say original 180mls wide ie 90mls either side of a centre line, the drifting is hopefully only in one direction so you lose the other 90mlies.
Right now the width of search area would be about the same as day 1 but increasing as every day goes by,
If it landed on the centre line you get 10x20 =200, compared to original 180, if it landed on eastern edge you get 90+200 = 290
however if it landed on west edge 180 of the 200 is the original area