摘要
地震资料含有各种类型多次波,而传统成像方法仅利用地震一次反射波成像,在地震成像前需将多次波去除.然而,多次波携带了丰富的地下结构信息,多次波偏移能够提供除反射波外的额外地下照明.修改传统逆时偏移方法,用包含一次反射波和多次波的原始记录代替震源子波,将SRME方法预测的表面多次波代替一次反射波作为输入数据,可将表面多次波成像.多次波成像的挑战和困难在于大量串扰噪声的产生,针对表面多次波成像中的成像噪声问题,将最小二乘逆时偏移方法与多次波分阶思想结合起来,发展可控阶数的表面多次波反演成像方法,有望初步实现高精度的表面多次波成像.在消除原始记录中的表面多次波后,通过逆散射级数方法预测得到层间多次波,将层间多次波作为逆时偏移方法的输入数据可将其准确归位到地下反射位置.数值实验表明,多次波成像能够有效地为地下提供额外照明,而可控阶表面多次波最小二乘逆时偏移成像方法几乎完全避免成像噪声.
Seismic data contains various kinds of mu and internal multiples. Conventional seismic imaging ltiple reflections, including surface-related utilizes reflections only and the multiples, being traditionally treated as noise, are removed before imaging. Actually, multiples penetrate into the subsurface several times and contain abundant reflection information. Multiples sometimes can provide higher fold and better illumination for the subsurface than primaries. Multiples usually travel with longer wave paths and cover larger areas than primaries in the media, and multiples can penetrate into the earth to illuminate the shadow zones which primaries cannot reach. Consequently, migration of multiples can sometimes provide better subsalt images compared to conventional migration which uses primary reflections only. We modify conventional reverse time migration （RTM） so that surface-related multiples can be used as constructive reflection energy for subsalt imaging. This approach replaces the impulsive source wavelet with the recorded data containing both primaries and multiples and uses predicted multiples as the input data instead of primary reflections. In the reverse time migration process, multiples recorded on the surface are extrapolated backward in time to each depth level, and the observed data with both primaries and multiples are extrapolated forward in time to the same depth levels, followed by a cross-correlation imaging condition. For RTM of all-order multiples, however, the main challenge is that undesired cross-correlation between forward and backward propagated seismic waves generate so many crosstalk artifacts. The crosstalks may distribute in the whole image profile, which can destruct the true image of reflectors and mislead the interpreting result of a migrated image. We apply the least squares migration scheme to these two successive orders of multiples. Our method is denoted as the least- squares RTM using controlled-order multiples （LSRTM-CM）. LSRTM-CM can provide a very clean section by reducing almost all artifacts. With limited iterations used, LSRTM-CM can provide a very interpretable image section, as LSRTM-CM can further enhance the image in detail by suppressing the erosstalk artifacts, balancing the amplitude, and improving the resolution. Internal multiples are usually generated below coal seams, salt domes, basalt flows and other geological formations with high velocity contrasts, and behave differently compared to primary and surface-related multiples. We propose a reverse time migration method which is capable of handling internal multiples. The method first extracts internal multiples from the wavefield, followed by extrapolating those multiples backwards in time through the subsurface. For this method, internal multiples are separated by inverse scatter series and used as data in reverse time migration. Numerical tests show that a wider coverage and a more balanced illumination of the subsurface can be achieved by migration of multiples compared with conventional migration of primary reflections.
作者
刘伊克
刘学建
张延保
LIU YiKe, LIU XueJian, ZHANG YanBao(Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, Chin)
出处
《地球物理学报》
SCIE
EI
CAS
CSCD
北大核心
2018年第3期1025-1037,共13页
Chinese Journal of Geophysics
基金
国家自然科学基金项目(41430321
41730425)
中国科学院战略性先导科技专项(B类)(XDB01020300)联合资助
