Detection of fractured zones, faults, and cavities by high resolution muon tomography in the Buda Hills

The high-penetration cosmic-ray muons can be used for scanning and exploring the structure of large geological formations, up to several hundred meters of thickness. Since the attenuation of the muon flux as it passes through the object depends on the density of the rock mass being scanned, measurements with high resolution muon detectors (muography) in suitable arrangement allow reconstruction of three-dimensional density distribution (muon tomography). The estimated density distributions can be used to infer the geological structure of the screened rock mass if the density inhomogeneity reflects it. The muographic method is extremely efficient because the mapping is performed along approximately straight lines, similar to X-ray or CT scans, since the deflection of high-energy muon trajectory due to interaction with the rock material is almost negligible. This ensures that density inhomogeneities can be determined with much sharper contours and higher accuracy than with other geophysical methods. In this paper we present a muography based high precision identification of a fractured zones and cavities which explorations are challenging to surface geophysical methods. The applicability of muon tomography is demonstrated in an area which is built up by Mesozoic carbonates (Buda Hills, Hármashatárhegy Range in Hungary) with variable topography, where the dominant directions and trends of the fault systems and fracture network are well known, which allows the tomographic results to be controlled. This case study demonstrates that muographic method is capable to obtain an accurate 3D picture of the density anomalous fracture system in carbonates for a rock thickness up to 100 m, that can very useful and unique contribution to geological interpretation and geological model building.