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Underground sensing is of interest in many applications, to find buried objects,fresh water layers, minerals, chemicals and possibly mines. Due to high attenuation in most soils, microwave-based underground sensing is most appropriate. Mywork was absolutely inclined to sensing of the depth of water layer, i.e detecting the soil water interface.

A Subsurface radar or ground-penetrating radar (GPR) is used in detection of burried objects below ground.The concept of (GPR) has been in research since the early 20th century. This techniques offers rapid, high resolution investigation of underground objects and structures by recording microwave radiation that passes through the ground and isreturned to the surface from several underground layers or objects.

Im 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 microwaves propagate at velocities that are dependent upon the dielectric constant of the subsurface medium. Changes in the dielectric constant that aredue 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 the depthat which the energy was reflected. Thus interpretation of this reflected energ

y yields information on structural variation of the subsurface amd also depth can be calculated from time elapsed between the transmission.

I was influenced to work in this field after reading a German patent by Leimbachand Lowy. They were the very first level of scientists to work in the design ofa GPR and publish research papers. Their technique consisted of burying dipoleantennas in an array of vertical boreholes and comparing the magnitude of signals received when successive pairs were used to transmit and receive.

The basic operation involves transmitting a modulated signal over the ground andreceiving the signals reflected or scattered from the layers below the ground.Then received signal is sampled and processed using several sgnal processing techniques and later analyzed. Since the received signal is random and discritized

for processing, advanced statistical analysis is required to process the signal.

The selection of antenna is very critical for proper signal analysis. A number of factors starting from the approx. depth of penetration, type of objects to identify, i.e their electromagnetic properties, the optimum operating frequency, etc must be considered before selecting the desired antenna. The problem is smaller the wavelength, the better the resolution. So very high frequency signal needsto be used but the higher frequency signals will not penetrate as deep as the lower frequencies.Limitizing the operating in the 100 MHz to 1 GHz frequency range considering thepurpose a penetration depth of about 2 to 3 meters can be acheived into the different types of soils. I have used a bow-tie antennaas which is a linearly polarized ultra wide band antenna. Its plane reflector has both a respectable bandwi

dth 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

The dielectric constant, i.e, the measure of the ability of a material to storea charge from an applied electromagnetic field and then transmit that energy isan important parameter in detection.

The refected signals from different layers of the ground with varing dielcetricconstants are scattered signals. Thus a scatterging parameter or scattering matr

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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 noisesas 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 algorithmslike genetic algorithm, imaging techniques can be used to detect unknown objectsor layers and to image the subsurface features in 3-D or 2-D with higher resolution.