f Combining gravity and electromagnetic methods for mapping sink-hole potential risk within a highway crossing over a salt diapir

Knowledge of the exact location of subterranean cavities and the determination of their dimensions is very important for planning highway paths in karstic areas (Mellett and Maccarillo, 1993). The irregularity of karst geometry makes it difficult to obtain an accurate picture of subsurface conditions based solely on drill hole information. Therefore, systematic geophysical survey must be designed for detecting and delineating potential risk areas. The study area is placed in Ebro Basin (NE, Spain) at an alluvial plain of the Cardener River that overlays the Súria anticline. The Súria anticline is a complex structure represented at the surface by two structures of opposite vergence: a south-verging anticline in the north and a northdirected thrust in the south. Both structures are mined for potash 450-850 m below the surface (Sans and Vergés, 1995). The core of the Súria anticline is constituted by the Cardona Formation, which consist of water-soluble evaporitic minerals (halite, polyhalite and anhydritegypsum).