In the Île-de-France region, seismic vibrator lorries operated on 280 km of roads to the west and south of Paris.
© S3 - Robert Famy
By improving knowledge of the subsurface, the Geoscan programme run by the French Environment and Energy Management Agency (ADEME), the French Directorate-General for Energy and Climate Change (DGEC) and BRGM aims to provide practical support to developers of deep geothermal energy projects to help them identify the most suitable sites. The programme is run in conjunction with the local authorities concerned (the Ile-de-France and Sud Provence-Alpes-Côte d'Azur regional authorities, the Bouches-du-Rhône département and the Aix-Marseille-Provence metropolitan area).
Two subsurface exploration surveys were carried out in 2024, one in the Île-de-France region and the other in the Arc basin between Aix-en-Provence and Fos-sur-Mer. Their purpose was to improve the mapping of the underground structure using the seismic reflection method (see box), which produces seismic profiles revealing the architecture of the strata, the existence of faults and the presence of aquifers. These data are essential for identifying and characterising potential geothermal reservoirs.
Down to a depth of 10,000 metres
These surveys involved the use of seismic vibrator lorries in urban areas. "The seismic profiles were acquired mainly at night, both to limit inconvenience to the public and to reduce the noise in the data caused by traffic," explains Alexandre Stopin, geophysicist at BRGM. In the Arc basin, the survey also included an operation to collect data by barge on the Etang de Berre. "In this way we were able to produce images of the subsurface to depths of 3000 metres in the Île-de-France region and 10,000 metres in the Arc basin," explains the researcher.
However, the content of the two projects differs significantly. "In Île-de-France, there is already an extensive range of available data. The aim here was therefore to reprocess old seismic profiles, acquire new data in the west and south of the area and correlate these results with the large number of available boreholes in order to better assess the quality of the geothermal reservoirs. Previous exploration dating from the 1980s to the 1990s targeted hydrocarbons," adds Camille Maurel, hydrogeologist at BRGM. "In Geoscan, we are studying the same geological environments but with geothermal targets located at slightly different depths. The primary objective is to characterise the porosity, permeability and depth of aquifers in order to assess their temperature. These properties are essential for geothermal energy."
Chiffres clés
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1000.00km2explored in the Arc basin
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1900.00km2explored in Île-de-France
The seismic reflection campaign on the Arc basin was carried out between 21 October and 20 November 2024 on roads and on the Etang de Berre.
© S3 - Robert-Famy
A resource for numerous projects
On the other hand, the available knowledge is more fragmentary in the Arc basin. The aim was therefore to produce an initial geophysical grid covering this structurally complex area. Geological field surveys were carried out to supplement this approach with direct observations.
"Now that we have finished processing the collected data, we are in the process of using and interpreting it," says Alexandre Stopin. While the results are intended primarily for deep geothermal energy stakeholders, they are of wider interest and will be made available to local authorities and the scientific community. As the two experts point out, "the knowledge generated by the Geoscan surveys is a valuable resource for many projects relating to the subsurface."
Operating principle of the thumper trucks deployed for the Géoscan project.
© Géoscan
What is seismic reflection?
This investigation method is similar to an ultrasound scan: mechanical waves are emitted underground and reflected by the different geological layers. The signals that return to the surface are recorded by geophones and used to build a two-dimensional image of the subsurface: this is referred to as a seismic profile.
Issue No.1 of BRGM mag, BRGM's magazine, was published in November 2025.
© BRGM
BRGM mag no.1: The subsurface, a key to the future of our regions
BRGM launches BRGM mag. This new magazine for the general public aims to explain why the subsurface is so vitally important and the essential contribution made by the French geological survey in responding to today's main environmental, energy and sovereignty concerns.
Borrowing from journalistic codes, BRGM mag has adopted a deliberately flexible range of features, to be able to adapt to its latest news, with varied forms (articles, interviews, computer graphics, reports, FAQs, etc.), as well as a popularised and educational editorial style to make the information accessible to as many people as possible.
This first issue examines the increasingly central role of the subsurface in spatial planning and ecological transition. Because an essential part of our future is playing out beneath our feet.
