I don't know for certain, but I have a few theories:
Discworld: Terry Pratchett's Discword series is about a fictional world that is a very large disc, spinning like a record and supported by a couple of giant elephants standing on the back of an even more giant turtle.
World Turtle: Throughout human history there have been a few cultures that believed the world was supported on the back of a giant turtle. It's entirely possible that some organization has such a design as their logo, but it could be anything from the Stone Masons (or similar) down to a local concrete company.
Turtle Expedition: WHile these folks use a different logo, it's possible that it could be a logo for a similar organization.
A few quick Google Image searches for Turtle Earth, Turtle Logo, Turtle Planet and Turtle Globe didn't turn up anything like that, so I really have no actual info to give you.
Def