Technical term – surface detection
Objects in the soil are detected from the soil surface with non-destructive techniques.
UXO and metal objects are detected by magnetometry. Passive detectors have a detection range of 3 m, even 4 m for large objects like aerial bombs.
For utility lines, underground structures and storage tanks, we use GPR and/or radio detection.
A surface detection doesn’t require any groundwork and is therefore a relatively cheap operation (as opposed to depth detection).