Before an offshore wind farm can be built, developers need to know much more than whether the area has strong winds.
They also need answers to questions such as: How deep is the water? What does the seabed look like? Is the ground made of sand, clay, or rock? Are there boulders, wrecks, or unexploded ordnance? What kinds of marine life are present?
This is why different types of Offshore Survey Vessels begin working years before major construction starts.
Environmental and seabed data collection may begin five years or more before the planned operation of an offshore wind farm. 

Terminology Note: Offshore Survey Vessel is used as a broad, reader-friendly term. In practice, survey vessels are usually identified more specifically by their mission, such as Geophysical Survey Vessel, Geotechnical Survey Vessel, Hydrographic Survey Vessel, or Environmental Survey Vessel.

One “Survey Vessel” Can Mean Many Different Things
Offshore Survey Vessel is a broad term rather than one standardized vessel type.
 • A Geophysical Survey Vessel uses technologies such as multibeam sonar, side-scan sonar, magnetometers, and seismic profiling to map the seabed and identify potential hazards. These surveys are generally non-intrusive and are often carried out early in project development. 
 • A Geotechnical Survey Vessel, by contrast, physically investigates the seabed through drilling, sampling, and in-situ testing. For fixed offshore wind projects, geotechnical data may be required tens of meters below the seabed to support monopile or jacket foundation design. 
Other vessels may conduct hydrographic or environmental surveys, examining water depth, sediment processes, and marine ecosystems.

Why Is One Survey Not Enough?
Different stages of development require different information.
Early surveys may cover a wide area to understand general seabed conditions. Later investigations become more detailed around turbine locations and foundation sites.
Cable routes also need their own surveys, because engineers must understand the seabed along the entire planned route.
Survey work therefore becomes progressively more detailed as the project moves from early planning toward final design and construction.

Survey Vessels Bring Back Answers Before Construction Begins
Survey vessels may not look as dramatic as WTIVs or Heavy Lift Vessels, but the data they collect influence major engineering decisions.
Their results help determine:
 • Where turbines can be placed, how foundations should be designed, where cables should run, and which areas should be avoided.
 • Seabed surveys support foundation optimization, wind farm layout, and the reduction of installation risk. 
So an offshore wind farm begins long before the first turbine appears above the sea.
Before construction vessels arrive, offshore survey vessels have already been turning an invisible underwater environment into maps, models, and engineering data.


References
 • Guide to an offshore wind farm | An informative resource for offshore wind
 • Guidelines for Providing Geophysical, Geotechnical, and Geohazard Information Pursuant to 30 CFR Part 585