MSN controller described an observed event when heavy separation was possibly an issue involving a B757 and PA28 crossover.

Narrative

MSN approach is split into east and west airspace. I was working west and a Supervisor giving a certification check was working east; I was also the CIC. When I noticed the B757 heading towards the small VFR aircraft I told east that I didn't think that was good; neither Developmental or the Supervisor took action; and targets merged with exactly 500 ft between them; the B757 passing right over the top of small aircraft. I didn't realize that the area they were over; Sun Prairie; WI was 970 ft field elevation; MSN field elevation is approx 850 so I thought it was OK; but it wasn't when you consider the elevation of the surface; which puts them into the 2500 ft from the surface of the earth and requires wake turbulence separation if less than 1000 ft apart and directly behind so lateral separation was not applied. I found out the field elevation the next day; I alerted them to the situation in the first place because I just didn't think it was good judgement. Recommendation; I would have needed to realize it sooner even though I wasn't working and forced the issue.