Mission: structure this data, extract the most relevant information and transform this wealth of information into a 3D digital model of the near subsurface of the Île-de-France region with a precision never achieved for such a large area. The S-PASS project is thus working to pave the way for new uses of the urban subsurface.
When metro construction sites become geological laboratories
According to Isabelle Halfon, head of the S-PASS project, the opportunity is historic. With the titanic construction site of the Grand Paris Express—this automated metro network extending around Paris—200 kilometers of tunnels are being dug, and the thousands of reconnaissance borings that were carried out beforehand represent as many windows opened onto the subsurface of the Île-de-France region. Each borehole drilled to prepare these construction sites generates precious geological and geotechnical data that it would be unthinkable not to capitalize on.
This is one of the major challenges of the S-PASS project launched in December 2023. Its ambition: to improve geological knowledge of the near subsurface in the territory of the Greater Paris Metropolis, by exploiting this considerable mass of data and valorizing it through a 3D digital model. This model will integrate the Parisian underground quarries and will be enriched by the underground structures of the RATP metro, a project partner.
The urban subsurface already concentrates multiple uses: building foundations, water and communication networks, public transport. But it also represents an energy resource that is still insufficiently exploited—geothermal energy—and could become, in the face of climate change and surface saturation, a new space to develop for tertiary developments. Provided it is well known.
(Find a fascinating exchange on the subject between Isabelle Halfon and Dominique Perrault, architect and urban planner in podcast).
SONGE: a database at the heart of the system
It all begins with SONGE, a database managed by the Société des Grands Projets (SGP) which manages the Grand Paris Express project. A distinctive feature of this database: it requires all service providers—geotechnical companies that prepare the tunnel construction sites—to systematically deposit their data there, which is quite innovative, because ordinarily, this data remains dispersed and often lost once the construction sites are completed.
The BRGM, a public institution whose mission includes collecting, storing and structuring all data on the French subsurface, quickly expressed its interest. Two agreements were signed in April 2023 between SGP and BRGM. The first concerns the provision of SONGE data to feed the Subsurface Database (BSS), a public platform accessible to all stakeholders who need it: scientists, developers, consulting firms. The second gives BRGM direct access to SONGE for its research projects, particularly the Geological Reference Framework of France (RGF).
But between receiving raw data and making it publicly available online, considerable work is necessary. This mass of data presents very high heterogeneity in their formats. They must be structured to make them exploitable and accessible via the Infoterre platform and the future FranceGéotechnique application, developed by BRGM.
A continuous flow of data until 2030
Concretely, an exchange between the project and SGP is organized every quarter to define the next batches of data to be transferred. Since the Grand Paris Express works are organized by sections on several lines, the reception schedule depends directly on the progress of the different work packages. In 2024, data from the works on the southern part of the future line 15 began to be sorted and analyzed—the first milestone of a process that will extend until 2030, the lifetime of the S-PASS project.
The project also benefits from other partnerships. RATP, a project partner, has provided BRGM with its own geological and geotechnical data distributed along the network of the 14 Parisian metro lines. Advantage: this data covers the interior of the city of Paris, where that of SGP concerns rather the peripheral perimeters. Together, these sources allow complete coverage of the territory of the Greater Paris metropolis.
Geological map including the sources of data from BRGM's BSS (Subsurface Database), SGP and RATP.
BRGM
From construction site to 3D model
Where does this data concretely come from? Mainly from coring carried out during studies prior to the works and from geotechnical tests in situ or in the laboratory. The cores—cylinders of rock or sediment extracted from the subsurface—are examined and described by geologists. Certain boreholes are subject to gamma-ray logging. A probe is lowered into the borehole, measuring the natural radioactivity of the soil at different depths. The curve obtained makes it possible to identify a certain number of geological layers, each having its characteristic radiometric signature.
It is a postdoctoral researcher at BRGM who analyzes this data. These profiles allow a very precise lithostratigraphic analysis. It is then necessary to compare the data collected at all survey points to create geological environment maps. Several hundred reference boreholes are thus analyzed and interpreted over the entire sector of interest.
These maps represent the distribution of various geological formations of the subsurface of the Île-de-France region and their associated lithologies: clays, limestones, sands, marls. They make it possible to trace the sedimentary history of the deposits and to understand how these layers are distributed in space. Essential information for urban development and infrastructure projects. Building on clayey formations does not pose the same challenges as on compact limestone. Drilling a tunnel in sand requires different techniques than in hard rock.
This work is part of scientific component No. 1 of the S-PASS project dedicated to geological and geotechnical knowledge of the Parisian subsurface. It is entitled "3D geometries and lateral variations of facies"—in other words, understanding how the different formations and lithologies are distributed in three dimensions. It is led by Justine Briais, sedimentary geologist at BRGM.
This analysis feeds the 3D geological modeling work led by Sandrine Grataloup, and attached to component No. 2 of the S-PASS project. The 3D model, currently under construction, corresponds to the interpretation in the form of continuous geological surfaces of the point analyses carried out at the reference boreholes.
Indeed, the subsurface is not a homogeneous stack of very regular layers. Geological formations present lateral variations that are sometimes significant over short distances: a clay layer can thicken here, thin there, disappear elsewhere. These variations condition the mechanical, thermal, hydrogeological properties of the subsurface. Knowing them makes it possible to anticipate behaviors, avoid unpleasant surprises, optimize technical choices.
What is unprecedented is the extent of the territory studied coupled with the fineness of resolution. We are applying usual methodologies, but in a framework never achieved in terms of data density in the Île-de-France territory.
A virtuous circle between research and territorial development
Beyond the challenge of knowledge and modeling of the subsurface, S-PASS illustrates a fruitful synergy between the world of research and the actors of territorial development. The developers (SGP, RATP) provide the data collected for their own operational needs. The researchers structure them, analyze them, valorize them. Then this data, once compiled and made public, will benefit all development actors to envision future projects that are more innovative, better adapted to the specificities of the subsurface. The pooling of this data constitutes a common good.
The visualization of the subsurface through a 3D digital model will allow public awareness of the underground space. It is also a tool facilitating the implementation of subsurface development policy that is reasoned and concerted. In the context of climate change, whose effects will be particularly marked in large metropolises over the coming decades, the subsurface can offer alternatives to urban sprawl. The possibilities are numerous, provided they are conceived in accordance with the geological realities of the terrain.