Using the Water Table Reflection to Add Topography
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Using the Water Table Reflection to Add Topography

The interface represents a very large change in electrical properties. The reflection from this interface can be one of the strongest interfaces seen with GPR. Water and air represent the most extreme relative dielectric constant (K) values. Dry sediments with air in the pore space has a low bulk dielectric permittivity of about 5. Below the water table, the air is replaced bywater in the pore space so the bulk dielectric permittivity is much higher; typically around20 to 30. Reflection strength depends on the difference between material permittivities, so the strong water table reflection amplitude often dominates the GPR reflection section .

Given that the water table provides a strong,easily interpretable reflector and if we assume the water table is a flat, horizontal boundary (which it usually is) the depth of the water table on the GPR section mimics the elevation change along the GPR profile line. Extracting the depth of the water table along a GPR line provides a means to compensate the GPR section for thetopography.

The EKKO_Project Interpretation module is ideally suited for this task. Adding a ‘polyline’ interpretation along the water table reflector (Figure 2) provides depth to the water tablereflector which are extracted to a spreadsheet. The “Position” and “Depth” columns(shown in green) are extracted and saved as atopography (.top) file and attached to the GPRline. Topography files are special “positioning” files that EKKO_Project recognizes and are used to automatically interpolate an elevation value for every GPR trace in the GPR line.

Elevation information allows the GPR line to be plotted with an elevation axis in the LineView module. Correcting a GPR line for the topography provides a more representative image for further interpretations about the structures imaged by the GPR