I saw a weird pattern in the bricks at an old fort in St. Augustine

I was walking around the Castillo de San Marcos last month and noticed some of the coquina bricks have these deep, perfectly round holes in them. They're not from cannonballs, they look drilled. A park ranger said they're from some kind of core sampling done in the 1950s, but he didn't know why they picked those specific spots. It made me wonder what they were looking for and if they ever published the results. Has anyone else seen these or read about that study?