Millimeter wave technology drills 100 meters into granite

SF6 is a horrifically powerful greenhouse gas, so I doubt it could be used. Its GWP is somewhere around 23,000 on a 100 year timescale.

Yeah, they say in their launch video for Project Obsidian (https://www.youtube.com/watch?v=xmrna_r_b3A) that they'll drill the first 3km using conventional rotary drilling and mmWave beyond that.

I'd be curious if anyone (perhaps the parent) knows why – my assumption is that it's more expensive and/or not as reliable to drill higher up with mmWave, not least because the ground might be uneven materials, etc., and then it becomes something predictable and harder to rotary drill lower down, incl. as you would spend more time doing things like replacing bits low down and sending things up and down?

If the goal is to simply purge the content of the hole, compressed air is typically sufficient. That said, the wider the hole, and the deeper it is, the harder it is to lift material on air.

To be clear though, I'd love to have one of these rigs on my old project and compare rate of progression and hole quality. Particularly when establishing the hole in sedimentary gravels and clays. I imagine casing will still be required.

One thing that I'd be concerned about is the ability to collect samples if most of the material is being vaporised or melted. Similarly, the cooking of the side of the hole on the way down could make geophysical responses much more difficult to interpret. Sonic velocity would probably increase, televised would probably be harder to interpret, harder to spot hydrothermal infill in sedimentary cover, would it affect gamma tools (probably not)

Edit: also wondering how the hole holds up around aquifers. Does the super heating cause wall instability immediately above the non geothermal aquifers as superheated steam is created? How does this affect the hole stability if we are not casing?

Edit 2: if we are not casing, how does the hole hold up around aquifer sands, loose fill, fractured or brecciated mass?

Edit 3: Also! Do we ream open the top of the hole to down past the last aquifers before the geothermal horizon? If not, how are we stopping stopping aquifers interplay and interaquifer contamination?

Isn't the idea here to gasify the rock by essentially microwaving it? With ultrasound, wouldn't you still need to remove the leftover rocks?

The video mentions that they did all the drilling so far at 100kW and are expecting to start doing 1 MW within the year.

I wonder what their transmission voltage is. They're talking about a 1km deep shaft. That feels like a lot of conductor to get to 1MW, unless you can send at 20kV or something high. Reciprocally though if you're not transmitting major force through a drillshaft, perhaps it still is a major net win for cost.

Figuring out heat management down there feels like it would likewise be pretty tricky! Again I wonder though how that would compare to the heat generated from drilling and how much management/circulation that requires.