Theoretical Analysis and Practical Verification of the WORP Technique Applied for the GPR Investigation of a Flood Levee
Theoretical Analysis and Practical Verification of the WORP Technique Applied for the GPR Investigation of a Flood Levee

Summary/Abstract: The ground-penetrating radar (GPR) method is one of the geophysical electromagnetic methods. The standard technique of the GPR surveys is the short-offset reflection profiling (SORP) technique, which can theoretically be described in the same way as the zero-offset (ZO) measurements. During the surveys performed using the SORP technique, the offset (i.e. the separation between the transmitter and receiver antennae) is not adjusted to the depth of the objects located in an examined medium. In more advanced measurements, performed using the wide-offset reflection profiling (WORP) technique, the offset is adjusted to the supposed depth of the underground objects which should be detected using the GPR method. The theoretical background and comparison of the SORP and the WORP surveys were described in the paper. The article also presents theoretical analyses regarding to the shape of the radiation pattern generated by the GPR antennas located on the ground surface, i.e. on the border of two media (i.e. air and geological medium) with different electromagnetic properties. The variability of shapes of the radiation patterns as well as the variability of the reflection coefficients for electromagnetic waves with transverse electric (TE) and transverse magnetic (TM) polarizations for different offsets, affect the quality of GPR recordings, which was analysed in the paper theoretically as well as through the field tests. The terrain measurements were performed on a selected part of the Vistula river flood levee in Krakow (Poland), where geotechnical sounding indicated the existence of the loose zones. In order to increase the detection possibilities of the GPR method, surveys were performed after precipitation, which created a temporary two-layer medium, i.e. near-surface, water-saturated zone and deeper located, dry zone. The results of the WORP surveys confirmed the theoretical analysis and allowed to record more readable radargrams for larger offsets than in the case of the short offset, which facilitated further interpretation of the recordings.

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