华北地区地震环境噪声特征研究

利用华北流动地震台阵观测的垂直分向连续波形数据, 通过计算功率谱密度和相应的概率密度函数, 对华北地区地震环境噪声特征进行了分析研究. 结果表明, 东部平原和沉积盆地2 Hz以上的高频环境噪声水平与全球新高噪声模型（NHNM）相近, 周期3——18 s的平均噪声水平低于NHNM和新低噪声模型（NLNM）的平均值; 山区及西部高原的高频噪声水平明显低于NHNM, 周期1——18 s的噪声水平大多明显低于NHNM和NLNM的平均值; 不同区域18 s以上周期的噪声水平差异相对较小. 流动地震台阵部分台站的环境噪声存在明显的昼夜变化, 个别台站噪声水平明显高于周边台站, 表明这些台站受人类活动干扰较大. 不同台站的噪声水平分析表明, 将台站布设在摆坑内, 能在一定程度上降低高频和低频段的噪声水平. 台站环境噪声特征的研究结果可为流动地震台阵观测数据质量的定量评估, 观测期间的台站优化调整等提供重要依据.      

粉质粘土台基处理方式对地震台背景噪声影响分析

计算不同频段范围内背景噪声记录的加速度功率谱密度,在不同台基处理方式下,研究粉质粘土层流动地震观测台站的噪声特征,分析噪声抑制效果,发现:在0.1—3 Hz频段,增加台基深度、钢钎固定铁板等方式对噪声影响较小;在3—49.8 Hz高频段,2.5 m深混凝土台基、1.5 m深钢钎固定铁板台基等较0.5 m深混凝土台基噪声降低约2—4 d B,1.5 m深钢钎固定铁板台基噪声水平较优;春季白天噪声水平较冬季高5—14 d B,春季夜间噪声水平低于冬季。     

黑龙江省测震台站背景噪声及动态范围分析

通过对黑龙江省测震台站台基背景噪声数据的计算和分析,得到当前各参评台站台基背景噪声地动速度均方根值（RMS）、台基噪声等级以及有效测量动态范围,并对存在突出问题的PSD曲线进行分析。通过计算各台站功率谱密度,得出台站在不同频段受干扰的情况。分析认为,黑龙江省29个台站环境地噪声水平为Ⅰ级,碾子山地震台环境地噪声水平为Ⅱ级（受当地采矿业机械震动影响）,七台河地震台环境地噪声水平为Ⅳ级;除七台河地震台观测动态范围小于130 dB,其他台站均大于130 dB。     

山西龙祠地震台毗邻的麦圪岭小山地形对背景噪声的影响研究

大规模流动地震台阵观测已经成为高分辨率深、浅部结构成像的重要手段和发展方向,台基条件和背景噪声是影响流动地震观测质量的关键因素.山西龙祠地震台为国家综合观测基准台,除测震(区域数字测震、国家数字测震)外,还包括地磁(FHD)、地电(地电阻率、地电场)、地形变(模拟金属水平摆、数字水平摆、数字垂直摆、数字钻孔地应变、定点水准)、数字气象等多种观测手段.     

西昌流动地震台阵背景噪声特征分析

利用西昌流动地震台阵2013年1月至4月观测的垂直分量的地震连续波形数据,计算了各台站评价环境噪声的功率谱密度概率密度函数,对地震台站环境背景噪声特征进行了分析。计算结果表明,西昌流动地震台阵多数台站环境的噪声水平较低,平均噪声水平在0.01~10 Hz频段低于全球新高噪声模型(NHNM)的参考值,不同台站在0.1~1 Hz频段的噪声水平基本相同,更接近于全球新低噪声模型(NLNM)的参考值。当地震计架设在基岩上并采用密封措施后,可以有效防风和保温,并且降低了气压变化对地震计的干扰。     

小北湖火山台迁建前后观测数据对比分析

小北湖火山台原址位于原始森林保护区,按照林区防火要求,2019年选址迁建。选取迁建前后小北湖火山台观测数据,通过计算不同频段范围内背景噪声记录的加速度功率谱密度,分析了迁建前后地震观测台站的噪声特征。结果表明,在1—20 Hz频段内,迁建后功率谱噪声值普遍降低10—20 dB;从RMS值来看,UD向降低22.0%,EW向降低58.2%,NS向降低62.8%,达到Ⅰ级台基水平,迁建后井下环境抗干扰能力更强,观测效果更好。     

乌加河地震台、乌力吉地震台台基噪声特征对比分析

利用2020年5月1—7日乌加河地震台、乌力吉地震台波形数据,应用噪声功率谱概率密度（PDF）方法,计算2个台站的台基噪声,分析了2种观察环境下的台基噪声特征及影响因素。结果显示,在小于0.1 Hz频段乌力吉地震台台基噪声值明显大于乌加河地震台,说明地震计在小于0.1 Hz频段受环境温度影响的特征较显著;在大于1 Hz频段2个台站台基噪声值均有台阶性升高频段,这是由在该频段台基噪声受人为活动影响所致。     

山东数字测震台站台基背景噪声分析

介绍了山东数字地震台网基本情况,计算了40个测震台站台基背景噪声,利用Welch方法计算噪声功率谱密度（PSD）,进而计算地震台台基1—20 Hz地动噪声均方根值（RMS）和有效动态观测范围。根据计算结果,依照《地震台站观测环境技术要求》,对山东测震数字台网40个参评测震台站进行背景噪声级别分类,并分析不同台站背景噪声水平较低的原因,以期为测震台网的优化建设提供数据支持。     

南京市测震台网台站背景噪声分析与应用

地震观测台站背景噪声水平是影响台网地震监测能力的重要因素。通过分析南京市属台站的噪声水平和背景噪声功率谱概率密度函数分布,发现2017年与2018年因受较多因素干扰,背景噪声水平整体高于2016年,结合仪器故障、溧水观音寺施工及六合台体应变施工等事件,探讨了如何及时发现观测环境的变化及仪器故障对台基噪声数据的影响。结果表明,研究台站背景噪声的变化,对了解台站数据记录质量、监控台站观测环境以及地震观测系统健康状态具有一定的参考意义。     

北京市测震台网数字地震台站台基背景噪声分析

基于北京市测震台网连续三分量地震计波形数据,计算28个测震台站台基噪声,利用Welch方法计算噪声功率谱密度（PSD）,进而计算地震台台基1-20 Hz地动噪声均方根值（RMS）和观测动态范围。结果表明,依照《地震台站观测环境技术要求》,北京市测震台网中有11个Ⅰ类台、9个Ⅱ类台、6个Ⅲ类台、2个Ⅳ类台。通过分析北京市测震台网数字地震台背景噪声水平,为测震台网的规划建设提供数据支持。     

宽频带流动地震台站中地震计浅井式安装研究

在井下地震观测中,各种地表背景噪声具有显著衰减作用,针对中国地震科学台阵现有大量非专用浅井式设计的Guralp CMG系列地震计,以及目前多个新获批项目在高背景噪声区域的观测需求,设计一种野外实用浅井观测方式,在北京国家地球观象台进行不同深度、不同井壁材质的观测对比实验,分析研究观测数据的背景噪声功率谱密度,结果表明,随着浅井深度的增加,地震背景噪声在垂直、水平向得到改善,改善程度随频率变化有所不同。分析认为,在华北现有地质环境下,6 m浅井是较为经济的观测方式,性价比高、占地小、施工简便,可用于后续多个项目的宽频带流动地震台阵观测。     

S-wave Velocity Profiles for Earthquake Engineering Purposes for the Cologne Area (Germany)

Local S-wave velocity-depth profiles are a key factor in seismic hazard assessment, as they allow the amplification potential of the sedimentary cover to be evaluated. Ambient seismic noise is mainly composed of surface waves, and therefore contains vital information about the S-wave velocity structure, allowing polarization or dispersion curves to be obtained from single station or array noise recordings. At two sites in the area of Cologne, Germany, the extended spatial correlation method was applied to such recordings and apparent phase velocity curves in the frequency range of interest for earthquake engineering were obtained. Using this data, a linearized inversion, the simplex downhill method, and a genetic algorithm yielded similar S-wave profiles. However, the latter method is recommended since it is less dependent upon a good starting model. Importantly, the presence of low-velocity layers in the Cologne area made it necessary to consider in the frequency range of interest higher modes in the inversion procedures. Finally, independent information on the total thickness of the sedimentary cover permitted the estimation of a 2D S-wave velocity profile crossing the Cologne area. Here, the H/V ratio inversion using 20 single-station noise recordings was used, with the results in good agreement with a geological profile.     

Ambient noise levels and detection threshold in Norway

Ambient seismic noise is caused by a number of sources in specific frequency bands. The quantification of ambient noise makes it possible to evaluate station and network performance. We evaluate noise levels in Norway from the 2013 data set of the Norwegian National Seismic Network as well as two temporary deployments. Apart from the station performance, we studied the geographical and temporal variations, and developed a local noise model for Norway. The microseism peaks related to the ocean are significant in Norway. We, therefore, investigated the relationship between oceanic weather conditions and noise levels. We find a correlation of low-frequency noise (0.125–0.25 Hz) with wave heights up to 900 km offshore. High (2–10 Hz) and intermediate (0.5–5 Hz) frequency noise correlates only up to 450 km offshore with wave heights. From a geographic perspective, stations in southern Norway show lower noise levels for low frequencies due to a larger distance to the dominant noise sources in the North Atlantic. Finally, we studied the influence of high-frequency noise levels on earthquake detectability and found that a noise level increase of 10 dB decreases the detectability by 0.5 magnitude units. This method provides a practical way to consider noise variations in detection maps.     

地震流动观测实用技术研究

根据地震流动观测实践经验,归纳总结了地震流动观测在提升地震监测能力,开展地震科学研究方面的作用和意义。着重介绍根据不同地区日照情况和设备功耗水平,确定流动地震台站关键设备参数的方法;针对流动观测特点设计制作地震流动观测防护装置、专用底座和集成装置;通过开展软土场地和水泥地面对比观测,分析两种观测环境下背景噪声特点,为改进和发展地震流动观测实用技术拓宽思路。      

Performance of superconducting gravimeters from long-period seismology to tides

The first phase (1997–2003) of the Global Geodynamics Project (GGP) has now been completed. Data from superconducting gravimeters (SGs) within GGP have shown great capabilities in a wide spectrum of geophysical applications from the tidal studies to the long-period seismology. Here, we compare the noise levels of the different contributing stations over the whole spectrum. We use three different processing procedures to evaluate the combined instrument-plus-site noise in the long-period seismic band (200–600 s), in the sub-seismic band (1–6 h) and in the tidal bands (12–24 h). The analysis in the seismic band has demonstrated that SGs are particularly well suited for the studies of the long-period normal modes and thus are complementary to long-period seismometers. In the sub-seismic band, the power spectral densities, computed over a period of 15 continuous days for every GGP station, cross the New Low Noise Model of Peterson from T = 16 min to T = 4.6 h. SG data are therefore appropriate for studying long-period seismic and sub-seismic modes. In the tidal bands, the noise comparison is realised by a least-squares fit to tides, local air pressure and instrumental drift, leading to gravity residuals where we estimate a standard deviation and average noise levels in different tidal frequency bands. Tidal gravity observations using SGs have also shown to be an independent validation tool of ocean tidal models, and they are therefore complementary to tide gauge and altimetric data sets. Knowledge of the noise levels at each station is important in a number of studies that combine the data to determine global Earth parameters. We illustrate it with the stacking of the data in the search for the gravity variations associated with the sub-seismic translational motions of the inner core, the so-called Slichter triplet.     

新疆和田台阵PSD与PDF分析

和田台阵是我国第一个自主建成并运行至今的小孔径台阵,承担着监测印巴地区核试验以及我国西部地震活动的重要使命.台阵波形数据中充斥着背景噪声,直接影响着数据质量.为了评估台阵噪声水平,本文利用Welch平均周期法对9个子台记录的数十万条噪声样本进行功率谱估计,对出现的谱异常进行了总结归纳,通过绘制概率密度函数图以及单频曲线来研究背景噪声变化范围和规律,最后针对台阵降噪提出了建设性意见.研究结果表明,中心台长周期噪声功率谱密度随季节变化显著,具有周期性;受温度和气压影响,水平分量长周期噪声变化幅度较大,局部频段超出新高噪声模型,建议改善仪器安装条件,或者利用数学方法进行校正.所有子台短周期噪声变化规律与长周期相反,受到采石场影响,谱密度曲线在4~8 Hz之间出现形态规则的高频尖刺,A1、B3、B4子台最为明显,可以通过窄带滤波或者聚束予以压制.本文取得的研究成果为台阵运维提供重要依据,除此之外,总结出的不同地震频谱特征也为地震解释工作提供重要参考.     

首都圈数字地震台网对微弱爆破信号的检测能力

利用首都圈数字地震台网接收人工地震信号,进行地下结构研究具有重要意义.但人工震源释放的能量小,激发的地震波以短周期为主,因此本文较全面地研究了地震台网对短周期微弱信号（1～20 Hz）的检测能力：(1) 分析了台网的背景噪声,结果表明基岩台址的地震台噪声比沉积盖层台址的地震台噪声低约13 dB,这相当于近1个震级的检测阈值;夜间的噪声比白天低约5 dB;噪声有逐年增高的趋势,2006年比2001年噪声提高约4 dB.(2 )分析了在台网内进行的药量为25 kg的陆地井下爆破实验,一次爆破相当于0.69级（M_L）的天然地震,有18个地震台可辨认爆炸产生的Pg、Pm或Pc波;离爆破点218 km的基岩台,仍可以接收到振幅只有1.6 nm 的Pm波,这个结果可为地震勘探实际工作提供参考.(3) 研究了台网外核爆试验的信号特征,2006年发生在朝鲜的地下核试验是一次检验台网检测微弱信号能力的好机会.波形记录经1～5Hz滤波后,台网中噪声小的18个基岩台可以清晰辨认核爆破产生的P波或Lg波,P波平均振幅为16 nm,计算的平均震级为m_b4.3,和NEIC给出的震级相同;分析还表明背景噪声是影响台站信号检测能力的主要因素之一.     

Very broadband seismic background noise analysis of permanent good vaulted seismic stations

This paper describes the results of a preliminary study conducted to analyze seismic background noise at sites of recently deployed very broadband stations of the Egyptian National Seismological Network (ENSN). The main purpose of the study is to assess the effects of permanent seismic vault construction and also to establish characteristics and origin of seismic noise at those sites. Another goal of this study is to determine the time needed for noise at those sites to stabilize. The power spectral densities of background noise at short period band (SP), very broadband (VBB), and ultra long period band (ULP) for each component of each broadband seismometer deployed in the different investigated sites are calculated. A MATLAB code has been developed that manages data processing and data analysis and compares the results with the high-noise model (NHNM) and low-noise model (NLNM) of Peterson (1993). Based on the obtained analysis, the noise stability and the efficiency of each station to record regional and teleseismic events are measured. The results of this study could be used in the future to evaluate station quality, to improve those processes that require background noise values, such as automatic association, and to improve the estimation of station and network detection and location thresholds.     

中国大陆地区宽频带地震台网台基噪声特征

利用中国地震台网880个宽频带地震台站2014年1月至2015年12月的垂直分量连续波形记录,通过计算每个台站的噪声功率谱密度和概率密度函数,对中国大陆地区的台基噪声水平进行了初步分析。结果表明:中国大陆的噪声水平在不同频带呈现不同的分区特性,其中高频带（0.1—1 s）噪声水平在西部和北部较低,而在中东部地区较高,京津冀和东南沿海地区最高;城市噪声水平高于人口稀少地区,表明高频带噪声与交通、工业等人文活动强度具有较高的相关性;在盆地及其边缘地区,高频带噪声水平高于周边山区,可能与沉积层的放大效应有关;微震噪声（1—20 s）主要来自于中国东部海岸线的海洋活动作用,其强度由东南沿海向内陆地区逐渐变弱。基于噪声的分区特征,利用概率密度函数的第5和第95百分位数获取了31个地震台网的高低噪声基线,相比全球新高低噪声模型,该基线能够对台站观测状态进行更有效的判别。