It was recently asserted to me that we have no idea what Planet Earth really looks like below our feet. I responded that (mainly oil-industry) seismic data covers a vast area and so actually we have a fairly good idea. The response was that this seismic coverage is very localised in particular areas.
Some statistics would be useful in this discussion but I'm unable to find any definitive sources.
To be specific, lets assume we are interested in the top 3 kilometers of the crust, and that a minimum 2D seismic spacing of 2 km would be required.
On that basis, what percentage of the earth's surface is covered by reflection seismic data?