تفسير خودكار پروفيلهاي گراني سنجي با استفاده از نسبت گراديان داده ها

Automatic interpretation of gravity profiled using data gradient ratio

روش­های تفسیر خودكار بی­هنجاری های میدان پتانسیل به طور قابل ملاحظه­ای سبب كاهش بار تفسیری ژئوفیزیكدانان می شوند و امروزه كاربرد گسترده­ای پیدا كرده­اند. این روش­ها به دو دسته كلی روش­های مدل­سازی و تحلیلی دسته بندی می­شوند. از پركاربردترین روش­های تحلیلی می­توان اویلر دیكانولوشن، ورنر دیكانولوشن و اخیراً روش­ تیلت – عمق (Tilt-Depth) را نام برد. روش تیلت – عمق بر پایه استفاده همزمان زاویه تیلت برای تخمین مرز و عمق توده­های مولد است. در این تكنیك روابط گرادیان قائم و افقی بی­هنجاری مغناطیسی مدل كنتاكت در رابطه مربوط به زاویه تیلت جایگذاری و رابطه ای برای تخمین عمق تولید می­شود. در این مقاله این روش در مورد مدل­های گرانی­سنجی استوانه افقی و كره مدفون تعمیم و توسعه داده می­شود. كارایی روش نیز با استفاده از مدل­های مصنوعی كره و استوانه افقی در حالت­های مختلف بررسی می­گردد. همچنین اثرات نویز و همجواری توده­ها نیز بر كارایی روش بررسی شده است. این روش روی دو پروفیل تهیه شده از نقشه بی­هنجاری گرانی معدن سنگ آهن شواز در استان یزد به كار برده شده و نتایج آن با روش تحلیل طیف انرژی مقایسه شده است.

Summary One of the most important problems in the interpretation of gravity or magnetic data is to obtain information about the sources position (geometry and depth). Potential field automatic interpretation techniques can significantly decrease the interpretation workload of a geophysicist and are widely used. Automatic interpretation methods can be classified into two major groups: modeling and analytical. Euler Deconvolution, Werenr Deconvolution and recently, Tilt-Depth method are the commonly used (practical goals) analytical methods. The basic idea in Tilt-Depth method is simultaneous application of tilt angle for edge and depth estimation of magnetic contact model. In this method, the vertical and horizontal gradients of magnetic contact substituted in tilt equation lead to an equation for depth estimation. This paper generalizes the tilt-depth method to gravity data using the horizontal cylinder and the buried sphere models.   Introduction Salem et al, (2007) introduced the tilt-depth method for the magnetic anomaly over a contact. Previously Miller and Singh had developed the tilt angle as a method of enhancing images of the vertical derivative of potential field data. The tilt-depth method only depends on mapping specific contours of the magnetic tilt angles. The zero contours delineate the spatial location of the magnetic source edges whilst the depth to the source is the distance between the zero and either the –45° or the +45° contour or their average. The tilt-depth method adds to the arsenal of geophysical methods currently in use to estimate magnetic source depths, many of which use second- and/or third-order derivatives. These include methods based on Euler’s equation and the local wavenumber, both of which calculate the source depths for a range of source-body geometries, and, more recently, for the simultaneous estimation of both source depth and source type.   Methodology and Approaches In this paper, the tilt-depth method will be both generalized (by applying it to gravity models) and extended (by using all values of the ratio of the field gradients, not just a single value). The gravity models used are 2D horizontal cylinder and buried sphere. In this regard we developed a MATLAB code for applying the proposed method to synthetic and real data. In this code the selection of the ratio of the vertical to horizontal derivatives are done on the basis of the signal to noise ratio of the dataset. Also for the consistency of result the vertical derivative is calculated using Hilbert transform. The final equation was solved by Newton method.   Results and Conclusions The efficiency of the proposed method tested using various synthetic gravity models. The sensitivity of methods to noise and interface was tested using synthetic data. On the basis of observations the method is sensitive to noise, but if the data continued upward before applying the algorithm or using of the stable derivative operator the inconsistency of the result decreases seriously. For overcome in body overlapping phenomena we suggest anomaly windowing or insulating by means of Bott (1966) algorithm. This method applied on 2 gravity profiles from Shavaz Iron ore in Yazd province. Then the results compared with power spectrum depth analysis. Accordance to this comparison the proposed method could produce the same result as power spectrum. In this case w upward gravity data to 2m in order to decrease noise content.