Analysis of Ambient Seismic Noise Recorded by Downhole and Ocean-Bottom Seismometers on Deep Sea Drilling Project Leg 78B
Analysis of Ambient Seismic Noise Recorded by Downhole and Ocean-Bottom Seismometers on Deep Sea Drilling Project Leg 78B
复制标题
深海钻探项目78B段井下和海底地震仪记录的环境地震噪声分析
DOI：
10.2973/dsdp.proc.78b.112.1984
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发表时间：
1984
期刊：
影响因子：
--
通讯作者：
T. Jordan
中科院分区：
文献类型：
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作者：
R. G. Adair;J. Orcutt;T. Jordan
海洋地壳深处的环境地震噪声是利用海洋地震系统的数据来表征的，海洋地震系统是一个垂直分量、数字记录、短周期地震仪系统，是深海钻探项目第78 B航次部署的钻孔仪器的一部分。仪器包在地下室516 m处的395 A孔中无阻尼。在0.16和2.2 Hz之间获得了可靠的微震噪声水平估计值;仪器噪声在该频带之外占主导地位。所观察到的微震噪声在1小时的时间尺度上是准稳态的，但不是10个。虽然频谱形状是稳定的，噪声幅度增长，随着时间的推移超过26小时，观察期3至5分贝。钻孔噪声水平与当地涌浪高度同时增加，表明存在因果关系。实验早期获得的位移功率密度估计值在0.21 Hz时的峰值为4 × 10 nm/ Hz，并以80 dB/decade从0.33 Hz时的1 × 10 nm/Hz降至1.9 Hz时的1 nm/Hz。在395 A号钻孔附近的海底观测到的噪声水平比钻孔中观测到的噪声水平高出一个系数，该系数随频率从0.2 Hz时的10 dB增加到2 Hz时的28 dB。这是一致的噪声传播的基本模式斯通利波附近的沉积物/海水界面被困。如果在第78 B航次期间观察到的海底和海底以下的噪音之间的关系是一般性的，那么海底钻孔噪音水平可能接近安静大陆地点的噪音水平。引言在制定部署海洋地震仪系统的最佳战略时，需要了解海底附近的环境地震噪声。然而，目前还没有关于海洋地壳内噪音的已发表研究，关于海底噪音的研究也很少，尤其是可靠的绝对噪音水平。大多数可用的测量是在20世纪60年代由高级研究计划局赞助的VELA制服项目期间进行的。人们希望海底的噪音水平与陆地上的噪音水平相当（Prentiss和Ewing，1963年），但事实上，海底的噪音水平要高得多（布拉德纳和Dodds，1964年; Schneider和Backus，1964年; Schneider等人，1964;莱瑟姆和萨顿，1966;莱瑟姆和诺鲁齐，1968）。这些研究表明，噪声作为捕获在海底/海底界面附近的倏逝波（斯通利波）传播（布拉德纳等人，1965年;莱瑟姆和萨顿，1966年;莱瑟姆和诺鲁齐，1968年），因此，预计海洋地壳内的噪音水平会降低。在海底植入仪器所需的技术和专业知识直到最近才得到开发，主要是由深海钻探项目（DSDP）开发的。钻孔地震仪已经在几次DSDP巡航中用于倾斜地震实验（Stephen等人，1980年，Stephen等人，1983年），但这些数据不适合于地震噪声的定量分析。由美国国防高级研究计划局赞助的一种井下地震仪系统，即海洋地震系统（MSS）。D、Salisbury，M. H、例如，Init.共和党人DSDP，78 B：华盛顿（美国政府印刷局）. 2地址：地质研究部，加州大学斯克里普斯海洋研究所，圣地亚哥，拉霍亚，CA 92093。在78 B航次期间在395 A孔（图1）中使用，目的是记录高质量的噪声和地震数据。本章介绍了海洋地壳内环境地震噪声的第一个定量特征。此外，使用附近海底地震仪（OBS）采集的数据直接比较海底和海底以下的噪声特性（见图1）。MSS的钻孔仪器包括健康状况传感器和两个垂直分量短周期地震仪（Teledyne Geotech S-700型），一个配置在另一个下面约50厘米。每个地震检波器的输出分为三个通道，六个通道的增益交错重叠，产生144 dB的动态范围（图2）。数据流经过过滤以防止混叠，使用10位数字字以75个样本/秒的速度进行数字化，并通过电缆传输到船上的记录器。该电缆还用于回收钻孔仪器包。地震计和滤波器组合级的位移响应计算结果如图3所示。响应在11 Hz附近达到峰值，并以12 dB/倍频程滚降到较低频率，以15 dB/倍频程滚降到奈奎斯特频率。从Glomar Challenger下放钻孔包至395 A孔（516 m地下室）的609 m次底部深度，此处塌方堵塞了55 m底部的通道。包裹放置29.5小时，但数据仅在最后26小时内记录，其中包括折射实验（见图4）。虽然地震检波器没有固定在洞里，但邻近的未破裂的大块围岩无疑促进了适当的耦合。(The围岩特征是根据第78 B航次早期进行的井径测井和声学反射测井推断的。）通过在海底铺设0.7 km的松弛电缆来平息传送到包中的船舶运动。在折射实验中，在折射拍摄之间和休息期间记录了有意和无意（当炸药未能引爆时）的噪音样本。相关操作和事件的时间如图4所示。得克萨斯大学地球物理研究所的地震学家从林奇号上扔下了四台三轴海底地震仪。每个OBS被编程为在折射激发期间以8.352ms的采样间隔记录40 s的数字数据
Ambient seismic noise at depth in the ocean crust is characterized using data from the Marine Seismic System (MSS), a vertical-component, digitally recording, short-period seismograph system which was part of the borehole instrumentation deployed on Deep Sea Drilling Project Leg 78B. The instrument package rested undamped in Hole 395A, 516 m sub-basement. Reliable estimates of microseismic noise levels were obtained between 0.16 and 2.2 Hz; instrument noise dominated outside this band. The observed microseismic noise was quasi-stationary on a time scale of 1 hr., but not 10. Although spectral shapes were stable, noise amplitudes grew with time over the 26-hr, observation period by 3 to 5 dB. The borehole noise levels increased concurrently with local swell height, suggesting a causal relationship. An estimate of displacement power densities obtained early in the experiment had a peak value of 4 x 10 nm/ Hz at 0.21 Hz, and decreased at 80 dB/decade from 1 × 10 nmVHz at 0.33 Hz to 1 nm/Hz at 1.9 Hz. Noise levels observed at the seafloor near Hole 395A were greater than those observed in the borehole by a factor which increased with frequency from 10 dB at 0.2 Hz to 28 dB at 2 Hz. This is consistent with noise propagating as a fundamental-mode Stoneley wave trapped near the sediment/seawater interface. If the relationship observed between noise at and below the seafloor during Leg 78B is a general one, ocean-bottom borehole noise levels could approach those at quiet continental sites. INTRODUCTION A knowledge of ambient seismic noise near the seafloor is needed in formulating optimal strategies for the deployment of marine seismometer systems. There are, however, no published studies of noise within the ocean crust and few of noise at the seafloor, especially of reliable, absolute noise levels. Most available measurements were made during the VELA Uniform Project sponsored by the Advanced Research Projects Agency in the 1960s. It was hoped that noise levels at the seafloor would be comparable to those on land (Prentiss and Ewing, 1963), but in fact they were found to be significantly higher (Bradner and Dodds, 1964; Schneider and Backus, 1964; Schneider et al., 1964; Latham and Sutton, 1966; Latham and Nowroozi, 1968). These studies suggest that the noise propagates as an evanescent wave (Stoneley wave) trapped near the ocean bottom/seafloor interface (Bradner et al., 1965; Latham and Sutton, 1966; Latham and Nowroozi, 1968), so reduced noise levels are expected within the ocean crust. The technology and expertise needed to implant instruments in the seafloor has only recently been developed, primarily by the Deep Sea Drilling Project (DSDP). Borehole seismometers have been used in oblique seismic experiments on several DSDP cruises (Stephen et al., 1980, Stephen et al., 1983), but the data are not suited for the quantitative analysis of seismic noise. A downhole seismometer system sponsored by the Defense Advanced Research Projects Agency, the Marine Seismic System (MSS), was deHyndman, R. D., Salisbury, M. H., et al., Init. Repts. DSDP, 78B: Washington (U.S. Govt. Printing Office). 2 Address: Geological Research Div., Scripps Institution of Oceanography, Univ. of California, San Diego, La Jolla, CA 92093. ployed in Hole 395A (Fig. 1) during Leg 78B with the intent of recording high-quality noise and seismic data. This chapter presents the first quantitative characterization of ambient seismic noise within the ocean crust. In addition, noise characteristics at and below the seafloor are directly compared using data acquired with a nearby ocean-bottom seismograph (OBS) (See Fig. 1). INSTRUMENTATION AND OPERATIONS The borehole instrumentation of the MSS consisted of state-ofhealth sensors and two vertical-component, short-period seismometers (Teledyne Geotech model S-700'), one configured approximately 50 cm below the other. Each seismometers output was split into three channels, and the gains of the resultant six channels were staggered with overlap to yield a dynamic range of 144 dB (Fig. 2). The data streams were filtered to prevent aliasing, digitized at 75 samples/s using a 10-bit digital word, and transmitted via cable to a shipboard recorder. The cable was also used to recover the borehole instrumentation package. The computed displacement response for the combined seismometer and filter stages is shown in Figure 3. The response is peaked near 11 Hz, and rolls off at 12 dB/octave toward lower frequencies and at 15 dB/octave to the Nyquist frequency. The borehole package was lowered from the Glomar Challenger to a sub-bottom depth of 609 m in Hole 395A (516 m sub-basement), where cave-ins blocked access to the bottom 55 m. The package was in place for 29.5 hrs., but data were recorded only during the final 26 hrs., which included a refraction experiment (see Fig. 4). Although the seismometers were not clamped in the hole, the adjacent unfractured and massive wall rock doubtless promoted proper coupling. (The wallrock character was inferred from caliper and acoustic reflection logs conducted earlier during Leg 78B.) Ship motions conveyed to the package were quelled with 0.7 km of slack cable payed out on the seafloor. Noise samples were recorded between refraction shots and during breaks in the refraction experiment, both intentional and unintentional (when explosives failed to detonate). The times of relevant operations and events are shown in Figure 4. Four triaxial ocean-bottom seismographs (OBSs) were dropped from the Lynch by seismologists from the University of Texas (UT) Institute of Geophysics. Each OBS was programmed to record 40 s of digital data with a sampling interval of 8.352 ms during the refraction shots