Application of 2D full-waveform inversion on exploration land data

Abstract

Estimating subsurface seismic properties is an important topic in civil engineering, oil and gas exploration, and global seismology. We have developed an application of 2D elastic waveform inversion with an active-source on-shore data set, as is typically acquired in exploration seismology on land. The maximum offset is limited to 12 km, and the lowest available frequency is 5 Hz. In such a context, surface waves are generally treated as noise and are removed as a part of data processing. In contrast to the conventional approach, our workflow starts by inverting surface waves to constrain shallow parts of the shear wavespeed model. To mitigate cycle skipping, frequency- and offset-continuation approaches are used. To accurately take into account free-surface effects (and irregular topography), a spectral-element-based wave propagation solver is used for forward modeling. To reduce amplitude influences, a normalized crosscorrelation (NC) objective function is used in conjunction with systematic updates of the source wavelet during the inversion process. As the inversion proceeds, body waves are gradually incorporated in the process. At the final stage, surface and body waves are inverted together using the entire offset range over the band between 5 and 15 Hz. The inverted models include high-resolution features in the first 500 m of compressional and shear wavespeeds, with some model updates down to 4.0 km in the first parameter. The inversion results confirmed by well-log information, indicate a better fit of compressional to shear wavespeeds ratios compared with the initial model. The final data fit is also noticeably improved compared to the initial one. Although our results confirm previous studies demonstrating that an NC norm combined with a source time function correction can partly stabilize purely elastic inversions of viscoelastic data, we believe that including an attenuation depth model in the forward simulation gives better results.