Integrated remote sensing data and field investigations for geological mapping and structural analysis in the Er-Rich area (High Atlas, Morocco)

The Er-Rich region is a focal area for understanding the geological evolution of the central-eastern High Atlas, which it covers almost entirely along a north-south transverse line. It is a hinge region between the two major tectonic structures of the High Atlas (the North and South Atlas faults), which reveal a framework of Meso-Cenozoic carbonate, detrital and magmatic rocks.

Previous studies have highlighted the complexity of mapping in this area. To date, no detailed geological map has been produced for this study area, with the exception of the old provisional 1:200,000 map of the Midelt-Rich High Atlas. Remotely-sensed mapping initiatives have also been carried out in the region, except that they do not provide a final interpretation as a geological map, supported by geological maps covering neighboring regions. A detailed geological map of the Er-Rich region, based on the results of remote sensing and field data, is therefore needed in the area. For this purpose, remote sensing geological mapping techniques have been applied to two types of satellite data: 1) Landsat 8 OLI (Optical Land Imager) multispectral optical data, and the Spot 5 panchromatic band acquired by the HRG-2 (High Geometric Resolution) instrument; 2) Sentinel-1 SAR data with dual polarisation (HV-HH).

All the data underwent several pre-processing or correction stages using appropriate software, in particular radiometric and atmospheric correction for Landsat 8 OLI (Optical Land Imager) images using ENVI software. The corrected product of the three Landsat 8 OLI scenes covering the region were then spatially enhanced using the Spot 5 panchromatic band to produce a multispectral image with a high spatial resolution of 5 m using ENVI software. The Sentinel-1 radar data were pre-processed using SNAP toolbox software by applying a series of corrections.

The results obtained by applying the Optimum Index Factor (OIF) method and Principal Component Analysis (PCA), allowing us to select the most significant colored compositions. Moreover, this combination enabled us to delineate with great precision the large outcrops of carbonate rocks (limestones, marl), siliciclastic rocks (conglomerates, sandstones and silts) and magmatic rocks (igneous intrusions).

The lineaments were extracted manually by visual interpretation of Sentinel-1 radar images, after applying directional filtering folowing four general orientations (N0, N45, N90, N135), enabling us to generate a synthetic structural map of the region.

The results obtained were compared with data from geological maps of adjacent areas and approved by field observations, leading to the production of a high-precision geological map, compiled with pre-existing geological literature.