The most accurate procedure for determining soil velocity is to identify GPR reflections to objects of known depths. The pipe test allows such a determination. Using the wall of an open excavation a metal pipe is inserted into it parallel to and at a known distance below the ground surface. Here, a one-inch (2.54 cm) diameter soil corer, with handle protruding, is used as a pipe, placed 50 cm below the surface. The GPR antenna is then moved over the pipe and a short profile of reflections is recorded.

Because metals reflect nearly 100% of radar energy a metal pipe is ideal for this test, but its diameter should be large enough to be detected with the antenna wavelengths employed (a low frequency antenna with longer wavelengths will require a larger diameter pipe). Conyers and Goodman (1997:110) assert that the long axis of the antenna must be placed parallel to the pipe in order that a maximum amount of radar energy can hit the narrow target and that image enhancements may be required to "see" the reflection in the data, but I have not noticed this problem at depths of a meter or less.

This figure shows the resultant GPR profile obtained with a GSSI 400 MHz antenna. The tick marks at top represent meter marks obtained with a survey wheel, using 50 scans/m. The range setting was 40 nS (10^-9 seconds) two-way travel time (TWTT). The ground surface reflection is approximately at 3 nS while the pipe, located at the peak of a characteristic hyperbolic reflection, is at approximately 13 nS.

Here are the relevant data:

Depth = 0.5 m
Time = (13-3)/2 = 5 nS   {divide by 2 to get one-way travel time}

Soil Velocity:  V = 0.5 m / 5 nS = 0.1 m/nS

This figure represents the average radar velocity through the first 50 cm of soil.

Soil velocity may also be estimated, but less accurately, through analysis and measurement of hyperbolic form.

Relative Dielectric Permittivity (RDP)

With an estimate of soil velocity the relative dielectric permittivity (K) may be computed:

K = C / V

where C = speed of light in a vacuum (0.2998 m/nS) and V = velocity of radar energy as it passes through a material (in m/nS), so:

K = 0.2998 m/nS / 0.1 m/nS = 2.998,

and

K = 8.99

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