Near Pilsen.

Report SPELEOLOGY, GEOPHYSICS 2017
The excavation of the railway tunnel from Ejpovice to Pilsen was a very demanding construction. The almost 100 m long railway set of wagons led by an embossing shield bit through the often not very solid subsoil. Immediately behind the gable, the tunnel was lined with reinforced concrete slabs. During downtimes or due to geological changes in the rock, the material sometimes fell over the cross-section of the tunnel body. These cavities then tend to spread upwards. Our task was to detect these anomalies with a georadar in time, monitor their spread and, if they reached the upper layers, they were remediated. When an anomaly was found, the measurement with the ROTEG georadar was further specified by electroresistive measurement. Two tunnels were dug side by side. In the years 2015 to 2017, we measured almost 100 km of profiles here. Special attention was paid to places with roads and buildings. 3 m long antennas with a frequency of 50 MHz were most often used for measurements. Data were processed by the Reflex program. Later in 2021, I returned to the original data and processed it with our Spirio program, it’s a pity we didn’t have it then, anomalies would be much easier to find.The profiles were most often compared before and after the excavation, the time interval was sometimes several months. Here is an anomaly found at a depth of 16 m.
If the cavities spread just below the surface, it looked like this. Above is a radargram with only higher frequencies, we see more details. Below is a radargram with all frequencies, dominated by low ones with a higher energy of reflection.

And when a spoon from an excavator hit this place a bit, there was room for tens of tons of material, where the room is already partially covered with lighter material.

Appendix 2/2021
Processing in Spirio program.