Subsurface Profiling Using Roadside MASW Survey: Influence of Multiple Sources and Offline Distance

Roadside MASW survey utilizes traffic-generated surface wave signals for subsurface characterization and, thus, can be a useful geophysical method, especially in urban areas. However, such signals originating from vehicular movements over road surface irregularities, or sources, produce complex field records of multi-source and multi-azimuthal characteristics. Such sources are termed intra-line if they exist within the receiver spread and outer-line when they exist outside the receiver spread. In a roadside survey, the receiver spread is placed outside and parallel to the centreline of the road, thereby creating an offline distance with respect to the sources on the road. In this study, experimental investigations are conducted to determine the influence of the presence of intra-line or outer-line sources and offline distances of source positioning on the dispersion imaging of roadside MASW records. Artificial hurdles were placed deliberately at different positions on an adjacent road to determine the influence of intra-line and outer-line sources. Furthermore, receiver arrays were placed at varying offline distances with respect to the centreline of the adjacent road to assess the effect of offline distance on the dispersion imaging and subsequent shear wave velocity profile. The study finds that the dispersion images obtained from the intra-line source have better resolutions compared to those obtained from the outer-line source. Further, the presence of multiple sources during the data acquisition does not necessarily shed any detrimental influence on dispersion imaging as long as there is no contamination and mutual interferences of the raw wavefield records. As the offline distance increases, the intensity of the traffic-generated source signal diminishes. It is observed typically for the studied site that beyond an offline distance of 15 m, there remains no recognizable energy to obtain a distinct dispersion image. A comparative study of the shear wave velocity profiles obtained from a borehole, roadside, active, and passive remote MASW surveys revealed an agreeable match, thereby indicating the usability of the roadside MASW survey, especially when offline distance is not enormously large.