ارزیابی روش‌های ژئوفیزیکی در شناسایی گسل‌های پنهان کواترنری واقع در رسوبات آبرفتی (مطالعه موردی: بوستان چیتگر)

در این تحقیق برای تشخیص گسل های پنهان کواترنری در رسوبات آبرفتی واقع در جنوب غربی بوستان جنگلی چیتگر (منطقه 22 تهران)، از روش های ژئوفیزیکی رادار نفوذی به زمین (gpr) و ژئوالکتریک استفاده شده است. با مطالعه پروفیل (gpr) مناطق دارای گسل شناسایی شدند. نتایج به دست آمده از 2 پروفیل ژئوالکتریک، حاکی از وجود دو زون با مقاومت الکتریکی پایین و مقاومت الکتریکی بالا در کنار یک دیگر است. افت مقاومت الکتریکی در رسوبات منطقه می تواند در ارتباط با عواملی مانند وجود سنگ های هوازده نشده، هوا در داخل رسوبات، دانه ریز بودن رسوبات و با جنگلی بودن منطقه مورد مطالعه به علت آبیاری درختان و نفوذ آب باران در زیرزمین باشد. مقاومت الکتریکی بالا نشان دهنده خردشدگی سنگ ها در اثر فعالیت گسل ها، رسوبات دانه درشت و خشک است. نتایج حاصل از روش های مورد استفاده در این تحقیق نشان دهنده، وجود یک زون گسلی با طول تقریبی 140 متر است. با توجه به روندهای ساختاری اغلب گسل های شناسایی شده، ادامه داشتن امتداد گسل ها به سمت قسمت های بالایی بوستان چیتگر بسیار محتمل است. با توجه به پتانسیل لرزه خیزی و احتمال آسیب پذیری سازه های مسکونی بویژه در شمال و شمال غربی منطقه مورد مطالعه، نتایج این تحقیق حائز اهمیت است. استفاده از آن برای تحلیل مخاطرات زمین ساختی و ارتقای کیفی نقشه های موضوعی توصیه می گردد.

Evaluation of Geophysical methods for identifying hidden Quaternary faults in alluvial sediments (case study: Chitgar Park)

AbstractIn this research, Geophysical methods Ground Penetration Radar (GPR) and Geoelectric have been used to detect hidden Quaternary faults in alluvial sediments located southwest of Chitgar Forest Park (District 22 of Tehran). The study of profiles (GPR) identified areas such as fault. The results obtained from two Geoelectric profiles indicate that there are two zones with low electrical resistance and high electrical resistance. Reduce the electrical resistance in the sediments of the area can be related to factors such as the existence of nonweathered rocks, the air inside the sediments, finegrained sediments and being a forest of the study area, due to irrigation of trees and rainwater penetration underground. High electrical resistivity indicates crushing of rocks by fault activity, coarsegrained and dry sediment. The results of the methods used in this research indicates that there is a fault zone with an approximate length of 140 m. Considering the structural trends of most of the faults identified, it is very likely that the faults will continue along the upper part of the Chitgar Park. Due to the seismic potential and vulnerability of residential structures, especially in the north and northwest of the study area, the results of this research are important. Its use is recommended for analyzing tectonic hazards and improving the quality of thematic maps.Keywords: Electrical resistivity, Fault zone, Ground Penetrating Radar, Landform, tectonic hazards. *Corresponding author: Email: r_mehrniya@pnu.ac.ir, Phone: +98 9122574607 Fax: +98 21 22440925 Address: Payame Noor university of Tehran, Artesh boulevard, Emam Ali freeway, Tehran, Iran. IntroductionIn spite of intense erosion, extensive vegetation, human activities and presence of higher thickness of sediments which make it difficult to access Quaternary earthquakes documentation, over the past two million years, we have been witnessed tectonic movements and the vast majority of changes in landforms which most of them are important in Seismological engineering.There are large and small faults in the megacity of Tehran, which mainly have sheared off and moved the TertiaryQuaternary sedimentary sequences. One of the main faults in Tehran is North Tehran fault , which is the most dangerous and most controversial seismic fault with East to West direction. This fault located in the north of Tehran between the mountains and the piedmont of Central Alborz.The study area is located in 22nd District of Tehran Municipality (in the southwestern of Chitgar Forest Park) with UTM coordinate (Easting: 517658.75/ Northing: 3955301.99). This area is located in the Bseries alluvial sediments of Tehran plain. Geologically, it includes unconsolidated sediments such as boulder, cobble, gravel and sand which were deposited and transported from the southern slopes of the Alborz Mountains. Based on of Tehran geological map (1.100.000), the geologic units Qt2 (Low level piedment fan and valley terrace deposits) and Qplc (Fluvial conglomerate, Piedmont conglomerate and sandstone) are recognized. In addition, part of the sediments recognized, layers of old soil and pseudoLaterite are seen. These layers indicate climate changes and the effect of warmer weather.Due to the factors such as being covered some of the faults with Quaternary alluvial Sediments, Extensive urban development of the city of Tehran (especially 22nd district) and outcrop of subsidiary faults in alluvial Sediments of Azadshahr fault (most important factor), Geophysical methods usage such as GPR (Ground Penetrating Radar) and Electrical Resistivity are considered.Method and MaterialIn this research, the GPR data was acquired by the MALA RTA50 (Rough Terrain Antenna) System with an antenna of 50 MHz. The maximum penetration of RTA50 is about 30 meters. The profile is adjusted with the time record system. Two geoelectric profiles were taken along the trench. For processing GPR data and geoelectric profiles from Reflex W and RES2DINV software were used.DiscussionAccording to the GPR profile, faults, fractures and displacement of the layers were identified. Generally, a fault zone is observed a distance of 110 to 380 meters. PoleDipole array method was used to investigate the geoelectrical resistivity of the studied area. The Data acquisition includes two 160 meters profiles with a distance of 60 meters from each other. The results obtained from two Geoelectric profiles indicate that there are two zones with low electrical resistance (2838 ohms) and high electrical resistance (137233 ohms) in the first profile (At a distance of 7585 meters) and low resistance (15.223.6 ohms) and high resistance (208328 ohms) in the second profile (At a distance of 8090 meters) they exist. Reduce the electrical resistance in the sediments of the area can be related to factors such as the existence of nonweathered rocks, the air inside the sediments, finegrained sediments and being a forest of the study area, due to irrigation of trees and rainwater penetration underground. High electrical resistivity indicates crushing of rocks by fault activity, coarsegrained and dry sediment.Conclusion The results of the methods used in this research indicates that there is a fault zone with an approximate length of 140 m. Also, the data processed for GPR profile indicative the effects of the faults and changes in the stratification of layers at distance mentioned and proves the results of the electrical resistivity method. Changes and function of active faults but hidden in the study area can be closely related to the location of this area adjacent to the main North Tehran fault and the effects of this great fault. Considering the structural trends of most of the faults identified, it is very likely that the faults will continue along the upper part of the Chitgar Park. Due to the seismic potential and vulnerability of residential structures, especially in the north and northwest of the study area, the results of this research are important. Its use is recommended for analyzing tectonic hazards and improving the quality of thematic maps.