تحليل طيفي داده هاي مغناطيس هوايي به‌منظور اكتشاف ظرفيت هاي زمين‌گرمايي استان آذربايجان شرقي

Spectral Analysis of aeromagnetic data for exploration of geothermal potentials of East Azerbaijan province

نقطه كوري، دمايي است كه در آن سنگ هاي پوسته زمين خاصيت مغناطيسي خود را از دست مي دهند. در مناطقي كه طرفيت زمين‌گرمايي دارند و يا مناطق آتشفشاني جوان انتظار مي رود كه عمق نقطه كوري به‌طور قابل‌توجهي كم باشد. ارزيابي تغييرات نقطه كوري يك منطقه مي تواند اطلاعات با ارزشي از توزيع دمايي ناحيه اي در عمق و تمركز انرژي زمين‌گرمايي در زير سطح عرضه كند. ايده به‌كارگيري داده‌هاي مغناطيس هوايي براي برآورد عمق نقطه كوري خيلي جديد نيست و تاكنون در جاهاي گوناگون دنيا در نواحي فعال زمين‌ساختي و ژئوديناميكي از آن استفاده شده است. در اين تحقيق از داده هاي مغناطيس هوايي استان آذربايجان شرقي براي برآورد عمق كوري استفاده شده است. براي اين كار، ابتدا نقشه تبديل ميدان به قطب تهيه شد. سپس ابعاد بهينه بلوك انتخاب شد و بلوك‌بندي روي نقشه تبديل ميدان به قطب صورت گرفت. عمق بالايي عميق ترين بلوك مغناطيسي با برازش خط راست به دومين بخش بلندترين طول موج طيف توان شعاعي به‌دست آمد. همچنين عمق مركزي عميق ترين بلوك مغناطيسي با برازش خط راست به بخش بلندترين طول موج طيف توان شعاعي برآورد شد. با توجه به عمق بالايي و مركزي بلوك ها، عمق نقطه كوري براي هر بلوك محاسبه شد. عمق كوري برآوردي براي منطقه مورد بررسي بين 9/42 تا 18/92 كيلومتر تغيير مي كند. در مناطقي كه تمركز چشمه هاي آب‌ِگرم بيشتر است، عمق نقطه كوري به‌طور قابل‌توجهي كمتر از بقيه مناطق است و اين نواحي را مي توان درحكم نواحي اميدبخش براي اكتشاف منابع زمين‌گرمايي محسوب كرد.

The Curie point (approximately 580C for magnetite at atmosphere pressure) is the temperature at which spontaneous magnetization vanishes and magnetic minerals exhibit paramagnetic susceptibility. The depth at which temperature reaches the Curie point, is assumed to be the bottom of the magnetized bodies in the crust. Curie point temperature varies from region to region depending on the geology of the region and mineralogical content of the rocks. Therefore, one can normally expect shallow Curie Point Depths (CPD) at regions which have geothermal potential, young volcanism and thinned crust (Aydin and Oksum, 2010). The assessment of the variations of the Curie depth of an area can provide valuable information about the regional temperature distribution at depth and the concentration of subsurface geothermal energy (Tselentis, 1991).     There are two basic methods which have been used in the examination of the spectral properties of magnetic data to estimate the magnetic basement depth. The first is the method of Spector and Grant (1970) and the second is the method of Bhattacharyya and Leu (1975, 1977). Spector and Grant (1970) showed that the expectation value of the spectrum of an ensemble model was the same as the average depth to the top of an ensemble of the magnetized rectangular prism. The second method estimates the depth to the centroid of the body by using the interpretation of a single anomaly. This method is useful when no spectral peaks occur on the amplitude spectra (Li et al., 2010). Following this, a joint method was developed by Okubo et al. (1985) who combined and expanded the ideas of the methods to the purposes of geothermal exploration.     CPD estimation (Zb) is obtained in two steps as suggested by Okubo et al. (1985). First, the centroid depth (Z0) of the deepest magnetic source is estimated from the slope of the longest wavelength part of the spectrum divided by the wavenumber, where P(k) is the power density spectrum, k is the wavenumber, and A is a constant. Second, the average depth to the top boundary (Zt) of that distribution is estimated from the slope of the second longest wavelength spectral segment (Okubo et al., 1985), where B is a constant. The equation Zb = 2Z0 Zt(Okubo et al., 1985) is used to estimate the depth to the bottom (Zb) of the inferred CPD from the centroid (Z0) and the top depth (Zt) estimated from the magnetic source for each subregion.     The study area was divided into eleven overlapped blocks for the purpose of spectral analysis. Center of blocks are shown on the residual field map (Figure 3). Each block covers a square area of 140 km by 140 km. Power spectral analysis was conducted on the RTP values of each of the blocks by plotting the logarithm of spectral energies against the wavenumber. The CPD estimation procedure as suggested by Okubo et al. (1985) was carried out for the eleven blocks to obtain the Curie point depth for each block. Figure 5 shows Curie depth values and thermal springs of the study area. In this figure, Curie depth varies between 9.42 and 18.92 km. For the blocks of 3, 6 and 7, Curie point depth is significantly less than the other blocks, which could be due to the presence of high temperature hot springs in this area. According to the geological information and Curie point depth values, A, B, C and D area are recommended for more geothermal investigation.