In our previous article, we introduced the Geophysical Survey Vessel, which uses sonar, magnetic sensors, and sub-bottom profiling to “see” the seabed without physically disturbing it.
But when engineers need to know how strong the seabed soil actually is / or how it will behave under the weight of an offshore wind turbine—remote sensing is not enough. 
That is where the Geotechnical Survey Vessel comes in.

Taking a “Soil Sample” from Beneath the Sea
Offshore wind turbine foundations must withstand long-term loads from the turbine itself, as well as wind, waves, and currents. Engineers therefore need detailed information about the soil and rock layers beneath the seabed.
A geotechnical survey vessel is equipped with offshore drilling systems that can create boreholes at selected locations and recover soil or rock samples from below the seabed.
Depending on the investigation, the vessel may also perform rock coring, wireline sampling, and other forms of seabed sampling.
The recovered samples are then examined in onboard or onshore laboratories to determine properties such as soil type, strength, density, and other engineering characteristics.

Testing the Ground Without Bringing It Back
Another important technique is the Cone Penetration Test (CPT).
Instead of removing soil, CPT pushes an instrumented cone directly into the seabed. Sensors measure parameters such as cone tip resistance, sleeve friction, and, in some systems, pore-water pressure.
In simple terms:
Borehole sampling brings the soil back for examination, while CPT tests how the soil behaves in place.
By combining these methods, engineers can develop a much more complete understanding of the ground conditions beneath an offshore wind farm.

How Can a Vessel Drill Accurately at Sea?
One major challenge is that the drilling platform itself is floating.
Many modern geotechnical survey vessels therefore use DP2 Dynamic Positioning systems to maintain their position while drilling or testing. Thrusters, positioning sensors, and computerized controls help keep the vessel aligned with the investigation point.
Drilling and CPT equipment may be deployed through a central moon pool or over the vessel’s side, depending on the vessel design.
Some vessels are also equipped with onboard laboratories, allowing engineers to perform preliminary testing immediately after samples are recovered and adjust the investigation program if necessary.

What Are the Data Used For?
Geotechnical data plays a critical role in foundation design.
Engineers combine borehole logs, CPT results, laboratory testing, and geophysical survey data to create a site ground model. This information helps determine how monopiles, jackets, or other foundations should be designed and installed.
So, if a Geophysical Survey Vessel is used to “look” at the seabed, a Geotechnical Survey Vessel physically tests what the seabed is made of and how strong it is.
Long before a giant offshore wind turbine rises above the waves, its foundation design may begin with something as small as a tube of seabed soil and a set of CPT measurements.

Terminology Note
Geotechnical Survey refers to direct ground investigation methods such as drilling, sampling, and CPT.
By contrast, Geophysical Survey refers mainly to remote and generally non-intrusive methods such as sonar, magnetometry, and sub-bottom profiling.
A simple way to remember the difference is: Geophysical surveys “see” the seabed; geotechnical surveys physically test it.


References
 • Guide to an offshore wind farm | An informative resource for offshore wind (Image source)
 • Fugro Wins Major Offshore Wind Contract in Taiwan 3.2 | Fugro
 • Guidelines for Providing Geophysical, Geotechnical and Geohazard Information | BOEM