Operation Sailor Hat was a 1965 test involving the detonation of several hundred tons of conventional explosive to see how equipment and the environment would respond to the magnitude of blast one might expect from nukes. There are lots of still shots and videos out there from this test, including this video.
Another test, Minor Scale, took place in 1985 but used nearly ten times as much explosive. Why are pics/video of this test not available (other than the still photo in the Wikipedia article)?
Because they are classified?
Possible, but that seems unlikely, considering there’s an awful lot of text-based information available about Minor Scale. The Department of Commerce offers a 321-page report about it. Pictures and video are entertaining, but I suspect not nearly as informative as a text-based engineering analysis; I would have expected pics/video to be unclassified, and detailed text about the event to be classified. 
Pictures or video may simply contain images of something that was itself classified. It may have nothing to do with the results of the explosion, but everything to do with something they put in the way of the explosion. Even just revealing its existence, shape, or that it was important to test.
I do hope someone ran a few really long lines of seismic sensors across the area. If you want to see deep into the crust, explosions like this are a tremendous opportunity.