What percentage of the earth's surface is covered by 2D or 3D reflection seismic data?
Perhaps an easier question would be - what percentage of the earth has discovered oil/gas fields! This data should be easier to find than the surveys themselves, and can probably give you a rough estimate. 

For an idea of the publically available seismic surveys conducted around western Australia, this site (https://wapims.dmp.wa.gov.au/wapims) can show you all of their stored survey data. Not sure where you could get other data though, sorry!