In New York city I saw some workers moving around a black metal box along the ground that was being pushed along by what looked like wheel chair wheels. It had wiring attached to it. I was some kind of depth analysis device. Later they put chalk marks all along the front and sides of the building on the sidewalk. What is the procedure for?
It could be any one of a number of things, but in a city street ground penetrating radar (GPR) would be a likely candidate.
If it’s a “depth analysis device”, it got to be ground penetrating radar. The metal detectors in general do not give you the depth information on underground targets. From your description, it sounds like a GSSI GPR system with a cart with 2 or 3 wheels.
Our company uses ground penetrating radar equipment, similar to what you described, to locate various subsurface targets. Since you didn’t see any police on the site they were probably not looking for buried evidence or bodies. Most likely the chalk marks were marking underground utilities prior to excavation.
It was most certainly GPR. Sensors & Software Inc, out of Mississauga, ON, makes a fantastic GPR system for locating buried pipes/conduits. I’ve hired a consulting company that uses their equipment and was very impressed with the data quality and durability of the machine.