A helicopter flight for the ReunEM geophysical survey of La Réunion © BRGM - René Carayol

Understanding the near subsurface to support sustainable and responsible development

02.04.2019
Because the near subsurface (0 to 100 m) is the interface between humans and the planet, it is the focus of a great many fundamental issues and the subject of many research projects.

Knowledge of the near subsurface (from 0 to 100 metres in depth) is essential to manage a great many fields of human activity. This is a major challenge for society in the context of global change. © BRGM - Vertigo 3D

Knowledge of the near subsurface (from 0 to 100 metres in depth) is essential to manage a great many fields of human activity. This is a major challenge for society in the context of global change. © BRGM - Vertigo3D

Characterising and managing resources (water, materials), managing risks (landslips, earthquakes, flooding, clay soil shrinking and swelling, etc), subsoil planning (for utilities networks, agriculture, construction of roads, buildings and infrastructure) are central to human activities. All are major challenges for contemporary societies as we grapple with global change.

To take up these challenges, the BRGM has been strengthening its research and expertise in near-surface geology for several years. As well as acquiring new knowledge, we have been developing innovative imaging and subsoil analysis methodologies and new approaches in the field of predictive geoscience.

+ 70% of mainland France is covered by shallow geological formations

Predictive mapping methodologies

Several projects on this topic were conducted in 2017, under the scientific programme for the French Geological Reference Platform (RGF).

A predictive mapping methodology to produce representations of alluvial terraces has now been developed. Based on image processing and analyses validated with ground data, the methodology draws on the IGN 25 m resolution digital terrain model and its extensions (elevations, slopes and curves). The methodology was tested in a Pyrenean piedmont zone, with the results of a ground campaign in the Baïse and Ousse basins used to compare the predictive maps with geological reality. The BRGM is now planning to pursue the study on a larger scale. We are developing a graphic interface to automate the process and thus accelerate the production of predictive maps.

Another project was conducted in the crystalline Maures range (southern France), which is subject to numerous - and costly - landslips, in order to map susceptibility to these phenomena. By cross-referencing characterised weathering profiles with the different slope angles and data on the structure and nature of the weathered rock, an exploratory methodology was devised to anticipate zones where landslips are likely to occur.

Finally, a study conducted during a second year Master’s internship produced maps of sensitivity to run-off flooding. The principle involves cross-referencing topographic data with data from airborne geophysical surveys (gamma spectrometry), which provide information on the lithology of surface formations, based on the expertise of geologists specialising in surface formations.

The site chosen is in the Loiret area, where severe flooding occurred in 2016. The method was calibrated using the large quantity of data available on these events (radar imagery, measurements, etc.).

Landslip favourability map of the crystalline Maures range, incorporating a weathering parameter and the geometry of rock structures (cleavage). © BRGM - C. Allanic

Landsplie favourability map of the crystalline Maures range, incorporating a weathering parameter and the geometry of rock structures (cleavage). © BRGM - C. Allanic

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