GPR field how-to

Equipment needed in the field:

General instructions:

  1. Determine an appropriate setup. For a target of dimension d (in m), the frequency f (in MHz) should be about 150/d √(εr), where εr is the relative permitivity (dielectric constant), which is generally 3-30 for us. So 1 m sized targets might work well with about a 100 MHz antenna, 0.1 m targets better with 800 MHz. Spacing between stations should be no more than 75/f √(εr) or about 1.5 m for the 100 MHz antenna or 0.2m for the 800 MHz (closer if higher permitivity). The sampling interval should be no less that 8 times the antenna frequency to avoid aliasing: this is about 6.4 GHz (0.15 ns) for 800 MHz antenna and 800 MHz (1.25 ns) for 100 MHz antenna. Generally a sweet spot for depth will be about 500/f, so an 800 MHz antenna will work best about a meter down while a 100 MHz will do better at greater depth. Our equipment doesn't allow us to alter the separation between source and receiver.
  2. Connect antenna(s) to controller and controller to PC. If used, connect measuring wheel (on one INSTAAR unit, connects to the antenna sled and then passes through that cable to controller)
  3. BE SURE THAT YOU ARE NOT PULLING ON THE CABLE CONNECTING CONTROLLER TO ANTENNA(S). There are usually stress relief attachments on the cables (little carabiners, etc.)
  4. Usually the final setup will have the controller in the GPR backpack, cable going through one of the holes, with the parallel cable going to the computer on the computer stand in the front.
  5. Turn on the controller, then the antennas, and finally boot the laptop. R, T, and D lights should all be flashing on both the antenna and the controller.
  6. Launch (usually) the GroundVision software. Ideally you see the F5 button on the toolbar on the screen with a red dot. Pressing this should prompt for starting a measurement. Adjust the settings for triggering the instrument according to the approach being used (string, wheel, etc.) and acquisition parameters appropriate for the environment and antenna used. On the first run, probably need to set the zero offset. Should be able to start survey at this point.
  7. Make sure that cell phones are kept away (they send and recieve on a common bandwidth to GPR).
  8. Adjust the filter settings to make subsurface signals apparent. This is usually the most challenging part of the field exercise, but note that filters can be changed after the data is returned to CU.
  9. If testing with the keyboard trigger selected, note that the Enter key is the trigger key (other keys do not work).
  10. In general, GPR is acquired on profiles. Distances from either the surveyor's string or the wheel are displayed on the screen. Notes should include the presence of any objects that might be of interest in interpretation (e.g., fence posts, trees, boulders, places the antenna came off the ground, etc.) and their position along the profile.
  11. Field notes should show where profiles were acquired, how they are oriented (start and stop as well as compass azimuth), and which file names the profiles are saved as.
  12. To acquire data, start GroundVision software (latest version present). Below the menu bar is a toolbar; about in the middle should be a red dot next to "F5". This (supposedly) indicates that communications to the control unit are working correctly. Pressing this button will start acquisition. When pressed, you are prompted for a filename and can control the acquisition parameters, including the trigger type (distance, time, keyboard). More details can allow choice of number of points, sampling frequency, etc. Normally the defaults for a specific antenna are appropriate. The usual wheel we use is the 100 MHz wheel. Acquisition is stopped with the black square (F6) in the toolbar.
  13. At this point, run a short trial along an area of interest. Examine the profile, in particular using the subtract mean trace or remove background filter and the contrast tool to see how deeply coherent energy is found. Adjust the acquisition parameters to avoid collecting a lot of useless data; also might consider what degree of folding is necessary.
  14. After one profile is acquired, you can either add on to that profile or make a new profile (leftmost tool in toolbar). Generally a separate file is advised.
  15. Appearance on the screen depends a lot on the filters chosen. This is usually unchangeable during acquisition but can be set before and after acquisition from the Profile menu's Edit Filter List command.

Troubleshooting:

Storage:

Typically not our problem any more. Return to lender with batteries charged and equipment properly stowed in boxes.

Please send mail if you encounter any problems or have suggestions.

GEOL4714/5714 home | C. H. Jones | CIRES | Dept. of Geological Sciences | Univ. of Colorado at Boulder

Last modified at Thursday, October 10, 2024 5:13 PM

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