[Cerium]

I worked on a seismic monitoring system for civil engineering structures. We recorded data at 200 Hz (though all the interesting content is in the zero to 15 Hz range). Once I tried this experiment with some of the data from a bridge in a harbor. It was very cool, you could hear so much going on in the bridge.

Ambient seismic vibration is something of a hidden world around use. With a very sensitive accelerometer you can measure the shaking in a concrete pad due to cars a block a way. Looking in the frequency domain things like when rush hour is and which day is the weekend pop out like a sore thumb.

[mhh__]

In between the acceptance of electronic intelligence and the dawn of everything being computer based, spooks spent a lot of time inventing ways to get this kind of info from dumb sensor data.

At one point MI5 could apparently lift crypto keys from the noise of the cipher machine through a wall. Obviously being MI5 this was to spy on the French and not the Russians, but still.

[mateo1]

Also depending on the scale, seismic surveys are seriously affected by nearby roads and construction, where "nearby" can mean several km in some cases.