نوع مقاله : مقاله تحقیقی (پژوهشی)
1 گروه علوم زمین، دانشکده علوم پایه، واحد علوم و تحقیقات، دانشگاه آزاد اسلامی، تهران، ایران
2 استاد، مؤسسه ژئوفیزیک، دانشگاه تهران، تهران، ایران
عنوان مقاله [English]
A determinant factor in generating qualified seismic images of subsurface layers is the coherency of reflection events in pre-stack seismic data. Accordingly, this level of coherency could be reduced due to the complexity of propagation media or operational errors during seismic acquisition. It will be challenging to process acquired data if these incoherencies are not compensated for. In mountainous seismic acquisition projects, displacement of the source is one of the more common causes of errors. Such displacement could be happened for receiver points as well, due to the access or operational difficulties in seismic acquisition. In the processing stage, there are some methods for improving coherency based on the assumption that incoherencies are stationary both in time and offset. In particular, it is assumed that all reflection events shifts are static in the time domain and predictable in the offset domain. Thus, they can be compensated by conventional static and dynamic corrections. Nevertheless, there are some violent circumstances where the shifts will no longer be predictable. The displacement of source or receiver stations without resurveying of them and the presence of velocity anomalies in wave propagation media are some examples. These often occur simultaneously in mountainous seismic terrain. Conventional static and dynamic corrections cannot properly rectify such non-stationary inconsistencies in the mentioned circumstances. Although applying the residual static correction to compensate for such incoherencies can improve coherency for some reflector’s events, especially those with good amplitude and frequency content, for the others it can actually make it worse, especially for deep reflectors. For precise seismic reflection imaging, the magnitude of non-stationary shifts plays a key role. Small shifts will result in a reduction of the frequency content of the final stack section. For bigger size of these shifts, the quality of stacked reflections will be reduced and then can be eliminated, consequently. As a well-known fact, in seismic operations, logically increasing frequency content in order to get a better resolution usually involves spending significant time and money. However, with the mentioned operational errors, this is essentially wasteful. To avoid such operational errors, the activities of different crews of the seismic project should be thoroughly checked, in particular, the survey and drilling crews for the accuracy of elevation and coordinates of the shot points. Using both synthetic and real data, this paper attempted to explain how reflection event incoherencies frequently occur during mountainous seismic acquisition, and then proposed an algorithm for compensating those incoherencies with a non-stationary time shift correction in pre-stack seismic data.