Practical guidelines for near-field mitigation on array-based active surface wave testing

Abstract

SUMMARY This paper aims to develop practical guidelines for near-field mitigation for active source surface wave testing. To this end, extensive field measurements were performed considering different factors, including depth to impedance contrast, source offset, source type, surface wave type (Rayleigh or Love) and transformation technique used for data processing. According to the results, near-field effects are independent of surface wave type and depth to impedance contrast. For sites with a very shallow impedance contrast, the cylindrical frequency domain beamformer transformation technique outperforms others in terms of dispersion resolution by significantly mitigating near-field effects. On the other hand, for sites with a very deep impedance contrast, the four transformation techniques provide the similar dispersion resolution when only considering near-field effects. It is also revealed that the normalized array centre distance (NACD) criteria required to mitigate near-field effects is a function of source type. Using 10–15 per cent as the accepted error boundary, a NACD of 1.0 or greater is recommended for low-output impulsive sources such as a sledgehammer source, whereas, for high-output harmonic sources such as a vibroseis, a normalized array centre distance of 0.5 is recommended. A standard deviation on the NACD of approximately 0.3 was observed in much of the data indicating near-field effects are site-specific and can affect the surface dispersion velocity at wavelengths longer or shorter than the recommended NACD. These NACD criteria should not be violated when using a limited number of source offsets (1 or 2). However, if the multiple source offset approach (≥3 source offsets) is used where some of the source offsets meet the criteria, the near-field criteria can be violated for some of the source offsets, given that the near-field effects can be mitigated using the composite dispersion data generated from different source offsets if those offsets produce acceptable dispersion data.