Karst investigations using microgravity and MASW; Application to Orléans, France

Nicole Debeglia (1) , Adnand Bitri (1) , Pierre Thierry (1)

1 BRGM - Bureau de Recherches Géologiques et Minières

An integrated geophysical approach to detecting and characterizing karst structures in an urban environment was applied experimentally to partially explored karst conduits located in Orléans, France. Microgravity was performed to detect voids, in conjunction with multichannel analysis of surface waves (MASW) for the purpose of identifying areas of mechanical weakness. Microgravity detected negative anomalies corresponding to known conduits and succeeded in identifying the probable extensions of this network in unexplored areas. Control boreholes located on these extensions encountered several levels of water‐saturated voids, probably belonging to the shallowest part of the karst system, overlying the main conduits. Buried urban networks, accurately located by ground‐penetrating radar (GPR), were shown to have no significant gravity effect. Simulations using the compact inversion approach to characterize the size and density of environmental disturbances confirmed this conclusion. In this context, the gravity method has been shown to be suitable for detecting near‐surface (<25 m deep) karst features. The MASW method, which analyses Rayleigh‐wave propagation, can determine the mechanical behaviour of superficial formations and serve as an indicator for subsurface heterogeneities such as voids or fractures. At the Orléans site, MASW provided evidence of perturbed zones superimposed on gravity anomalies, characterized by the appearance of several dispersion modes, velocity inversions and the attenuation of seismic markers. One of these features was characterized by low velocities and was interpreted as an area of mechanical weakness, confirmed by pressure measurements in the boreholes. Repeated gravity measurements, or time‐lapse microgravity, were conducted on the anomalous areas to ascertain gravity reproducibility and detect possible temporal variations due to subsurface mass redistribution that may indicate site instability. A two‐year experiment revealed low‐amplitude gravity changes that were recorded in the two sensitive zones. However, their significance is still unclear and these changes need to be validated by further repeat experiments.

Domaines: Sciences de la Terre

Nicole Debeglia, Adnand Bitri, Pierre Thierry. Karst investigations using microgravity and MASW; Application to Orléans, France. Near Surface Geophysics, 2006, 4 (4), pp.215-225. ⟨10.3997/1873-0604.2005046⟩. ⟨hal-03752337⟩