Investigation, as far as I’ve seen locally and on Youtube is the least favored mission type because the completion time to reward ratio is underwhelming.
I propose a fix to this perception is that you make it unique - Hadron rewards you 0 ordo for doing any tier of investigation. Instead, have a resource multiplier as payment 
Difficulty / Resource Mod
1 / X2
2 / X4
3 / X6
4 / X8
5 / X10
6 / X12 (hi intensity adds 2 to the multiplier)
Numbers tweakable of course but still an idea for making this mission stand out as a choice.