The estimation of shear-wave statics using in situ measurements in marine near-surface sediments
Gavin Winsborrow, Dei G. Huws and Everhard Muyzert
Journal name: Geophysical Prospecting
Issue: Vol 53, No 4, July 2005 pp. 557 - 577
Info: Article, PDF ( 688.45Kb )
Shear-wave statics in marine seismic exploration data are routinely too large to be estimated using conventional techniques. Near-surface unconsolidated sediments are often characterized by low values of Vs and steep velocity gradients. Minor variations in sediment properties at these depths correspond to variations in the shear-wave velocity and will produce significant static shifts. It is suggested that a significant proportion of the shear-wave statics solution can be estimated by performing a separate high-resolution survey to target near-surface unconsolidated sediments. Love-wave, shear-wave refraction and geotechnical measurements were individually used to form high-resolution near-surface shear-wave velocity models to estimate the shear-wave statics for a designated survey line. Comparisons with predicted statics revealed that shear-wave statics could not be estimated using a velocity model predicted by substituting geotechnical measurements into empirical relationships. Empirical relationships represent a vast simplification of the factors that control Vs and are therefore not sufficiently sensitive to estimate shear-wave statics. Refraction measurements are potentially sensitive to short-wavelength variations in sediment properties when combined with accurate navigational data. Statics estimated from Love-wave data are less sensitive, and sometimes smoothed in appearance, since interpreted velocity values represent an average both laterally and vertically over the receiver array and the frequency–depth sensitivity range, respectively.
For the survey site, statics estimated from near-surface irregularities using shear-wave refraction measurements represent almost half the total statics solution. More often, this proportion will be greater when bedrock relief is less.