What is the GPR’s fuzzy zone? This is the shallow zone that is difficult to image with a GPR. This means that when the GPR puts out a signal, there is a portion of the immediate subsurface that is almost un-imagible (so I made up a word!). Many of LearnGPR’s students come to me with frustration as to why there is an area directly below the surface that they cannot locate buried targets. I tell them that the size of this zone will vary and will sometimes be smaller and sometimes larger.
Here are three reasons why this zone exists:
1) Antenna Spacing – the distance between the antennas has an affect on the size of the fuzzy zone. The greater the spacing between the transmitter and the receiver, the larger the fuzzy zone.
2) Wavelength – The length of the wave (from crest to crest) will effect the size of the fuzzy zone. The longer the wavelength, the larger the fuzzy zone. This is why it can be difficult to estimate thickness of 4 cm asphalt using a 250 MHz antenna.
3) Pulse duration – This refers to how long the antenna puts out a pulse. The longer the time it takes an antenna to put out a pulse, the larger the fuzzy zone. When the pulse is emitted by the transmitter, it will inundate the receiver with energy. This can mask the first X nanoseconds of response where X is the pulse duration.
All three of these reasons are related to antenna size and frequency. The lower the frequency, the larger the antenna. Antenna spacing is generally the same distance as the size of the antenna itself. So a larger antenna will require a greater spacing between transmitter and receiver creating a larger fuzzy zone. A lower frequency will create signals with longer wavelengths and thus a larger fuzzy zone. Finally, the larger the antenna, the longer the pulse duration.
So lower frequency antenna will have larger fuzzy zones. As examples:
a 2600 MHz antenna will have around a 1 cm fuzzy zone
a 400 MHz antenna will have around a 15 cm fuzzy zone
a 200 MHz antenna will have around a 30 cm fuzzy zone
So, don’t try and image a 4 cm pipe buried 5 cm below the ground surface with a 200 MHz antenna. While the fuzzy zone for a given antenna is not an area where responses are unattainable, but it is an area that is more difficult to image. If the locations of deep and shallow targets are required for a project, you may want to consider using multiple antenna.
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