Geophysical studies

Geophysics is the study of the Earth’s physical characteristics using indirect measurement techniques to determine the physical properties of the subsurface, reveal its structure and composition, and identify the presence of heterogeneities. It relies on non‑destructive methods that provide a continuous and extensive view of the ground, whereas point‑based investigations (boreholes, soundings) offer only localized information. In the context of infrastructure projects, geophysics is an essential tool for anticipating soil behaviour and the risks associated with its variability prior to the design and construction of infrastructure.

Understanding geophysical investigations: Key words and definitions

Geophysics relies on several types of methods, each sensitive to a specific physical property of the ground. In the context of field studies, the following methods are considered: 

Seismic methods

They analyze the propagation of seismic waves through the ground to determine its stratigraphy, the type of geological layers, their mechanical properties, and the site’s level of seismic sensitivity. 

Electromagnetic methods

They measure, among other things, the resistivity or conductivity of the ground, which is useful for detecting lithological variations, water‑saturated zones, geological discontinuities, or cavities. 

Ground‑penetrating radar (GPR)

This method provides vertical imaging of the first metres of the subsurface, ideal for locating geological interfaces and discontinuities as well as buried infrastructure. 

Why conduct geophysical surveys?

For an infrastructure project of this scale, understanding the subsurface is a major issue to ensure the stability, durability, and the safety of the infrastructure. Geophysics makes it possible to:

  • Identify the topography of geological interfaces and the mechanical properties of the different layers.
  • Identify risk zones (cavities, compressible soils, discontinuities, etc.).
  • Reduce geotechnical uncertainties and associated costs.
  • Improve the overall understanding of the alignment before the design phase.

It therefore serves as an essential complement to conventional investigations, providing a continuous and integrated view of the ground.   

forest landscape
forest landscape

What do geophysical surveys involve? 

A geophysical survey is carried out in several stages. It begins with defining the objectives and selecting the methods best suited to the geological context and project constraints. Fieldwork generally consists of deploying, on the ground or in the water, along straight survey lines, various setups extending over several tens of meters. These setups are composed of sensors (geophones, electrodes) spaced at regular intervals. 

Subsequently, field surveys are conducted to characterize the site and compile relevant information on the plant communities present and the habitats they comprise. Characterization includes the delineation of plant communities, identification of species present within the different vegetation strata, and a description of site soils and hydrology.  Drone-based surveys may also be conducted to document certain environmental characteristics and complement field observations.