The seismometer-detached ocean bottom seismograph and its experiments in the Gakkel Ridge,Arctic Ocean
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摘要:
中国第十二次北极科学考察在密集浮冰覆盖的北冰洋加克洋中脊完成了国际首次大规模主动源海底地震仪探测,成功回收了43台海底地震仪中的42台,其中5台分体式海底地震仪均成功回收。本文介绍了针对该航次中面临的海底地震仪回收定位难题研发的一款新型分体式海底地震仪,其主要特点为:① 集成超短基线信标和声学应答系统,实现双重定位保障;② 使用分体式结构增强地震计与海底的耦合,有效提高信噪比;③ 选用国产地震计和浮力材料,实现核心部件国产化。分体式海底地震仪在北冰洋加克洋中脊的试验结果显示:地震背景噪声能量低,且记录的远震波形、两个近源微地震波形和三分量主动源地震记录均具有清晰可见的震相信息,表明该分体式海底地震仪能够满足冰下海底勘探需求。
Abstract:The ocean bottom seismograph (OBS) is an indispensable instrument for investigating the deep crust-mantle structure. China’s 12th Arctic scientific expedition marked a significant milestone in marine seismic exploration by achieving the first large-scale active exploration along the challenging Gakkel Ridge in the Arctic Ocean, where ice coverage poses formidable obstacles. Impressively, the expedition successfully recovered 42 out of 43 deployed OBSs, with all five seismometer-detached OBSs retrieved intact. This paper introduces a seismometer-detached OBS specifically developed to overcome the unique challenges encountered during the expedition, particularly in terms of OBS recovery and positioning. Its main features include:
1) The newly developed seismometer-detached OBS integrates cutting-edge technology to enhance its functionality and performance under harsh Arctic conditions. One of its key innovations is the combination of ultra-short baseline beacons and advanced acoustic response short baseline array positioning systems, which improves positioning accuracy through dual positioning technology. The positioning accuracy error of the short baseline array is within ±100 m, and the mature Sonardyne ultra-short baseline array loaded on the Xuelong-2 polar scientific research icebreaker can achieve a positioning accuracy up to one thousandth of the water depth. The use of such a dual positioning system can ensure accurate positioning of the instrument even in areas with dense ice layers. This ground-breaking design significantly improves the accuracy and efficiency of OBS recovery operations, enabling researchers to locate and recover instruments with unprecedented reliability and precision.
2) Referencing the current status of seismometer-detached OBSs both domestically and internationally, a titanium alloy seismic instrument chamber with a buoyancy of 17 kg in water has been designed. This chamber is incorporated into the seismic instrument separation structure, independently installed inside the OBS hull via flexible cables. Upon reaching the seafloor, it is timed to release, optimizing the coupling between the seismic instrument and the seabed. This structure also serves to reduce the impact of bottom currents on the seismometer. By enhancing this coupling, the OBS effectively improves the signal-to-noise ratio for recording seismic data, thus minimizing ambient noise and interference. Consequently, researchers can obtain clearer and more accurate seismic records, thereby facilitating a deeper understanding of the Earth’s geological processes and tectonic activities.
3) The OBS utilizes advanced domestic seismic instruments and buoyancy materials. On one hand, this provides new options for the materials used in OBS development, to some extent alleviating the problem of insufficient component production, which is advantageous for the large-scale production and industrialization. On the other hand, it enhances the flexibility of the OBS’s design, enabling the possibility of loading multi-functional modules onto the OBS. Additionally, using domestically produced seismometers makes it easier to optimize and develop hardware and software according to scientific research requirements. This signifies a significant step towards achieving self-sufficiency in marine instrument technology. This domestic production capacity not only enhances China’s scientific research capabilities, but also promotes innovation and technological advancement in the field of marine instruments, establishing independent intellectual property rights for key technologies of marine seismographs.
China’s 12th Arctic scientific expedition has yielded promising results, with the seismometer-detached OBS demonstrating exceptional performance in recording seismic signals. It was capable of collecting seismic data in the low-level horizontal seismic ambient noise environment along the Gakkel Ridge of the Arctic Ocean. Notably, the OBSs have exhibited low horizontal seismic ambient noise levels, underscoring their suitability for seismic exploration in ice-covered marine environments. Additionally, these OBSs successfully collected the waveform records of a small teleseismic, two micro-earthquakes and three-component active seismic exploration. This validates to some extent the effectiveness of the seismometer-detached structure in improving the signal-to-noise ratio, indicating that this type of OBS can meet the requirements for submarine exploration under the ice.
The successful development and application of the seismometer-detached OBS provide valuable experience for future submarine seismic exploration in extreme environments. Looking ahead, there are opportunities for further enhancement in various aspects such as real-time data transmission, longer battery life, and integration of more modularized sensors. The use of flexible buoyancy materials also liberates the submarine seismograph from the limitations imposed by traditional glass chambers. This enables the configuration of different types of modularized sensors, thus paving the way for the development of a versatile submarine observation platform with powerful functionality.
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引言
一般结构抗震设计时,仅考虑水平向地震作用的影响,往往忽视了竖向地震作用对结构产生的影响(王国权,2001;周正华等,2007)。在我国抗震设计中,仅遇到高烈度区或近场的大跨结构、悬臂结构和超高层工程结构才考虑竖向地震动的作用,将竖向地震动影响系数的最大值取为水平向的0.65倍(中华人民共和国住房和城乡建设部,中华人民共和国国家质量监督检验检疫总局,2010)。然而,近年来发生的几次强地震,在近场产生了较强的竖向地震动,部分竖向峰值加速度远远超过水平向峰值加速度。许多研究人员对这一特征进行了深入研究,其结果显示竖向与水平向的加速度谱比为一周期函数,而且与场地条件相关,短周期内的谱比大于2/3而长周期的谱比小于2/3 (Bozorgnia,Niazi,1995;Dimitriu et al,1999;周正华等,2003;Ambraseys,Douglas,2003;耿淑伟,陶夏新,2004;徐龙军,谢礼立,2007;贾俊峰,欧进萍,2010;李恒,秦小军,2010;谢俊举等,2010)。
近年来,南北地震带的川滇甘陕地区发生了大量的破坏性地震(汶川地震、玉树地震、芦山地震等),该地区布设了大量的强震台站,因而获得了大量的强震动数据。随着各种特殊结构物的大量兴建,竖向地震作用的研究受到了工程界的广泛重视(徐龙军,谢礼立,2007;李恒,秦小军,2010;齐娟等,2014)。目前,直接以我国大量强震动记录为数据基础进行的竖向地震动研究相对来说较少,为此,本文将以南北地震带川滇甘陕地区的强震动记录(M≥4)为研究对象,统计分析不同场地、震中距和震级等因素下的竖向与水平向的反应谱比,以期为工程结构的竖向抗震设计提供一定的参考依据。
1. 强震动记录来源与分组
本文选取南北地震带川滇甘陕地区最大峰值加速度(peak ground acceleration,简写为PGA)≥10 cm/s2 的强震动记录作为统计资料,剔除明显不合理的强震动记录并进行基线校正处理,最终获得研究区域内有详细场地条件的强震动记录802组。为了更好地统计分析,按照大震近场、大震远场、小震近场、小震远场的分类对所获取的数据进行分类(赵培培,2017;赵培培等,2017),其柱状图结果绘于图1。由于我国场地类别划分中Ⅱ类场地的分布范围较广,因此所得的Ⅱ类场地上的强震动记录较多,又由于该地区发生大震的次数有限,所以图1中小震远场Ⅱ类场地的强震动记录较多,这可能会对统计分析结果产生一定的影响。图2给出了不同类别场地上强震动记录随震级、震中距的变化趋势。
2. 竖向与水平向反应谱比的总体特征
本文先计算出所用强震动记录中阻尼比为0.05的加速度谱。为了更好地分析竖向与水平向反应谱比的总体特征,对每组强震动记录计算谱比,然后再对谱比进行平均,其结果如图3所示。可见:竖向与水平向的谱比并不是规范规定的一个定值0.65,而是随着周期而变化;谱比P(T)总体的平均值为0.57,比规范所规定的0.65要偏小些;0—0.1 s周期内的谱比曲线存在一峰值0.71,远高于0.65;其余大部分周期内的谱比值均小于0.65。因此,竖向反应谱不应简单地取为水平向反应谱的一定值,而应考虑其随周期的变化。
3. 竖向与水平向反应谱比与场地类别的关系
大量的研究表明,竖向与水平向的谱比主要受场地类别的影响(耿淑伟,陶夏新,2004;贾俊峰,欧进萍,2010;李恒,秦小军,2010)。本文按照小震、大震、场地类别将同场地类别的强震动记录谱比进行平均,得到平均谱比曲线,如图4所示,其中小震的谱比曲线周期最长取到3 s,大震的谱比曲线周期最长取到10 s。表1列出了小震、大震情况下不同场地的平均谱比分段均值。
表 1 不同场地各周期段的谱比平均值Table 1. Average spectral ratio for several periods on different sites周期T/s 谱比均值 Ⅰ类 Ⅱ类 Ⅲ类 小震 <0.1 0.57 0.74 0.58 [0.1—1.0) 0.53 0.56 0.38 [1.0—3.0] 0.56 0.67 0.42 [0—3.0] 0.55 0.65 0.41 大震 <0.1 0.65 0.63 0.49 [0.1—1.0) 0.63 0.54 0.36 [1.0—3.0) 0.87 0.76 0.46 [3.0—10.0] 1.25 0.97 0.54 [0—10.0] 1.11 0.88 0.51 从图4可看出,场地类别对竖向与水平向谱比的影响比较显著,不同场地的平均谱比曲线的形状基本相似,Ⅰ类和Ⅱ类场地基本呈两边高、中间低的现象,但不同场地的谱比变化规律有所不同。小震时,Ⅱ类场地的谱比值基本上要高于Ⅰ类场地;0—0.1 s周期内,Ⅱ类场地的谱比值要高于 《建筑抗震设计规范》 (以下简称为“规范”)(中华人民共和国住房和城乡建设部,中华人民共和国国家质量监督检验检疫总局,2010)所规定的0.65,且峰值可达0.9,Ⅰ类场地和Ⅲ类场地的谱比值基本低于0.65;0.1—1.0 s周期内,除个别Ⅲ类场地的谱比值稍高于0.65外,其余情况下的谱比值均远远小于“规范”所规定的0.65;1.0—3.0 s周期范围内,Ⅱ类场地的谱比值大部分要高于0.65,其余情况均小于0.65,Ⅲ类场地的谱比值在周期大于0.1 s时产生了剧烈的上下振荡现象,造成这种现象的原因可能是该场地类别上的强震动记录数量较少而引起的统计偏差。大震时,随着场地变软,全周期段的谱比值不断地减小;Ⅲ类场地的谱比值在全周期内均小于0.65;周期大于1.0 s的范围内Ⅰ类和Ⅱ类场地的谱比值随周期的增大呈增大趋势,而且远远高于“规范”所规定的0.65,Ⅰ类场地的峰值高达1.41,Ⅱ类场地的峰值也在1.05。从大震统计的结果来看,该结果与谢俊举等(2010)关于汶川主震竖向与水平向谱比值的统计分析结果基本相近。
从表1可看出:小震时,Ⅱ类场地在<0.1 s和1.0—3.0 s周期范围内的谱比均值高于0.65,其余情况下的谱比均值均小于0.65;大震时,不同周期段的谱比均值均随着场地变软而降低;全周期段的平均谱比值除Ⅲ类场地外均远远高于规范规定的0.65;除小于0.1 s周期外,Ⅰ类和Ⅱ类场地的中长周期平均谱比均值远高于0.65。
4. 竖向与水平向反应谱比与震级的关系
按照震级区间 [ 4—5.5), [ 5.5—6.5), [ 6.5—7.5)和≥7.5分组,求取每个震级区间内的谱比均值,从而得到同场地不同震级区间内的平均谱比P(T)曲线,如图5所示;将图5中不同震级区间的谱比曲线进行分段求其平均值,结果列于表2。
表 2 不同震级区间的平均谱比均值列表Table 2. Average spectral ratio of different magnitude intervals场地类别 周期T/s 平均谱比均值 4.0≤M<5.5 5.5≤M<6.5 6.5≤M<7.5 M≥7.5 Ⅰ类 <0.1 0.53 0.67 0.69 0.63 [0.1—1.0) 0.49 0.67 0.63 0.60 [1.0—3.0) 0.50 0.79 0.83 0.87 [3.0—10.0] 0.43 0.81 1.06 1.19 [0—10.0] 0.45 0.79 0.97 1.07 Ⅱ类 <0.1 0.74 0.74 0.61 0.64 [0.1—1.0) 0.57 0.54 0.55 0.54 [1.0—3.0) 0.64 0.74 0.75 0.76 [3.0—10.0] 0.59 0.79 0.99 0.95 [0—10.0] 0.60 0.76 0.90 0.87 Ⅲ类 <0.1 0.57 0.58 – 0.40 [0.1—1.0) 0.46 0.36 – 0.38 [1.0—3.0) 0.50 0.37 – 0.45 [3.0—10.0] 0.54 0.59 – 0.44 [0—10.0] 0.53 0.53 – 0.44 从图5可看出,震级对不同场地条件下的竖向与水平向反应谱比有一定的影响。Ⅱ类场地下,反应谱比在周期小于1.0 s的范围内随震级的增大呈减小的趋势,在大于1.0 s的周期内随着震级的增大呈增大的趋势;Ⅱ类场地下,不同震级区间的谱比曲线均在0.1 s周期以内出现一峰值,而后迅速下降至谷底,谷底位置基本处于0.3 s周期,谷底过后则随着周期的增大谱比值有上升的趋势;Ⅱ类场地下,不同震级区间的谱比曲线表现为“马鞍形”,在>3.0 s的周期内,谱比值随着震级的增大呈增大的趋势。当M≥5.5时,Ⅰ类与Ⅱ类场地不同震级的平均谱比在周期>1.0 s时均远远大于0.65,而且平均谱比峰值远高于1.0。由于Ⅲ类场地上的强震数量过少,所以在分析震级对谱比的影响时存在一定的偏差,尚待收集更多的Ⅲ类场地资料进一步分析。
由表2的统计结果可以看出:Ⅰ类和Ⅱ类场地在M≥5.5的各个震级区间的平均谱比除个别外基本高于0.65,尤其是长周期段的谱比均值远高于“规范”规定的0.65定值,有些谱比均值甚至大于1.0;Ⅰ类和Ⅱ类场地在M≥5.5的整个周期内的谱比均值要远高于0.65;Ⅰ类和Ⅱ类场地的平均谱比均值随震级的增大而增大;Ⅲ类场地上的谱比均值基本在0.65以下。
5. 竖向与水平向反应谱比与震中距的关系
由于Ⅰ类和Ⅲ类场地的强震数据较少,本文仅就强震动数据比较丰富的Ⅱ类场地,对小震和大震下不同震中距的平均谱比进行统计分析,各分组的平均谱比曲线如图6所示;将不同震中距的平均谱比进行分段求其平均值,其计算结果列于表3。
表 3 不同震中距R下的平均谱比值Table 3. Average spectral ratio within different epicentral distance R ranges周期T/s 平均谱比值 R<20 km 20 km≤R<60 km 60 km≤R<100 km R≥100 km 小震 <0.1 0.67 0.75 0.77 0.69 [0.1—1.0) 0.49 0.57 0.57 0.62 [1.0—3.0] 0.55 0.68 0.70 0.70 [0.0—3.0] 0.54 0.65 0.66 0.67 周期T/s 平均谱比值 R<50 km 50 km≤R<100 km 100 km≤R<200 km R≥200 km 大震 <0.1 0.71 0.75 0.64 0.53 [0.1—1.0) 0.52 0.53 0.55 0.55 [1.0—3.0) 0.84 0.78 0.82 0.67 [3.0—10.0] 0.87 0.96 1.13 0.89 [0.0—10.0] 0.83 0.89 1.01 0.81 由图6可见:不同震中距的平均谱比曲线的形状基本一致,不管是小震还是大震,其谱比曲线均呈中间低、两边高的变化趋势,而且均在小于0.1 s周期内出现一峰值,在0.1—1.0 s周期内出现一波谷,在周期大于1.0 s后随着周期的增加不同震级的平均谱比值也在不断增大;小震情况下,不同震中距的平均谱比在短周期内的谱比峰值均在0.8以上,小震远场在周期大于1.0 s时的谱比峰值均高于0.65;大震情况下,0.1 s周期内、200 km震中距以内的谱比峰值高于0.65,周期大于1.0 s的平均谱比峰值均高于1.0,远远高于“规范”所规定的0.65定值,说明该地区的大震在周期大于1.0 s时具有较大的竖向地震作用。
从表3可知:不同震中距的平均谱比均值在0.1—1.0 s周期内均低于0.65,在周期>1.0 s时小震远场的谱比均值高于0.65,在全周期内的谱比均值基本处于0.65左右;大震在0.1 s周期内、震中距小于100 km时的谱比均值大于0.65,在周期>1.0 s时不同震中距的谱比均值均远远高于0.65,全周期内不同震中距的谱比均值均高于0.8,远远高于“规范”所规定的0.65,这说明“规范”中规定的0.65有待商榷。
6. 讨论与结论
本文以我国南北地震带川滇甘陕地区的M≥4.0强震动资料为基础,对竖向与水平向加速度反应谱比按照总体特征、场地、震级和震中距进行了统计分析。结果显示:竖向与水平向反应谱的谱比并不是某一定值,而是随周期的变化而变化;竖向与水平向反应谱比值会受到场地、震级和震中距等因素的影响,平均谱比值在大震时随着场地条件的变软而变小,Ⅱ类场地在周期大于1.0 s时的平均谱比值随震级的增大而增大,不管何种情况下,Ⅱ类场地的平均谱比曲线均呈两边高、中间低的特征。对谱比的分段求平均值可知,无论是何种分组情况,平均谱比均值在0.1—1.0 s周期内基本低于“规范”所规定的0.65定值, Ⅰ类和Ⅱ类场地下周期大于1.0 s时的大震谱比均值远远高于0.65,Ⅱ类场地下周期大于1.0 s时的小震远场以及5.5≤M<6.5时的谱比均值均大于0.65; Ⅰ类和Ⅱ类场地下,全周期段的谱比均值基本上大于0.65。然而,大部分的抗震规范取水平向地震作用的0.65倍作为竖向地震作用,这显然不太合适,因此本文建议根据现有的竖向强震动记录,按照水平向反应谱的标定方式来确定竖向地震作用,从而将其应用于结构的竖向抗震设计。
此外,相对于Ⅱ类场地, Ⅰ类和Ⅲ类场地的强震动记录较少,因此关于Ⅰ类和Ⅲ类场地的统计分析有待于积累更多的强震动记录。
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图 6 加克洋中脊分体式OBS15台站位置(a)及其记录的2021年海地MW5.8地震波形(b)
波形图使用0.04—0.2 Hz带通滤波进行了处理。下行小图分别为可识别的初至P波和PcP,PKiKP,初至S波和SKS和ScS的放大图。使用IASP91模型对上述地震波到时进行了预测,见图中红色和蓝色实线
Figure 6. The location of the seismometer-detached OBS15 station in the Gakkel Ridge (a) and the waveforms (b) of the Haiti MW5.8 earthquake in 2021
The waveforms were processed with bandpass filtering (0.04−0.2 Hz). The figures in the second row are enlarged views of identifiable P-wave and PcP,PKiKP,S-wave and SKS,ScS waveforms,respectively. The arrival times of these seismic waves were predicted using the IASP91 model,indicated by the red and blue solid lines in the figures
图 2 短基线阵及其定位结果
(a) 短基线阵定位系统在雪龙2号月池车间的安装情况;(b) 短基线阵试验定位三维结果
Figure 2. The short baseline array and its positioning results
(a) The installation of the short baseline array positioning system in the moon pool workshop of the Xuelong-2;(b) The three-dimensional results of the short baseline array experimental positioning
图 3 超短基线在北冰洋加克洋中脊分体式海底地震仪回收过程中的三维实时定位结果
图(a)和(b)分别为分体式海底地震仪OBS9和OBS15的上浮轨迹
Figure 3. The three-dimensional real-time positioning results during the recovery process of the seismometer-detached OBS with ultra-short baseline in the Gakkel Ridge of Arctic Ocean
Figs. (a) and (b) show the ascending tracks of the seismometer-detached OBS9 and OBS15
图 4 分体式海底地震仪及其在加克洋中脊的试验
(a) 2021年中国第十二次北极科学考察布设和回收的海底地震仪位置和炸测位置示意图;(b) 分体式海底地震仪的投放;(c) 分体式海底地震仪的回收
Figure 4. The seismometer-detached OBS and its experiments in the Gakkel Ridge
(a) The locations of OBS deployment and recovery and air-gun shooting lines during the Chinese 12th Arctic scientific expedition in 2021;(b) Deployment of the seismometer-detached OBS;(c) Recovery of the seismometer-detached OBS
图 5 (a) 分体式OBS3垂直分量记录于2021年8月10—24日在北极加克洋中脊的功率谱密度分布;(b,c) 分体式OBS3两个水平分量的功率谱密度分布
图中彩色细实线代表每小时记录对应的功率谱密度,不同颜色代表分布在不同能量区间的概率,黑色粗实线为平均值,灰色粗实线为Peterson (1993)提出的全球背景噪音模型参考线
Figure 5. (a) The vertical component records of the OBS3 in the Gakkel Ridge of the Arctic collected from August 10 to 24,2021 presented in terms of power spectral density probability density functions;(b,c) The power spectral density distribution of the two horizontal components of the OBS3
In the figures color thin solid lines represent the power spectral density corresponding to each hourly record,with different colors indicating probability distribution in various energy ranges. The thick solid black line represents the average,and the thick solid gray lines serve as the reference lines for the global ambient noise models proposed by Peterson (1993)
图 7 加克洋中脊分体式OBS3 (a)和OBS17 (b)台站记录的微震信息
图中波形已经带通滤波(3—12 Hz)处理,红色和蓝色实线标注了使用IASP91模型对上述地震波到时进行的预测
Figure 7. Microseismic information recorded by the seismometer-detached OBS3 (a) and OBS17 (b) along the Gakkel Ridge
The waveforms have been bandpass filtered (3−12 Hz). The arrival times of the seismic waves were predicted using the IASP91 model,indicated by the red and blue solid lines
图 8 布放在加克洋中脊的分体式OBS3台站主动源三分量地震记录的折合时间剖面(折合速度为6.0 km/s)
(a) x分量;(b) y分量;(c) z分量
Figure 8. Active source three-component seismic record sections with reduced time of the OBS3deployed along the Gakkel Ridge of Arctic Ocean (The reduced velocity is 6.0 km/s)
(a) x component;(b) y component;(c) z component
表 1 宽频带分体式海底地震仪组件及其技术参数
Table 1 Components and technical parameters of the broadband seismometer-detached OBS
地震计频带范围 60 s—50 Hz 标定信号DAC 16位 地震计满量程 10 mm/s (单峰值) 标定信号类型 脉冲,正弦波可选,参数可设置 地震计灵敏度 2000 V/(m/s)(双端) 标定信号启动方式 定时、指令 ADC 24位 数据记录 32 GB×3 数据采样率 50 sps,100 sps,200 sps,500 sps,
每个采集通道可单独设定采样率电子罗盘动态精度 ±1o 数字滤波器 线性相位FIR,最小相位FIR 电子罗盘调平范围 45° 数据采集动态范围 >135 dB 数据通信接口 LAN以太网 授时 GPS,北斗 数据通信协议 TCP,IP 守时 芯片级原子钟 最大工作水深 6 km 水听器频带范围 10—2000 Hz 工作时间 6个月—1年 方位角精度 俯仰范围±30°±0.1°,±(30°—45°)±0.2° 磁场测量精度 10 nT 表 2 短基线阵和超短基线信标的技术参数
Table 2 The technical parameters of short baseline arrays and ultra-short baseline beacons
深度级 频段 收发机波束角 测距精度 短基线阵 4000 m12 kHz 半指向性 100 m 超短基线信标 7000 m19—34 kHz 半指向性 <15 mm -
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