Propagator and wave-equation inversion for near-receiver material properties

Near-receiver material properties are required for the separation of the recorded wavefield into upgoing and downgoing P and S waves, and are also important for static time-shift corrections. However, it is difficult, especially in land seismics, to obtain reliable estimates for these local material properties using conventional techniques. We compare three methods for estimating these material properties using a 3-D geophone configuration. The first two methods are based on inversion of the wave equation and can be used on almost all of the recorded wavefield. However, they require that the wavefield is recorded by a dense 3-D receiver group to allow the computation of either spatial wavefield derivatives or interpolants. The third approach is based on the inversion of the vertical wavefield propagator. We present a procedure for estimating this propagator using only two multicomponent geophones, one buried and one positioned at the surface. Propagator estimation and inversion avoids the explicit computation of wavefield derivatives, and is therefore less sensitive to measurement errors than both wave-equation inversion schemes. However, in the form presented it requires the identification of arrivals of incoming waves that are isolated in time, and can only be applied to such data. Noise tests demonstrate that the propagator inversion provides accurate estimates for P- and S-wave velocities of a near-surface low-velocity layer, and is robust with respect to signal-generated near-surface reverberations. In case of a near-surface velocity gradient, velocities are obtained which are consistent with effective medium velocities.