DTH is only usable on hard ...

DTH is only usable on hard rocks that may include metamorphic, most Triassic and pre-Triassic rocks and basement rock.  It is completely unsuitable for soft poorly indurated rock such as marl, evaporites, soft chalk and Tertiary to recent sediment.  An Ingersol Rand TH 20 can drill a 500 foot bore hole in hard siliceous Permian limestone can be drill in two days with a total crew of 2.  However, I have seen the TH20 operated by the Regie Sondage in Tunisia take 9 months on the same boreholes.  Tell me what you are drilling and I will tell you if it is appropriate.  I have direct experience with DTH in many places in Africa and around the United States.  Otherwise use mud rotary or reverse rotary methods.  Best you buy a copy of "Groundwater and Wells" by Driscoll, 1986.  Available from Abebooks.com for abot $88.