Presentation a new and efficient imaging condition in Reverse Time Migration

Document Type : Research Article

Author

Dept. of Mining, Lorestan University, Khoramabad, Iran

10.29252/anm.2019.1367

Abstract

Summary
Reverse time migration (RTM) as a new seismic imaging method solves the two-way wave equation and has been implemented through three main steps including forward and backward wave-field 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 ray-based and one-way 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.
 
Introduction
Seismic imaging is based on numerical solutions to wave equations, which can be classified into ray-based (integral) solutions and wave field-based (differential) solutions. In complex geological structures such as subsalt media, the velocity variation leading to complex multi-pathing reflections. Hence ray-tracing may fail to image the subsurface properly and cannot image steeply dipping reflectors corresponding to the velocity model. On the other hand, one-way wave propagation extrapolates wave-fields 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 (two-way) 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 one-way wave equation-based migration algorithms. Nevertheless, RTM has its limitations. The major drawback is the low frequency artifacts produced by the image condition (zero cross-correlation 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 Approaches
To suppress the RTM artifacts, the imaging condition as the heart of RTM was developed. A new presented imaging condition includes the separated down-going and up-going wave-fields 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 Conclusions
RTM 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.

Keywords

Main Subjects


الگوریتم‌های تصویرسازی عمقی (مهاجرت عمقی) عموماً در دو گروه روش‌های پرتو- مبنا[i] و معادله موج-مبنا[ii] طبقه‌بندی می‌شوند و هر گروه نیز می‌تواند به زیرمجموعه‌هایی تقسیم شود. روش‌های پرتو- مبنا شامل روش‌های مهاجرت کیرشهف[iii] و اشعه[iv] است، حال آنکه گروه معادله موج- مبنا شامل مهاجرت یک طرفه معادله موج[v] (ادامه فروسو) و مهاجرت دو طرفه (کامل)[vi] معادله موج صوتی است[1]. مورد دوم تحت عنوان مهاجرت زمانی معکوس (RTM) شناخته می­شود و از طریق حل کامل معادله موج میسر می­شود. از جمله مزایای روش‌های پرتو- مبنا، حجم محاسباتی و هزینه کم‌تر آنها نسبت به روش‌های معادله موج- مبنا است، اما این نوع روش‌های تصویرسازی در مناطق با زمین‌شناسی پیچیده و دارای ساختارهای پرشیب با مشکل مواجه می‌شوند[2]. در مقابل روش‌های معادله موج- مبنا دارای حجم محاسباتی بالا بوده، اما در مناطق با زمین‌شناسی پیچیده کارایی بهتری در تصویر نمودن ساختارهای زیرسطحی، به ویژه ساختارهای پرشیب از خود نشان می‌دهند[3]. روش‌های مهاجرت معادله موج یک طرفه در مواردی که امواج با یک زاویه خاص از جهت اصلی (معمولاً جهت عمودی) انتشار می‌یابند، خوب عمل می‌کنند، اما در مورد امواجی که با زاویه نزدیک 90 درجه یا بیش‌تر انتشار می‌یابند (انتشار تقریباً افقی)، با شکست روبرو می‌شوند؛ بنابراین روش مهاجرت معادله موج یک طرفه نمی‌تواند بازتابنده‌های با شیب تند در محیط‌های زمین‌شناسی پیچیدهرا به خوبی تصویر کند[3-5].

از طرف دیگر، مهاجرت زمانی معکوس (RTM)، معادله موج صوتی کامل (دو طرفه) را به ازای انتشار میدان موج حل می‌کند. بنابراین انتشار موج در همه جهات را شبیه‌سازی می‌کند و در نتیجه محدودیت‌های زاویه‌ای را برطرف نموده و همه انواع موج از جمله امواج منشوری[vii] و وارون شده[viii] را نیز تصویر می‌کند. به همین دلیل، کارآمدی آن به عنوان الگوریتم اصلی تصویرسازی در موارد مناطق زمین‌شناختی پیچیده بسیار اهمیت می‌یابد[6]. این مسئله زمانی بیش‌تر اهمیت می‌یابد که عملیات اکتشاف مواد هیدروکربوری با ساختارهای زمین‌شناسی بسیار پیچیده‌ای همانند ساختارهای پر­شیب و یا برآمدگی‌هایی مثل گنبد نمکی مواجه گردد، مسئله‌ای که فراتر از حیطه عملکرد الگوریتم‌های مهاجرت معادله موج یک طرفه است[6]. علاوه بر مزیت‌های روش RTM، نوفه‌های با دامنه بلند و فرکانس پایین از شاخصه­های این روش است که کیفیت تصویر را به شدت کاهش می‌دهد. از این رو حذف و یا کاهش این نوفه‌ها یکی از چالش‌های عمده این روش است[7].



[i] Ray-based

[ii] Wave equation-based

[iii] Kirchhoff

[iv] Beam

[v] One-way wave equation migration

[vi] Two-way wave equation migration (full)

[vii] Prismatic waves

[viii] Overturned

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