Before an offshore wind farm can be built, engineers first need to answer a basic but critical question: 
What exactly does the seabed look like?
The ocean hides the seafloor from view, while layers of sediment, rock, and buried objects may lie beneath it. This is where the Geophysical Survey Vessel comes in. 
Equipped with specialized sensors, the vessel acts like an offshore reconnaissance platform, helping engineers map the seabed and investigate what lies below the surface.

Not X-Rays—But Sound and Magnetic Fields
Calling it an “X-ray” is only a metaphor.
Geophysical surveys are generally non-intrusive, meaning they can collect information without drilling into the seabed. Common techniques include multibeam sonar, side-scan sonar, sub-bottom profiling, and magnetometry.
 • A Multibeam Echo Sounder sends out a fan-shaped array of sound pulses toward the seabed. By measuring how long the echoes take to return, it can calculate water depth and build a detailed three-dimensional bathymetric map.
 • A Side-Scan Sonar, meanwhile, creates acoustic images of the seabed by measuring the strength of reflected sound. It is especially useful for identifying seabed textures, objects, wrecks, and other potential obstacles.

Can We See Below the Seabed Too?
Yes, at least to a certain depth.
A Sub-Bottom Profiler uses lower-frequency acoustic signals that can penetrate the upper layers of the seabed. Reflections from different sediment or geological boundaries allow engineers to interpret shallow subsurface layering.
A vessel may also tow a Magnetometer, which detects local variations in the magnetic field. Large ferrous objects can create magnetic anomalies, helping survey teams identify buried metallic objects, wreckage, or potential unexploded ordnance (UXO) that may require further investigation.

Why Does the Vessel Sail Back and Forth in Straight Lines?
Geophysical survey vessels usually follow a planned grid of survey lines across the wind farm site and proposed cable routes.
By repeatedly scanning the area along closely spaced lines, the survey team can build a consistent dataset covering seabed topography, surface features, and shallow geology.
The resulting information is used in GIS systems and supports decisions such as wind turbine positioning, foundation design, cable routing, and the planning of later geotechnical investigations.
Typical manned geophysical survey vessels may be around 30 to 70 meters long and can remain offshore for extended survey campaigns. In recent years, smaller autonomous survey vessels have also begun to take on some of these tasks.
So, one of the earliest steps in developing an offshore wind farm is not installing equipment—it is first making the invisible seabed visible.

A Geophysical Survey Vessel is, in effect, the offshore wind farm’s early-stage scout, using sound, magnetism, and data to create a detailed map of the underwater world.

Terminology Note
 • In this series, Geophysical Survey refers to remote, generally non-intrusive methods used to investigate the seabed and shallow subsurface.
 • By contrast, Geotechnical Survey refers to direct ground investigation methods such as drilling, sampling, and Cone Penetration Testing (CPT).
That distinction makes the two topics a natural pair: geophysical surveys“look”at the seabed, while geotechnical surveys physically test it.
 

References
 • Guide to an offshore wind farm | An informative resource for offshore wind (Image source)
 • NOAA Ocean Exploration