2D Electrical Resistivity Imaging to delineate the exact boundary and true thickness of granular aquifers

Abstract

Electrical resistivity imaging (ERI) is one of the best geophysical techniques for studying subsurface and groundwater aquifer investigations. In clastic sediments, determining the exact boundary between different layers on the inverse section of ERI is not an easy task due to gradual changes in resistivity between surrounding layers and the granular aquifers. Therefore, the main aim of the study is to indicate a more precise boundary of the granular aquifers (true thickness of aquifers) on the inverse section. Two different models were used: the field model and the laboratory model, in the field model, four different locations were selected. They are located in (Kalar, Chamchamal, Bazyan, and Piramagroon), within the Sulaymaniyah Governorate, NE Iraq. Two-dimensional (2D) electrical resistivity imaging was carried out along four profiles of 355 m length with a five-meter electrode spacing. The Wenner-Schlumberger array was used for recording the data. For the lab model, a laboratory geological model was constructed has length, height, and width equal to 192, 65, and 45 cm, respectively. Two synthetic profiles were carried out on this model, they have lengths of 190 cm with electrode spacing of 10 cm. The field results indicated that aquifers in clastic sediments on inverse sections always showed anomalies with apparent thicknesses larger than their true thicknesses, which were obtained from the geological column of the wells. In the four studied locations, aquifers' average true thickness is approximately 68.2% of the apparent thicknesses on the inverse sections. The result of the laboratory geological model also showed that the true thickness of the unsaturated gravel is approximately equal to 59.7% of the apparent thickness of the anomaly, which is formed by the dry gravel layer. Also, the true thickness of the saturated gravel is about 57.1% of the apparent thickness of the anomaly, which is formed by the saturated gravel layer. Moreover, the geological model showed that the higher resistivity layer (dry gravel) creates a narrower transitional resistivity zone between clay and gravel than saturated gravel of lower resistivity.