Probably some radar or ultrasound distance sensor would suffice. Maybe even ToF of the laser.
I don't think that's anywhere near to the top of the issues they are going to run into.
I don't think that's anywhere near to the top of the issues they are going to run into.
However, anything about radar, ultrasound, or laser ToF would require electronics at the head of this waveguide and a way to communicate data to the surface. From what they're saying, the downhole environment of this thing is going to be very high temperature. Physically vaporizing 100% of the rock to make hole tends to do that. Conventional oilfield electronic tools already have trouble getting the MTBF above a few hundred hours at current downhole temperatures, which are much cooler. It seems likely that no electronics would survive at all at the temperatures they're planning on running.