Sitting and performing Salaah in a coal mine

I work on a coal mine, where the roof is not high enough to allow one to stand up straight. We work morning shift for a week and night shift the next week. Dhuhr is delayed until returning to surface when working morning shift, however salaah is performed underground when working afternoon shift. As one is required to stand upright, is it required to a) sit and pray (and then repeat the parayer when on surface) or b) stand and pray (even though one stands slightly crouched)?