Engineers and archaeologists are drilling test boreholes across the site between Friarsgate and The Broadway in the city centre.
The holes will be up to 40m deep to allow Pre-Construct Archaeology Ltd to build up a picture of what lies under the surface. It is in the heart of the Roman, Saxon and medieval city.
The one pictured is on the grass outside the former Friarsgate GP surgery.
Developer Henderson has submitted its revised planning application to the city council. It wants to build flats and shops and drop plans for a new bus station and offices.
Planning permission was originally granted in 2009.