Near surface cavities mapping using integrated geophysical techniques: Case study of Gadwalian dam, North Pakistan

Integrated geophysical techniques have always been useful in identifying subsurface features. In the present study, three integrated geophysical methods of ground-penetrating radar (GPR), resistivity and Multi-channel Analysis (MASW) of surface waves have been utilized for near surface studies and identification of subsurface cavities near the Gadwalian dam. Acquisition of four inline and crossline profiles through GPR (100 MHz shielded antenna), two resistivity profiles through Imaging system and six seismic refraction profiles (MASW) have been made on the selected site having potential subsurface cavities. The processing and interpretation of GPR data through different software's exhibit variations in amplitude/diffraction patterns and several cavities have been identified in the GPR profiles. Sweetness attributes applied on the GPR data set also validate the cavities and unsaturated/saturated fractures. Slate beds and cavities have also been identified on pseudosections obtained through resistivity imaging. Further studies on 2D shear velocity (Vs30) profiles using MASW analysis for interpretation suggested that the absence of low velocity layer in shallow surface and velocity increase with respect to depth.