Optimal GPR antenna selection

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Courtesy of Geoscanners AB

It is common knowledge to anyone practicing Ground Penetrating Radar (GPR) technology that lower center frequency antennas mean higher penetration and lower resolution. In the same way it is correct to say that higher center frequency antennas mean lower penetration and higher resolution. This is without any doubt an axiom that provides us with a rule of thumb on what to use and what not to use for our surveys. The question I am trying to explore in this particular application is quite different: what is the actual threshold for choosing between a higher or a lower frequency antenna?

Answering to this question is not straight forward and never easy, but I'll try to explain it with a simple example the cons and pros of using one or the other type of antenna. I used for this experiment two antennas manufactured by Geoscanners AB, a GCB-200 with a center frequency of 200MHz and a GCB-500 with a center frequency of 500MHz. I will have to go into the details for both of the antennas because they are very relevant to the conclusions we are going to reach later on, so please, be patient.

The power spectrum for each antenna are presented in figures 1 and 2 respectfully so you can better understand the concepts explained below.
The GCB-200 has a -10dB bandwidth starting at 121MHz and up to 357MHz giving us 239MHz as the -10dB center frequency and a 98% bandwidth at the -10dB levels. Penetration of six meters or more can be expected in moderate conditions with this antenna. The transmitted pulse duration for the antenna we used was 4.8ns and the relative dielectric permittivity of the media we surveyed equal to 6. With this figures we can easily calculate that the expected vertical resolution should be around 30 cm.

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