Data Interpretation

Where to measure your GPR target’s depth (Ground Penetrating Radar)

By October 2, 2017 One Comment

I recently returned from a trip to Australia where I educated a bunch of Utility Locators in GPR. I did a presentation at a conference, a workshop, a 3 day bootcamp, and a coaching session. Throughout this amazing trip I had three people ask me the same question…which part of the hyperbola should I use to measure the depth of my target.

This is a super important questions since one of the most important benefits of GPR is its ability to measure depths, but this issue has some problems. One of the people (thanks Josh) whom I had this discussion with pointed out an article to me where the author asked 6 (I believe) different manufacturers where to mark the ground surface and the author received 6 different answers!!! WHAT??? That is nuts.

So who is correct?

The truth is that all of them could be correct. The way that I learned it was that you should measure the First Break. The initial part of the signal that indicates a reflection. So, that’s what I have done for as long as I can remember. But, you could measure it from any other place too. The key to correctly measuring depths to a target is to use consistent markers for where you measure your ground surface and where you measure your target.

The ground surface is your reference point anyway. So if you measure the peak amplitude of the central band for your ground surface…then you MUST measure the peak amplitude of the central band of your target. If you measure the First Break for your ground surface, then you MUST measure the First Break of your target. In either case you should end up with the same depths.

So where should you measure? It doesn’t really matter all that much as long as you remain consistent.

Leave a comment below about how you have measured depths until now. is an online training platform for Ground Penetrating Radar that helps civil engineers, utility locators, and others learn about GPR through fun and engaging courses and coaching.

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Join the discussion One Comment

  • Innes Fisher says:

    Thanks Dan, another informative video. I’m not sure how different manufacturers determine ‘time-zero’, but this would normally be done within the collection application I assume and the end-user doesn’t get to set their own ‘time-zero’ measurement point. When template fitting to determine the velocity and therefore get the most correct depth you’d need to know for your particular setup where the time-zero was set and therefore where to measure on any reflection to ensure you use the correct relative position. It would be interesting to work out the sort of error in depth introduced by not picking the correct point for depth measurement (i.e. first break vs mid break vs end of first break). We talk often about ‘margins of error’ in giving GPR depths to clients so perhaps any difference is going to fall within an acceptable margin of error in any case.