in a frenzy searching for something

In time-honored New York fashion I was standing at the window-shelf of a pizza joint grabbing a slice at 72nd and Second last week when I noticed a hyperactive security camera observing the intersection.

See it bolted to the corner of the building on the left, the south west corner?

But it was jerking around in a frenzy searching for something and the installation looked kinda temporary to me. Turns out it’s not security. They are digging the new 2nd Avenue subway there at the moment.

So I went outside and asked the boss of the digging crew. It’s a “seismometer” he told me. It measures movement in the earth. 

http://en.wikipedia.org/wiki/Seismometer

The green and white laser spins on that axis you can see below the shelf. The white box above is an all-weather computer that monitors the data and relays it all back to the mainframe in Queens. The little silver disc is the coverplate of the electrical hookup box, to power the spinning servo motor and the computer.

Before any digging occurs the advance crew mounts optically perfect reflectors in various locations on all the nearby buildings, then the vectors are established with the laser and the databank is set. During digging the little laser spins around looking up and down for each of the reflectors. When it finds them, it compares the shoot-angle to the databank. He told me there were “dozens” at this particular dig site.

If a foundation is affected and a building shifts as little as 1/1,000th of an inch the little laser knows, an alert is sent to the mainframe and the giant underground digger---also linked-in to the computer---is auto-shutdown.

Now we know.