Application of the seismic reflection method to abandoned mine detection in Óbidos, Portugal

The municipality of Óbidos wanted to investigate the suitability of an area in Casal Caxinas, for construction of one or two story building. The surrounding area was an active gypsum mine, now closed for a few decades. An old mine map exists but serious doubts are cast over its validity since, for example, no gallery connects the shaft well to the extensive net of galleries, and some references seem misplaced. Several areas a few tens of meters from the planned construction site collapsed but the map shows no galleries at the site. The spot has been covered by several meters of mine wastes, that show some irregularities and fissures that can be caused by natural subsidence due to gypsum withdrawal or by galleries collapse. To verify the landfill competence and thickness and detect the eventual presence of old mine galleries, a reflection seismic survey was carried out. The water table was expected to be a few meters deep and the fact that the landfill materials were clayish prevented the use of georadar. The depth of the galleries (over 20 meters deep) and the short space available in some areas of the construction site (less than 60 meters) also did not allow the use of geolectric methods. A grid of seismic reflection lines was realised and the results interpreted with a few geothecnical mechanical soundings. Several meters of alluvium were detected beneath 5.5m of waste material, while the water table showed at about 7 meters depth. None of the soundings exceeded 20 meters and so no cavity was detected but also no signs of decompression in the alluvium were detected, indicating that no collapse occurred, even if galleries beneath the site of drilling exist. To the South of the study area, the presence of galleries is strongly suggested by the seismic data and a new map of the mine is created showing the limit of the galleries several meters further North. However, space is still available for the building construction and though the landfill material competence is weak, construction is possible provided some measures are taken.