معرفی یک شرط تصویرسازی جدید و کارآمد در روش مهاجرت زمانی معکوس

روش مهاجرت زمانی معکوس (reverse time migration) به عنوان یک روش تصویرسازی لرزه‌ای نوین، با حل دو طرفه معادله موج و از طریق یک فرایند سه مرحله‌ای شامل برون‌یابی پیشرو و پسرو میدان‌موج چشمه و گیرنده و اعمال یک شرط تصویرسازی مناسب اجرا می‌شود. این روش همه انواع موج را بدون هیچ محدودیت زاویه‌ای مدلسازی می‌کند، این امر با توجه به ضعف روش‌های تصویرسازی پرتو مبنا و حل یک طرفه موج در تصویر نمودن درخور ساختارهای موجود در محیط‌های زمین‌شناسی پیچیده، بسیار حائز اهمیت است. علی‌رغم برتری‌های گفته شده، نوفه‌های فرکانس پایین که عمدتاً در زاویه‌های بازتاب بزرگ تولید می‌شوند (60 تا 90 درجه)، محدودیت عمده روش rtm به شمار می‌آید که تصویر مهاجرت یافته را پوشش داده و کیفیت آن را کاهش می‌دهند. لذا هدف از مقاله حاضر آن است که با بهبود شرط تصویرسازی به عنوان قلب روش rtm، ضمن حذف نوفه‌های فرکانس پایین، اطلاعات مفید مربوط به زاویه‌های بازتاب 60 تا 90 درجه را حفظ و از آنها در جهت تولید یک تصویر با کیفیت بهتر استفاده نماید. این کار با ارائه یک شرط تصویرسازی نوین و با اضافه نمودن یک تابع وزنی بر اساس زاویه‌های بازتاب انجام شده است. در نهایت نتایج روش rtm با استفاده از شرط تصویرسازی جدید ارائه و با نتایج برخی روش‌های سنتی و مدرن مشابه مقایسه شده است.

Presentation a new and efficient imaging condition in Reverse Time Migration

SummaryReverse time migration (RTM) as a new seismic imaging method solves the twoway wave equation and has been implemented through three main steps including forward and backward wavefield extrapolation from the source and receiver and employing a proper imaging condition. RTM models all types of wave without any dip limitation. This is very important regarding the drawbacks of raybased and oneway wave equation imaging methods in properly imaging the complex geological media. Despite the above superiorities, low frequency artifacts especially in large reflection angles (60 to 90 degree) are the main drawback of RTM which cover and reduce the migrated image quality. Therefore, the aim of this paper is to improve the imaging condition as the heart of RTM to suppress the low frequency artifacts and use the useful information of the large reflection angle domain (60 to 90 degree) and produce a high quality image. This was achieved by presenting a new imaging condition including a weighted function based on the reflection angles. Finally, the RTM results using the new proposed imaging condition was presented and compared with the results of some conventional and modern similar methods. IntroductionSeismic imaging is based on numerical solutions to wave equations, which can be classified into raybased (integral) solutions and wave fieldbased (differential) solutions. In complex geological structures such as subsalt media, the velocity variation leading to complex multipathing reflections. Hence raytracing may fail to image the subsurface properly and cannot image steeply dipping reflectors corresponding to the velocity model. On the other hand, oneway wave propagation extrapolates wavefields vertically and cannot accurately model waves that propagate nearly horizontally. they fail to handle waves propagating at wider angles, especially those near or beyond 90°. RTM directly solves the full (twoway) acoustic wave equation and incorporates all type of waves propagating in different directions. Hence, it has proved to be the preferred imaging algorithm in many geologically complex basins. RTM can image the complex geological media properly which is beyond the limits of oneway wave equationbased migration algorithms. Nevertheless, RTM has its limitations. The major drawback is the low frequency artifacts produced by the image condition (zero crosscorrelation at lag) or by strong velocity contrast which is the main topic of this paper to be developed to suppress the RTM artifacts. Methodology and ApproachesTo suppress the RTM artifacts, the imaging condition as the heart of RTM was developed. A new presented imaging condition includes the separated downgoing and upgoing wavefields and a new weighted function based on the reflection angles. It  is implemented to suppress the low frequency artifacts for large reflection angles and maintain the useful information for the same reflection angle domain through an advance procedure. Results and ConclusionsRTM results using the presented imaging condition indicates that the low frequency artifacts was suppressed properly and the subsurface geological structures was imaged as well as possible in final migrated image I comparison the other seismic imaging methods.