I am processing GPR data and notice that the same diffractions in a given profile coorespond to different velocities depending on whether I am using GPR-SLICE or ReflexW. In both programs, I fit a hyperbola to diffractions to determine the velocities. We are talking 0.1 m/ns difference! Any ideas why?
Hi Christine,
I haven't get any chance to work on GPR-slice yet but I guess there are Algorithm difference in both softwares. I would like to highlight few other ways to calculate velocity which you may know already:
1. GSSI Dual Frequency system has an option for hyperbola fitting in Acquisition software itself where you can focus on the hyperbola by adjusting curvature and dielectric constant to get the Velocity.
2. By ground-truth survey: Collect a profile over a known depth utility, open data in Reflexw and adjust the velocity in plot option to shift the hyperbola produced by utility at that known depth which will automatically update the travel time scale also.
3. GSSI system has an option to select soil type before starting acquisition. For example: if u choose Dry sand, it considers the dielectric value 4, for asphalt it takes 5, 5.5 for dry sand gravel and so on.
So you can choose the soil type according to your site, calculate the velocity by inserting the dielectric value in the formula v = velocity of light/sqrt(dielectric const.) m/ns and input the same velocity value in ReflexW for processing.
Thank you, Pranjal. I'm working with the MALA ProEx system and do not assume any velocity or soil type during acquisition. Ground-truthing is not an option.
Yeah I guess Mala doesn't have Soil customization option.
Ultimately our concern is, depth. What I do while processing is, suppose we are getting three hyperbolae which fits at different velocities. I choose the lowest velocity among them (usually 0.1m/ns) to process that section and present the depth as 'MPD' (Minimum Possible Depth). So if a utility is present at 0.5m in real, by applying minimum velocity, it will show it at little lower depth (for example 0.4m) which is more safe for the client to know before any drilling/construction work.
Which software did you find best to process 2D GPR data?
ReflexW, RADAN or GPR Slice?
Each of these software has advantages and disadvantages, but in general, they are not significantly different. I personally prefer the MATGPR package because it is free and can be easily used.
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