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Resistivity methods mainly electrical resistivity tomography 2D and resistivity sounding 1D represent the main geophysical tools to delineate seawater intrusion into freshwater aquifers. Thirty 2D imaging sections were applied with Wenner-Schlumberger array (NW-SE direction), distributed on six profiles (perpendicular to the coast) in southern part of the coastal aquifer in Gaza Strip (Rafah Governorates), supported by 4 Vertical electrical soundings points with Schlumberger array, to map shallow-depth seawater intrusion into freshwater aquifer and study the subsurface geologic formation. Analysis and interpretation of the electrical resistivity tomography 2D and 1D sounding results reveal that seawater intrusion zone appears at the western part of the study area and extends along Rafah coast with a difference distance and depths. The depths are ranging between (few meters to 20 m) in the western sections, while they are betwee 40 to more than 65 m in the eastern section. The resistivity data results were correlates well with the 3 boreholes data and 3 chemical analyses water samples.

In this study quantitative interpretation of some 14 Vertical Electrical Sounding (VES) points using the Schlumberger array and data from 10 boreholes were applied to study the aquifer geoelectrical/hydrogeological parameters in the southern governorates of Gaza Strip. The quantitative interpretation of (VES) results analyzed in order to find empirical relations between different aquifer parameters. The results represent the predicted hydraulic conductivity (K), transmissivity (Tr), TDS, earth resistivity, water resistivity, porosity and Dar-Zarrouk parameters (Longitudinal conductance and Transverse resistance). The results show the earth resistivity is strongly affected by the chemistry and salinity of the groundwater. The relation between longitudinal resistivity (ρL) and groundwater salinity (TDS) was achieved for groundwater aquifer. The geoelectric (Dar-Zarrouk) parameters determined and interpreted by using the integration of the results, which indicates that longitudinal conductance has a high values at the western part of the study area represents an increase in groundwater movement horizontally, that is a strong evidence of seawater intrusion. Porosity was estimated to be in the range of 38–52% with an average value of 46%. Transmissivity values for the study area were calculated. This result helps to delineate the contact between the saline and fresh groundwater aquifer in the study area.

Integrated geophysical, hydrogeological and geochemical investigations employing DC resistivity tomography and resistivity sounding were carried out in Southern part of the coastal aquifer in Gaza Strip (Khan Younis and Rafah Governorates) to investigate subsurface geologic formation and delineation seawater intrusion into freshwater aquifers in the coastal environment of the study area. Due to the lack of geophysical studies in the Gaza Strip for seawater intrusion, hence, this study aims to apply hydrogeophysical investigation, and the work focuses to develop an approach to study seawater intrusion and its effects on groundwater quality, and that contributed to map the subsurface salinity distribution. Fourteen (14) Vertical Electrical Sounding (VES) measuring points (Schlumberger array) were carried out using the instrument Syscal Junior R1, in order to determine the number of the underlying layers, depths and their thicknesses with the total length of the expanding electrodes with maximum (AB/2) is equal to (200m). The field curves are interpreted automatically by using the (IPI2win) program. Eighty three (83) 2D Resistivity spreads was carried out using Wenner-Schlumberger array with a NW-SE direction, distributed on seventeen (17) profiles, and the numbers of the employed electrodes were (24) and the spacing between them is (5m). The total length of survey line is equal to (125m), and the profile expand range from (0.8- 1 km) from west to east. The measurements of 2D imaging survey were interpreted by using (Res2dinv) program. The resistivity data were consistent and showed good correlation with 12 borehole lithologic cross sections, (10) agriculture wells for chemical analysis and previous geophysical studies, proving the efficiency of combining two methods in solving environmental problems. Quantitative and qualitative interpretation was applied to study the type of the electrical field curves, which were classified to KQ, QQ, and Q in the investigated area. Moreover, two cross sections H1 and H2, where H1 extends along the western side of the study area and close to the coastal line, and cross section H2 was selected to parallel cross section H1with 400m east of H1. In general, resistivity measurements showed that the top soil at H1 about (300 Ωm) are characterized with electrical resistivities lower than that of H2 section about (1000 Ωm). This may be due to soil type and water salinity. The second layer (sub aquifer) at H1 geoelectrical section is characterized by resistivities lower than that of H2 section, because it is near coast line. The fresh water aquifer almost is absent in H1 section. All the aquifer layers are saturated with saline water except the upper most layers. Seawater intrusion appeared at all the section at few meter depths in Rafah to about 30 m depth in Khan Younis, but fresh aquifer layers in H2 cross section is characterized by medium resistivity values and much layer thickness. This means that the aquifer is still have fresh water. Either two resistivity profiles (A1, A2) were constructed, with N-S trending and parallel to each other with distance about 400 m. In the first profile, The model shows that seawater intrusion (> 2.8 Ωm) in the aquifer mapped at shallow depths (3m to more 6m) in different sections near coast line. But in the second profile, the geoelectrical image shows that seawater intrusion cannot observed eastwards the first sections by the present 2D configuration. This is due to the limited depth of penetration because of the limited user cable length. The inverse models of 2D resistivity imaging and 1D sounding showed decreasing in resistivity values with the depth due to the salinity of groundwater. According to geochemical analysis for water samples, it revealed that maximum values of EC, TDS and CL is higher in the western part of the study area (9370μs/cm, 5829mg/l and 2900 mg/l) respectively and decreases in the east. Empirical relations between different aquifer parameters TDS, earth resistivity, and water resistivity were achieved, that show the earth resistivity is strongly affected by the chemistry and salinity of the groundwater. The geoelectric (Dar-Zarrouk) parameters determined and interpreted by using the integration of the results, which indicates that longitudinal conductance has a high values that shows an increase to the western part of the study area which reflects the validity of the electrical current in parallel to the layers, as this represents an increase in groundwater movement horizontally, that is a strong evidence of seawater intrusion. By using Archie's law, the porosity was estimated to be in the range of 0.38-0.52% with an average value of 0.46%. Finally, a map with the possibility of salt/brackish and fresh water interface was generated, which showed that the area suffers from acute seawater intrusion and could be aggravated if there is continuous groundwater abstraction, and subsurface structural geological map of the studied area deduced from electrical resistivity survey.

Vertical Electrical Sounding (VES) proved to be an important tool for mapping the freshwater-saltwater interface. Seawater intrusion is serious problem especially in coastal areas. 14 Vertical electrical soundings with Schlumberger array were carried out (perpendicular to the coast) in southern part of the coastal aquifer in Gaza Strip (Khan Younis and Rafah Governorates) to study the subsurface geologic formation and delineation of seawater intrusion into freshwater aquifer. Lithology and hydrochemistry data from 12 bore holes were used to integrate the results. The results reveals that 50% of the total sounding are interpreted of 3 layer modes, while the other 50% are of 4 layers model. Interpreted 2D geoelectrical models (in two cross sections parallel to the coast line) showed that seawater intrusion with electrical resistivity less than 2Ωm is dominant along the section parallel to coast line at 400m east of it and for shallow depths (3-10m depth), while in the other section (1000m) east of the coast line seawater intrusion is detected in some places while the other parts still have brackish/fresh groundwater with resistivity more than 10Ωm at different depths (26-50m). ملخص: اثبتت طريقة المقاومة الكهربائية العمودية (VES) فعاليتها فى تحديد الحدود الفاصلة بين المياه العذبة و المالحة . تعتبر مشكلة تداخل المياه المالحة فى مياه الخزان الجوفى العذب مشكلة ذات اهمية و خاصة فى المناطق الشاطئية. لقد تم تنفيذ 14 مجسة كهربائية عمودية بأستخدام تشكيل شلمبرجير بشكل قطاعات عمودية على على الساحل الجنوبى لقطاع غزة (محافظات خانيونس و رفح) و ذلك بهدف دراسة التركيب الجيولوجى للخزان الجوفى و تحديد مدى تداخل مياه البحر فيه . كما تم الاستعانة بالمعلومات الليثولوجية و الهيدروكيميائية من 12 بئرمياه فى مناطق الدراسة. اظهرت النتائج ان 50% من منحنيات المقاومة الكهربائية المقاسة تمثل نموذج لثلاث طبقات جيولوجية بينما 50% تمثل اربع طبقات. تم اختيار قطاعين متوازيين و موازين لشاطىْ البحر, يبعد الاول 400 م عن الشاطىْ بينما الاخر يبعد 1000م عنه لدراسة تغيرات المقاومة فى بعدين. أظهرت النتائج ان مياه الحر ذات المقاومة الكهربائية أقل من 2 أوم.متر تسود على طول القطاع القريب من الشاطىْ و لاعماق ضحلة (3-10 م), بينما فى القطاع الثانى و الذى يبعد 1000م شرقا تم تحديد أمكن تداخل مياه البحر المالحة فى بعض الاماكن و لم يتم تحديدها فى أماكن أخرى مما يفسر على انه لا زال تلك المناطق تحتوى على مياه عذبة/ مالحة ذات مقاومة كهربائية اكبر من 10 أوم.متر و على أعماق تتراوح بين 26-50 متر.