Normal moveout stretch mute on shallow-reflection data
Ruei-Siang Ke
Abstract
These papers demonstrate the potential consequences of overlooking the significance of allowable stretch ratio when performing normal moveout corrections on shallow-reflection data. Extreme velocity gradients occasionally present within near-surface materials can inhibit optimal common midpoint (CMP) stacking of near-surface reflection arrivals. For example, abrupt increases in velocity are observed routinely at the bedrock surface and at the boundary between the vadose and the saturated zone.
Two shallow reflection data sets with drastically different near-surface geologic settings conclusively show the significance of subtle changes in allowable stretch mute. In addition, a example of large velocity gradient, a shallow seismic reflection survey acquired in eastern Minnesota, USA, designed to image targets from about 5 to 50 m experienced the large velocity gradient problem.
A proper normal moveout stretch mute can reduce distortion of reflection wavelet spectra caused by nonvertical incidence recording to less than 10 percent. Stretched reflection wavelets improperly muted can be misinterpreted on CDP stacked sections as stacked refractions or subtle stratigraphic features. In some settings, due to shallow extreme-velocity contrasts, the velocity gradient may not allow corrections for nonvertical incidence to be performed on reflections above and below this surface within the same processing pass. One solution to the problem is to use segregated processing in which the long-offset and shortoffset data are processed separately and then recombined after NMO is applied.
References
Miller,R.D., 1992, Normal moveout stretch mute on shallow-reflection data, Geophysics, 57, 1502–1507.
Richard D. Miller and Jianghai Xia, 1998, Large near-surface velocity gradients on shallow seismic reflection data, Geophysics, 63, 1348–1356