Ground penetrating radar (GPR) is an effective locating tool with the ability to detect metallic/non-metallic utilities or search for unknowns in suitable soil conditions. In the last few years, advances in technology have led to lower cost systems with more advanced features which make modern GPR systems user friendly, compact and portable.
The following short survey design guide will provide GPR operators with the appropriate operational methods for maximum success in the field.
1. Define Your Problem
Defining your problem should ultimately answer the question: Is GPR suitable to solve this problem and to help make your field campaign as efficient as possible?
It should be noted that GPR is best used in conjunction with other locating tools (i.e. EM methods, probing, etc.) and especially in areas of highly conductive soils that may have high clay content or saline conditions.
2. Questions to Help Determine Suitability
- What is the depth of the target?
- Is the target within the range of GPR in ideal conditions?
- What is the geometry of the target (size, width, length, strike, dip, etc.)?
- What are the target’s electrical properties?
- Is there enough contrast between the medium and the target?
- What is the in situ material?
- Will I get the depth penetration?
- Is the medium homogeneous or at risk for clutter?
- Is there a presence of water/water table?
- What is the survey environment like?
- Is there enough space to perform the survey?
- Is there any background noise or high power electrical line in the area?
- Utility lines are often strongly polarizing
3. Determine Survey Grid
When using 2D survey techniques, determine the direction of the utility line(s) you wish to investigate. Once the direction is known, your survey lines should be perpendicular to the long axis. If the direction is unknown, use orthogonal grid or determine the direction through additional test lines in different directions.
Line spacing should be determined by the smallest anomaly of interest, and lines in one direction will normally suffice.
If you are looking for objects of limited dimensions (point objects) and if high confidence is needed, the line spacing should be shorter than the smallest dimension of the targets. This would be a good time to perform a large overview grid followed by smaller focused grids, once the targets have been identified.
4. Select Appropriate Antenna(s)
Antenna frequencies for utility location generally range from 200 to 900 MHz. The lower the frequency, the deeper the penetration you will achieve and the higher the frequency, the better resolution you will achieve. Choosing an antenna in the middle of this range is generally acceptable.
5. Set Up the System
- Make sure your batteries are fully charged and spares are available for a full day of surveying.
- Check that all connections are clean and secure.
- Try to set up the system in a dust- and rain-free environment.
- Select the correct wheel calibration.
- Check the signal position, which should be approximately 30 samples before first arrival.
- This is an important step to zero the data to ground level.
- Choose the sampling frequency to be around 10 times the antenna frequency, i.e. 2,500ns for an antenna frequency of 250 MHz.
- The time window should be greater than the estimated investigation depth +30 percent.
- Use automatic stacking, if available, which will give a better signal-to-noise ratio.
- Set the point distance or trace interval, according to needed resolution.
- This value is typically .08 to .2 ft per microsecond, for normal walking speed.
- If a higher frequency antenna is required, for example, a utility under a sidewalk, let the antenna stay turned on at least 5 minutes prior to the survey to avoid signal drift.
6. Conduct Test Lines
- A test line may help determine if it is possible to perform a full-scale survey. It will also determine if a grid can be made more efficiently.
- Test lines will help you determine if the correct antenna is chosen.
- In conductive soils, you may need a lower frequency antenna in order to get the required depth to the target.
- Test lines will also allow the operator to manipulate the settings and filters, to reduce background noise levels for optimal performance.
- A velocity determination should also be performed, typically by hyperbola fitting, which will give a reasonable time-to-depth conversion.
7. Adjust System for Optimal Data
Based on the test lines, adjust the settings if necessary.
- By lowering the number of samples, this will increase the speed of the system.
- Make sure the velocity settings are correct, using hyperbola fitting or another available function.
- Choose the profile direction, again surveying perpendicular to the direction of the utility. Choose a different frequency antenna, if needed.
- Use the lowest frequency antenna that can resolve the target to avoid noise or clutter.
Ideally you want to be perpendicular to the direction of the utility. This will give you the best possible image of the utility.
Begin scanning and once you see a hyperbola in the data, back the unit up until the vertical cursor is centered on the hyperbola. At this point, you are directly on top of the utility. You would then place a mark on the ground or if your system is equipped with GPS, place a coordinate mark on the data. Continue this procedure until you have completed the survey.
A properly executed survey design will determine if the area is suitable for GPR surveys and provide you with the optimal setup of the system. By utilizing these simple techniques described above, ground penetrating radar operators should have a greater chance of success in locating underground utilities in the field.
Jesper Emilsson is GPR product manager for Mala Geoscience.