In a notice to mariners, Vattenfall said the survey would begin next week from the Terramare 1 vessel.
Ramora has previously carried out UXO clearance at RWE Innogy's Gwynt y Mor project in the Irish Sea.
Construction at the extension project, located 7 kilometres off the UK's south-east coast, is expected to begin later this year.
Preparation of the seabed began in October. Contractors Bohlen & Doyen used a water jet tool to clear the area of seabed along the cable route.
The project will feature 15 MHI-Vestas V112-3.3MW turbines, and is expected to be fully commissioned in 2016.