Near-offset effects on Rayleigh-wave dispersion measurements : Physical modeling
Résumé
Surface-wave profiling techniques using active sources and linear arrays are often performed with short source receiver distances, compared to the involved wavelengths. Dispersion measurements however are usually performed by assuming body-wave amplitudes to be negligible and the recorded wave-field to be dominated by plane Rayleigh-waves. The estimated dispersion curves may then be corrupted by near-field effects. In this instance, both numerical and physical modeling has helped illustrate such effects,which are typically identified as a systematic underestimation of measured phase velocity at low frequencies. A normalized representation, based on theoretical phase velocities and spread length, has shown the apparent invariability of near-offset effects : the underestimation occurred as soon as the measured wavelength exceeded 50% of the spread length; homogeneous and normally-dispersive media provide the same limitation, regardless of the spread length value.