In land seismic acquisition (both on the surface and in boreholes) a common seismic source is vibroseis: A vibrating mass on a baseplate provides a seismic signal whose frequency is varied slowly with time. For example, the frequency might be swept between 10 and 100 Hz over 8-20 seconds. The two-way travel time to the target (for surface seismics), or one-way time (for borehole seismics) will be much less than this--in exploration typically 1-4 seconds. The duration of the trace transmitted from each geophone must be at least the sweep time plus the travel time. If each trace is to be individually recorded far more data points will be required than for an impulsive source, where the trace duration is the travel time.
Because of this in most circumstances the raw traces are not recorded. The traces are cross-correlated with a trace representing the nominal sweep and (if a number of sweeps have been made from one site or closely spaced sites), these cross-correlations are summed. Only the summed cross-correlations between zero and the travel time are recorded.
It is therefore an object of the invention to provide methods and apparatus for compressing seismic data, in particular seismic data which represents vibroseis traces.