پردازش و تفسير داده‌هاي مغناطيس زميني مرتبط با منابع زمين‌گرمايي با استفاده از دو روش اويلر و AN-EUL، شمال شرق محلات

Processing and interpretation of ground magnetic data corresponding to geothermal resources using Euler and AN-EUL method, north-east of Mahallat

چشمه هاي آبگرم متعدد در منطقه محلات و همچنين نوع زمين‌شناسي منطقه، نشان‌دهنده وجود يك سامانه زمين گرمايي بزرگ در اعماق منطقه است. طي فعاليت هاي صورت گرفته از 1374 در ايران، چندين ناحيه اميدبخش از لحاظ پتانسيل زمين گرمايي از سوي مركز انرژي هاي نو وزارت نيرو معرفي شده كه محلات يكي از مناطق مهم در اين زمينه است. به‌منظور شناسايي دقيق نواحي مستعد زمين گرمايي، عمليات مغناطيس سنجي زميني در طول 10 نيم‌رخ‌‌ به‌نسبت بلند، جمعا به طول حدود 160 كيلومتر با فاصله ايستگاهي 40 متر و فاصله نيم‌رخي 5/1 كيلومتر، در محدوده اطراف چشمه هاي آبگرم محلات صورت گرفت. در مجموع بيش از 4000 ايستگاه مغناطيس سنجي برداشت شد. در اين پژوهش داده هاي حاصل از عمليات مغناطيس سنجي زميني با استفاده از دو روش بسيار كاربردي اويلر و روش تلفيقي سيگنال تحليلي و اويلر (AN-EUL) مورد تفسير قرار گرفته است. نتايج برآورد عمق منبع اصلي بي هنجاري به‌دست آمده با اين دو روش، وجود يك منبع زمين گرمايي با عمق حداقل يك كيلومتر در منطقه را برآورد مي كند. مقدار شاخص ساختاري به‌دست آمده از اين دو روش پردازشي، مويد اين است كه منبع بي هنجاري احتمالا داري شكلي شبيه به يك استوانه قايم است. نبود قطب منفي مغناطيسي بارز در منطقه احتمالا حاكي از گسترش اين توده در عمق است. همچنين نتايج اين پژوهش نشان مي دهد كه روش اويلر علاوه بر برآورد عمق بي هنجاري، به بررسي روند تغيير عمق در نقاط متفاوت بي هنجاري هم مي پردازد ولي روش AN-EUL فقط در نقاط خاصي مي تواند برآورد عمق را عملي سازد.

Ground magnetic survey was carried out in Mahallat region, Iran, as the first part of exploration and development of geothermal energy program conducted by Arak and Tehran universities. The magnetic method is useful in mapping near-surface volcanic rocks that are often of interest in geothermal exploration. The magnetic method has come into use for identifying and locating masses of igneous rocks that have relatively high concentrations of magnetite. Strongly magnetic rocks include basalt and gabbro, while rocks such as granite, granodiorite and rhyolite have only moderately high magnetic susceptibilities. The surface manifestations of thermal activity in the area are in the form of hot springs, hydrothermal deposits, thermal alteration, vast travertine outcrops and young volcanic rocks. In the geological dividing of Iran, Mahallat area is located in the volcanic zone of Central Iran. This zone has been one of active zones during the different geological periods that is located in Central Iran with triangle form. This area, from permeability aspect and by regarding to expansion of calcareous and dolomite units and also presence of cracks and joints, has a good condition. Also, with regard to presence of hot water springs and regard to geologic settings in the area, we can define it as one of important and suitable geothermal potential in Iran. Existence of many hot springs in Mahallat and also its special geology show a big probable deep geothermal system in the area. During activities performed since 1996 in Iran, few encouraging area with geothermal potential have been introduced by the Renewable Energy Organization of Iran and one of the corresponding areas is Mahallat. In order to delineate susceptible area with geothermal potentials accurately, ground magnetic survey was performed around hot springs of Mahallat. Total magnetic field data was collected along 10 profiles, with station distance of 40 m and profile distance of 1.5 km. During this survey, measurements were performed at over 4000 stations. In this research, magnetic data of magnetometry operations are interpreted by two practical methods: standard Euler deconvolution and AN-EUL. The standard Euler method is based on the Euler equation, and using this method in the depth estimation of magnetic anomalies inserts the geology of the region into the calculations. AN-EUL is a new automatic method for the simultaneous approximation of depth, geometry and location of magnetic sources. The principle advantage of this method is its combining both the analytic signal and the Euler Deconvolution methods. In this method, the determination of the source location is based on the position of the maximum value of the analytic signal amplitude. Results of depth estimation of the main source of the anomaly, from these two methods, have estimated existence of geothermal resource, with probable depth of more than 1000 m and structural index of 1.8. Estimated structural index from these two methods shows that source of anomaly has a conical cylinder shape. Absence of negative magnetic pole in the area imply of extension of anomaly in depth. Results of this research also show that Euler method in addition to estimation of anomalous depth is also able to investigate trend of depth variations in different position of the anomaly, while AN-EUL can only estimate depth in special position. Gravity, resistivity and MT surveys and also exploratory drillings over the interpreted geothermal anomalies in the area are suggested for the future assessment of geothermal energy in the region on an industrial and scientific scale.