Theoretically, the nonlinear oscillation generates ultra-harmonics, sub-harmonics, ultra-sub-harmonics and chaotic oscillation. The field experiment with special specifications and the comparison with simulation study enabled us to confirm that the seismic vibrator source generated nonlinear oscillation including chaotic oscillation. While the upsweep method that is used in conventional surveys can suppress the harmonic ghost due to ultra-harmonics, sub-harmonics and so on remain as source generated noise in the correlated record.The nonlinear characteristic is influenced by the following factor: the non-linearity of the ground, viscosity friction of earth and the hold down weight of vibrator source. In order to decrease the amplitude level of nonlinear source generated noise, an adequate hold down weight should be selected. On the other hand, we can evaluate non-linearity and viscosity friction by analyzing nonlinear oscillation pattern.
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