abstract:(a) is an imaginary line representing the True vertical depth, while line (b) is the [[borehole itself, and its length is called the measured depth.]]
It just became possible, thanks to a laser camera developed by physicists at Heriot Watt University in Edinburgh, UK. You can pretty much see how it works from the images above -- laser beams are bounced off the target and the duration of their return journey is measured precisely enough to detect millimeter depth changes even at extreme distances.
