二維全波形逆推研發與其在海域震測的應用
Development of Two-Dimensional Full Waveform Inversion and Its Application of Marine Seismic

本論文為自行發展的平行化重合前全波場逆推技術。針對台灣西南海域近海床淺部地層構造及天然水合物，透過結合傳統震測資料的前處理與不同方式的速度分析所建立初始速度模型，利用線性化疊代演算過程，進行細部的速度構造解析。逆推的理論基礎是以逆時逆推為運算核心，結合理論波場的正演波傳波場和理論與觀測資料間殘差波場的逆時波傳波場間相互交對比過程，獲得殘差波場對速度變動量間的關係，並以最速下降法或共軛梯度法進行最佳化，進而推估深度‒速度構造剖面。

在理論敏感度算核的研究中，可透過逆時逆推演算法理論的驗證瞭解在有限頻寬下震波對應於波徑路徑周圍的速度異常敏感度，並進而運用於全波形逆推進行速度的逆推、調整與更新。由棋盤格模型測試，可進一步瞭解演算法對速度異常體的大小與速度變異量的解析能力。在含高雜訊或含地表地形變化的陸上震測與海床地形變化的海域震測資仍可進行且穩定的收歛。演算法本身可同時處理因地形起伏、側向速度變化、及風化層所引發之淺層效應。實際海底地震儀施測方式的敏感度算核分析，由其有限波徑周圍的建設性干涉的結果，可顯示波形逆推方法的解析能力。海底地震儀的垂直向速度分量記錄比壓力分量記錄更適合進行波形逆推處理，能提供更高解析度的海床下地層速度構造。

針對2008年的仿三維震測於永安海脊區域的MCS881-42測線的海底地震儀資料，以3至12Hz進行多尺度全波形逆推。結果顯示海底仿擬反射的深度範圍在200至500公尺之間，速度為1.7公里/秒，游離天然氣的速度則在1.3至1.5公里/秒之間。在永安海脊的西側，由於海底地震儀施放位置集中，逆推過程的因波徑的建設性干涉程度良好。海床下地層速度變化、地層不整合面、斷層及海底仿擬反射的位置都明顯清晰可見。永安海脊處下方的有三條可能是流體移棲路徑的斷層存在，且斷層交匯在深度位置2公里處。

逆推方法還有改善的空間：(1)需縮小海底地震儀的置放間距，提高其資料在空間的採樣；(2)受放置方式與海流造成的三維效應，可考慮在不同的資料集合下進行速度的逆推工作；(3)最佳化方法可進一步朝類牛頓法、牛頓法或非線性的方法發展；(4)全波形逆推方法可擴展應用到三維的聲波、彈性波、黏彈性、電磁及重力等使用不同物理參數的課題上。

Abstract

An in-house development of parallel prestack full waveform inversion scheme which utilizes iterative approach for velocity structure investigation and refinement related to gas hydrate exploration at offshore SW Taiwan. The generalized linear inversion uses optimization strategies including steepest descent and conjugate gradient methods. For full waveform inversion (FWI), the initial reference model can be created either from conventional semblance analysis or through tomography study. The computation involved in FWI is based upon adjoint-state wavefield method (AW) by cross-correlating the synthetic forward propagating wave fields with backward propagating residual wave fields. Reverse-time wave field extrapolation is the key computational engine to construct sensitivity kernel and to estimate the amount of velocity perturbation for the final depth-velocity profile.

In study of sensitivity kernels, through synthetic verifications the velocity adjustments are directly associated with predominant frequency and wave-paths which is different from ray-centered approach. Checkerboard tests provide critical test that illustrate the limits on seismically defined scatter size and amount of velocity perturbation. The method is well behaved through tests including the effects related to noise level, near-surface, topography changes, strong lateral velocity variations and arbitrary survey geometries. For illumination/sensitivity analyses of realistic MCS/OBS marine acquisitions, particle velocity records are more sensitive than the pressure records for detecting and updating lateral velocity structure through full waveform inversion.

Field data application is performed for a set of OBS data collected along MCS8810-42 during a pseudo-3D survey around Yuan-An ridge in 2008. Implementation of full waveform inversion is done with dominant frequency of 3-12 Hz. FWI results indicate that BSR is located around 200-500 meter below sea-floor. Average velocity for BSR is 1.7 km/s with free gas velocity varying between 1.3-1.5 km/s. Because of constructive interference and wav-paths effects, the rather closely deployed OBS instrumentations can clearly detected and depicted the lateral velocity variation, unconformity, faults, and BSRs. Three faults intersect at depth of 2 km provide a useful fluid migration paths and form fairly distinct BSR signatures.

Webmin is a web-based interface for system administration for Unix.

Using the Webmin YUM repository

If you like to install and update Webmin via RPM, create the /etc/yum.repos.d/webmin.repo file containing :

[Webmin]
name=Webmin Distribution Neutral
#baseurl=http://download.webmin.com/download/yum
mirrorlist=http://download.webmin.com/download/yum/mirrorlist
enabled=1

You should also fetch and install my GPG key with which the packages are signed, with the commands :

wget http://www.webmin.com/jcameron-key.asc
rpm --import jcameron-key.asc

You will now be able to install with the command :yum install webminAll dependencies should be resolved automatically.

-- from official website: http://www.webmin.com/

My name is Quincy.

2012年10月3日星期三

2012年9月27日星期四

My Ph.D. dissertation

Mendeley

2012年9月26日星期三

Webmin

Webmin is a web-based interface for system administration for Unix.

Using the Webmin YUM repository

2012年10月3日 星期三

2012年9月27日 星期四

My Ph.D. dissertation

Mendeley

2012年9月26日 星期三

Webmin

Webmin is a web-based interface for system administration for Unix.

Using the Webmin YUM repository

2012年10月3日星期三

2012年9月27日星期四

2012年9月26日星期三