Frequency

 

GPR has a range of antennae frequencies from 125 to 1200 MHz. There is a significant contrast between data collected with these difference frequencies. Low frequency antennae can penetrate deeper into the ground, but the resulting image is lower resolution. High frequency antennae provide a very high resolution image, but can’t ‘see’ as deeply into the ground. The antennae used for each project needs to be thoughtfully chosen because of this tradeoff.

For the survey we chose to use two different antennae frequencies; 225 MHz and 450 MHz. These two antennae can be seen below. These antennae were chosen because anything lower would not have provided the resolution needed to locate the burials. Thedifference in the data collected is visible in the following examples which are all taken from Transect 8 in the grid.


The 225 MHz antenne is in the background and the 450 MHz in the foreground.

 

225 MHz antennae

450 MHz antennae


Using the higher frequency antennae more reflections can be seen. In 225 the MHz profile 2 or possibly 3 reflections can be seen. In the 450 MHz profile at least four hyberbolas are visible, and possibly more. As indicated on the side of the profile however the 225 profiles reached 3.5 meters deep opposed to the 450 which only reached 2 meters.


225 MHz 3D Cube

400 MHz 3D Cube

 

The 3D visualizations also demonstrate the differences in the antennae. There is a lot more detail in the second cube. However, with the high frequency antennae, the higher resolution is more likely to image small features in the subsurface that may not be of concern and clutter the image. On many of our profiles tree roots can be seen near the surface. The roots make other reflections more difficult to interpret. It is important to weigh these factors when choosing the antenna frequency for surveys.

 


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