User's Manual

pulseEKKO PRO 6-Surveying Techniques
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6.11 Common Mid-Point (CMP) Survey
A Common Mid Point (CMP) survey is performed to measure GPR velocity in the survey area. A CMP
survey is best done in an area with flat lying reflectors and relies on changing the antenna separation to
increase the signal path length from the mid point reflector (Figure 6-10). The resulting data can then be
processed using a semblance analysis routine to extract average velocities to the reflectors (Figure 6-11).
The extracted velocities can then be used when adding a depth axis for other GPR survey lines collected
in the area of the CMP.
Figure 6-10: CMP setup. CMP surveys are used to measure the GPR velocity to a reflector or series of reflectors. Each trace is
collected with the antennas at a larger separation resulting in an increased travel path length and therefore increased travel time.
The data can be analyzed with a semblance analysis routine to extract the average velocity to the reflector.
Conducting a CMP survey efficiently normally requires two people and may require the use of the optional
20m fibre optic cables (available from Sensors & Software) rather than the standard 5m fibre optic cables.
Follow this procedure when collecting CMP data:
1) Conduct a CMP survey after the reflection survey lines have been collected in the area
because it is best to identify a location with at least one strong, flat-lying reflector.
2) Place a tape measure (Section 10.2: P117) on the ground in the direction that the antennas will
be expanded about the central point. Try to make the central point correspond to a round
number on the tape measure, i.e. 5m, 10m, 20 feet, so that moving the antennas to the new
location for each trace is easy for the operators.
3) Set the antennas on the ground at the usual antenna separation distance for the selected
frequency (Section 3.2.4: P37). The antennas should be centred on the central point.