Geophysical Surveys For Groundwater In The Gaza Strip

The Gaza Strip (Gaza) is an area 40 km long and 8 km wide along the eastern shore of<br>the Mediterranean Sea. The primary sources of groundwater in Gaza are shallow aquifers<br>within the Kurkar formation, a transgressive-regressive sedimentary section consisting<br>primarily of interbedded sands and clays. The Kurkar rests on a marine clay called the Saqiye,<br>which forms the base of the aquifer. The quantity of fresh water that can be withdrawn<br>depends on a number of factors, such as recharge, geology, and hydraulic properties.<br>Moreover, since the aquifer is in contact with seawater, interfaces between fresh and saline<br>water are in dynamic equilibrium, and the distribution of the interface is a critical parameter<br>defining the resource. The water table in Gaza is at an elevation of about sea level.<br>Groundwater characterization studies of the Kurkar aquifer have been an ongoing<br>project in Gaza, funded by USAID. Recently, geophysical surveys were used to better define<br>the shallow stratigraphy and groundwater salinity distribution within the Kurkar aquifer.<br>Various non-intrusive geophysical methods were tested for this purpose, however the best<br>results were derived from an integrated interpretation of time domain electromagnetic<br>soundings (TDEM) and seismic reflection data. The scope of work consisted of acquiring,<br>processing, and interpreting 24.7 km of seismic reflection data, and 84 TDEM soundings. A<br>small amount of Induced Polarization, resistivity and seismic refraction data was also acquired<br>but did not aid significantly in the interpretation.<br>The salinity of ground water within a geologic unit can often be inferred from<br>resistivity, since it is strongly influenced by salt concentration that lowers groundwater<br>resistivity. However, clay horizons also often have low resistivities, and can be misinterpreted<br>as saline water. On the other hand, seismic reflection data images the lithologic boundaries<br>between sand and clay layers, and is insensitive to groundwater salinity. Thus if a lithologic<br>boundary is coincident with a resistivity boundary it is more likely to correspond to a clay layer.<br>On the other hand a low resistivity region which is not defined by a seismic reflector may<br>simply reflect a change in the resistivity of the pore water. Therefore, by combining the two<br>methods it is possible to better interpret the cause of low resistivity values. Several wells close<br>to seismic lines were available with geologic logs allowing the correlation of the geophysical<br>data with the geologic logs.