Ultrasonics can - in principle - achieve the intended resolution when using a respective high frequency (MHz range). The involved electronics isn't simple in this case. But I agree with Keith, that it most likely won't work for a real world problem. In any case, we have to look at the exact measurement setup and enviromental conditions. Saying just 30 cm/0.1 mm isn't enough. It's also a big difference, if you only want to demonstrate the measurement principle, e.g. for an academic project, or design a ruggedized industrial measurement system. For the latter, you would surely go for optical methods, if actually needing a contactless sensor.