snapster:
So the project will use a couple of ping sensors somewhat continuously for, say, 1 hour per day, and the whole thing will go into low-power mode for the remaining time. During that 1 hour, it will be using probably 60-100 mA current, and in the remaining time, it ought to use very little current. So alkalines will be insufficient, as they won't be able to satisfy the high-load times for long.
Now we have that information, it sounds like an ideal application for a small gel lead acid battery.