AN ACFT CONDUCTING PIPELINE PATROL EXPERIENCES CONFLICT WITH ANOTHER ACFT APPARENTLY CONDUCTING UNAUTHORIZED FORMATION FLIGHT.

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WHILE PATROLLING PIPELINE (15 NM; 046 DEG; DEN VOR); I WAS FLYING S OBSERVING PIPELINE. I OBSERVED TFC (AN SMA Y) 12 O'CLOCK 2 MI APPROX 200 FT ABOVE. MY REACTION WAS TO DSND AND REVERSE COURSE. AFTER REVERSING COURSE; I WAS IN A POS TO PATROL A PIPELINE THAT GOES E FROM THAT LOCATION. I CONTINUED E ON THAT PIPELINE. BY RAISING MY R WING; I COULD LOOK OVER MY R SHOULDER AND KEEP OTHER ACFT IN SIGHT. THE OTHER ACFT TURNED IN MY DIRECTION AND WAS APCHING ME FROM THE 4 O'CLOCK POS. I CONTINUED MY PATROL E UNTIL THE OTHER ACFT WAS ABOUT 500 FT TO MY REAR AND 50 FT BELOW. AT THAT TIME I INITIATED A CLB IN AN ATTEMPT TO EVADE THE OTHER ACFT. THE CLB CAUSED THE OTHER ACFT TO FLY PAST. I CONTINUED TO CLB AND DEPARTED AREA. I DID NOT SEE OTHER ACFT AFTER HE PASSED BELOW. THE GREATEST POTENTIAL FOR AN ACCIDENT IN THIS SITUATION IS WHILE PATROLLING PIPELINE. THE PATROL PLT MAY MAKE SUDDEN TURNS IN EITHER DIRECTION -- IN THIS CASE; A R TURN WOULD HAVE PROBABLY CAUSED A MIDAIR. I THINK THE OTHER ACFT WAS TRYING TO GET MY ACFT NUMBER. IN SO DOING HE PUT BOTH ACFT IN DANGER.