Gear to stay stationary during rappel

I'm a researcher doing work on rock climbing routes. I will be rappelling from fixed anchors, but need to know the best way to keep myself stationary and free up both of my hands while I record data on the rock face. Obviously having a belayer would be ideal, but would clipping to a prusik or ascender do the trick? Any other suggestions? 

Use a Gri Gri and tie a safety back up when stopped.

Leg wrap

Wrap the rope around your leg, and/or use a third hand (Klemheist / equivalent).

You have a couple of options. fixing the rope with a reepschlur and rappelling on a single strand with a grigri is very easy, just tie backup knots when you stop to log data. This has the benefit of being easy to rig to ascend if you need to go back to look at something. Or you can rap normally with a third hand back up with a autoblock, look up videos on extending a rappel. 

Have you ever rappelled before?

My wife and I are birders as well as climbers. We've birded canyons in Costa Rica by rapping down and hanging just as you describe. We use our regular ATCs, or sometimes (for the fat static line we borrow from friends there) figure 8s, with autoblocks ( en.wikipedia.org/wiki/Autob… - we use the Machard knot version) of 6mm accessory cord. We can hang at leisure hands-free to glass cliff- or canopy-dwelling birds.

Extend the rappel and use a prusik/auto-block/third hand. Easiest and most intuitive method. Not a huge fan of rapping on a gri gri and tying and untying a knot every time you stop can be tedious.

Take a class from a professional.  There are many options, but it would be tough to figure them out from internet posts.

FrankPS wrote: Have you ever rappelled before?

Most important detail right there.

Grigri will work. There are also devices specific to rappelling that allow no-hands hanging. Be aware that some of these are better suited for static ropes. And as already said, always tie-in with a "catastrophe knot" or other safety before going hands-free.

http://www.petzl.com/US/en/Sport/Belay-devices-and-descenders

You may get better answers to your question on a caving forum.

Great trick from rgold: wrap the brake strand around your body (I.e. over your harness).  Way easier and way more secure than leg loops, IMO.

nothing beats the grigri!

Petzl ID...

Take a class and practice in a controlled environment with a live belay before you have to do it when it counts.  There are many ways to successfully and unsuccessfully rappel, and since you will be doing something other than rappelling, you will need to be really smooth and efficient.  Be safe, have fun!