Right, and they would keep your jeans clean and dry - but they wouldn't meet my needs in detecting or caving. I'd settle for a rubber hard shell cap, that is stiff, yet giving slightly, making terrific traction on a smooth surface like ice...and makes a lava tube hands-and-knees crawl, fairly comfortable...! Protecting a good-sized area of the knee - they're SWAT team style - check out
Galls Gear Tactical Knee Pads
and another, almost same, but quality
Blackhawk Advanced Tactical Knee Pad V2 @ TacticalGear.com
this one has the rivets holding the hard-shell in place, that are better distributed, perhaps lasting longer, than the Galls set I use now.