Harnessing harmonics for imaging thin shallow reflectors

Christopher B. Harrison, Gary F. Margrave, Michael P. Lamoureux, Arthur Siewert, Andrew Barrett, J. Helen Isaac

Harmonics have traditionally been treated as noise to be attenuated out of the vibrator seismic data. Much time and energy have been dedicated to the removal of these harmonics in all stages of seismic delving, from the acquisition phase, to data processing and even vibrator engineering. This desire to remove the frequencies associated with higher order harmonics comes from the correlation process which causes ghost forerunners or tails at positive and negative correlation times (depending on type of sweep) if harmonically "contaminated" sweeps are used as correlation operators. However, these higher frequencies, if sampled properly and harmonics extracted precisely, prove to be highly useful as correlation operators to image thin near surface reflectors. In this paper we utilize harmonics decomposed from baseplate recorded sweeps and ground force as correlation operators. The results indicate that the baseplate harmonics reveal thinner near surface reflectors while the ground force reveals more numerous coherent near surface reflections.