Application of Geophysical Survey to Study the Rising of Ground Water Level (A Case Study of Beiji Refinery

Abstract

At Beiji Refinery the level of ground water raised from 13m below ground surface to less than 1m within fifteen years. Number of boreholes was drilled to monitor the level and movement of ground water. Geophysical techniques (Seismic refraction and resistivity) have been used to detect subsurface cavities and channels in the refinery. It has been found number of subsurface cavities and holes most of them related to the presence of manholes and subsurface pipes and cables. The cavities are shallow (less than 5m deep) and had been formed due to the leakage water from these manholes and pipes which lead subsequently to the leaching of soil that is contain secondary gypsum. The most important result, however, which is not known before and lead to the clarification of ground water movement in the refinery is that the refinery had been built over a subsurface channel (old meander of Tigris river). This channel had been filled with river sediments (Pebbles, Sand, silt and clay). These sediments are presents as lentils. The direction of the subsurface channel is from east towards the west bending southwards, where the refinery had been built at the bending area where the thickness of the river sediments in this part is over 20m. It has been found that the water leakage from manholes and pipes are accumulating in the bending area where it forms as subsurface lake since it is the lowest part in the channel. From this area the water discharges to the southern refinery. The subsurface channel has been followed outside the refinery where it has been observed number of springs, cross-bedding and imbricate structures. An advice has been forward to the refinery to dig channels in the spring’s area to help discharging subsurface water.