Pascal Bigarre

Measurement of natural stresses in a Provence mine (Southern France)

Abstract Stress measurements were carried out in the Arc syncline using drifs in a lignite mine. Eleven sites were investigated using the flat jack and hydraulic fracturing (or stimulating) methods. Two stress states were found to coexist, one isotropic, the other highly anisotropic. The orientations of the principal stresses are not homogeneous and an orientation ranging from E-W to NE-SW predominates locally. This does not accord with the regional stress field. The vertical stresses are systematically underestimated.

Mining Induced Seismicity - Monitoring of a Large Scale Salt Cavern Collapse

To improve our understanding in large scale ground failure phenomenon induced by old underground mining works, a field experiment was undertaken in collaboration with the SOLVAY mining company: a solution mine was instrumented in 2004 previously to its collapse which was triggered in February 2009, as part of the mining scheme. This solution mine is located in the Lorraine salt basin (France). To monitor the cavern collapse, a multi-parameter system featuring high resolution microseismic linked to ground surface leveling (tacheometer and GPS-RTK) was used. The data transmitted for on-line processing offered daily insight of the evolution of the underground cavity. The early signs of unstable evolution were detected during spring 2008: shifts in microseismic background regime end recurrent microseismic episodes were associated to a general upwards process of rock failure of the roof cavern, with no ground surface movement detected. The high microseismic regime of the cavern has convinced the operator to trigger the collapse by brine pumping. Three main microseismic regimes were then observed, each being well correlated with changes in both the surface subsidence rate and the brine level in the cavern.

Aseismic Mining Subsidence in an Abandoned Mine: Influence Factors and Consequences for Post-Mining Risk Management

In the Lorraine area of eastern France, decades of iron-ore mining from 1850 to 1997 have left vast underground cavities beneath or in the vicinity of urban areas. Several major collapses occurred in the southern part of this iron-ore basin in the 1990s, after the mine closure and the flooding of underground mine workings. Following these large-scale collapses, the French government initiated a strategy of post-mining risk management to prevent and control risks associated with these ground failures. The high-risk zones are secured either by reducing the vulnerability while the moderate risk zones are monitored for public safety purposes by using in situ monitoring. This monitoring relies mainly on real-time microseismic systems, to detect precursors to a rapid large-scale collapse. After the progressive closing and then flooding of the northern iron basin ending in 2008, subsidence was observed in a town of the Lorraine basin in autumn of 2009. However, this local subsidence, with a low velocity of few centimeters per month, was not clearly detected by the borehole microseismic monitoring station located nearby. Only some microseismic events were recorded, which could not be unambiguously related to the beginning of the subsidence event. To better understand this lack of microseismic precursor a geophysical investigation was launched. A calibration blast experiment was carried out from a remaining old underground access in order to answer to the following questions: (1) what is the seismic wave attenuation field?; (2) what is the minimum source power that can be detected by the sensors?; (3) what is the impact of the geology, the faults corridor and the integral pillar extraction zone on the wave propagation field? The results of this study show strong anelastic attenuation of the seismic waves though the monitored overburden most likely related to the extensive fault system intersecting the study site. Strong attenuation might explain the lack of detected microseismicity during the subsidence event. In order to clarify this issue, a mobile GPS monitoring system was designed and tested to address this type of situation in future.

3D acoustic and microseismic location of collapse events in complex, 3D geological structures

Passive microseismics is a well developed technique that has gained importance in petroleum exploration operations as well as in geohazard assessment. When applied in complex geological environments, it requires advanced processing capabilities to ensure useful accuracy in the source location and characterization. Here we investigate a fast marching method to determine the travel-time field, rays and ray take-off angles in complex 3D media, for application with a direct-search event location method. We then illustrate and discuss the potential of the chosen methodology in the mining context. This methodology allows improvements in acoustic monitoring of large-scale underground mines by taking into account the intrinsic characteristics of propagation of the acoustic waves. Ongoing work on a dataset collected during the monitoring of a large-scale salt cavern collapse is also discussed. We expect that the use of an evolving 3D model will help to reduce the location errors and improve the dataset analysis, improving risk management for time-varying collapse events.