Abstract

Seismic refraction and resistivity imaging methods were used to investigate a Dam site in Southwest Saudi Arabia to delineate the source and pathway of groundwater seepage in the site. The selected methods have the possibility to give an image of the subsurface and map lateral and vertical variations in the subsurface geology of the site. For this purpose, 48-channels seismograph with geophone spacing of 5 m, near-offset distance of 10 m and a total far-offset distance which varies between 230 and 280 m was used. Three seismic lines were conducted and the acquired data were inverted to velocity sections using tomographic inversion. Velocity sections show that the subsurface is classified into four distinct seismic layers; starting with the topmost unconsolidated alluvium that is underlain by a second layer of saturated and/or compacted alluvium sediments. The third layer is associated with fractured basement, while the fourth layer is correlated to the hard massive bedrock with a relatively high velocity. These results indicate that the site is affected by faulting that resulted in two depressions which extends in the form of buried structural channels filled with porous alluvium and fractured greenstone. These depressions are considered zones of permeability and represent a favourable pathway for groundwater flow. These results and the zones of seepage are confirmed and verified using resistivity imaging, where a low resistivity zone of thickness up to 22 m is observed.This low resistivity zone is interpreted as a layer of alluvial sediment saturated with groundwater, which may indicate possible seepage flow.

Key words: Seismic refraction, resistivity imaging, site investigation, Saudi Arabia.