Proceeding of the 8 th Conf. Geology of Sinai for Development, Ismailia, 2007, pp.29-39 Delineation of Subsurface Structures in the area of a Hot Spring, Central Sinai, Egypt based on Magnetotelluric and Magnetic Data M. Mekkawi, M. Elbohoty and E. Aboud National Research Institute of Astronomy and Geophysics (NRIAG), 11722 Helwan, Cairo, Egypt. ABSTRACT An intensive geophysical study has been performed for hot spring, central Sinai, in order to illustrate the role of magnetic and magnetotelluric interpretation in the detection of the major subsurface structural elements affecting both the sedimentary section and the underlying basement complex. More attention was given to the hot spring to spot more light on the subsurface structural features that control the occurrence of this spring. For that, five magnetotelluric (MT) stations have been carried out with about 5.0 km distance. Magnetotelluric components (Hx, Hy, Hz, Ex and Ey) have been recorded simultaneously. According to the magnetotelluric results, the area is tectonically active and there is a conductive anomaly situated just beneath the hot spring at a depth of 2.0 km. The modeling revealed that, a possible connection between this anomaly and the conductive sediments, which can be considered as the source of the phenomena. A detailed land magnetic survey was carried out along several profiles crossing the hot spring. The necessary reduction concerning the daily variation, the regional gradient and the time variation were applied, and then the total intensity anomaly map was constructed. This step was followed by application of the filtered and 3-D Euler deconvolution techniques. The results of magnetic interpretation revealed that, the hot spring is tectonically controlled by faulting having a major NW-SE alignment and extends to a depth of about 2.0 km. The strong magnetic anomalies surrounding the hot spring can be attributed to the occurrence of subsurface basic intrusion of high magnetic contents. Also the study revealed that, the area has been affected by two main tectonic trends having the directions NE-SW and NW-SE. The depth estimation to the basement complex indicated that, it ranges between 1.0 and 2.0 km, and volcanic basaltic intrusions & dykes may occur between 400 and 600 m. 1. INTRODUCTION The study area is located in the Gulf of Suez region which represents one of the most prolific and prospective oil provinces in Egypt. It is located in the central part of Sinai Peninsula, between latitudes 29°25΄&29°38΄N and longitudes 33° 11΄& 33° 22΄E (Fig. 1). The area includes a number of oil fields such as Assel oil field. The geothermal activity observed in the area is a result of the presence of a hot spring (Abouswira), which is located at latitude 29°33΄05΄ N and longitude 33°15΄14΄E. The temperature of the hot water and vapor reaches more than 120 C. The conducting formations should be treated as the main source of the electrical conductivity. To spot more light on the hot spring, five magnetotelluric soundings (MTS) have been carried out crossing this spring in which the interval between the two points was 1 km. The total intensity magnetic data was also acquired by a land magnetic survey (Fig.3). The main target of this study is to analyze the magnetic and magnetotelluric data in the light of the geological information, in order to define the significant tectonic patterns that are responsible for the structural development of its geological units. To achieve this goal, various methods and techniques of analyses were applied to interpret the acquire data of the study area. 2. GEOLOGICAL BACKGROUND The study area is a triangular excavation which represents the synclinal trough to the southwest of El- Halal anticline massif. The head of this triangle is directed northward while its base measures a length of about 25 km facing Gebel Kharim in the south (Fig. 2). The length of this plain attains about 50 km. Its ground elevation decreases northward from 300 m at its base to about 190 m at its apex. The surface of these plains is dotted by numerous mesas and buttes, formed of early Eocene limestone with cherty bands. These surfaces are dominated by washed, dark angular to sub-angular cherty and limestone fragments (Garfunkel and Bartov, 1977). Locally, these plains are covered by active sand dunes, which form several lines of isolated branches. The area is controlled by three different tectonic provinces related to northern part of the African plate (Moustafa, 1993). These are the northern Egypt fold belt, NE-SW (Syrian arc system) and the Suez rift which is younger than north Egypt fold belt. This is dominated by NW to NNW oriented normal faults. Opening the rift started in the early Miocene map. The area shows surface deformation related to the opening of the Suez rift. This area has been dissected by E-W and NW-SE oriented faults (Raid, 1977, Abd-Allah, 1993 and Khalil and Moustafa, 1994).