Detection of burried water layer

8/8/2019 Detection of burried water layer

1/26

DETECTION OF BURRIED WATER LAYER Page 1

INTRODUCTION

Underground sensing is of interest in many applications, including detection of buried

conduits, fresh water layers, minerals, chemicals and possibly unexplored ordnance or mines.Due to high attenuation in most soils, microwave-based underground sensing is most appropriatefor targets on or near the airsoil interface. There is often the need to perform quick wide-area

surveillance, to circumscribe regions likely to contain mine fields or former bombing ranges.

Still deep penetrating radars are also very commonly used in geological surveys for the purposes

of mapping the ground profile and to detect possible water bodies or underground water layers.

The terms subsurface radar or ground-penetrating radar (GPR) refers to a wide range of

electromagnetic techniques designed primarily for the location of objects or interfaces buried

beneath the earths surface such as pipes, cables, land mines, and hidden tunnels. They are alsoused for detecting the presence and depth of water layer beneath the soil. These techniques offer

rapid, high resolution and non-invasive investigation of underground objects and structures byrecording microwave radiation that passes through the ground and is returned to the surface fromseveral underground layers or objects.

In a typical GPR, a transmitter sends a microwave signal into the subsurface which will

be reflected by the buried objects or interfaces beneath the earths surface. The microwavespropagate at velocities that are dependent upon the dielectric constant of the subsurface medium.

Changes in the dielectric constant that are due to changes in the subsurface materials cause the

radar wave to be reflected. The time taken for the energy to return to the surface is related to thedepth at which the energy was reflected. Thus interpretation of this reflected energy yields

information on structural variation of the subsurface.

The operational effectiveness of the subsurface radar depends on the (i) efficient couplingof electromagnetic radiation into the ground; (ii) adequate penetration of the radiation through

the ground having regard to target depth; (iii) ability to obtain from buried objects, or other

dielectric discontinuities, a sufficiently large scattered signal for detection at or above the groundsurface; and (iv) an adequate signal bandwidth in the detected signal with regard to the desired

resolution and noise levels. A GPR system should have low and short coupling between

transmitting and receiving antennas to avoid false detection.

A wide range of time-domain applications such as impulse Ground Penetrating Radar

(GPR) requires an ultra-wideband antenna system capable of transmitting properly short

transient pulses. Ultra-wideband (UWB) applications have stimulated a surge of interest inantenna design by providing new challenges and opportunities for antenna designers. The main

challenge in UWB antenna design is achieving wide impedance bandwidth while still

maintaining high radiation efficiency. Metal-plate antennas are preferred in most situations. The

classic solution is to obtain an omni-directional pattern using a thin wire dipole or its counterpartmonopole version with a ground plane. However, the wire dipole and monopole suffer from

narrow impedance bandwidth, but it can be widened by using flat metal rather than a thin wire

structure.


2/26


OBJECTIVE:

Our chief aim is to understand the hidden subsurface hydrogeological conditions

accurately and adequately. Since the base of any geophysical detection methods is the contrast

between the physical properties of the target and the environs, the better the contrast or anomaly,better would be electromagnetic response and hence the identification. So, the efficacy of any

technique lies in its ability to sense and resolve the hidden subsurface hydrogeological

heterogeneities or variation.

The importance of groundwater for the existence of human society cannot be

overemphasized. Groundwater is the major source of drinking water in both urban and rural

India. Besides, it is an important source of water for the agricultural and the industrial sector.

Water utilization projections for the year 2000 put the groundwater usage at about 50%. Being an

important and integral part of the hydrological cycle, its availability depends on the rainfall and

recharge conditions.

Therefore, here we are giving a modest effort to understand different aspects of groundwater level detection with the help ofan economical wide band antenna. In the past, we used todepend more on surface water sources like rivers, ponds and streams. But, with depletion of

water bodies due to extreme summer and other factors, we have shifted our focus to ground

water.

The situation, if not reversed through conservation measures, would lead to collapse ofentire agriculture sector and drinking water supply system of the affected area in the country in

near future. Hence our study is aimed to help to identify the regions where the water layer buried

underground is depleting fast so that measures like Ground water recharge could be taken

accordingly.

LITERATURE SURVEY:

Ground penetrating radar (GPR) is an electromagnetic technique useful for mapping

layering in soils and rocks and for detecting underground objects due to changes in the electrical

properties of materials. The technique has been in existence for many years, but in the beginning

little was known about propagation, penetration and the interaction of electromagnetic energy in

earth materials. Consequently, trial and error methods were used, and experience was

accumulated by various workers, studying numerous applications under different conditions.

The first description of the use of electromagnetic signals to determine the location of

buried objects was found in a German patent by Leimbach and Lwy in 1910. Their techniqueconsisted of burying dipole antennas in an array of vertical boreholes and comparing the

magnitude of signals received when successive pairs were used to transmit and receive. The

main feature of this technique is the use of continuous wave (CW) together with diffractioneffects of buried objects or underground features.


3/26


The first ground penetrating radar survey was performed in Austria in 1929 to sound the

depth of a glacier. The technology was largely forgotten (despite more than 36 patents filedbetween 1936 and 1971 that might loosely be called subsurface radar) until the late 1950's when

U.S. Air Force radars were seeing through ice as planes tried to land in Greenland, but misread

the altitude and crashed into the ice. This started investigations into the ability of radar to see

into the subsurface not only for ice sounding but also mapping subsoil properties and the watertable. In 1967, a system much like Stern's original glacier sounder was proposed, and eventually

built and flown as the Surface Electrical Properties Experiment on Apollo 17 to the moon. In

1972, Rex Morey and Art Drake began Geophysical Survey Systems Inc. to sell commercialground penetrating radar systems.

Thus began an explosion of applications, publications, and research, fostered in great

part by research contracts from the Geological Survey of Canada, the U.S. Army Cold Regions

Research and Engineering Laboratory (CRREL), and others. There are now over 300 patents

that might loosely be related to ground penetrating radar, around the world. Ground penetrating

radar is sometimes called georadar, ground probing radar, or subsurface radar.

Ground penetrating radar uses electromagnetic wave propagation and scattering to image,

locate and quantitatively identify changes in electrical and magnetic properties in the ground. It

may be performed from the surface of the earth, in a borehole or between boreholes, from

aircraft or satellites. It has the highest resolution in subsurface imaging of any geophysical

method, approaching centimeters under the right conditions. Depth of Investigation varies from

less than a meter to over 5,400 meters, depending upon material properties.

Detectability of a subsurface feature depends upon contrast in electrical and magnetic

properties, and the geometric relationship with the antenna. Quantitative interpretation through

modeling can derive from ground penetrating radar data such information as depth, orientation,

size and shape of buried objects, density and water content of soils, and much more.

The Ultra-wideband antenna has its roots in the original spark-gap transmitters that

pioneered radio technology. In 1898, Oliver Lodge introduced the concept of syntony, the idea

that a transmitter and a receiver should be tuned to the same frequency so as to maximize the

received signal. In this same patent, Lodge discussed a variety of capacity areas, or antennas

that will be quite familiar to modern eyes: Lodge disclosed spherical dipoles, square plate

dipoles, biconical dipoles, and triangular or bow-tie dipoles. He also introduced the concept of

a monopole antenna using the earth as a ground.

In this project a bow-tie antennaas a linearly polarized ultra wide band antenna is used.The following topics give a brief insight into the type of antenna used, its basic construction and

the reason why such an antenna was chosen.


4/26


OVERVIEW OF SUBSURFACE RADAR

The very basic functionality of an underground detection system or say subsurface radarcan be very easily demonstrated with the help of the following block diagram.

The transmitting and the receiving antennas are kept at very much the same elevationabove the ground at which the object or scatterer is buried. The pulses being transmitted is

generated by the source and modulator block. The backscattered wave is received by the receiver

antenna. This received backscattered wave is then sampled and digitized to allow storage and

further processing of the data. Ultimately the images of subsurface features are constructed fromthe data, by means of advanced signal processing techniques and displayed onto the display unit.

From the bloc diagram it is quite evident that efficient design of the antenna plays a very

important part in the success of the detection. As has been discussed earlier, antennas with wide

frequency band are particularly suitable e.g. a conventional bow-tie antenna.

Figure 1: Block diagram of GPR system


5/26


BOW-TIE ANTENNA:

In recent years UWB (Ultra Wide Band) systems have received growing attention, due tothe concrete possibility to develop commercial short-range wireless systems with extremely high

data rates. We present a novel design for a UWB antenna backed by a plane reflector which hasboth a respectable bandwidth and a large front to back ratio. The antenna is formed by two

metallic patches with bow-tie shape, and two parallel stripes which run between the bow-tie tips,and are finally connected to the metallic patches.

Design parameters such as the antenna flare angle and linear dimensions, as well as thefeed-points coordinates and the distance between the antenna and the reflector have been

optimized numerically to obtain the proper frequency transmission required for our purpose.

The above fig. shows simulated unit cell of the regular array of the bow tie antenna. In

fig. 2.a the element type is a PEC made Bow-Tie element placed 166 mm on top of a groundplane. Fig 2.b shows the Feeding model of the bow tie antenna with the feeding point lying at the

centre of the unit cell. The unit cell is fed with a voltage source having an impedance of 200

ohm. Fig. 2.c shows the photograph of the antenna that has been manufactured andexperimentally characterized.

Since the aim of this project is to detect the presence of buried objects or water layer

beneath the soil, it demands the detailed study of the properties of soil and the changes observed

in signals when travelling through soils of varying properties.

c) Manufactured unit

Figure 2: Basic structure of bow tie antenna


6/26


PROPAGATION OF RADIO WAVES THROUGH

DIFFERENT SUBSURFACE MATERIALS:

The GPR method is based on the transmission of electromagnetic pulses, which then

propagate as waves, into the ground and measuring the time elapsed between their transmission,reflection off buried discontinuities, and reception back at a surface radar antenna.Each physical

or chemical change in the ground through which the radar waves pass will cause some of that

energy to be reflected back to the surface, while the remainder continues to propagate deeperuntil it finally dissipates. Buried discontinuities where reflections occur are usually created by

changes in the electrical or magnetic properties of the rock, sediment or soil, variations in their

water content, lithological changes, or changes in bulk density at stratigraphic interfaces.

Reflections also are generated when radar energy passes across interfaces betweenarchaeological features and the surrounding matrix. Void spaces in the ground, which may be

encountered in burials, tombs, tunnels, caches, or pipes, will also generate significant radar

reflections because of asimilar change in radar wave propagation velocity.

The following diagram properly shows the way the transmitted pulses go through several

change of wavelengths and amplitudes. As the short pulse gets reflected from several layersinside the surface of earth, the wavelength gets increased and the amplitude gets decreased due toattenuation.

Figure 3: Underground wave propagation and reflection from subsurface discontinuities


7/26


RELATIONSHIP BETWEEN VELOCITY OF WAVE

PROPAGATION & RDP:

Relative dielectric permittivity (RDP), also called the dielectric constant, is a measure of

the ability of a material to store a charge from an applied electromagnetic field and then transmit

that energy. It takes the electrical and magnetic properties of buried materials into account. In

general, the greater the RDP of a material, the slower radar energy will move through it. Relative

dielectric permittivity is a general measurement of how well radar energy will be transmitted to

depth. It therefore measures velocity of propagating radar energy and also its strength. For most

archaeological applications, RDP values and measurements of the velocity of radar travel in the

ground are used synonymously, as it is very difficult to measure or predict most of the other

components of radar wave behavior used in the complex calculation of RDP.

For most archaeological studies, RDP and velocity are used interchangeably as a way to

determine velocity of radar wave propagation in the ground. For instance, the RDP of fresh water

is very high (about 80), but radar energy can easily be transmitted through it without being

attenuated only at a very slow velocity.

It is always important to have some understanding of the RDP (or velocity) of the material in the

groundat each site being studied, as it will be used to convert radar travel times to depth. Therelative dielectric permittivity of air, which exhibits only negligible electromagnetic attenuation,

Figure 4: Relative Dielectric Permittivity Vs Velocity graph


8/26


is approximately 1.0003, and is usually rounded to 1. Most soils and sediments found at

archaeological sites have RDP values that range between 3 and about 25. In a totally dry state,most naturally occurring materials in the ground have an RDP that varies little, usually between

about 3 and 5. But if just a small amount of water is added to the material (which is almost

always the case in natural conditions, even in the driest of deserts), the RDP will increase,

sometimes dramatically.

If data about the types of material in the ground are not immediately available, the RDP

of the ground can only be estimated using a number of field methods which relates RDP andradar velocity of a material, is shown below:

K =relative dielectric permittivity (RDP) of the material through which the radar energy passes.

C = speed of light (.2998 meters per nanosecond).

V=velocity of the material through which the radar passes (in meters per nanosecond).

The amplitude of reflections generated at an interface between two materials with known RDPs

can be calculated using equation.

R _ coefficient of reflectivity at a buried surface

K1 _ RDP of the overlying material

K2 _ RDP of the underlying material

Ground material can be described by its electrical properties such as electric permittivity

(), electric conductivity () and magnetic permeability (). Since most ofthe earths material isnon-magnetic, the permeability of the medium can be approximatedas the permeability of free

space (=0). The conductivity of the medium determinesthe amount of energy lost from

currents induced in the medium by the propagating wave.Losses can arise from both conductionand displacement currents.Some of the factors ofsubsurface radar application are its attenuation

( ) and the velocity of wave propagation () as a function of frequency. The relationship

between these parameters and the apparent permittivity () is given by the equation

= + j = j ()1/2

The table below shows the dielectric constant of some common earth materials. The

dielectricconstant of the medium depends principally on the water content. The reason is that atlower frequencies, water has a relative permittivity of about 80, while for the solid constituents

of most soils relative permittivity is in the range of 2 to 6. The measured relative permittivityof

soil was found to be in the region of 4 to 40.


9/26


From the table we can observe that in general, wet materials exhibit higher loss than dry

ones at a given frequency. The highest-amplitude radar reflections usually occur at an interface

of two relatively thick layers that have greatly varying properties.

DEPTH AND FREQUENCY OF OPERATION:

Subsurface radar systems usually operate in the megahertz range. The waves thatpropagate into the ground will have wavelengths on the order of 1 meter or less. The horizontal

and vertical resolutions of the radar images are dependent upon the wavelength of operation,

such that the smaller the wavelength, the better the resolution. The higher frequency source will,therefore, yield better resolution, but the higher frequency signals will not penetrate as deep as

the lower frequencies. For a given signal detection threshold the maximum depth of investigation

decreases rapidly with increasing frequency. Thus a careful choice of frequency of operation

Table1: Dielectric constant and velocity of wave propagation of different materials


10/26


must be made based on the expected target depth and the system design goals. The following

table shows the typical required penetration depths of a range of materials, and their appropriateupper operating frequencies. A consideration which applies when choosing an operating

frequency for detecting localized objects, rather than interfaces between thick layers, is the fact

that the backscattered amplitude is also frequency dependent and, apart from any resonance

effects, decreases with decreasing frequency.

It can be concluded from the table that subsurface radar operating in the 100 MHzto 1GHz frequency range has a penetration depth of about 2 to 3 meters into the differenttypes of

soils (clay, loam and sand). This will be the typical operating frequency range ofmost subsurface

radars that are used for mine detection. For our study we needed much deeper penetration depthsand the target was interfaces between soil and water layers buried underground. So we designed

and simulated an antenna with a much lower frequency band and higher penetration.

RESOLUTION OF SUBSURFACE FEATURES:

Subsurface resolution is mostly a function of the wavelength of propagating radar energyand the geometry of the buried materials in the ground of interest. Low-frequency antennas

(those of 10 to 120 megahertz) generate long wavelength radar energy that can penetrate up to 50

meters or more in certain conditions but are capable of resolving only very large subsurfacefeatures. In pure ice, antennas of this frequency have been known to transmit radar energy many

kilometers, and they are commonly used to determine the thickness of glacial ice or the

orientation of sub ice bedrock surfaces (Bogorodsky et al. 1985; Delaneyet al. 2004). In contrast,the penetration depth of a 900-megahertz antenna is about 1 meter, and often less, in typical

Table 2: Penetration depth and max. operating frequency of different materials

Material Penetration Depth Max. Operating Frequency

Cold pure fresh water ice 10 km 10 MHz

Temperate pure ice 1 km 2 MHz

Saline ice 10 m 100 MHz

Fresh Water 100 m 100 MHz

Sand (desert) 5 m 1 GHz

Sandy soil 3 m 1 GHz

Loam soil 3 m 500 MHz

Clay soil 2 m 100 MHz

Salt (dry) 1 Km 250 MHz

Coal 20 m 500 MHz

Rocks 20 m 50 MHz

Walls 0.3 m 10 GHz


11/26


ground conditions, but its generated reflections can resolve features down to a few centimeters in

diameter. A tradeoff therefore exists between depth of penetration and subsurface resolution .

The ability to resolve buried features is largely a function of the wavelength of energy

reaching them at the depth they are buried. A rule of thumb is that the minimum object size

that can be resolved is about 25 percent of the downloaded wavelength reaching them in theground. Downloading of radar energy alwaysoccurs as energy passes in the ground and decreases

in frequency, increasing the propagating wavelength of the radar waves. Determining what the

wavelength of any frequency radar wave might be in the ground is further complicated byadditional changes in wavelength as energy passes through materials with different RDPs.

SCATTERING PARAMETERS (S11)

Scattering parameters, which are commonly referred to as s-parameters, are a

parameter set that relates to the traveling waves that are scattered or reflected when an n-portnetwork is inserted into a transmission line.

Two-port network showing incident waves (a1, a2) and reflected waves (b1, b2) used in s-parameter

definitions.

The independent variables a1 and a2 are normalizedincident voltages, as follows:

=

=

=

=

Dependent variables b1, and b2, are normalized reflected voltages:

=

=

=

=

The linear equations describing the two-port network arethen:

b1 = s11 a1+s12a2

Figure 5: S-parameter in two port network


12/26


b2 = s21 a1+ s22a2The s-parameters s11, s22, s21, and s12are:

when Input reflection coefficient with the output port terminated by a

matched load (ZL=Z0 sets a2=0)

when Output reflection coefficientwith the input terminated by a

matched load (ZL=Z0 sets a2=0)

when Forward transmission (insertion)gain with the output port

terminated in a matched load.

when Reverse transmission (insertion)gain with the input port

terminated in a matched load.

Notice that :

=

=

,

And

Where

is the input impedance at port1.

The scattering parameter S11 is the ratio of the reflected voltage over the incident voltage

(i.e., reflection coefficient). The decibel value is S11dB =20log10 (S11). Values of S11 less than

10 dB are generally considered acceptable, but values of 15 dB or less are more desirable.

The objective is to accurately determine the distance of the reflecting surface from the probe.

This distance is calculated by measuring the time-domain scattering parameter S11 for everysample:


13/26


Where, 0 is a ratio of the reflected wave from the object and the incident wave from the

antenna, is the phase constant, and l is the distance from the probe to the object at every step of

the scanning process.

Advantages of S parameters:

1. An important advantage of s-parameters stems from the fact that traveling waves, unliketerminal voltages and currents, do not vary in magnitude at points along a lossless

transmission line. This means that s-parameters can be measured on a device located atsome distance from the measurement transducers, provided that the measuring device and

the transducers are connected by low-loss transmission lines.

2. The above equations show that they are simply gains and reflection coefficients, bothfamiliar quantities to engineers.

3. As the S-parameter gives an easy measure of the ratio of the reflected and incident wavepower, this value can be very easily used to detect the presence of any buried object orinterface of water layer or any other discontinuity of dielectric parameter buried

underground.

DATA COLLECTION & SIGNAL PROCESSING:

Subsurface detection of discontinuities involves rigorous survey of the area under

consideration after dividing it into sectors and grids. The radar is then moved along the sectors

and the grids to achieve large set of data. Then these set of datas are used to process 2-D or 3-D

image of the subsurface features. Advanced data accusation and signal processing techniques are

needed for these purposes. In our study the basic objective was to detect the presence of the

water layer. So we did not opt for imaging of subsurface features. We simulated for some known

cases with and without the water layer. Now the simulated results were used as signatures. Any

data collected for cases, where it is not known whether water layer is present under the ground or

not, can be compared with the signatures using signal processing techniques to decide about the

presence or absence of the water layer.

Correlation Technique:

A very efficient mathematical tool to detect whether two set of datas are similar in nature

or not is Correlation.The degree of association or the strength of relationship between thethe two

bivariate variable datas x , y assumed by the bivariate (X,Y) is called the correlation between the

two random variables X and Y.


14/26


If y has a tendency to increase as x increases or vice-versa, we say x and y are positively

correlated.

Again if y has a tendency to decrease as x decreases or vice-versa, we say x and y are negatively

correlated.

If the values of x or y are not affected by the changes in the values of y or x, then we say x and y

are uncorrelated or zero correlated.

Thus a measurement of the amount of correlation between two variables becomes

necessary to get the idea of association or similarity between two variables. Another powerful

mathematical tool called the Pearsons Product Moment Correlation Coefficient or simply

Correlation Coefficient is used. It is defined as:

Correlation Coefficient, ()

Where, = Covariance of x and y, = Standard deviation of x and y respectively.


15/26


Figure 6: Bow tie antenna with dielectric sphere in WIPL-D Simulation

EXPERIMENTAL SET-UP

The aim of the project was to detect the presence of buried objects or interfaces of

different layers. To achieve this, a bow tie antenna was placed at a certain distance above thesurface of the soil. A microwave signal was transmitted into the soil. Since the same antenna was

used as transmitter and reflector, the reflected signal from any subsurface object or interfaces ofvarying dielectric was received using the same antenna. The signal thus obtained was analyzed

using signal analysis techniques.

Our study was basically limited to the simulation and optimization of the antenna and to

model the real world detection situation using different simulation tools, and to study the

different impacts of practical situations by modeling them. We had two choices; to study the

models in either frequency domain, by using WIPL-D or in time domain, by using CSTMicrowave Studio. But we chose to study the impact of having different dielectric materials as

target, using frequency domain analysis. The modeling of the real world condition was doneusing the time domain methods.

Frequency Domain Simulation:

For the purpose of numerical modeling of the simulation set up method of moment based

software WIPL-D was used. We considered three variable parameters for our study. The power

radiated inside the ground changes its course due to the Relative Dielectric Permittivity, Distancetravelled before colliding with the target and the size of the target under test. To study the

different effects these parameters had on the S-parameter we used several set-ups.


16/26


As in the figure of the actual set-up shown above, we placed the antenna in the free space and

then placed a scatterer or target in front of the antenna, so that the antenna radiation getsreflected by the target and reaches back to the antenna which acts as receiver this time. Then we

kept changing one of the parameters at a time keeping the other two constant. First we changed

the value of the dielectric constant several times and simulated for each case. Then the distance

of the object from the antenna was changed and at last the radius of the target was changed.Simulations were done for each cases and the changes were properly recorded.

Time Domain Simulation:

Due to lack of actual experiment set-up it was more than necessary to simulate the real

world condition using a proper model. For that purpose proper models were designed and

simulated using time domain software CST Microwave Studio. At first the antenna was kept

10mm over the ground with no buried discontinuity. Then the same set-up was used with a smallsphere buried inside the ground. Again the antenna parameters were tested keeping the same set-

up, but with an water layer buried inside the soil layer and the sphere being removed. The

folloeing figures show the actual set-ups used for simulation using CST.

Figure 7: Bow tie antenna


17/26


Figure 8: Bow tie antenna with discrete waveguide port as generator

Figure 9: Bow tie antenna with buried object (conducting)


18/26


Figure 9: Bow tie antenna with underground water layer


19/26


20/26


2) With Varying Distance Of The Target From The Antenna:The distance between the antenna and the target sphere of certain dielectric material were

change several times and the impacts of these changes were observed.

Figure 12: Change in S-parameter with change in distance of the target from the antenna


21/26


3) With Varying Radius Of The Target:The radius of the target was changed several times and the effects on the parameters of

the received signal were observed.

Figure 13: Change in S-parameter with change in radius of the target


22/26


Discussion

The realistic modeling of ground penetrating radar (GPR) system has to deal with many

aspects such as broadband antennas, lossy and dispersive media in the ground, ground surfaceroughness, and natural clutter like rocks and twigs. But here moisture content in the soil, theground surface roughness and the soil in-homogeneities are all not considered. Here the GPR is

simulated assuming no ground surface roughness and no soil in-homogeneities. As the

transmitted energy from the bow tie antenna is reflected from various buried objects or distinctcontacts between different earth interfaces, we get different simulation results.

WIPLD SIMULATIONS:

With antenna in the free space we can see that the resonant frequency is about 95 MHzwithin a frequency band of 60 to 140 MHz and it is clearly visible that the scattering parameter

S11 is around -12db at the frequency of resonance. Here the lower value of of the scatteringparameter means that reflected power is less.

Now consider the case1, where the antenna is placed with an object. Here scattering

parameter S11 is now increasing for the objects with increase in their dielectric permittivity. The

value of scattering parameter of metal is the greatest among the other objects as the reflected

signal is more in case of metal. So we can conclude that the S-parameter value gives a direct

indication of the kind of object being used. This change of S-parameter with change of object

properties can be easily used to decide the presence of a certain material by comparing them

using signal processing techniques.

Let us consider the case 2 where the object is placed with the antenna and the distance

between them is varied. As the distance of the bow tie antenna is increasing the scattering

parameter of the antenna is decreasing from -8db to -12db at the resonance frequency. This

shows that the reflected power of the bow tie antenna from the ground is getting lower with

increasing distance. This is due to the fact that microwave signals get attenuated with distance.

So the lesser is the incident signal or download, the lesser would be the reflected signal power.

For the third case the radius of the target object was changed keeping the other two

parameters constant. As the radius of the object was increased, the scattering parameter too kept

increasing. For the change in radius from 0.2m to 0.8m the S11 value increased from -11.5dB to -10dB nearly.

CST SIMULATIONS:

In the first case the bow-tie antenna was placed in air. For this case the value of S 11 was

nearly -42dB at the center frequency.


23/26


In 2nd case the bow tie is on the ground. In the simulation graph of received voltage

versus time the scattered signal is plotted. From the time domain analysis a strong reflected

signal from the ground due to the discontinuity effect of the air-ground interface can be seen

continuing called clutter signal. To find the clutter in a practical situation, the radar can be

moved away from the target and readings can be obtained at several positions on the ground. The

average reading would give a reasonable value for the clutter provided the ground is fairly

homogeneous. According to the s parameter analysis, the scattering parameter S11 is -38 dB at

center frequency, which is greater than the scattering parameter S11 of free space.

Now consider the third case in which bow tie is on the ground with a conducting object.

In the time domain analysis it can be seen that the received scattered signal with clutter got

delayed unlike in the previous case with only the ground reflecting the signal. According to the s

parameter analysis, the modulus of the scattering parameter S11 here is around -30db. Thereflected signal power increased due to the reflection from the ground-object interface.

Now consider the fourth case in which bow tie is on the ground with water layer buriedinside. From the time domain analysis plot, it can be seen that the scattered signal is has

continuingly decreased in amplitude and has gone through several increases in the wavelength.

This proves the presence of an interface of a material with relative dielectric permittivity

different than that of the earth. According to the s parameter analysis, the the scattering

parameter S11 is -32 dB which is indicating a reflection from the ground water interface layer.

The S-parameter plot also shows two center frequencies.

COMPLICATIONS IN SUBSURFCE DETECTION:

Background Noise:

A complication that affects resolution of reflections in the ground is background noise, which is

almost always recorded during GPR surveys. Ground-penetrating radar antennas employ

electromagnetic energy of frequencies that are similar to those used in television, FM radio, and

other radio communication bands, so there are almost always nearby noise generators of some

kind. With the wide bandwidth of most GPR antennas, however, it is usually difficult to

completely avoid such external transmitter effects, and any major adjustments in antenna

frequency may affect survey objectives.

Focusing and Scattering Effects:

Reflection off a buried surface that contains ridges or troughs, or any other irregular features, can

either focus or scatter radar energy, depending on the surfaces orientation and the location of the

antenna on the ground surface. If a reflective subsurface plane is slanted away from the surface

antennas location or is shaped so that the surface is convex upward, most energy will bereflected away from the antenna, and no returning energy, or a very low-amplitude reflection,

will be recorded. This is termed radar scatter. The opposite is true when the buried surface is


24/26


tipping toward the antenna or the surface is concave upward. Reflected energy in this case will

be focused, and a very high-amplitude reflection derived from a portion of the buried surfacewould be recorded.

Air Waves:

The wide field of energy transmission from most GPR antennas can produce unwanted

reflections that occur from features that may not be in the ground, especially with lower-

frequency antennas that are not well shielded. When using unshielded antennas in areas of high-tension power lines or nearby buildings, reflections are likely to be produced from these features,

creating what are called air waves in reflection profiles. These reflections are often high in

amplitude and can obscure meaningful reflections from within the ground.


25/26


CONCLUSION

This project deals with microwave imaging an object buried in the earth or interfaces

within the soil. For reducing the difficulty of measurements in practical cases, it uses the S11parameters of a radiation antenna rather than data of scattered electromagnetic field. S11

parameters not only provide a simple yet standard platform for signal analysis, but also is aneffective tool for measurement of many other parameters like, gain, bandwidth and resonant

frequency.

There are three considerations that are essential for the success of the methods used in

solving the purpose of this project. First and foremost, conditions must be conducive for

microwave propagation in the ground. Conditions include the type of soil, its density, dielectric

property etc. For example if the soli itself has various layers of contrasting dielectric, then itbecomes rather difficult for the methodologies to hold good, since we assume a specific

dielectric is present throughout the length, breadth and depth of the soil.

Secondly, we assume the features i.e. either the buried objects or interfaces within the

soil that needs to be detected has a certain dielectric that is quite different from the soil itself.

Again if the dielectric of the soil and the item is not considerably different, its presence cannot be

differentiated from the background sediment or soil.

Thirdly, we fix a certain range of operating frequency band for the antenna to function.

This selection depends on parameters like skin depth, position of antenna above ground surface,

depth of penetration etc. There are restrictions posed on the range due to antenna size andspecifications that we had optimized to enable accurate signal processing and analysis. Thus it is

assumed that features i.e. either the buried objects or interfaces within the soil is/are present at adepth that can be resolved with the equipment available.

It is also necessary to mention the fact that cross correlation functions have been used as

the sole means to find the degree of resemblance between two signals. Since correlation demandssignals with matching time co-ordinates, data have been measured, calculated and analyzed

keeping this in mind.

The project uses simulations that correspond to real life techniques for imaging an object

buried in the earth or interfaces within the soil. Since the level of complicacy is far greater in

practical application of this project, the various problems that may be encountered in real life

testing, could not be encountered or resolved. All parameters that have been used for formulatingthe simulations strive to achieve an optimized performance with respect to the antenna used.


26/26

SCOPE OF FUTURE WORK:

In the last century a lot of work has been done in the area of ground penetrating radar but

still some complications and restrictions exist. Projects in these areas are greatly funded by the

defense services because of its high application in mine detection from a distance without

requiring any invasive methods. Though our study was greatly concentrated on deep penetrating

low frequency applications of GPR, the antenna could have been easily modified to use it as high

frequency GPR for land mine detection.

Our study was to detect the presence of buried water layers. This can also be modified to

detect the presence of water on other side of mine walls. So, it can actually help the mine

workers to take necessary actions by detecting danger early without invasive methods.

We limited our observations in the simulation using numerical methods, but the actual

situations can be vastly different. So experimentally verifying our datas in the future wound

enable us know more and more aspects of subsurface microwave propagation.

In real world applications several noises would be encountered due to the presence of

other microwave devices working in the same band. Again reflections may occur from the

presence of the surveyor himself. Advanced modulation and demodulation techniques like

CDMA need to be realized to negate the effect of the noises as much as possible.

We can also create a large data base of several signatures with different conditions,

different heights and different targets. Then advanced imaging algorithms like genetic algorithm,imaging techniques can be used to detect unknown objects or layers and to image the subsurface

features in 3-D or 2-D with higher resolution.

To conclude, practical verification, fabrication of the antenna with practically tested

parameters, extensive survey and data collection, creation of a signature bank, and implementing

advanced signal processing and imaging algorithms would be our next tasks to improve upon the

current study.

Documents

Detection of burried water layer