GSSI UtilityScan, Quick Start Manual

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UtilityScan Quick Start Guide Page 25
Calibrating for Depth with ScanMax
GPR works by measuring the two-way travel time (TWTT) of the energy pulse to the target and back. GPR energy moves
very rapidly so the TWTT is expressed in nanoseconds (ns) which are billionths of a second. The velocity of GPR energy
in soils, sediments, asphalt, rock, etc. can change both with depth and across an area. Since it changes, it can be very
difficult to get accurate depth readings with the GPR method. This means that you should always provide a margin of
error to your client when quoting depth to target. In homogenous materials the margin of error may be very tight, but if
you do not know the subsurface conditions, you should quote a margin of error of at least 10% of the overall depth.
The value used to express GPR velocity in materials is the dielectric constant. If you do more research on the GPR
method, you may see this referred to as Relative Dielectric Permittivity (RDP) or by the mathematical shorthand of E
r
. It
is a unit-less measure describing how easily GPR energy moves through a material. The range is 1 (air, close to the speed
of light) to 81 (water, 1/9 the speed of light).
The UtilityScan has two methods of calibrating for depth: Set Depth and Set Dielectric.
Set Depth
is used when you have a target at a known depth. You will need to collect some data as close as possible to the
point of known depth. This is also known as adding “ground truth” and can result in very accurate depth calculations in
homogenous ground conditions.
Set Dielectric
uses a software tool called migration to calculate the dielectric based on the shape of the hyperbola. You
do not need to know the actual depth of the target. In homogenous ground conditions, migration can have equivalent
accuracy to ground truth, but in general you should quote a larger depth margin of error than you would if you used Set
Depth.