Increasing the number and signal-to-noise ratio of OBS traces with supervirtual refraction interferometry and free-surface multiples
Increasing the number and signal-to-noise ratio of OBS traces with supervirtual refraction interferometry and free-surface multiples
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DOI：
10.1093/gji/ggs087
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发表时间：
2013-01
影响因子：
2.8
通讯作者：
P. Bharadwaj;X. Wang;G. Schuster;K. Mcintosh
中科院分区：
文献类型：
--
作者：
P. Bharadwaj;X. Wang;G. Schuster;K. Mcintosh
超虚干涉测量的理论被修改，使得自由表面相关的多次折射可以被用于通过与NS成比例的因子来增强主折射事件的信噪比（SNR），其中NS是针对指定折射倍数的后临界源的数量。我们还表明，折射多次波可以转换成主折射事件记录在虚拟水听器位于实际水听器之间。因此，通过海底地震（OBS）台站的粗采样记录的数据原则上可以被转换成具有P倍于OBS台站的虚拟勘测，其中P是可见的自由表面相关的多次折射的阶数。关键的假设是折射到达是那些头波，而不是纯粹的潜水波。用合成的OBS数据和台湾近海的OBS数据验证了该方法的有效性。结果表明，远偏移距道的成功重建达到了120 km的源-接收器偏移距。主超虚拟轨迹将可拾取的初至波的数量从大约1600增加到大于3100，用于源仅在记录站的一侧的OBS数据集的子集。此外，与一阶自由表面折射多次波相关联的首波允许创建在位于实际OBS站之间大约一半处的虚拟OBS站处记录的六个新的公共接收器道集。与原始调查相比，这使OBS站点的数量增加了一倍，并将可拾取的轨迹总数从约1600条增加到6200多条。总之，我们对OBS数据的结果表明，折射干涉测量有时可以使可用道数增加四倍以上，增加源-接收器偏移，用更密集的OBS站分布填充接收器线，并提供更可靠的初至拾取。这种方法的一个潜在的可能性是，由干涉测量提取的长偏移距折射波不一定是来自更深折射界面的首波。提取的波至可能来自较浅的界面，因此仅提供关于该部分地下的冗余信息。尽管如此，我们的层析成像的例子显示了这些到达的价值，在减少伪影和提高分辨率的层析成像。
SUMMARY The theory of supervirtual interferometry is modified so that free-surface related multiple refractions can be used to enhance the signal-to-noise ratio (SNR) of primary refraction events by a factor proportional to √ Ns, where Ns is the number of post-critical sources for a specified refraction multiple. We also show that refraction multiples can be transformed into primary refraction events recorded at virtual hydrophones located between the actual hydrophones. Thus, data recorded by a coarse sampling of ocean bottom seismic (OBS) stations can be transformed, in principle, into a virtual survey with P times more OBS stations, where P is the order of the visible free-surface related multiple refractions. The key assumption is that the refraction arrivals are those of head waves, not pure diving waves. The effectiveness of this method is validated with both synthetic OBS data and an OBS data set recorded offshore from Taiwan. Results show the successful reconstruction of far-offset traces out to a source‐receiver offset of 120km. The primary supervirtual traces increase the number of pickable first arrivals from approximately 1600 to more than 3100 for a subset of the OBS data set where the source is only on one side of the recording stations. In addition, the head waves associated with the first-order free-surface refraction multiples allow for the creation of six new common receiver gathers recorded at virtual OBS station located about half way between the actual OBS stations. This doubles the number of OBS stations compared to the original survey and increases the total number of pickable traces from approximately 1600 to more than 6200. In summary, our results with the OBS data demonstrate that refraction interferometry can sometimes more than quadruple the number of usable traces, increase the source‐receiver offsets, fill in the receiver line with a denser distribution of OBS stations, and provide more reliable picking of first arrivals. Apotentialliabilityofthismethodisthatlong-offsetrefractionarrivalsextractedbyinterferometry might not necessarily be head waves from deeper refraction interfaces. The extracted arrivals might be from a shallower interface, and so only supply redundant information about that portion of the subsurface. Nevertheless, our tomography example shows the value of these arrivals in reducing artefacts and increasing resolution in the tomogram.