Engineering Geophysical Study of Unconsolidated Top Soil Using Shallow Seismic Refraction and Electrical Resistivity Techniques

Abstract

A near-surface geophysical study was conducted at University of Peshawar (UOP), Khyber Pakhtunkhwa, Pakistan, using an integrated approach including seismic refraction and electrical resistivity survey (ERS) techniques in order  to image the shallow subsurface in terms of main geological and geophysical properties covering the study area. Seismic longitudinal wave velocities (Vp) were determined within four meters beneath ground surface which indirectly provided us with critical subsurface information about depth of layers, morphology and stratigraphic sequence without borehole information. The results of the seismic refraction survey along profile AB, showed two-layers separated by a refractor having gentle slope and P-wave velocity values (223m/sec & 316 m/sec) for overlying and underlying layers respectively indicating loose soil filled in top four meters underneath the surveyed seismic profile. Seismic refraction data demonstrated shallow subsurface structure characterized by longitudinal wave velocities less than 330 m/s.Apparant resistivity data was acquired along two profiles  (CD & DE) using four shlumberger vertical electrical soundings with maximum spread length of 10m. Electrical resistivity survey validated the results obtained from seismic refraction data analysis by detecting bi-layer near-surface geologic model at all VES stations with distinct characteristics. These geoelectric layers included top soil/dry unconsolidated surface material ranging in thickness from 1.11m to 1.3m with true resistivity values (38.08 - 52.70 ?m) and less resistive (13.13-18.38 ?m) clayey layer. Integrated geophysical approach showed that overburden terrain in the target zone is characterized by a relatively thin superficial layer (dry unconsolidated sediments) underlain by a clay layer having high porosity and saturation. Based on seismic velocities (223m/sec & 316 m/sec) and resistivity values (13.13 - 52.70 ?m), it is derived that sub surface soil conditions within studied depth interval are poor and should be considered seriously as this may put the high rise buildings at risk.

Keywords: Shallow seismic refraction, Compressional wave velocity (Vp), time-term inversion, electrical resistivity survey (ERS), top soil.