Geologist can't find new mines unless they're exhausted before he searches

I had suspected this on a number of occasions before but hadn't actually confirmed to myself until today.

I had a marble mine that was going to exhaust within 20 minutes. I sent the geologist out, which would take him 30 minutes to refill the exhausted mine. The marble mine exhausted right on time, but 10 minutes later, the geologists reports that he couldn't find any new marble mines.

Seems to me that it would make more sense to be able to time it to keep the mines going as long as possible by timing the search to complete with the time to exhaust. The way it is, the mine has to stay exhausted the complete length of time that it takes a geologist to find them.

I haven't figured out the timing for sure, but you can send them before it is exhausted...just not sure how much before. It's easiest with coal mines because it takes so long to find them. I've experienced what you're talking about, but I've also successfully found them doing what you're talking about too.