EFFECTS OF SCATTERED SURFACE WAVES FOR IMAGING THE NEAR SUBSURFACE

Abstract

Scattering by surface waves is a topic that has received considerable attention in exploration seismology due mostly to its negative impact when processing seismic reflection data. In near surface studies a better understanding of the problem can potentially lead to methods for mapping shallow obstacles and mapping heterogeneities, fact that has been acknowledged in a number of studies and publications (e.g., Riyanti et al. 2005; Luke and Calderón-Macías, 2008). In the context of geotechnical engineering, surface wave scattering finds applicability in problems such as mapping cemented lenses, faults, fissures, tunnels and caves. From the perspective of seismic imaging, it is of interest to study and understand the differences and/or similarities between imaging body wave diffractions (e.g., Landa and Keydar, 1998) and imaging scattered surface waves (e.g., Riyanti et al., 2005). When applying migration of P- or PS-waves (incident P waves converted to S waves upon reflection) to map reflectors in which surface waves haven't been attenuated, because surface waves are recorded at early (small source-receiver offsets) and late (larger source-receiver offsets) arrival times, the surface waves obscure shallow and deep reflectors. Imaging with a velocity that approximates the shear wave velocity from the very near surface approximately moves the surface wave energy at or near the source location in space (this concept has been used for ground roll attenuation in reflection data processing). Strong lateral heterogeneities that are shallow or accentuated topographic lows and highs can have an important amplitude and phase effect on the direct surface wave. These “distortions” in the wavefield have the potential for detection of lateral changes in the near surface. The present work focuses on studying this phenomena by proposing a model of the scattered surface waves and studying its effects for imaging purposes. Applications of these concepts to real data are also discussed.