CN101266271A - Method and device for measuring dielectric constant of soil by electromagnetic waves - Google Patents

Abstract

The invention discloses a device and a method for testing the dielectric constant of high-conductivity soil. The sensor is connected with the electromagnetic wave reflector through the coaxial cable, and the electromagnetic wave transmitter is connected with the PC. It can avoid the electromagnetic wave dissipation caused by the high conductivity of the material in the traditional electromagnetic wave method, and can test the dielectric constant of the high conductivity soil without being affected by the salinity of the soil. The invention can realize the dielectric constant test of chemically reinforced soil, high organic matter soil, high water content and high plasticity clay and other high-conductivity soils, and is a safe, fast and accurate method for testing the dielectric constant of soil. Lay the groundwork for testing the moisture content of high-conductivity soils.

Description

Method and device for measuring dielectric constant of soil by electromagnetic waves

technical field

The invention relates to a detection device using electromagnetic wave reflection, in particular to a testing method and device for testing the dielectric constant of soil with electromagnetic waves.

Background technique

In engineering, the electromagnetic wave method can be used to test the water content of the soil on site. The American field water content test standard ASTM6780 uses TDR (electromagnetic wave time domain reflection method) to test the water content of the soil. First, the dielectric constant of the soil needs to be measured, and then the water content of the soil is obtained through the indoor calibration equation. The traditional TDR method to test the dielectric constant of soil is the travel time method, which is based on the different propagation speeds of electromagnetic waves in soils with different dielectric constants, and first obtains the propagation time of electromagnetic waves in the tested soil. The propagation time is determined according to the first reflection point and the end reflection point of the electromagnetic wave in the sensor. For high-conductivity soil, the electromagnetic wave will have a large energy loss when propagating in the soil, causing the dissipation of the electromagnetic wave. In this case, it is difficult to accurately determine the end reflection point. Therefore, the traditional TDR method is difficult to test the dielectric constant of high-conductivity soil and obtain its water content. And in actual engineering, the water content of the chemically reinforced soil of high conductivity is a very important physical parameter again, so the present invention has introduced a kind of testing device and the method of new testing high conductivity soil dielectric constant, for Test its moisture content to lay the groundwork.

Contents of the invention

The object of the present invention is to provide a testing device and method for testing the dielectric constant of soil with electromagnetic waves, especially for testing the dielectric constant of high-conductivity soil.

The technical scheme that the present invention adopts is as follows:

One, a kind of testing method of electromagnetic wave testing soil body dielectric constant, the step of this method is as follows:

1) Compact the soil to be tested in the compaction barrel according to the compaction method of the geotechnical test standard, so that the height of the soil is equal to the compaction barrel, and insert a stainless steel probe in the center;

2) Put a stainless steel ring on the upper end of the compaction barrel, install the first screw rod in the center of the lower end of the stainless steel block on the coaxial measuring head, and install the second, third, and fourth screw rods on the same circumference of the lower end of the stainless steel block, The four screws exposed at the lower end of the stainless steel block are of the same length, and then the four test rods are connected to the four screws, and then the coaxial probe connected to the test rods is placed on the stainless steel ring, the stainless steel probe and the first test rod The rods are in contact, and the second, third and fourth test rods are in contact with the stainless steel ring;

3) Connect the coaxial cable and the TDR test instrument, turn on the PC, and use the PMTDR software that comes with the TDR instrument to collect waveforms, soil dielectric constant $\sqrt{K_{\mathrm{soil}}^{*}}=k\frac{1-{\mathrm{\ρ}}_3}{1+{\mathrm{\ρ}}_3},$ k is a calibration constant related to the instrument. In this method, k is taken as 1. In the formula, ρ ₃ is the reflection coefficient of the soil surface, which can be obtained by the following formula: ρ ₁ =ρ _1f (1) ${\mathrm{\ρ}}_2=\frac{{\mathrm{\ρ}}_{2f}-{\mathrm{\ρ}}_{1f}}{(1-{{\mathrm{\ρ}}_1}^2)}$ (2) ${\mathrm{\ρ}}_3=\frac{{\mathrm{\ρ}}_{3f}-{\mathrm{\ρ}}_{2f}}{(1-{\mathrm{\ρ}}_1^2)(1-{{\mathrm{\ρ}}_2}^2)}$ (3), where ρ _1f is the reflection coefficient of the interface between the coaxial probe and the coaxial cable on the TDR test waveform, ρ _2f is the reflection coefficient of the interface between the test rod and the coaxial probe on the test waveform, and ρ _3f is the test waveform The reflection coefficients of the upper soil surface, and their values are as follows. Using the method of obtaining the first derivative of the waveform described by Svitzky and Golay in 1964, the TDR test waveform is derived, and the abscissa is time, and the ordinate is reflection The waveform of the first derivative of the coefficient, the time t ₁ , t ₂ and t ₃ at the end of the reflection of the three interfaces of the coaxial probe, the test rod and the soil are obtained on the waveform, and then the value of the ordinate on the corresponding TDR test waveform is found , which are the values of ρ _1f , ρ _2f , and ρ _3f ; get ρ ₁ through formula (1), substitute ρ ₁ into formula (2) to get ρ ₂ , and then substitute ρ ₁ and ρ ₂ into formula (3) Find ρ ₃ .

2. A test device for electromagnetic wave testing of soil dielectric constant:

The sensor is connected to the electromagnetic wave transmitter through the coaxial cable through the BNC connector, and the electromagnetic wave transmitter is connected to the PC; the sensor includes a coaxial probe and a coaxial test barrel, wherein:

1) Coaxial measuring head: including a stainless steel block, four test rods and four screw rods, a stepped hole with a small upper end and a larger lower end is opened in the center of the stainless steel block, the small hole is equipped with a BNC connector, and the large hole at the lower end of the stainless steel block is Fill the resin block and install the first screw in the center. The second, third, and fourth screws are installed on the same circumference of the lower end of the stainless steel block. The four screws exposed at the lower end of the stainless steel block are equal in length. Each is equipped with test rods of equal length;

2) Coaxial test barrel: including stainless steel probe, hollow compaction barrel, stainless steel ring and insulating base, the center of the compaction barrel filled with test soil is inserted into the stainless steel probe, and the stainless steel probe is connected to the first center of the lower end of the stainless steel block. The root test rod is coaxial, the compaction barrel is placed on the insulating base for positioning, and the upper end of the compaction barrel is covered with a stainless steel ring.

The beneficial effects that the present invention has are:

It can avoid the electromagnetic wave dissipation caused by the high conductivity of the material in the traditional electromagnetic wave method, and can test the dielectric constant of the high conductivity soil without being affected by the salinity of the soil. The invention can realize the dielectric constant test of chemically reinforced soil, high organic matter soil, high water content and high plasticity clay and other high-conductivity soils, and is a safe, fast and accurate method for testing the dielectric constant of soil. Lay the groundwork for testing the moisture content of high-conductivity soils.

Description of drawings

Fig. 1 is a schematic structural diagram of a testing device of the present invention.

Fig. 2 is a schematic diagram of the structure of the coaxial probe and the test rod.

Fig. 3 is a schematic diagram of the structure of the coaxial test bucket.

Fig. 4 is a structural schematic diagram of the probe positioning mold.

Fig. 5 is a diagram after derivation of the ordinate of a typical TDR test waveform.

Figure 6 is a typical TDR test waveform.

Figure 7 is a calibration curve of the k value of the test device.

In the figure: 1. PC, 2. Electromagnetic wave transmitter, 3. Coaxial cable, 4. BNC connector, 5. Sensor, 6. Coaxial probe, 7. Test rod, 8. Coaxial test barrel, 9. Stainless steel block, 10, resin block, 11, screw rod, 12, spring, 13, stainless steel probe, 14, compaction barrel, 15, stainless steel ring, 16, insulating base, 17, soil surface, 18, probe positioning mold

Detailed ways

As shown in Fig. 1, Fig. 2, Fig. 3, Fig. 4, sensor 5 of the present invention is connected with electromagnetic wave launcher 2 through coaxial cable 3 through BNC joint 4, and electromagnetic wave launcher 2 is connected with PC machine 1; Described sensor 5 includes a coaxial probe 6 and a coaxial test bucket 8, wherein:

1) Coaxial measuring head 6: including a stainless steel block 9, four test rods 7 and four screw rods 11, in the center of the stainless steel block 9 there is a stepped hole with a small upper end and a larger lower end, and a BNC connector 4 is installed in the small hole, stainless steel The large hole at the lower end of the block 9 is filled with a resin block 10 and the first screw rod 11 is installed in the center, and the second, third, and fourth screw rods 11 are installed on the same circumference of the lower end of the stainless steel block 9, exposed at the lower end of the stainless steel block 8 The four screw rods 11 are equal in length, and each screw rod 11 is equipped with equal length test rods 7 respectively;

2) Coaxial test barrel 8: including stainless steel probe 13, hollow compaction barrel 14, stainless steel ring 15 and insulating base 16, the center of compaction barrel 14 filled with test soil inserts stainless steel probe 13, stainless steel probe 13 Coaxial with the first test rod 7 at the center of the lower end of the stainless steel block 8, the compaction barrel 14 is placed on the insulating base 16 for positioning, and the upper end of the compaction barrel 14 is covered with a stainless steel ring 15.

The diameter of the stainless steel block 9 in the coaxial measuring head 6 is 140-180 mm, and the height is 50 mm-80 mm. The resin block 10 is made of polyoxymethylene resin material, and the length of four equal-length screw rods 11 is 30-40 mm. The second, Three or four screws are evenly distributed on a circle with a radius r of 60 to 70 mm, any one of the second, third and fourth screws is elastically connected to the stainless steel block 9 through a spring, and can move up and down in the vertical direction; equal length The length L of the test rod 7 is 200-300mm.

The height h of the compaction barrel 13 in the coaxial test barrel 8 is 110-120mm, and the inner diameter D is 100-110mm; the diameter d of the stainless steel probe 13 is 8-10mm; the height c of the stainless steel ring 15 is 30-40mm, and the insulation The base 16 is a polyoxymethylene resin material, and the compaction bucket 14 is fixed by the bracket on the insulating base 16;

As shown in Figure 1, the sensors are respectively connected to the electromagnetic wave transmitter through the BNC connector through the coaxial cable, and the electromagnetic wave transmitter is connected to the PC; the sensor includes a coaxial probe and a coaxial test barrel; wherein the coaxial probe : Including stainless steel block, four test rods and four screw rods, in the center of the stainless steel block, there is a stepped hole with a small upper end and a larger lower end, the small hole is equipped with a BNC joint, the large hole at the lower end of the stainless steel block is filled with a resin block and in the center The first screw is installed, and the second, third, and fourth screws are installed on the same circumference of the lower end of the stainless steel block. The four screws exposed at the lower end of the stainless steel block are of equal length, and each screw is equipped with a Coaxial test barrel: including stainless steel probe, hollow compaction barrel, stainless steel ring and insulating base, the center of the compaction barrel filled with test soil is inserted into the stainless steel probe, the stainless steel probe is connected to the center of the lower end of the stainless steel block The first test rod is coaxial, the compaction barrel is positioned on the insulating base, and the top end of the compaction barrel is covered with a stainless steel ring.

Described electromagnetic wave launcher is the product TDR100 of American Campbell Scientific company.

The coaxial cable is a model RG58A/U coaxial cable, and its length is 1-2 meters.

Data acquisition and processing were performed using PMTDR software developed by Campbell.

When testing, fix the compaction barrel first, compact the soil to be tested into the compaction barrel according to the geotechnical test standard compaction method, and level the soil in the compaction barrel with a scraper, so that the height of the soil is equal to the compaction barrel Then put the probe positioning mold 18 (as shown in Figure 4 (a), (b)), make the lower surface of the mold close to the soil surface 17, then squeeze the stainless steel probe 13 with a small hammer; take off the mold, put Upper stainless steel ring 15; ensure that the stainless steel ring 15 is on the top of the compaction barrel 14 and in close contact with it, place the coaxial measuring head 6 connected to the four test rods 7 on the stainless steel ring 15, and ensure that the first test rod is on the stainless steel probe The upper end is in close contact, and the second, third, and fourth test rods are in close contact with the stainless steel ring; connect the coaxial cable and the TDR test instrument, turn on the PC, and use the PCTDR software to collect waveforms. The connection of the test device structure is shown in Figure 1, a typical The test waveform is as shown in Figure 6. According to the method for obtaining the first derivative of the waveform described by Svitzky and Golay (1964) for the test waveform, a waveform whose horizontal axis is time and vertical axis is the first derivative of the reflection coefficient is shown in Figure 5 shown. Analyze the waveform in Figure 5 and find three obvious peaks in the 20-30ns of the waveform, the first peak appears around 24ns, and the direction of the peak is upward; the second peak appears around 25ns, and the peak is convex The rising direction is upward; the third peak appears around 27ns, and the rising direction of the peak is downward. The end points of the three peaks (as shown in Figure 5) are the time t ₁ , t ₂ and t ₃ at the end of the first reflection, the second reflection and the third reflection of the TDR pulse, and then find the corresponding TDR test waveform The upper abscissa is the value of the ordinate corresponding to the points t ₁ , t ₂ and t ₃ , that is, the values of ρ _1f , ρ _2f , and ρ _3f . ρ ₃ can be obtained by the following formula: ρ ₁ = ρ _1f (1) ${\mathrm{\ρ}}_2=\frac{{\mathrm{\ρ}}_{2f}-{\mathrm{\ρ}}_{1f}}{(1-{{\mathrm{\ρ}}_1}^2)}$ (2) ${\mathrm{\ρ}}_3=\frac{{\mathrm{\ρ}}_{3f}-{\mathrm{\ρ}}_{2f}}{(1-{\mathrm{\ρ}}_1^2)(1-{{\mathrm{\ρ}}_2}^2)}$ (3). The dielectric constant of soil can be obtained by the following formula: $\sqrt{K_{\mathrm{soil}}^{*}}=k\frac{1-{\mathrm{\ρ}}_3}{1+{\mathrm{\ρ}}_3},$ k is a calibration constant related to the instrument, k can be approximately 1, or can be obtained by a calibration method. The calibration method is as follows, using a solution with known dielectric constant (deionized water is 79.90, absolute ethanol is 17.70, butanol is 25.20 ), test its TDR waveform according to the test method described in this paper, and obtain the surface reflection coefficient ρ ₃ of the respective solutions, and then use the function (y=kx) to $\sqrt{K^{*}}~\frac{1-{\mathrm{\ρ}}_3}{1+{\mathrm{\ρ}}_3}$ Perform linear fitting to obtain the slope k, as shown in Figure 7. Then k is the calibration constant of the instrument.

Claims (4)

1, a kind of method of testing of electromagnetic wave test soil body dielectric coefficient is characterized in that the step of this method is as follows:

1) soil body that will test is hit the solid yardage method according to the soil test standard and in hitting real bucket, hit reality, make soil body height and to hit real bucket concordant, squeeze into the stainless steel probe at the center;

2) on hitting real bucket, put stainless steel ring, first screw rod is equipped with at the center of the piece of stainless steel lower end on the coaxial gauge head, on the same circumference of piece of stainless steel lower end five equilibrium be equipped with second and third, four screw rods, expose at four screw rods of piece of stainless steel lower end isometric, then four reference test bars are received on four screw rods, the coaxial gauge head that will connect reference test bar then is placed on the stainless steel ring, and the stainless steel probe contacts with first reference test bar, second and third, four reference test bars contact with stainless steel ring;

3) connect concentric cable and TDR Test instrument, open PC, the PMTDR software collection waveform that utilizes the TDR instrument to carry, dielectric constant of soil body $\sqrt{K_{\mathrm{soil}}^{*}}=k\frac{1-{\mathrm{\ρ}}_3}{1+{\mathrm{\ρ}}_3},$ K is the demarcation constant relevant with instrument, and k gets 1 in the method, ρ in the formula ₃Be the soil body surface reflection coefficient, can try to achieve by following formula: ρ ₁=ρ _1f(1) ${\mathrm{\ρ}}_2=\frac{{\mathrm{\ρ}}_{2f}-{\mathrm{\ρ}}_{1f}}{(1-{{\mathrm{\ρ}}_1}^2)}$ (2) ${\mathrm{\ρ}}_3=\frac{{\mathrm{\ρ}}_{3f}-{\mathrm{\ρ}}_{2f}}{(1-{\mathrm{\ρ}}_1^2)(1-{{\mathrm{\ρ}}_2}^2)}$ (3), ρ wherein _1fBe the reflection coefficient of coaxial gauge head on the TDR Test waveform and concentric cable interface, ρ _2fBe the reflection coefficient of reference test bar on the test waveform and coaxial gauge head interface, ρ _3fReflection coefficient for soil body surface on the test waveform, their obtaining value method is as follows, utilize Svitzky and Golay described method of waveform being asked first order derivative in 1964, to the differentiate of TDR Test waveform, obtaining a horizontal ordinate is the time, ordinate is the waveform of the first order derivative of reflection coefficient, time t when the reflection that obtains coaxial gauge head, reference test bar, three interfaces of the soil body on waveform finishes ₁, t ₂And t ₃, find the value of ordinate on the corresponding TDR Test waveform then, be ρ _1f, ρ _2f, ρ _3fValue; Through type (1) is tried to achieve ρ ₁, with ρ ₁Substitution formula (2) is tried to achieve ρ ₂, again with ρ ₁, ρ ₂Substitution formula (3) is tried to achieve ρ ₃

2, the device that is used for the method for testing of the described a kind of electromagnetic wave test soil body dielectric coefficient of claim 1, it is characterized in that: sensor (5) is connected with electromagnetic wave transmitter (2) through concentric cable (3) by BNC connector (4), and electromagnetic wave transmitter (2) is connected with PC (1); Described sensor (5) comprises coaxial gauge head (6) and coaxial test bucket (8); Wherein:

1) coaxial gauge head (6): comprise piece of stainless steel (9), four reference test bars (7) and four screw rods (11), have the upper end at piece of stainless steel (9) center little, the shoulder hole that the lower end is big, BNC connector (4) is housed in the aperture, potting resin piece (10) and first screw rod (11) is housed at the center in the macropore of piece of stainless steel (9) lower end, five equilibrium is equipped with second on the same circumference of piece of stainless steel (9) lower end, three, four screw rods (11), expose at four screw rods (11) of piece of stainless steel (8) lower end isometricly, under each root screw rod (11) isometric reference test bar (7) is housed respectively;

2) coaxial test bucket (8): comprise the real bucket of hitting of stainless steel probe (13), hollow (14), stainless steel ring (15) and insulator foot (16), fill the real bucket of hitting of the test soil body (14) center and insert stainless steel probe (13), stainless steel probe (13) is coaxial with first reference test bar (7) at piece of stainless steel (8) center, lower end, hit real bucket (14) and be placed on insulator foot (16) location, hit stainless steel ring (15) on real bucket (14) upper end cover.

3, the device of the method for testing of a kind of electromagnetic wave test soil body dielectric coefficient according to claim 2, it is characterized in that: piece of stainless steel (9) diameter is 140～180mm in the described coaxial gauge head (6), highly be 50mm～80mm, resin mass (10) is the polyoxymethylene resin material, isometric four screw rods (11) length is 30～40mm, second, three, four screw rods are evenly distributed on the circumference that radius r is 60～70mm, second, three, any screw rod is connected with piece of stainless steel (9) elasticity by spring in four screw rods, can move down by in the vertical direction; Isometric reference test bar (7) length L is 200～300mm.

4, the device of the method for testing of a kind of electromagnetic wave test soil body dielectric coefficient according to claim 2, it is characterized in that: hitting real bucket (13) height h in the described coaxial test bucket (8) is 110～120mm, and interior diameter D is that 100～110mm stainless steel probe (13) diameter d is 8～10mm; Stainless steel ring (15) height c is 30～40mm, and insulator foot (16) is the polyoxymethylene resin material, and it is fixing to hit real bucket (14) by the support on the insulator foot (16).

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