Mapping of Cavities using Ground Penetrating Radar

 

 

 

 

 

 

 

 

 

 

 

 

Figure: Cavities (green shadings) as shown by Radar image.

The ground penetrating radar operates at frequencies typically greater than 50 MHz to as much as 2GHz, and thus wave effects become important. The transmitter is an electric-dipole antenna, which generates pulse sources, with center frequency around a particular value for which the antenna has been tuned. The pulse source generates EM waves, which are transmitted directly through the air, and through the earth, some part of it being reflected. The receiver measures both the amplitude and arrival times of the directly transmitted and reflected pulse or pulse trains. Because the earth effectively acts like a filter the waveforms of the reflected pulses are modified. The data collection scheme and the interpretations are very similar to that of reflection seismology, which is based on the propagation of elastic waves. It is possible to use radar data to map the spatial distribution and discontinuities of dielectric permittivities and the conductivity and velocity of electromagnetic propagation inside the earth. The main disadvantage of radar is the limited depth of investigation it can achieve, because of heavy attenuation in the presence of a conductor. Typically, under the best circumstances, radar cannot be expected to penetrate more than a few tens of meters. Figure above shows an example of radar applied to the mapping of underground cavities. The imaged cavity is located at 10 to 30m horizontally as the green shading and is a very good representation of the great hall of the Harrison Cave systems in Barbados, West Indies.

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