While this is being improved all the time it is possible to give a reference for this. A major study was carried out in Germany that deals with this topic. Here it was shown that 16 mm rebars could be detected with a minimum cover of 20mm. The reference to the paper is shown below.

Detection of near-surface reinforcement in concrete components with ultrasound
Sarah Vonk1,*, and Alexander Taffe1
1HTW Berlin – University of Applied Sciences Berlin, Nondestructive Testing in Civil Engineering Department, 12459 Berlin

Link to paper