The multi-million pound programme, which runs until the end of the year, will see up to 100 people working on site in Anglesey during busy periods.
The contract for this round of studies has been awarded to Structural Soils Ltd, a specialist ground investigations contractor that previously carried out work for Horizon between 2009 and 2010, and again in 2011.
Structural Soils has also worked for EDF Energy at Hinkley Point in recent years.
The Wylfa Newydd ground investigation requires the drilling of around 400 boreholes and trial pits across the site. The material removed is then analysed to give Horizon a detailed understanding of the ground conditions that will inform more detailed planning of the preferred site layout.
Horizon site development director Charlie Tasker said: "The ground investigation work is another example of the rapid progress we're making. This part of the project will take us most of this year to complete and will give us a huge amount of technical detail about the ground we want to build on."
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