A method and apparatus for correlating two time series of digital quantities comprising means for converting one of the time series to a sign bit series and means for algebraically summing values of the second series which occur at times corresponding to the zero crossings of the sign bit series. The algebraic summation includes the modification of the signs of the summed values according to the direction of the zero crossing which occurs in the sign bit series. The preferred embodiment is a field correlator for use in geophysical seismic surveying to monitor data as it is recorded. The field correlator converts a digitized pilot signal into a sign bit only series, which is then correlated to a digitized record of a geophone spread output by summing only those samples of the geophone outputs which correspond to zero crossings of the pilot signal for a given lag value. As in standard correlation, the summation is repeated for each lag of the pilot relative to the detected signal so that an earth spikes trace is generated.