Analyzing and Filtering Surface-Wave Energy By Muting Shot Gathers

Abstract

Accurate estimation of the fundamental-mode dispersion curve is the most critical processing step of many shallow surface-wave methods. Use of multichannel analysis of surface waves (MASW), has proven very effective in separating different dispersive events that share the same seismic-frequency range. Yet, under certain circumstances, even when using such a data-redundant method, it may not be possible to separate the fundamental mode of the surface waves from higher modes. Dominant higher surface-wave modes, together with body- and guided waves, can impede the estimation of the fundamental mode. This is especially true when relatively short spread lengths are required for the survey for reasons such as higher lateral-resolution demands or presence of noise at the far offsets. A simple multichannel processing technique that mutes the interfering seismic waves in the shot records (offset-time [Formula: see text] domain) can be used to analyze and filter noisy surface-wave modes and thus significantly improve the range and resolution of multimodal dispersion curves in the phase-velocity-frequency domain. This is demonstrated on both synthetic and real shot gathers. One shortcoming of the muting method is the estimation of artificially high phase-velocity values at low frequencies. This artifact can be countered by employing dispersion-curve estimation in the same low-frequency range using the unmuted shot records. The proposed muting technique can be beneficial not only for the evaluation of the fundamental mode of the Rayleigh wave, but also for other types of dispersive seismic energy, such as higher Rayleigh-wave modes, Lamb, guided, and Love waves.