Magnetic and magnetotelluric data integration to determine the origin of Siwa Oasis Lakes, Western Desert, Egypt.

Abstract

Geological structures are the most critical parameter for understanding the distribution of groundwater, oil, and minerals. In Siwa Oasis and other oases with comparable geological settings in the northern Western Desert, Egypt, brackish groundwater seeps to the surface through fractures and permeable zones, sustaining phreatophytic vegetation and enabling human habitation in otherwise arid environments. The formation of these oases is closely tied to the subsurface structural and hydrogeological conditions. To determine the origin of these lakes, a geophysical investigation was conducted using magnetic and magnetotelluric (MT) data to identify whether the water leakage stems from deeper artesian aquifers or shallower marine-origin reserves. Magnetic data were used for the geological structural analysis and the depth estimation of basement rocks. The Magnetic Data Analysis includes constructing the Total Horizontal Gradient (THG), Tilt Derivative (TDR), and 2D Magnetic Modelling Constrained by borehole data to estimate basement depths. Magnetotelluric (MT) data analysis includes obtaining 3d resistivity models via inversion, and Cross-sections were extracted to trace fault zones and lithological contrasts. The Key Findings from this study are Surface Lineaments that are dominant in the Northeast (NE), North (N), and East-Northeast (ENE), with a minor East-West (E-W) trend. Shallow Trend Analysis (Tilt Derivative-TDR): Revealed structures in NE-SW, NW-SE, and E-W directions. Total Horizontal Gradient (THG) Map: Confirmed major structural trends in NE-SW, NW-SE, and E-W directions, with additional W-NW trends. The E-W trend correlates with most lakes in the study area. Depth estimation (2D magnetic modelling): Basement rock depths range between 3400 and 4600 m in the magnetic data coverage. The magnetotelluric (MT) model reveals a near-surface E-W direction with resistivity variations linked to lithology changes and groundwater presence.