Near-surface seismic refraction anomalies due to underground target models and the application in civil and environmental engineering

Identifying the precise depth and location of shallow underground utilities in civil and environmental engineering through geophysical means is quite challenging due to near-surface and cultural noise. This study investigates the influence of buried targets on seismic refraction tomography (SRT) measurements at a geophysical experimental site, aiming to evaluate the accuracy and reliability of SRT data. A pre-burial study was performed, resulting in seismic velocity distributions between 200 m/s and 800 m/s for the unsaturated upper layer at depth of 0.0 m–3.0 m, 800 m/s to 1300 m/s for the middle layer at depth of 0.5 m–2.5 m, and 1300 m/s to 2400 m/s for the lower layer at depth of 1.1 m–6.0 m, unveiling a complex stratigraphy that holds valuable insights for engineering endeavours. Parallel seismic layers observed in the pre-study were attributed to a short distance profile (40 m). The pre-burial and post-burial surveys showed consistent layer velocities and thicknesses. The position of larger modelled targets such as drums and clustered plastic buckets indicates significant distortion with depressed/projected displacements, suggesting SRT anomalies, which spatially coincide with the positions of the target buried. However, some of the buried targets have not yet been detected by the SRT technique, which may be due to several factors. The suspected seismic refraction anomalies due to the non-metallic and metallic buried targets tend to generate downward and upward curve nature anomalies, respectively. In addition, the highest displacement resulting from the refraction of seismic waves at various depths appears to be a combination of shear wave and body wave overlapped. Based on the sizes of the buried modelled targets, the study recommends a 0.50 m geophone spacing for investigating very shallow underground utilities in civil and environmental engineering using 2D SRT.