Petrel Sub-basin Marine Survey (GA0335/SOL5463) : Sub Bottom Profiler Processing Report

The Petrel Sub-basin Marine Survey (GA0335/SOL5463) was undertaken in May 2012 by Geoscience Australia in collaboration with the Australian Institute of Marine Science (AIMS), as part of the Australian Government's National Low Emission Coal Initiative (NLECI). Its purpose was to acquire pre-competitive geophysical and biophysical data on shallow seabed environments within two targeted areas to support assessment of CO2 storage potential. The geophysical acquisition consisted of multibeam sonar mapping of sea floor morphology and multi-channel sub bottom profiling of the shallow sub surface geology. The aim of sub bottom profiling was to investigate regional seal breaches and potential fluid pathways by providing high resolution images connecting the sea floor map to regional seismic reflection data acquired concurrently in the area.
The sub bottom profiler data were acquired aboard the AIMS research vessel (RV) Solander along 51 lines, totalling 654 line km in the Petrel Sub-basin of the Bonaparte Basin. Acquisition employed a Squid 2000 sparker as the source and a 24 channel Microeel streamer for the receivers. Group interval was 3.125 m and shot interval 6.25 m, resulting in 6 fold data. Record length was 500 ms with a sample interval of 0.25 ms.
Some problems in acquisition needed to be addressed in processing. Firstly, sea conditions were far from smooth for most of the voyage. Obvious relative motion occurred between the source and the streamer, and along the streamer itself, due to the ocean swell. In some cases, acquisition commenced while the vessel was still turning onto the line and the streamer was not straight in line behind the stern. Finally, malfunction of the sparker on some half dozen lines resulted in gaps in the coverage, which could not be filled in later, due to bad weather reducing the time for the survey.
Multichannel seismic reflection processing was able to compensate for some of the limitations of sparker acquisition. Mutes and filters were necessary to remove the worst of the noise, including leaked timing pulse and swell noise. Surface related multiple elimination (SRME) successfully attenuated the water bottom and later multiples. Non surface consistent trim statics were able to correct for the relative motion of the sparker and the streamer, thereby allowing alignment of reflections prior to stack, which improved the signal to noise. Minimum entropy deconvolution was a critical step in both suppressing ghosting and enhancing latent high frequencies in the data, thus improving the resolution. Migration was necessary to correctly image small channels by collapsing diffractions. Finally tidal static corrections were essential to remove mis-ties in high frequency data.
The processing stream has been well documented, along with scripts employed to handle the large amount of data efficiently and consistently. This record is a manual for a much more rigorous way of processing multi-channel sparker data, and details a work flow that can be implemented within Geoscience Australia and used for future surveys.
The final migrated seismic data proved to be very high resolution, allowing delineation of multiple episodes of channelling in the top 100 m of sediment. Comparison of the sub bottom profiles with older regional seismic reflection data showed just how much more detail is available in the region critical for mapping deeper faults and fluid pathways to features on the sea floor. Acquisition and processing of the sub bottom profiler data surpassed the survey expectations.

海燕次盆地海洋调查（GA0335/SOL5463）于2012年5月由澳大利亚地球科学局（Geoscience Australia）联合澳大利亚海洋科学研究所（Australian Institute of Marine Science, AIMS）共同实施，作为澳大利亚政府国家低排放煤炭倡议（National Low Emission Coal Initiative, NLECI）的组成部分。本次调查旨在针对两个目标区域采集浅海海底环境的竞争前地球物理与生物物理数据，以支撑二氧化碳封存潜力评估工作。地球物理数据采集工作涵盖海底地形多波束声纳测绘与浅层地下地质的多道海底剖面测量。海底剖面测量的目标为，通过建立海底地图与该区域同期采集的区域地震反射数据之间的关联，获取高分辨率图像，以研究区域盖层破坏及潜在流体运移通道。
海底剖面测量数据由澳大利亚海洋科学研究所科考船“索兰德号”（RV Solander）采集，共布设51条测线，在波拿巴盆地（Bonaparte Basin）的海燕次盆地内总测线长度达654测线公里。本次采集采用Squid 2000电火花震源作为发射源，搭配24道Microeel拖缆作为接收装置。道间距为3.125米，炮间距为6.25米，最终得到6次覆盖的数据。记录时长为500毫秒，采样间隔为0.25毫秒。
数据采集过程中存在若干问题，需在处理阶段予以解决。首先，航行大部分时段海况均不平稳，受海浪影响，震源与拖缆之间以及拖缆自身均出现明显相对运动。部分测线的采集工作在船舶仍处于转向阶段时即已启动，导致拖缆未与船尾保持直线对齐。最后，约6条测线的电火花震源出现故障，造成覆盖区域出现空白，且因恶劣天气压缩了调查时长，后续无法补测。
多道地震反射处理可弥补电火花震源采集的部分局限性。需通过切除与滤波去除最严重的噪声，包括泄露的计时脉冲及海浪噪声。基于地表的多次波压制（Surface Related Multiple Elimination, SRME）可有效压制水底多次波及后续多次波。非地表一致性剩余静校正可校正震源与拖缆之间的相对运动，从而在叠加前实现反射波的对齐，提升了信噪比。最小熵反褶积是压制鬼波、增强数据中潜在高频分量的关键步骤，进而提升了数据分辨率。偏移成像可通过压缩绕射波，正确刻画小型水道的地下图像。最后，潮汐静校正对于消除高频数据中的时差偏差至关重要。
本次处理流程已得到完整记录，同时配套了可高效、一致地处理海量数据的脚本代码。本数据集说明文档为多道电火花震源数据的高标准处理提供了操作指南，详细阐述了可在澳大利亚地球科学局内部推行、并可应用于未来海洋调查的工作流程。
最终的偏移成像地震数据具有极高分辨率，可清晰刻画沉积物表层100米内多期水道的分布特征。将海底剖面数据与早期区域地震反射数据进行对比后可见，在用于绘制深层断层及海底流体运移通道的关键区域，本次采集数据可提供更为丰富的细节信息。海底剖面测量数据的采集与处理效果超出了本次调查的预期目标。