Chief executive Alison Munro said: "This next key stage of our work will assess in much greater detail the potential impact of the proposals on local sites."
Landscape and ecology surveys will identify plants, habitats and animals and, if needed, do more detailed investigations on specific species. The company's environmental specialists will also look at ground conditions and physical features.
The firm said the work was an "essential part of the collection of baseline data" to help prepare further stages of the project. It will be used to inform an environmental impact assessment and later design work. Building is scheduled to start in 2017.
The £33bn proposal was approved in principle three weeks ago (HW, 20 January). Critics, including the Wildlife Trusts, said the 225km line would destroy vast tracts of landscape including 50 ancient woodlands and 160 wildlife sites.