D10 controller described a MVA event when an IFR aircraft was issued a VFR clearance and permitted to enter a higher MVA area; the reporter listing weather factors and less than optimal scanning as causal factors.

Narrative

I was working Aircraft X inbound to ADS. Along with this aircraft I had several other VFR pop up aircraft that had to be low level to remain VFR; typically around 1;500 feet. Aircraft X was at 2;000 on a vector for an ILS approach approximately 15 miles southeast of ADS. I observed the aircraft in proximity to a 2;200 foot MVA. I; for some reason; registered that the aircraft was VFR and instructed the aircraft to maintain VFR at or below 2;500 so that he could maintain obstacle clearance. The pilot complied and descended to 1;600 to VFR conditions. I pointed out the antenna and the pilot reported it in sight. It was then that I realized that the aircraft was in fact IFR and I climbed him immediately to 3;000. By that time the aircraft was inside the MVA at 1;600. Recommendation; this event could have been avoided had I had a better scan. It was complicated with bad weather and having to watch aircraft that were deviating and descending/climbing in areas where they would not normally be being worked by other positions.