Decompression Planning

[DRE]

The scenario described below will only work if you're trimix certified, so I encourage people who aren't to post their ways of possibly doing this dive.
Up front I must admit that there is a wide variety of possible scenarios available, dependent upon team size (whther there are support divers, surface supplied O2, etc.). Using support divers for a dive such as this might be a bit exaggerated, but it would be great practice for bigger dives.

With a team of 2 or 3 divers I'd be using TRI 21/35 for backgas. The reasoning behind it is twofold: I consider an END of 130ffw too deep to accurately take measurements or survey. With 21/35 I've got an END of 73ffw (considering the O2 to have the same narcotic effect as N2), which is fine for surveying. Also, the helium makes offgassing far more easier and better for the body compared to using N2, so I know I'll have an added deco advantage. I'd use EAN50 and O2 for deco starting at 70ffw and 20ffw respectively, which would yield me a total bottom time of 53 minutes.
The reason for using 21% O2 in the mix as compared to a higher O2 percentage has to do with two things: first, 21/35 is a standard gas that is used by many deep exploration teams. I believe in uniformity of execution, and it is far easier to match gasses for everyone on the team and mix these gasses over and over, than to use a custom blend for every single dive. Secondly, by keeping my bottom PO2 very low (1.0) I won't run into problems with regards to the CNS clock and could do this same dive 3 times in a day with a 3 hour surface interval without having to change the overall run time of the dive (ie I could run the same profile for each dive). Doing this dive 3 times in a day is maybe not the best idea (mainly because of the repeated submersion in a cold water environment), but at least 2 times shouldn't be a problem whatsoever.

Like I said, this might be a little bit more advanced way to be doing this dive, but there are other safe ways to do this one as well without using helium - so fire away!