User's Manual

pulseEKKO PRO 5-Data Collection Mode
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5.5.4 Calib - Velocity Calibration
GPR systems can be used to scan into many different materials including soil, rock, concrete, snow, ice
and wood. The radio wave emitted by a GPR system will travel at different velocities depending on the
material being scanned. As discussed earlier (Section 3.2.2.1: P35), depth estimates are only accurate if
the system has been properly calibrated to determine the velocity of the material being scanned.
The Calibration function is a tool for determining the velocity of the material being scanned. A velocity
value can be input directly or determined in one of two different ways depending on the situation:
Hyperbola matching and Target of known depth.
5.5.4.1 Hyperbola Matching
The most accurate way of determining the velocity of the material being scanned is to use the hyperbola
fitting method because it extracts the speed from data collected in the area. This method may not work in
all situations because it depends on having a good quality hyperbola (or inverted U) in the data.
A hyperbola is the characteristic GPR response from a small point target like a pipe, rock or even a tree
root. This phenomenon occurs because radar energy does not radiate as a pencil-thin beam but more like
a 3D cone. Reflections can appear on the record even though they are not directly below the radar system.
Thus, the radar system “sees” the pipe before and after going over top of it and forms a hyperbolic
reflection.
Figure 5-8: Hyperbolas in the data result from the conical shape of the GPR energy as it goes into the ground. Targets, like pipes,
are detected as the GPR approaches them (left), passes over them (middle) and after it has passed by them (right) because the GPR
energy propagates both in front and behind the instrument.
If the hyperbola has long tails on it, we can match the shape of the hyperbola and determine the velocity of
the material in the area.
With the hyperbola visible on the DVL screen, select the Calib button. This will superimpose a hyperbola
on the data. This hyperbola can be moved up, down, left and right using the appropriate buttons. The goal
is move the hyperbola until it lies on top of the hyperbola in the data (see Figure 5-9). Then, the user can
adjust the width of the hyperbola to make it wider (Vel+) or narrower (Vel-) until the shape of the hyperbola
matches the shape of the hyperbola in the data. After matching the hyperbola, the velocity value is
extracted and used for all subsequent data acquisition.