Geohazards of the seabed and how they are identified

What you need to know before starting any offshore shelf project

The seabed may appear calm and uniform, especially when viewed on satellite maps. However, for engineers designing drilling platforms, cable routes, or offshore wind farm foundations, the seabed is an area of high uncertainty. Beneath the sediment cover lie geological hazards that can jeopardize an entire project.
In this article, we explain what is considered a seabed geohazard, why these features pose risks, and which methods are used to detect them in advance.

What is considered a geological hazard?

1. Gas-charged sediments
Sediments containing high concentrations of methane or other gases. They often have reduced bearing capacity and can cause instability of structures. In addition, gas disrupts seismic wave propagation, degrading data quality.

2. Submarine landslides and unstable slopes
Even gentle seabed slopes can be hazardous, especially in areas with soft sediments. A submarine landslide can damage cables or pipelines.

3. Buried boulders and carbonate blocks
Hard inclusions within sediments can damage drilling or piling equipment and distort geophysical data.

4. Paleochannels
Ancient river channels now buried beneath younger sediments. They are often filled with loose materials that distribute loads unevenly beneath foundation structures.

5. Faults and fractured zones
These may be active or potentially mobile and are particularly important for facilities designed for long-term operation.

6. Man-made objects and debris fields
Fragments of metal structures, abandoned cables, and shipwrecks. All of these can obstruct drilling operations and subsea installations.

How are they detected?

Seismo-acoustic surveys (SBP, Chirp, Boomer)
These methods image the upper tens of meters of the seabed, allowing visualization of stratification and detection of gas pockets, blocks, and debris.

Engineering seismic surveys
Used to build detailed cross-sections and three-dimensional models of the subsurface structure. They are especially important in areas with potential paleochannels or submarine faults.

Side-scan sonar and multibeam echosounders
These systems identify seabed morphology changes, landslides, man-made objects, and other large-scale anomalies.

Geotechnical drilling and in situ testing
Provide information on the physical properties of sediments and confirm interpretations derived from geophysical data.

Modelling and interpretation
After data acquisition, specialists develop models to predict soil behavior under load, identify risk zones, and support decisions on design modifications.

Why is all this necessary?

Identification of geological hazards is a key element in the preparation of any offshore project. Underestimating geophysical investigations can lead to:

• structural settlement,
• costly repairs and redesign,
• construction delays,
• emergency situations.

Geophysical site investigations are a critical component of engineering safety. Understanding what lies beneath the seabed makes it possible to build reliably, safely, and without unpleasant surprises.