Detection, Characterisation, of Residual Mining Subsidence Using SAR Interferometry. Application to Nord/Pas-de-Calais Coalmine

The extraction of underground resources induces subsidence on the surface. Those deformations often create disorders in structures and infrastructures. In the case of coal mining, the major part of subsidence occurs during active mine, and the residual subsidence can cover a long period about 5 years after the end of the exploitation. The amplitude of this residual subsidence is usually very small compared to the total subsidence. Charbonnages de France is in charge of the monitoring of the area of Nord/Pas-de-Calais (France), known for its important mining resources at depth. The exploitation of mine in this area is completed since 1992. Charbonnages de France continues to use high precision levelling comparison for surveying the surface. This traditional method present many disadvantages and the interferometry technique could presents a serious help for quantify the residual subsidence. In this study, both differential SAR Interferometry (DINSAR) and Persistent Scaterrers Interferometry (PSI) are used to estimate deformation during a 12 years period (1992 to 2004) after the end of exploitation. 88 ERS scenes, distributed on two adjacent tracks, are processed, using DIAPASON software for DINSAR and GAMMA-IPTA for PSI. The area undergoes high temporal decorrelation due to the high amount of vegetation. The evaluation of atmospheric artefacts is also difficult in this area because of rapid climatic changes. Nevertheless, deformations are well detected; they present low amplitude with a maximum rate of only 1 cm/y during 5 to 7 years after the end of the exploitation. The results obtains by interferometry are then compared to traditional measurements for (n points). They show a good agreement with the traditional methods of levelling. A robust methodology can be developed to use Interferometry for surveying surface above deep abandoned coalmines.