennas and microstrip antennas. When the antenna sensor is in the air, the frequency response function T of the incident electromagnetic wave is a (f) is,

[0058]

[0059] In this embodiment, the receiving antenna frequency resonance point f a0 = Absorption coefficient S at 0.1 GHz a0 =0.9, taking the bandwidth parameter i related to the antenna sensor structure a0 =0.01.

[0060] like Figure 3 As shown, the implementation of the present invention is divided into the following five steps:

[0061] Step S1: Select the appropriate antenna sensor resonant frequency f according to the soil moisture range and depth to be detected a0 and the GPR center frequency f D0 ; In this example, select f D0 =0.1GHz, i.e. 100MHz. The resonant frequency of the antenna sensor is f a0 =0.1GHz;

[0062] Step S2: Use the ground penetrating radar to transmit a Gaussian pulse wave signal, receive the electromagnetic wave reflected by the antenna sensor, analyze the echo spectrum, and obtain the minimum value f of the spectrum signal. a ;

[0063] Step S3: bury the antenna sensor at the depth h of the ground to be measured, and use the ground penetrating radar to detect the echo spectrum reflected by the antenna sensor to obtain the minimum value f of the spectrum signal. d ;

[0064] Step S4: Based on the minimum value f of the spectrum signal reflected by the antenna sensor in the air and in the ground a The dielectric constant of the ground around the buried antenna sensor is calculated using the following formula:

[0065]

[0066] Step S5: Calculate the water content of the ground around the antenna sensor according to the Topp formula.

[0067]

[0068] Furthermore, in step S2, the mathematical expression of the signal transmission of the ground penetrating radar is as follows: Figure 4Take the following three steps to artificially generate the simulated received signal spectrum R a (f):

[0069] Step S21, generating the transmission signal spectrum S(f) of the ground penetrating radar, the electromagnetic wave pulse signal V g The mathematical representation of is,

[0070] V g =exp(-p 2 (t-t0) 2 ) (10)

[0071] Where p is a parameter determined by the time width of the pulse signal, and t0 is the time midpoint of the pulse signal. In this example, p is set to 0.25×10 9 , t0=1.4×10 -9 s, the time domain waveform of the Gaussian pulse signal is as follows Figure 5 (a). Figure 5 As shown in (b), using Fourier transform, the spectrum S(f) of the Gaussian pulse signal can be obtained as follows:

[0072]

[0073] Step S22, determine the frequency response functions At(f) and Ar(f) of the ground penetrating radar transmitting antenna and receiving antenna. The ground penetrating radar's transmitting and receiving antennas can be regarded as filtering the electromagnetic wave signal. And since the receiving antenna and the transmitting antenna are symmetrical, the frequency response function A t (f)A r (f) can be expressed as,

[0074]

[0075] Among them, f D0 is the center frequency of the ground penetrating radar, S D is the reflection coefficient of the frequency resonance point, i D It is a parameter determined by the bandwidth of the ground penetrating radar's transceiver antenna. Note that the present invention only focuses on the minimum point of the electromagnetic wave spectrum, so the signal amplitude does not affect the detection result. Therefore, for simplicity, in this example, S D =1,i D =0.1; Therefore, the frequency response function M(f) of the ground penetrating radar system itself is,

[0076]

[0077] Function M(f) is still a Gaussian function, and its center frequency f M0 and shape parameter i M0 Calculate as follows:

[0078]

[0079] Make an image of the frequency response function of the above M(f), such as Figure 6 shown.

[0080] Step S23, determining the frequency response function T of the antenna sensor in the air a (f), frequency response function T of the antenna sensor a (f)

[0081]

[0082] In this embodiment, the receiving antenna frequency resonance point f a0 = Absorption coefficient S at 0.1 GHz a0 =0.9, taking the bandwidth parameter i related to the antenna sensor structure a0 =0.01. a Under the action of (f), the received signal spectrum R of the ground penetrating radar a (f) Figure 7 shown. Figure 7 The spectrum signal received by the ground penetrating radar shown has only one minimum point f a , analyze the signal spectrum and get f a =0.1GHz.

[0083] Furthermore, in step S3, when the antenna sensor is buried at the depth h of the measured stratum, the electromagnetic wave receiving signal spectrum of the ground penetrating radar is artificially generated, and the steps are as follows:

[0084] Step S31: The path loss L caused by the attenuation and geometric diffusion of the electromagnetic wave signal p The formula is,

[0085] L p (dB)=6+20log 10 (d)+20log 10 (β0)+8.69α0d (17)

[0086] Among them, the path loss L p In dB, d is the electromagnetic wave transmission distance, the loss constant α0 and the phase change constant β0 can be obtained by equations (18) and (19), respectively.

[0087]

[0088] Where μ is the magnetic permeability of the soil, ε0 is the dielectric constant in a vacuum, K′ and K″ are the real and imaginary parts of the soil's relative equivalent complex dielectric constant, K* (K* = K′ + jK″). The soil's relative equivalent complex dielectric constant is considered independent of the electromagnetic wave frequency, f, within a narrow frequency band. Based on the path loss formula, the function P(f) can be expressed as,

[0089] P(f)=f B exp(Af+C) (20)

[0090] Among them, the parameters A, B, and C are related to the electromagnetic properties of the medium and the propagation distance d. In this example, given the parameters A = -20.72, B = 1.15, and C = 2.30, the spectrum of the signal attenuation coefficient is as follows Figure 8 shown.

[0091] Step S32: determining the frequency response function T of the antenna sensor to the electromagnetic wave incident from the ground penetrating radar when the antenna sensor is located in the measured stratum. d (f)

[0092]

[0093] In this embodiment, the frequency resonance point f of the antenna sensor in the formation is taken as d0 = Absorption coefficient S at 0.032 GHz d0 =0.9, taking the bandwidth parameter i related to the antenna sensor structure d0 =0.01. d Under the action of (f), the frequency response function R of the ground penetrating radar receiving signal is generated according to formula (22): d (f),

[0094] R d (f)=M(f)gT d (f)gP(f) (22)

[0095] In this case, R d The spectrum of (f) is as follows Figure 9 As shown, there is only one minimum point f in its spectrum signal d . Analyzing the spectrum signal, we can get f d =0.032GHz;

[0096] Furthermore, in step S4, the minimum value point of the spectrum of the reflected signal of the antenna sensor in the ground penetrating radar detection stratum is f d , and the minimum value of the spectrum of the reflected signal of the antenna sensor in the air is f a , then the dielectric constant of the soil around the antenna sensor is,

[0097]

[0098] In this case, f a =0.1GHz, f d =0.032GHz, then the relative dielectric constant is ε r =9.77.

[0099] Furthermore, in step S5, the soil dielectric constant is substituted into the Topp formula

[0100]

[0101] In this example, the soil volume moisture content θ can be obtained v =0.33.

[0102] The above are preferred embodiments of the present invention. Any changes made according to the technical solution of the present invention, as long as the resulting functions and effects do not exceed the scope of the technical solution of the present invention, shall fall within the scope of protection of the present invention.

Claims

1. A non-contact detection method for soil moisture content based on an antenna sensor, characterized in that: The steps include: Step S1: Select the resonant frequency f of the antenna sensor according to the detected soil moisture content range and soil depth. a0 and the GPR center frequency f D0 ; Step S2: Use ground penetrating radar to detect the spectrum response of the antenna sensor in the air and obtain the minimum value of the spectrum signal f a ; Step S3: Use ground penetrating radar to detect the spectrum response of the antenna sensor buried in the ground to obtain the minimum value of the spectrum signal f d ; Step S4: According to formula ε r =(f a / f d ) 2 Calculate the dielectric constant of the measured stratum; Step S5: Calculate the water content of the measured stratum using the Topp formula; In step S2, the ground penetrating radar transmits a Gaussian pulse signal, whose spectrum function is S(f). The frequency response functions of the ground penetrating radar transmitting antenna and receiving antenna are At(f) and Ar(f) respectively. Then the received signal spectrum R a (f) is, R a (f)=S(f)gAt(f)gAr(f)gT a (f) (1) T a (f) is the response function of the antenna sensor in the air to the electromagnetic spectrum signal of the ground penetrating radar. Find the spectrum function R of the received signal. a The minimum point of (f) is recorded as f a ; In step S2, the spectrum function S(f) of the Gaussian pulse signal emitted by the ground penetrating radar is, p is a parameter determined by the time width of the pulse signal. The transmitting and receiving antennas of the ground penetrating radar can be regarded as filtering the electromagnetic wave signal. Since the transmitting and receiving antennas are symmetrical, the frequency response function A is t (f)A r (f) is expressed as, Among them, f D0 is the center frequency of the ground penetrating radar, S D is the reflection coefficient of the frequency resonance point, i D is a parameter determined by the bandwidth of the GPR transceiver antenna; the received signal spectrum R a (f) is, When S a0 ≈1, R a The minimum point f of (f) a satisfy, f a ≈f a0 (5) In step S3, after the antenna sensor is buried at a depth h in the soil layer to be measured, a ground penetrating radar is used to detect the spectrum response of the antenna sensor buried in the stratum; After the antenna sensor is buried at a depth h in the soil layer to be measured, the ground penetrating radar also uses Gaussian pulse signals to detect the antenna sensor, as follows: Compared with the air, the response function T of the antenna sensor to the electromagnetic spectrum signal of the ground penetrating radar d (f) is expressed as, Where S d0 is the frequency resonance point f of the antenna sensor in the formation d0 Absorption coefficient, 1-S d0 is the frequency resonance point f of the antenna sensor in the formation d0 The reflection coefficient, i d0 is the bandwidth parameter of the antenna sensor in the stratum; the electromagnetic wave signal of the ground penetrating radar experiences attenuation and diffusion in the soil layer, and the corresponding frequency response function P(f) is, P(f)=f B exp(Af+C) (7) Among them, the parameters A, B, and C are related to the electromagnetic characteristic parameters of the soil layer and the propagation distance d, so the GPR receiving signal spectrum R d (f) is, R d (f)=S(f)gAt(f)gAr(f)gT(d)gP(f) (8) Substituting formulas (6) and (7) into formula (8), we get When S d0 ≈1, parameter A<150, the received spectrum signal R d The minimum point f of (f) d satisfy, f d ≈f d0 (9)。 2. The non-contact detection method for soil moisture content based on an antenna sensor according to claim 1, characterized in that: In step S1, the resonant frequency f a0 The antenna sensor can receive electromagnetic wave signals with a frequency of f a0 The electromagnetic waves nearby, the reflected signal frequency is far away from f a0 The electromagnetic wave, so the response function of the antenna sensor in the air to the electromagnetic wave spectrum signal of the ground penetrating radar is T a (f) is expressed as, Where S a0 is the antenna sensor at the frequency resonance point f a0 Absorption coefficient, 1-S a0 The antenna sensor is at the frequency resonance point f a0 The reflection coefficient, i a0 It is a bandwidth parameter related to the antenna sensor structure.

3. The non-contact detection method for soil moisture content based on an antenna sensor according to claim 1, characterized in that: In step S5, according to the dielectric constant ε of the soil r , use the Topp formula to calculate the soil moisture content θ of the measured stratum v ,

Images