Near-surface characterisation of weathered layer-using application of rock physics modelling in seismic measurement

Deep subsurface weathering has traditionally been measured by quantifying variations in weathered material properties through cores and boreholes. However, cores collected from the subsurface necessarily sample a minimal fraction of the subsurface. On the other hand, geophysical surveys can be used to obtain a representative perspective of the subsurface. To better understand the near-surface weathered layers, the paper presents an application of rock physics modelling with the help of field-based seismic refraction measurement. We acquired seismic refraction data in hillslope using 4.5 Hz natural frequency geophone with 5-meter geophone spacing. We estimated the subsurface compressional (P) wave velocity (VP) structure and mapped the seismic profile. The profile demonstrated saprolite thickness varying from 1 to 2 meters. To better identify the weathered layers (saprolite) in this section, we predicted porosity using the velocity-porosity relationship model. The porosity estimated through the model was validated by the porosity of the sample extracted from boreholes. This study shows how low-cost techniques can provide useful information about near-surface complexity. Therefore, from this study, it was concluded that proper identification of near-surface layers requires a combination of complementary field-based measurement and precise lab-based rock physics measurement.