Abstract
The objective ofthe present GPR survey was to investigate a segment of underground railway in Paris where subsidences had been observed. The railway sleepers are made of concrete reinforced by 7 rebars. The sleepers are regularly spaced every 60±2cm. With 400 MHz antennas part of the transmiued energy penetrates the ground and is reflected back to the receiving antenna by the subsurface mirrors, unfortunately backscattering from the regularly sleepers makes it difficult to see reflections from the subsurface. Taking advantage of the uniform spacing between sleepers a wavenumber notch filter was designed and applied. The resulting image made reflections due to a decompressed volume clearly visible. Drilling confirmed the presence of a decompressed volume likely to be a precursor of an underground collapse progressing towards the surface.
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