A method is provided for preparing an area of the ground surface to be milled by a milling machine. Prior to performing any milling operation on the area of the ground surface, a survey vehicle separate from the milling machine is used to traverse the area. The survey vehicle detects the presence of buried obstacles with a sensor and generates a sensor output signal indicative of the presence of the buried obstacle. The sensor output signal is received in a controller which generates a sprayer actuation signal and responds to the sensor output signal. A marking sprayer is actuated in response to the sprayer actuation signal and sprays the ground surface above the obstacle with a visible marking emitted from the sprayer. Subsequently, a milling machine separate from the survey vehicle may be utilized to mill the ground surface and by observation of the visible markings the operator may avoid impacting the milling drum with the buried obstacles.