Why and how it is that the autocorrelation of natural noise gives us a reflection seismogram.
There I was, stuck in a dreary lecture, staring up at the fire sprinklers near the ceiling. I would much rather be conjuring up prospects for the instrumented oil field. What if that water nozzle near the ceiling was a geophone, and I was down here in the reservoir? If that geophone was recording all the noise in this room, all the people chattering and banging about, from all that sound, would I be able to figure out the shape of this room and the location of all the furniture in it? It reminds me of that old question, ``Can you hear the shape of a drum?''
We are taught that sound waves and light waves are both waves, so why do we need to control the illumination in one case, but not the other? The idea of acoustic daylight imaging is that we should be able to make seismic images from the natural ambient seismic noises just as our eyes do with the natural ambient light.
One geophone is not enough. A line of geophones is not enough either. Imagine your eye had a retina with just one light sensing cell, or even a linear array of rods and cones. That still would not be enough. You need a retina with an areal array of sensors.
So, if the ceiling was covered with geophones, from those sounds, I could tell you the shape of this room and everything in it. I could even tell you if somebody moved any pieces of furniture. This is exactly what James Rickett and I showed the readers of The Leading Edge in August 1999 when we told them about the seismology of our sun, Sol.