Elastic Waveform Inversion with Modified Surface Boundary Conditions for Land Seismic Data

We present an elastic waveform inversion for low-frequency wide-aperture land seismic data. The presence of wavelength-sized contrasted layers limits the applicability of acoustic inversion. This happens above about 3 Hz when a few hundred meters thick carbonate layers interleave with shale/sand layers, as seen in the Middle East. Elastic inversion is challenging because of computational cost and, in land, because of ground roll. To retrieve the long-to-intermediate wavelengths of the P-velocity, we invert the weak diving waves and remove ground roll in the pre-processed data. We modify the surface boundary conditions and obtain an elastic modelling without surface waves by zeroing the normal derivatives of the shear stresses at the surface. An analysis of this modification shows that the reflection coefficients become independent of the velocity ratio and the PP-reflection coefficient is similar to the acoustic one. This modification allows us to develop approximate elastic modelling of diving/transmitted waves without implementing ground-roll suppression for the modelled data. Moreover, it allows us to use large velocity ratios to speed-up the inversion without degrading the resulting P-velocity model. This approach can be seen as intermediate between acoustic and elastic inversion with free-surface conditions. We illustrate it with synthetic and real data examples.