A Polar Coordinate Approach to Identify and Remove Higher Mode Rayleigh Waves

Abstract:

We present an approach to isolate and separate higher mode Rayleigh wave signals using active source multicomponent seismic data. Our approach allows for improved subsurface shear wave velocity estimates compared to established single component, multi-channel (MASW) methods. We show that the phase velocity vs. frequency relationship of the fundamental Rayleigh wave mode can become contaminated when higher mode Rayleigh waves interfere with the fundamental mode dispersion. Under many geological models, we observe higher mode contamination and these higher velocity modes can lead to low relative coherence along the fundamental mode dispersion path or an overestimation of shear wave velocities with depth. For a typical range of frequencies utilized in active source surface wave analysis (5-100 Hz), the fundamental mode propagates in retrograde motion at the surface. For many earth models, higher mode Rayleigh waves can propagate in prograde motion. By utilizing vertical and horizontal inline seismic components, we can measure particle motion direction and selectively remove the prograde higher mode Rayleigh wave signals via our polar mute approach. We show with numerical models and field results that by removing these higher modes, we can better isolate the fundamental Rayleigh wave dispersion to improve our confidence of shear wave velocity estimates with depth compared to a single channel approach.