Advances in 3-D infrared remote sensing gas monitoring. Application to an urban atmospheric environment

Remote sensing technologies are some of the most powerful tools for atmospheric monitoring of natural or anthropogenic ecosystems. Extensive developments were observed in the two last decades concerning both satellite, airborne, on-board and ground systems. The present paper focuses on an advanced 3D reconstruction of a gas cloud detected in the atmosphere of an urban area using a scanning infrared (IR) gas system (SIGIS2, Bruker). Several measurements were carried out from 3 different positions in order to monitor an atmospheric volume around 108 m3. The images generated by the imaging remote sensing system correspond to the 2-D projections of the 3-D gas cloud. All the 2-D data are fully georeferenced (x, y, z and t). Each pixel of the 2-D images is associated to an IR spectrum, which was approximated to a linear combination of reference spectra and expressed as a coefficient of correlation (0 to 1). Data with a correlation coefficient higher than 0.75 are selected for 3-D modeling. The method for 3-D reconstruction of gas clouds is based on the combination and relocation of all the oriented and georeferenced measurement data. The 3-D gas cloud is determined from the 2-D images in the volume of interest processing a 3-D interpolation using the gOcad® DSI procedure. This integrated approach was applied to a local case study in an urban area. It leads to the identification and the spatial demarcation of a cloud of SO2 with a total volume of 65 × 106 m3. The existence of this pollutant may be related to the presence of ancient underground tanks of gasoline, leaking because of a defect of waterproofness. Another source of SO2 can be the emission of gases stemming from diesel machines used for important public works in this urban area. This study demonstrates that the combination of scanning imaging IR spectroscopy with the measurement setup and the 3-D gOcad® processing can be used as a generic approach for 3-D reconstruction of gas clouds applied to any kind of ground emissive sites.