WHERE IS ZERO TIME?*

Mis‐ties are all‐too‐common results of seismic surveys made at the same place but at different times with different equipment or by different organizations. Even after removal of positioning or polarity errors, reflection times often appear to differ by several tens of milliseconds. Zero time appears to fluctuate.

How can zero time differ on surveys with only minor differences in acquisition or processing? What can be done to identify the true zero time for each survey?

The first step toward establishing zero time is to record the source pulse. It is well‐known that the different sources currently used in reflection seismic prospecting (propane‐oxygen explosions, compressed‐air discharges, explosives, steam bubbles, mechanical implosions, vibrations, etc.) yield different pressure wavefronts as the input to the seismic reflection system. By recording this wavefront we capture the basic pulse shape and we establish the initial time delay.

The second step is to process the recorded source pulse as if it were reflection data to establish the additional time and shape changes introduced by data processing. Then, display the recorded and processed source pulse as an auxiliary variable at the ends of the seismic section. From this display the interpreter can systematically establish the time shifts appropriate to each picked event. He can determine also whether the pick should be a peak or a trough. He can see why surveys which appear to tie for shallow reflections appear to mis‐tie for deep reflections.

The display of the processed source pulse constitutes a major interpretation aid which, in a readily useable form, increases the information content of the basic seismic section.