THE PLT REPORTED THAT SNOW HAD BEEN FALLING REGULARILY FOR THE PREVIOUS FEW DAYS BEFORE THE FLT. HE DEPARTED THE STAGING AREA WITH 12 DRILL CREW MEMBERS. AT THAT TIME, THE VISIBILITY WAS 2 TO 3 MI & A CEILING OF 800 TO 1000 FT. THE PLT FLEW 9 OF THE PASSENGERS TO A SEISMIC LINE ON AN 8000 FT RIDGE LINE, THEN FLEW THE OTHER PASSENGERS TO THE VALLEY FLOOR, WHERE THEIR EQUIPMENT WAS LOCATED. DURING ARRIVAL, THERE WAS A 500 FT CEILING, THE VISIBILITY WAS ABOUT 1 MI & SNOW WAS FALLING LIGHTLY. WHILE LANDING, THE ROTOR WASH KICKED UP CONSIDERABLE SNOW, BUT THE PLT WAS ABLE TO USE THE DRILL RIGS FOR A VISUAL REFERENCE. HE TOOK OFF AGAIN TO RETURN TO THE STAGING AREA, BUT AFTER FLYING APRX 1/4 MI, THE SNOW SHOWER INCREASED & THE VISIBILITY BEGAN DROPPING. HE DECIDED TO LAND & WAIT FOR THE WX TO IMPROVE. HOWEVER, AS HE APPROACHED THE GROUND, ALL VISUAL REFERENCE WAS LOST DUE TO SNOW KICKED UP FROM THE ROTOR WASH. SUBSEQUENTLY, THE HELICOPTER DRIFTED TO THE RIGHT, THE RIGHT SKID DUG INTO THE SNOW & THE HELICOPTER ROLLED OVER.