数据自相关多次波偏移成像

在常规偏移方法中一般都需要压制地震数据中的多次波,仅利用一次波信息成像,把自由表面反射的多次波视为噪声,但是在多次波中也包含着地下结构信息,应该将其充分利用到成像中来.事实上,已经有不少成像方法试图利用多次波信息,但是大部分方法都需要对多次波进行预测.本文提出了基于傅里叶有限差分偏移算子的数据自相关偏移方法.在这种偏移方法中,对含有一次波和多次波的地震数据,分别进行下行和上行延拓,然后直接利用常规的互相关成像条件成像.由于波场延拓采用了傅里叶有限差分算子,其计算效率高,能够很好地对复杂介质中的地震数据进行延拓.在数值试验中,使用了一个含散射点的三层模型和Marmousi模型.合成数据测试结果表明,这种方法可以对更大范围的地下构造成像,比常规的只利用一次波的傅里叶有限差分法照明度更好,并且在浅层可以提供更高的分辨率.我们提出的数据自相关策略易于实现且避免了繁杂的多次波预测,这对于复杂地下构造成像可能有着重大意义.

Imaging multiples by data to data migration

In conventional migration methods, free surface related multiples are regarded as noise and only the primary wave is utilized in imaging. However, the multiples also contain information of subsurface structures and can offer extra illumination in migration procedure. There are already numerous methods to take advantage of multiples, but most of them need multiples prediction, which is time consuming. To avoid multiples prediction and wavelet estimation, this paper presents a new multiples migration method called data to data migration.Based on the migration method that uses primaries and multiples simultaneously, this paper presents one-way wave equation migration of data to data migration. The FFD (Fourier finite difference) method is employed in wave field extrapolation. For the data containing primaries and free-surface related multiples, the data to data migration replaces the source wavelet function and the primaries in conventional FFD migration with the recorded data. The same crosscorrelation imaging condition is used. In the FFD migration method, the field data is transformed into the frequency domain. Compared to RTM(reverse time migration), it has higher computation efficiency and less low frequency noise. The FFD is more suitable for the implementation of data to data migration.Compared to conventional FFD migration that uses primaries only, the images generated by data to data migration based on FFD operator have better illumination and higher resolution in the shallow zone. The images of data to data migration contain artifacts produced by the corsscorrelation of different seismic events. In the deep zone, the migration results are inferior to conventional migration. One reason is the influence of artifacts; another is that multiples need longer receiving time. In the dataset, higher order multiples may not yet be received completely. The results of the conventional migration and data to data migration have different polarities. Assumed the wavelet and the primaries have positive polarity, and then the conventional migration result has a positive polarity. The first order multiple is reflected by the free surface once, so it has negative polarity. It is crosscorrelated with the primary that has positive polarity and generates the result with negative polarity. The second order multiple that has positive polarity is crosscorrelated with the first order multiple that has negative polarity and generates the result with negative polarity. And the m+1 order multiple is crosscorrelated with the m order multiple which have different polarity and generates the result with negative polarity. Therefore the final result of data to data migration has negative polarity and it has different polarity with conventional migration. The computational speed of FFD is much faster than RTM which makes FFD appropriate for the multiples data with long record length.Recorded data without direct wave are utilized in data to data migration procedure. The numerical examples verify its effectiveness. The FFD operator improves the computational efficiency greatly. The proposed approach has three key advantages: (1) Compared to conventional migration method, data to data migration has wider illumination area and better resolution of scattering points. (2) It can generate subsurface image without multiples prediction, which is time consuming. (3) It can offer fairly good image of shallow reflectors. Another point is wavelet estimation. For the field data, how to extract wavelet function is intricate and prone to error. Data to data migration avoids this complicated process,which may be significant for imaging subsurface structures in the processing of field data. We should also notice that although most energy are imaging correctly, there are still artifacts in the final result.Least squares migration and wide azimuth acquisition technology will partly decrease the artifacts.