Which direction should I collect my GPR transects in? Parallel to the sidewalk? Perpendicular? Both directions? Does it matter?
These are questions I get asked all the time. The answer to the last one is…YES…it does matter!
It primarily matters depending on what you are searching for or mapping. If you are looking for small isolated targets, you can get away with only collecting data in one direction. But, your transect spacing must be close enough so your gpr wave reflects off of your target on multiple transects. Otherwise it is impossible to know if it was your target or some other random variation in the subsurface. “Hitting” your target multiple times will give you some definition of the feature and help interpretation. An example would be a grave or a UST.
If you are locating linear features though you should do everything in your power to collect data in perpendicular orientations. The reason is that your pipe or rebar or retaining wall or whatever, could be oriented parallel to your transects and could be missed entirely or removed during post processing with a background filter (if you collected data right on top of it). Also, in many of these cases, you will have linear features running in different directions such as with locating utilities or scanning concrete and going in two directions will help you see these.
I would argue that in these cases with linear features, if you are limited to a certain number of transects, it is better to give up some transect spacing to ensure that you can collect data in multiple directions.
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