BRGM has been using the mobile laser scanner, a 3D modelling tool, since 2017. Several projects have since benefited from this highly precise technology to map underground voids and prevent the risk of collapse.
10 March 2021

3D modelling of underground cavities

In the "Minutes de l'innovation cat-nat et climatique", the MRN association (Mission Risques Naturels) highlights the mobile laser scanner, our 3D modelling tool. Silvain Yart, cavity risk engineer at BRGM, discusses the interest and possibilities offered by this tool in hard-to-reach areas such as underground cavities.

© Mission Risques Naturels

NATURAL DISASTER AND CLIMATE INNOVATION

EPISODE 5: 3D MODELLING OF UNDERGROUND CAVITIES

 

The French Geological Survey, BRGM, develops the use of mobile laser scanners to prevent the risk of collapse of underground cavities. We can now map in 3D underground cavities and model underground space that was inaccessible by conventional means. The scanners enable us to rapidly map large areas, such as underground quarries that are tens of hectares in area. This was impossible with previous tools.

 

What was the problem?

 

Today, in France, there are 190,000 cavities, natural or human-made. Many were made on the edge of historic urban centres. They were forgotten about and are now in urban zones. The collapse of these cavities is a clear threat to human life as well as to infrastructures. So identifying and diagnosing the state of these cavities is a major issue. It is one of BRGM's missions.

 

How does the material work?

 

The scanner has a laser sensor and an accelerometer. It rotates and measures the distance from the cavity walls over 360 degrees up to 300,000 times per second. The device is held by an operator and it scans as they move. So every nook and cranny can be scanned very rapidly. After scanning, an object recognition algorithm produces a 3D model of the cavity. This model is then used to gather useful information for risk assessment. This information supplements visual observations made on site. It enables us to locate with decimetre level accuracy the cavity in relation to surface infrastructure and to identify degraded areas on the cavity's walls, where the probability of a collapse is the most important.

 

In which context is this tool used?

 

We can now propose more reliable and rapid solutions to cavity-collapse issues. Usually, 3D scanning of an average-sized cavity represents a few thousand euros of a stability study's total budget. The limited cost and usefulness of the tool means this technology can be used for many of our services.

 

What are the benefits of this innovation for risk management actors and insurers in particular?

 

The flexibility and performance of these new tools have radically changed the way we work. There are two major advantages of this solution. First, for already identified cavities, it will improve our knowledge thanks to high-precision mapping. This will enable us to anticipate collapses and subsidence and thus provide better risk analysis. Secondly, when we find non-identified cavities, we will be able to act rapidly. The tool is very easy to use. And this agility will be essential in disaster management.

Zeb-revo - 3D mapping of underground cavities

The Zeb-revo mobile laser is a tool for mapping underground cavities in high-precision 3D. It can be used to model underground space that is inaccessible by other means, particularly in urban areas, in order to prevent the risk of collapse.

© BRGM 

BRGM: Geosciences for a Sustainable Earth

THE 3-D SCANNER, A CAVITY MAPPING TOOL

Caves, chasms, former quarries and cellars. In France, no fewer than 180,000 cavities lie beneath our feet, whether natural or manmade. Over time, many of them were forgotten and buildings were built above them. To prevent the danger of these cavities caving in, their stability must be monitored. That's what the risks and prevention managers at BRGM do by maintaining a database of abandoned underground cavities. They use a mobile 3-D scanner to map them. 3-D scanners use a laser beam to survey the area. The time it takes for the beam to return indicates the distance between the scanner and its target. The scanner takes over 43,000 measurements per second creating a web of points to represent the 3-D geometry of the scanned object down to the centimetre.

The laser 3-D scanner has two major advantages: First of all, it works underground, which is impossible for GPS systems. The second advantage is that it is a tool that works quickly, unlike traditional topographical techniques, which can be tedious. So we're able to do topographic studies of caves or quarries in a few hours or a few days. We couldn't do that in the past.

Here, beneath St Remi Basilica in Reims, 450 m2 of galleries were mapped in less than 2 hours.

The 3-D laser scanner will change how we work at BRGM. It's a tool that allows us to visualize the exact geometry of underground cavities. We'll also have data within the geometry concerning the cavity's weak zones, areas that have fractured or that are unstable. We recalibrate that data in terms of the surface risks, like buildings, for example, and we take the appropriate risk prevention measures, such as evacuating a home if we identify an immediate danger in terms of an unstable cavity near the surface.

Every year, the BRGM team maps over 15 hectares across France.

BRGM: Geosciences for a Sustainable Earth

Caves, chasms, but also old quarries or cellars: some 180,000 cavities have been identified in France. 

Identifying and monitoring underground cavities is one of BRGM’s tasks 

Whether of natural origin or dug by people, these cavities must be mapped in order to identify areas of weakness and prevent them from collapsing and taking homes or any other building on the surface with them. 

To this end, BRGM manages the  national database of abandoned underground cavities

The mobile laser scanner, a revolutionary tool for mapping underground cavities 

With a laser beam that performs nearly 43,000 measurements per second, the mobile laser scanner has revolutionised risk prevention. Used since 2017 by BRGM to map underground cavities in 3D, this tool allows surveys to be carried out in record time compared to conventional topography techniques.

3D model of an underground quarry in Orléans
  • 180000.00
    cavities  recorded in France

  • 15.00
    hectares of  cavities scanned each year

  • 43000.00
    measurements per second

  • 2.00
    major benefits: speed and mobility