Abstract
The near-surface rock structure that covers an abandoned marl mine nearby the village of Montevecchia (Italy) was investigated through a combination of seismic surveys. The methods selected for these investigations were refraction seismics and multichannel analysis of surface waves (MASW). A sort of transillumination experiment was also attempted. All the methods were successful and gave complementary information. Refraction seismics was used to characterize the upper low velocity layer and the second layer of the near-surface structure. The MASW method was necessary to assess the existence of a velocity inversion revealing the presence of a low velocity layer between the 2nd layer and a 4th high velocity layer covering the upper mine gallery. The transillumination experiment validated the presence of the 4th layer and gave an estimate of the average velocity that represents a lower boundary for the P-wave velocity within this layer. Both the refraction and transillumination data were analysed to derive average estimates of attenuation level and rock quality factor.
Highlights
Montevecchia is an old historical village settled on the flanks of a 500 m high isolated hill located north of Milan (Northern Italy; Fig. 1)
After a preliminary on-site inspection, we selected three techniques that were promising for the specific case: refraction seismics, multichannel analysis of surface waves (MASW), and transillumination experiments
The MASW method seems to indicate a thickness for the second layer of about 10 m
Summary
Montevecchia is an old historical village settled on the flanks of a 500 m high isolated hill located north of Milan (Northern Italy; Fig. 1). In 1958 a large sinkhole occurred on top of the hill because of a collapse involving all mine levels. No other maintenance works were done in the abandoned mine. An investigation program was planned by the regional authority to assess the safety conditions of the mine and the possible risk of other events like the 1958 sinkhole. Excluding direct inspections of the mine galleries for safety reasons (the upper gallery is unsafe and probably obstructed by the 1958 collapse, while all the other levels are flooded by water) all the geological and geophysical investigations had to be performed outside the mine except of a few measurements that could be taken at the entrance of the mine where the upper gallery departs from the main room of the mine
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