NS 2.6 Near-surface velocity gradients and their effect on shallow reflection data

Near-surface velocity gradients can be large enough to inhibit normal moveout corrections on a single pass of a complete shallow velocity function during routine shallow seismic reflection processing flows. Stretch artifacts that are the result of inversion and compression within the highest rate of velocity change portion of the shot gather can stack coherently on CMP sections. The constant velocity nature of a single shallow, low velocity reflection wavelet when corrected with a time varying velocity function can result in dramatic distortion of the waveform. Structural variations in the top of the high velocity surface can be sufficiently undersampled spatially even with 2 ft receiver intervals to result in moveout curves on shot gathers with sufficient non-symmetry to inhibit accurate correction to vertical incidence.