Near-offset effects on Rayleigh-wave dispersion measurements: Physical modeling

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.