The site was prepared for a 24-metre tall piling rig which was brought onto the site by the company on 20 August.
The rig is part of preliminary work for a £16.6m transformation of the 115-year-old theatre.
The machine will determine the composition of the ground beneath the structure, essential to inform the material to be used during building work.
Horsecross Arts and project partners Perth & Kinross Council successfully lodged plans to restore the B-listed Edwardian theatre auditorium to its former glory on 12 August.
A new 200-capacity studio theatre will be added for more intimate shows, live music and comedy events.
The newly-transformed venue will have increased workshop spaces for creative learning and community projects, including the Perth Youth Theatre which celebrates its 50th anniversary in 2016.
Due to be completed late 2017, the High Street entrance to the building – owned by the Council and operated by Horsecross Arts – will be retained, with new frontage created on Mill Street, improving access and facilities for visitors.
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Kevin Dickson, managing director of Robertson Construction Tayside, confirmed the equipment will be on site for two or three days, although the test piling itself should only take a few hours.
Mr Dickson said: "Determining the ground conditions is an essential first-stage of the project as it allows us to establish the most suitable type of piling to be used within this historic, inner-city environment.
"This is a particularly interesting project to work on because it will see one of Scotland's most historic buildings transformed into a state-of-the-art venue – without losing any of the character that made it so popular in the first place.
"We're looking forward to working closely with Horsecross Arts and Perth & Kinross Council as we restore Perth Theatre and give it back to the people of the city."
Horsecross Arts chairman, Magnus Linklater, said: "With the plans now formally approved, and site investigation work in full swing, we look forward to moving forward with this exciting project."
Council Leader Ian Miller added: "An exciting transformation awaits Perth Theatre and I'm delighted to see this important restoration work now beginning on site.
"Once completed, Perth Theatre will again make a valued contribution to Perth's vibrant cultural offering and is certain to be a popular attraction for locals and visitors alike."
Horsecross Arts has been pledged £15.1m for the Perth Theatre redevelopment.
Funding is being provided by project partners Creative Scotland, The Gannochy Trust, the Heritage Lottery Fund, the council, other trusts and foundations, individual donations, other supporters and funding bodies.
Horsecross Arts is now looking to raise an additional £1.5m to ensure that the newly transformed venue is fully equipped and furnished for a new century of theatre in Perth.
The venue is scheduled to reopen in 2017.
(LM/JP)
