استفاده از تحليل داده‌هاي گراني در تعيين مدل پوسته و مقايسه با مدل هم‌ايستايي ايري- هيسكانن در درياي عمان

Crustal and isostasic model for the Gulf of Oman using gravity data

براي تحقيق ساختار پوسته در درياي عمان ، از بي‌‌هنجاري‌هاي گراني و مدل هم‌ايستايي ايري- هيسكانن، استفاده كرده و ضخامت پوسته را به‌دست آورده‌ايم. نقشه بي‎هنجاري هم‎ايستايي ايران كه بر پايه مدل تعديل شده هم‌ايستايي ايري- هيسكانن تهيه شده است، نشان مي‎دهد كه چگالي بلندي‎ها 67/2، چگالي ميانگين پوسته 75/2، چگالي ريشه كوه‎ها 85/2، چگالي گوشته بالايي 35/3 گرم بر سانتي‌متر مكعب و ضخامت عادي پوسته 30 كيلومتر است. طبق مدل ايري- هيسكانن، چگالي پوسته ثابت است و تعادل هم‌ايستايي با تغيير در ضخامت پوسته در محل ارتفاعات (ريشه‌‌هاي عميق) و گودي‌ها (پادريشه‌‌‌ها) صورت مي‌گيرد. قاره‌‌ها از كف اقيانوس‌‌ها به دليل داشتن چگالي بيشتر و نيز ضخامت بزرگ‌تر، بالاتر هستند. بي‌‌هنجاري‌‌هاي گراني بوگه از داده‌هاي ارتفاع‌سنجي ماهواره‌اي روي اقيانوس‌ها محاسبه شده است.قدرت تفكيك مكاني اين داده‌ها 5 كيلومتر است. در اين تحقيق مدل پوسته و هم‌ايستايي ايري-هيسكانن جديدي براي منطقه مورد بررسي براساس روش معكوس به روش پاركر تعيين شده است ناحيه مورد بررسي در عرض جغرافيايي 20= تا 5/25= و طول جغرافيايي 60= تا 5/66= درجه كه كلاً پوشيده از آب است، قرار دارد. براي محاسبه ضخامت مقادير گراني و عمق، به‌صورت يك شبكه كيلومتر در آمده و براي محاسبات در هر دو روش از اين قدرت تفكيك استفاده شده است. تفاوت مشاهده شده در تعيين ضخامت پوسته حاكي از آن است كه منطقه به‌لحاظ هم‌ايستايي، جبران نشده است.

The mapping of the crust-mantle boundary surface is an important geophysical task, which the method of seismic profiling has dealt with profitably. There are, however, areas where the crustal structure is not known up to the present, and where the Moho has as yet not been determined by geophysical sounding. In such areas the isostatic theory may be applied to give a first estimate of the depths of the crust-mantle boundary. However, young orogenic regions are not necessarily in isostatic equilibrium. Therefore the isostatically calculated crust mantle boundary must be corrected. In our method, the long wavelength observed gravity anomalies are inverted in an iterative process to model the crust-mantle boundary, assuming thus that the mass responsible for the observed gravity anomalies is located at the level of the crust-mantle boundary. As in all gravity-inversion problems, the ambiguity inherent in the underlying mass distributions implies the choice of a particular starting model, defining the crustal equilibrium thickness, and the crust and mantle densities. This may be done adopting a standard crustal model, where mean values found in the literature are used. In cases where these are available, further geophysical knowledge on the crustal thickness from other sources, particularly seismic, gives a means to anchor the crustal equilibrium depth. Thickness of the crust is mostly determined using seismic data provided by recorded earthquakes. The gravity data also can be a very useful source for this purpose. In this paper, authors have estimated a new crust model in The Oman Sea using the new database. The study region is extended between latitude and . The estimated thickness of the crust is compared with the A-H model of Isostasy. According to Airy, the mountains are floating on a fluid lava of higher density, so that the higher the mountain, the deeper it sinks. (Airy Isostasy: constant density materials, topography underlain by roots, depressed Moho depth). Airy proposed this model, and Heiskanen gave it a precise formulation for geodetic purposes and applied it extensively. The gravity model computed by Bhaskara Rao et al., formula (1990) and is compared with the observed gravity dataset. To investigate subsurface structure from potential data such as gravity and magnetic data, various methods have been developed. Blakely (1995) divided them into three categories of forward method, inverse method, and data enhancement and display. Since the algorithm that allows the Fourier transform quite fast had developed, there have been many attempts to apply it to geophysical data processing, and one of the most important works in potential field was done by Parker (1973). He derived mathematical expansions and showed how a series of Fourier transforms could be used to compute the gravity anomaly caused by an uneven, non-uniform layer of material. Shortly after his work, Oldenburg (1974) deduced a method to compute the density contrast topography from the gravity anomaly reversely in two-dimensional Cartesian coordinate system by intuition from the Parkerʹs formula. anomaly. The inversion method used here is that proposed by Oldenburg (1974), in which the topography of a density interface generating a certain gravity anomaly is estimated using the equation described by Parker (1973). To do this, we need to know both the mean depth of the interface and the density contrast between the bodies separated by this interface. According to Parker (1973), the Fourier transform of the gravity anomaly and the sum of Fourier transforms of the topography causing such a gravity anomaly are related.