Systems for transverse electromagnetic mode in-situ soil testing

A slotted TEM transmission line and an in-situ TEM transmission line are utilized to determine both complex permittivity and permeability of soil. The permittivity and permeability information may be used by underground sensing techniques such as GPR and EMI to enhance information from these techniques. The in-situ probe provides that both complex permittivity and permeability can be measured simultaneously over a broad frequency range without disturbing the soil conditions.

FIELD

This disclosure is directed to measurement of electric and magnetic properties of soil.

BACKGROUND

Electromagnetic sensors are commonly used to obtain information about underground environments and objects. Ground Penetrating Radar (GPR), for example, has been utilized as an important tool for investigating many underground environments and objects. Electromagnetic induction (EMI) is also commonly utilized to detect objects that may be located underground. Accurate and meaningful interpretation of data from electromagnetic sensors requires knowledge of the electromagnetic properties of the soil. For example, because GPR signals penetrate through the soil, the electromagnetic properties of the soil are needed to obtain useful information from GPR sensors. The soil properties of many regions are determined for the most part by the water contents and density of soil, which significantly modify permittivity of soil, but not permeability. For this reason, traditional focus has been on measuring the permittivity of soil to predict GPR signal behavior in the soil, while permeability of soil was assumed to be uniform. However, soil in many regions may not have uniform permeability. For example, in iron-rich soil environments behavior of the electromagnetic (EM) wave has been found to also be affected by the iron contents of soil. Also, magnetic soil has been reported to adversely affect the performance of metal detectors. Thus, in such cases, the permeability of soil should also be considered to analyze the performance of EM sensors, such as GPR and EMI sensors. In such iron-rich environments, both permittivity and permeability of soil are essential to analyze GPR data and thus required to be measured simultaneously.

SUMMARY

Embodiments disclosed herein provide soil measurement devices that allow for the determination of soil permittivity and permeability, thereby providing for enhanced utilization of GPR and EMI. GPR and EMI may be used, for example, in locating unexploded ordinance hidden underground. In one aspect, a slotted transverse electromagnetic mode (TEM) transmission line is adapted as a soil sample holder and used to measure impedance of a sample over a desired frequency range. Such a slotted TEM transmission line device requires a certain amount of soil sample to be packed into the sample holder to measure the soil properties. This procedure, in some cases, causes a distortion of the original soil properties, because soil properties are a function of density and moisture. In another aspect, in order to measure the undisturbed soil properties, a three conductor transmission line is provided for in-situ soil measurement.

In one embodiment, the present disclosure provides a soil probe for use in determining permittivity and permeability of a soil sample, comprising: (a) a slotted TEM transmission line, (b) a first port interconnected to a first end of the slotted TEM transmission line; (c) a second port interconnected to a second end of the slotted TEM transmission line; and (d) a soil sample receiver in the slot of the TEM transmission line. The first and second ports of this embodiment are adapted to be connected to a network analyzer that provides electromagnetic signals thereto and receives electromagnetic signals reflected and radiated from the soil and received at said slotted TEM transmission line. A length of the soil sample receiver may be selected to be different than a half-wavelength multiple of signal frequencies received at said slotted TEM transmission line. In an embodiment, the first port and second port comprise coaxial connectors. Each of the coaxial connectors comprises a center conductor that is interconnected to said slotted TEM transmission line and the soil sample receiver is placed to minimize air gaps between the sample and each center conductor. In an embodiment, the slotted TEM transmission line receives electromagnetic signals in the frequency range from about 50 MHz to about 1 GHz through said first and second ports. Impedance of the slotted TEM transmission line may be measured over the frequency range to provide for simultaneous determination of permittivity and permeability of the soil sample.

In another embodiment, the present disclosure provides a soil probe for use in determining permittivity and permeability of a soil sample, comprising: (a) a TEM transmission line comprising a plurality of metal rods interconnected at a first end to a first ground plate, and removably interconnected at a second end to a second ground plate; (b) a first port interconnected to the first ground plate; and (c) a second port interconnected to the second ground plate. The metal rods are configured to be inserted into an in-place soil sample and the first and second ports adapted to be connected to a network analyzer that provides electromagnetic signals thereto and receives electromagnetic signals reflected and radiated from the soil and received at the TEM transmission line. The TEM transmission line may comprise three metal rods. A first metal rod is interconnected with the center conductor of each coaxial connector, and second and third metal rods are each interconnected with an outer conductor of each coaxial connector. The TEM transmission line receives and sends electromagnetic signals in the frequency range from about 50 MHz to about 1 GHz through said first and second ports. Impedance of the TEM transmission line may be measured over the frequency range to provide for simultaneous determination of permittivity and permeability of the soil sample.

In still another embodiment, the present disclosure provides a system for measuring reflection and radiation from a soil sample, the reflection and radiation usable to determine permittivity and permeability of the soil sample, comprising: (a) a soil probe comprising a TEM transmission line and first and second ports; (b) a transmitter that generates electromagnetic energy across a frequency band of interest and forwards the electromagnetic energy to the first and second ports; (c) an impedance measurement circuit that measures an input impedance of the soil probe over the frequency band and generates measured impedance data, the impedance data providing for simultaneous determination of permittivity and permeability of the soil sample. A network analyzer may include the transmitter and impedance measurement circuit.

DETAILED DESCRIPTION

The present disclosure recognizes that in order to obtain useful information from electromagnetic sensors used in ground penetrating applications, both permittivity and permeability of soil are needed in many instances. Aspects of the present disclosure disclose the measurement of electromagnetic properties of the materials at microwave frequency.

Several microwave methods have been developed and published in material science, and have been applied to estimate the soil properties at frequencies of interest. Generally, soil property measurement methods can be divided into non-resonant and resonant methods. Resonant methods utilize the fact that permittivity and permeability of a dielectric resonator with a certain dimension determines its resonant frequency and quality factor. This method is suitable for accurate measurement of soil properties at a single frequency. On the other hand, non-resonant methods utilize reflection of the EM wave from the soil interface and the transmission through the soil to estimate a general electromagnetic property of the soil over a frequency range. The non-resonant method can be divided into reflection method and reflection/transmission method. The reflection method can only measure one parameter which is permittivity of soil on the assumption of uniform permeability, while the reflection/transmission method can be used in the measurement of both permittivity and permeability. For general GPR environments, the resonance or the reflection method have traditionally been used to measure complex permittivity of soil of interest on the assumption that its permeability is unity. However, for iron-rich soil, the reflection/transmission method is necessary to obtain both complex permittivity and permeability.

For the reflection/transmission method, coaxial transmission lines are widely used as a sample holder to measure reflection (S11) and transmission (S21) of the soil sample due to its broadband frequency coverage. However, the coaxial lines introduce severe measurement uncertainty due to air gaps between the sample and the center conductors. In one embodiment, a slotted TEM transmission line is adapted as a sample holder to reduce this uncertainty because it provides access to both sides of the center conductor and thus can minimize the air gaps between the sample and the center conductor. Such a TEM transmission line method requires a certain amount of soil sample to be packed into the sample holder to measure the soil properties. This procedure inevitably causes an undesirable distortion of the original soil properties, since the soil properties are a function of its density and moisture contents. Therefore, in order to measure the undisturbed soil properties, certain embodiments provide a three conductor transmission line method for in-situ soil measurement.

With reference now toFIG. 1, a detailed structure of the novel in-situ soil probe and its calibration method will be discussed for an embodiment.FIG. 1shows, for an embodiment, a slotted TEM transmission line measurement apparatus20. In this embodiment, a slotted transmission line24has a known characteristic impedance, 50 ohm in this exemplary embodiment, when it is filled with the air. Such a slotted TEM transmission line may be a commercially available transmission line, such as a Hewlett Packard 805e transmission line available from Hewlett Packard, Corp, Palo Alto, Calif. The slotted TEM transmission line24includes a slot28, with and may have a soil sample to be tested inserted into a segment32of transmission line through the slot28. In this embodiment, a plastic sample holder36holds the soil sample in segment32. The slotted TEM transmission line24includes a first port40and a second port44that provide inputs to, for example, a two port network analyzer. The network analyzer in this embodiment collects scattering coefficients S11and S21, in the frequency range from 50 MHz to 1.0 GHz.

In one embodiment, the measurement procedure is as follows. First, the slotted transmission line24filled with air is connected at port two of a network analyzer through a coaxial cable. Then a standard two port calibration is performed at the calibration plane indicated by R inFIG. 1. In other words, the slotted transmission line24is included in the cable section connected to port two and then both port one and two are calibrated at the reference plane (R). The soil sample holder36is then filled with the soil sample and the two-port measurements are made. These measurements provide the scattering coefficients (S11and S21) of the slotted transmission line24with the soil sample. The permittivity and permeability of the soil sample can be calculated by the Nicolson-Ross (N-R) algorithm. The calculated permittivity and permeability may then be used in conjunction with EM data related to an area associated with the soil sample.

For example, assume the sample holder is filled with a soil sample whose permeability is μ=μ0μRand permittivity is ∈=∈0∈R. Then, the characteristic impedance of the sample holder section is modified and a new characteristic impedance is given by:
Z=√{square root over (μR/∈R)}Z0(1)
where Z0is the characteristic impedance of the TEM slotted transmission line with the air. The intrinsic reflection coefficient of a wave on the interface from the air-filled line is given by

Γ=Z-Z0Z+Z0=μR/ɛR-1μR/ɛR+1(2)
and the transmission coefficient is given by

T=exp⁡(-j⁢⁢ω⁢⁢d⁢μR⁢ɛRc)(3)
where ω is the angular frequency, c is the speed of light in the air and d is the length of the sample holder.

This intrinsic reflection and transmission coefficients can be expressed by the scattering coefficients measured by the network analyzer through the N-R algorithm. Measured scattering coefficients are given by

S11=Γ⁡(1-T2)1-Γ2⁢T2(4)S21=(1-Γ2)⁢T1-Γ2⁢T2(5)
Using N-R algorithm, the intrinsic reflection coefficient is given by

Γ=X±X2-1(6)withX=(S112-S212)+12⁢S11(7)
and proper sign in equation (6) should be chosen so that |Γ|≦1. The transmission coefficient is given by

T=(S11+S21)-Γ1-(S11+S21)⁢Γ(8)
Therefore, the complex permittivity and permeability can be calculated from [8], as

μR=K1⁢K2⁢⁢⁢ɛR=K1K2⁢⁢with(9)K1=(1+Γ1-Γ)2,⁢K2=-(cω⁢⁢d⁢ln⁡(1T))2(10)
It should be noted that the N-R algorithm does not work well at resonance frequencies of the sample holder, where the sample holder length is a multiple of a half wavelength when filled with soil. This fact should be taken into consideration when determining the length of the sample holder, and in the embodiment ofFIG. 1the length of sample holder36is selected to have a different length than a half-wavelength multiple.

With reference now toFIG. 2, another embodiment is described in which an in-situ soil test may be obtained. It is recognized that a laboratory measurement is not sufficient to provide the accurate soil properties at a specific time, i.e. the moment of GPR survey. This is because the soil properties vary with the weather conditions and density of the soil. Thus, an in-situ soil probe, which can measure both permittivity and permeability of a soil without disturbing the soil conditions, is useful for better interpretation of GPR data measured in iron-rich soil. Iron-rich soil is present in many environments, such as volcanic soil. In this embodiment, an in-situ soil probe50is constructed having a TEM transmission line54. The TEM transmission line54of this embodiment has three metal rods58, a first port62, a second port66, and two ground plates70. The two ports62,66, in this embodiment include coaxial connectors. The center conductors of both coaxial connectors for ports62,66are linked by the center metal rod58. In one embodiment, each metal rod has a ⅛ inch diameter. Ground plates70in this embodiment are connected to the outer conductor of the coaxial connectors for ports62,66. The ground plates70at both sides are connected by the other two metal rods58. In the embodiment ofFIG. 2, the ground plate70associated with the second port66is detachable from the metal rods58. This enables the soil probe to be inserted into the soil from the side without disturbing the soil.

FIG. 3illustrates an in-situ probe50inserted into a soil sample below the surface74of the soil. In this embodiment, the probe50is inserted at least two inches below the surface74of the soil in order to provide suitably accurate readings. It is to be understood that different configurations may require different soil depths. In the embodiment ofFIG. 3, a two port network analyzer78is connected to each port62,66of the in-situ probe50and S11and S21are collected at the network analyzer78.FIG. 4illustrates another embodiment with the in-situ probe50, a ground plate82associated with the second port66illustrated in an exploded view and having detachable tips86that connect metal rods58.

The in-situ soil probe50has a characteristic impedance of the TEM transmission line as modified by equation (1) when it is filled with the soil. Unlike the slotted TEM transmission line, the characteristic impedance of the three-conductor TEM transmission line in the air is not equal to the impedance of the ports (i.e. 50 ohm). Thus, in an embodiment the N-R algorithm is modified and a probe calibration method is provided to calculate the electromagnetic properties of the soil. Calibration and measurement procedures for the in-situ soil probe50are as follows for this embodiment. First, the two-port network analyzer78is calibrated at the terminal of the coaxial cable. Next the scattering coefficients of the three-conductor transmission line50in the air are measured for the probe calibration, from which the characteristic impedance of this transmission line in the air is obtained by

ZTLAir=Z0⁢1+Γ1-Γ(11)
where Z0is the characteristic impedance of the coaxial cable and Γ is the intrinsic reflection coefficient on the interface from the air-filled three-conductor transmission line50, which can be calculated by equation (6). Then, the in-situ probe is inserted into the soil and collects the scattering coefficients (S11and S21). From these measured scattering parameters and the impedance of the transmission line in the air, the complex permittivity and permeability of the soil can be calculated by modified N-R algorithm and it is given by

μR=J1⁢J2⁢⁢ɛR=J1J2⁢⁢with(12)J1=(Z0ZTLAir·1+Γ1-Γ)2,⁢J2=-(cω⁢⁢d⁢ln⁡(1T))2(13)
where d is the length of the three-conductor TEM transmission line50.

In one example, the apparatus ofFIGS. 2 and 3were used along with the described measurement methods in experimental studies conducted for iron-rich volcanic Hawaiian soil. Effects of water contents, density and iron contents of the soil on the electromagnetic soil properties were investigated by the laboratory method using the slotted TEM transmission line ofFIG. 1. The volcanic soil samples were collected at a typical red soil area in Oahu, Hi. Laboratory soil measurements were conducted for different levels of water contents of soil samples and different densities of soil samples.FIGS. 5 and 6show the real and imaginary parts of the permittivity and permeability of the volcanic Hawaiian soil with different level of the water contents and different densities. It is well known that the complex permittivity of a soil is proportional to the water contents and density of the soil, which are in agreement with the results as illustrated inFIGS. 5 and 6. On the other hand, the permeability of soil was not influenced by the water contents of soil. The real part of permeability of the volcanic Hawaiian soil indicated approximately from 1.03 to 1.05 over the frequency range, while the imaginary part indicated from 0.004 to 0.012 regardless of density and water contents, which were slightly higher value compared to those of dry sand without iron contents shown inFIG. 7.

With reference toFIG. 7, effects of iron contents on the soil properties are illustrated for three soil samples prepared by mixing the iron grains with sand in different volume fractions. It is believed that the permeability of soil is more sensitive to larger sizes of the iron grain. Thus, the iron grain size in the samples used to generateFIG. 7were selected to observe variations of the complex permeability, with a size range of 300-600 μm.FIG. 7plots the complex permittivity and permeability for each sample with different levels of the iron contents.FIG. 7confirms that both complex permittivity and permeability increase with iron content.

The in-situ soil probe described above was tested in the laboratory with soil samples to evaluate its performance by comparing its result with that of the slotted TEM transmission line ofFIG. 1. The results from these measurements are shown inFIG. 8. The real part of permittivity measured by the in-situ probe abruptly decreases with frequency increment from around 650 MHz. This is because frequency is approaching to the resonance frequency of the in-situ probe in the material. As discussed above, the N-R algorithm does not work well near the resonance frequency of the probe in the material. In cases where a GPR or EMI are to be operated near these frequencies, the length of the metal rods in the in-situ probe may be reduced or increased to avoid this result. It is also observed that difference in measurement results of two methods increases with frequency increment. This can be explained by non uniform characteristic impedance of the tested in-situ probe along the transmission line. Since the probe used in generating these data was manufactured manually by relatively inaccurate machines, spacing between three conductors was not perfectly uniform and their alignments were not perfect. This structural variation resulted in non-uniform impedance along the transmission line, which created increased reflection and radiation over the transmission line. These actions are amplified as the frequency increases and causes the error in measurement results.