What trace interval should I use for GPR investigations?
The trace (also called a scan) or point interval of your GPR investigations depends mostly on your application and can vary between 0.1 up to 100 cm. In most situations a trace interval between 0.5 and 15 cm is used.
For object detection, it is recommended to use a trace interval half the size, or less, of the object you want to detect. E.g., if your object has a diameter of 10 cm the trace interval should be 5 cm or less. For smaller objects, as e.g., rebars, the trace spacing can be as small as 0.1-0.5 cm.
For layer detection, the trace interval can generally be larger. In some cases, a larger trace interval will render data easier to interpret as it effectively filters smaller point objects (clutter). A larger trace interval will of course also produce a smaller data file which may be of importance for larger (very long profiles) projects.
In the examples, data is collected over the same pipe and layer but with different trace intervals for comparison.
The same two pipes measured with 2.3 and 23 cm trace interval.
The same layer measured with 4.5 and 90 cm trace interval.
A guide for suitable trace intervals for different applications are given in the table below.
|Application||Suitable Trace Interval (in cm)|
|Rebars and concrete||0.1-1 cm|
|Concrete – voids and fractures||0.1-1 cm|
|Smaller utilities, < 20 cm in diameter||1-3 cm|
|Larger utilities, > 20 cm in diameter||3-10 cm|
|Underground storage tanks||5-15 cm|
|Tree roots||1-3 cm|
|Bedrock layers||5-25 cm|
|Soils layers||5-25 cm|
|Asphalt layers||5-25 cm|
|Road and Rail Roads||5-25 cm|
|Ice and Snow – thickness||5-25 cm|
As you know you can also trig your GPR measurements by time. This is done for instance when you are working with a drone mounted GPR or doing measurements from a boat or different types of unshielded antennas where wheels cannot be attached. What distance interval to use depends on the speed of measurement. For walking pace (1 m/sec), 0.05 to 0.1 sec is often sufficient, giving you 10-20 measurement points per meter profile. For 10 m/s (36 km/h) it is suitable with 0.005 to 0.01 sec.
Of course, a dense trace interval will affect the measurement speed, but modern GPR system most often work with real-time-sampling technique, giving a high-speed data collection. This means that for data collection in walking pace, the trace interval will not limit your speed. But when doing highway investigations, or measurements after a skidoo, the trace interval can be important. For our HDR systems (as Ground Explorer and Easy Locator) some values of max speed for different time windows and trace intervals are shown in the table.
|Time window||Point interval||Max. speed|
|75 ns (GX750)||10 cm||125 m/s|
|300 ns (GX450)||10 cm||76 m/s|
|625 ns (GX160)||10 cm||89 m/s|
|812 ns (GX80)||10 cm||119 m/s|
Finally, the depth to the object is of course also important. Natural soil will act as a low-pass filter and deeper laying objects must therefore be larger in diameter than shallow objects in order to be detected by GPR. As a rough “rule of thumb”, the target diameter should be at least 10% of the depth to be successfully detected.
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