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。     

流动地震观测背景噪声的台基响应

大规模流动地震台阵技术发展为高分辨率深部结构成像提供了重要基础,背景噪声是影响流动地震观测质量的关键因素. 为掌握流动地震观测噪声规律,发展流动地震观测降噪技术, 编制流动地震观测技术规范, 我们开展了针对不同台基流动地震观测背景噪声的观测实验与分析. 其中,山西省临汾市五个地点架设了共22个对比观测台站, 进行了超过一年半的连续观测. 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, 研究了不同场地条件和环境噪声下流动地震观测台站的噪声特征及其台基响应,分析了不同台基处理方式对噪声的抑制效果. 结果表明:(1)高频人为噪声和长周期自然噪声是影响流动地震观测质量的主要噪声, 可以通过增加台基深度和改善台基处理方式等方法降低其影响; (2)增加台基深度能有效地降低长周期噪声和高频噪声, 2 m深坑能使高人为噪声台站各分量的高频频段和长周期频段分别降低5 dB和10 dB; (3)由于其不稳定性, 沙子台基的水平分量在长周期频段一般要高于摆墩台基5 dB, 流动地震观测中推荐使用摆墩台基; (4) 台站位置、台站内部温度和空气流动都是影响台站噪声的重要因素. 在此基础上提出了不同场地条件和噪声环境下的台基处理建议和适合国情的移动地震台阵台站建设参考方案, 有助于流动地震观测野外工作的标准化和规范化.     

辽宁新建台站勘选过程概述及结果分析

利用地震台址勘选中得到的观测数据,计算勘选台站的地脉动背景噪声RMS值,噪声功率谱密度等数据,通过结果分析得到了各个台站的环境噪声水平和台基等级分类情况,为正式建设台站提供数据支持。     

环境变化对乌加河地震台观测质量的影响

针对近年由城镇化建设引起的乌加河地震台观测环境变化、台站记录受到干扰的问题,选取2016—2018年乌加河地震台JCZ-1T地震计的记录数据,采用傅里叶变换,对环境变化引起的噪声频谱进行分析,计算台基平均噪声水平及动态范围变化,并与2015年未受干扰的观测数据进行对比,结果发现,乌加河地震台地脉动噪声水平发生明显变化,地噪声功率谱密度增大,观测动态范围减小.     

鹤岗三道林场地震台阵场址地质条件

从地震构造环境、地形地貌、场地工程地质岩性特征、场址环境噪声勘测等,详细阐述鹤岗三道林场地震台阵场址的地质条件,并参考测震台站DB/T 16-2006、中国数字测震台网技术规程JSGC-01、地震监测台网项目地震台阵场址勘选技术指南及地震测震台站观测环境技术要求GB/T 19531.2-2004等台站建设规范和标准,认为该地震台阵场址地质条件符合台站建设的相关要求,建成后将进一步提高鹤岗市及周边地区微震活动的监测能力。     

环境变化对乌加河地震台观测质量的影响

针对近年由城镇化建设引起的乌加河地震台观测环境变化、台站记录受到干扰的问题,选取2016—2018年乌加河地震台JCZ-1T地震计的记录数据,采用傅里叶变换,对环境变化引起的噪声频谱进行分析,计算台基平均噪声水平及动态范围变化,并与2015年未受干扰的观测数据进行对比,结果发现,乌加河地震台地脉动噪声水平发生明显变化,地噪声功率谱密度增大,观测动态范围减小。     

兰州地震台阵环境噪声调研分析

随着我国社会经济的发展,地震台站观测环境破坏日益严重。对兰州地震台阵环境噪声进行调研,定量分析并提供该台阵与国际标注各大干扰源的直线距离,以充分了解各干扰源对地震台阵的影响程度。结果发现,兰州地震台阵目前基本达到Ⅰ级台站环境噪声水平,满足台阵观测条件。     

青藏高原东部和周边地区地壳速度结构的背景噪声层析成像

利用连续地震背景噪声记录和互相关技术获得瑞利面波格林函数,进而反演获得了青藏高原东部和周边地区的地壳三维速度结构.地震数据源于北京大学宽频带流动观测地震台阵,国家数字测震台网数据备分中心提供的部分固定台站的连续记录及INDEPTH IV宽频带流动观测地震台阵.首先对观测数据进行处理和分析取得所有可能台站对的面波经验格林函数和瑞利波相速度频散曲线,反演得到了观测台阵下方周期从6~60s的瑞利波相速度异常分布图像.并且进一步反演获得研究区域三维剪切波速度结构和莫霍面深度分布.短周期(6~14s)相速度异常分布与地表地质构造特征吻合较好,在青藏高原和四川盆地之间存在一个明显的南北向转换带.而本文最重要的结果是周期大于25s的相速度异常分布图像显示,以昆仑断裂带为界,柴达木盆地和祁连山脉地区呈现与青藏高原截然不同的中地壳速度结构,反而与青藏高原东缘地区和川滇菱形块体速度结构相似.反演获得的剪切波速度在27.5~45km深度的切片也明显地揭示:青藏高原的松潘—甘孜地块和羌塘地块呈现均一的低速层;然而,柴达木盆地和祁连山脉地区则呈现较强的横向不均匀性,尤其是柴达木盆地的高速异常和四川盆地的高速异常相对应.这些结果为前人提出的青藏高原东北向台阶式增长模式提供了重要的地震学观测证据.与全球一维平均速度模型(AK135)相比较发现,本文测量和反演获得的研究区域内平均相速度和剪切波速度都比AK135模型慢很多,尤其是青藏高原的中地壳(25~40km)剪切波速度显著低于全球平均速度模型.进一步的层析成像反演证实松潘—甘孜和羌塘地块中地壳(27.5~45km)呈现大范围均一的低速层,为青藏高原可能存在大规模中下地壳"层流"提供地震学观测证据.在祁连山脉的27.5~45km深度观测到的明显低速异常体可能对应于该造山带下地幔岩浆活动导致的底侵作用,表明引起该地区地壳增厚的主要机制可能是来自地幔岩浆的底侵作用.     

地震台站台基噪声功率谱概率密度函数Matlab实现

选取2015年四川数字测震台网中筠连和华蓥山地震台记录的垂直分向连续波形数据,利用Matlab软件,计算地震台站台基噪声功率谱概率密度函数,分析地震台站环境噪声特征。结果表明,台站环境噪声功率谱密度概率密度分布对地震事件波形（体波、面波）、人为噪声（台站周围人为活动、车辆及机器噪声等高频干扰）、系统瞬变（数据丢失、地震计小故障）以及仪器标定信号等反映较好。使用台基噪声功率谱概率密度函数方法,有利于监测地震台站数据记录,提高观测数据质量。     

江苏省区域地表背景噪声特性的分析

利用welch方法,计算了江苏省"十五"数字地震台站地表背景噪声在0.01～20 Hz频带范围内的功率谱值,结果显示在周期10～16 8、4～8 s处分别存在两个明显的峰值.对比白天和夜晚时段台站三分向地表背景噪声的功率密度谱比值发现,地表台站三分向背景噪声在高频段(≥1 Hz)变化最为显著,在微震峰值频段(0.125～1 Hz)几乎所有台站之间的差异都不大,低频段(≤0.125 Hz)大部分台站垂直向白天时段的噪声水平比夜晚的值低,水平向则相反;但井下观测系统全频段内的比值变化都很小.此外,在2～16 Hz频率范围内,沿长江的苏南-上海地区的平均噪声水平高于苏中和苏北地区,比NLNM(低噪声模型)值高约45 dB左右;在0.125～1 Hz频率范围内,江苏中东部的噪声水平高于其他区域,推测这可能是与区域地质构造差异有关.     

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.     

Conversion and comparability of data presentations on seismic background noise

Data on seismic background noise are collected, amongst others, for assessing the suitability of sites for temporary or permanent seismic recordings. Site quality requirements depend on the task of seismic observations and thus on their resolution, dynamic range, bandwidth and frequency range. Till now noise data are collected with a wide range of instruments, both analog and digital, of different bandwidth, resolution and transfer functions. Accordingly, noise appearence in seismic records, amplitude- and frequency-wise, differs and the various kinds of noise spectra derived therefrom vary too. They are not easily comparable amongst each other and with older presentations of noise spectra derived from analog records. Also, when having determined noise power density spectra from digital records it is not so obvious what this means in terms of noise ground motion amplitudes or noise appearance in records of different bandwidth and vice versa. The paper does not aim at serving as a guide to station site selection but rather to present and comment the relationships to be used for the conversion of power and amplitude spectra into different kinematic units and for calculating from the spectral representations of seismic noise the related frequency dependent RMS or average peak amplitudes of different bandwidth and vice versa. For the new global high (NHNM) and low-noise model (NLNM) given by Peterson (1993) in dB of acceleration power density the related velocity and displacement power spectral densities are presented both graphically and tabulated. Examples for the application of the conversion relationships and the effect of bandwidth on noise and signal amplitudes are given. For a selected data set from a site-selection noise survey in NW Iran the suitability of some potential sites is assessed by comparison with the NLNM.     

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.     

New Seismic Noise Models Obtained Using Very Broadband Stations

It has been two decades since the last comprehensive standard model of ambient earth noise was published Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993). The PETERSON model was updated by analyzing the absolute quietest conditions for stations within the GSN (Berger et al. in J Geophys Res 109, 2005; Mcnamara and Buland in Bull Seism Soc Am 94:1517–1527, 2004; Ringler et al. in Seismol Res Lett 81(4) doi:10.1785/gssrl.81.4.605, 2010). Unfortunately, both the original model and the updated models did not include any deployed station in North Africa and Middle East, which reflects the noise levels within the desert environment of those regions. In this study, a survey was conducted to create a new seismic noise model from very broadband stations which recently deployed in North Africa. For this purpose, 1 year of continuous recording of seismic noise data of the Egyptian National Seismic Network (ENSN) was analyzed in order to create a new noise model. Seasonal and diurnal variations in noise spectra were recorded at each station. Moreover, we constructed a new noise model for each individual station. Finally, we obtained a new cumulative noise model for all the stations. We compared the new high-noise model (EHNM) and new low-noise model (ELNM) with both the high-noise model (NHNM) and low-noise model (NLNM) of Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993). The obtained noise levels are considerably lower than low-noise model of Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993) at ultra long period band (ULP band), but they are still below the high-noise model of Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993). The results of this study could be considered as a first step to create permanent seismic noise models for North Africa and Middle East regions.     

四川地区地震背景噪声特征分析

选取四川省数字测震台网2015年1月1日至2018年12月31日期间60个固定台站的三分量连续波形记录,计算了台站噪声加速度功率谱密度及相应的概率密度函数分布,统计了不同频率下的噪声功率谱密度值分布,对不同区域、不同频率下背景噪声水平的变化特征予以分析。结果表明:大部分地震台站的高频段噪声由于受到台站附近人为的、规律的作息生活和生产方式的影响,呈现明显的季节性变化和日变化,即夏季噪声水平升高,冬季降低,在农历春节期间达到全年最低值,地理空间分布特征不明显;第二类地脉动冬季噪声水平升高,夏季降低,季节性变化明显,平均变化为1—5 dB,且冬季峰值出现的频率向长周期方向移动1—2 s,呈现明显的地理空间分布特征,川东地区平均噪声水平最高,攀西地区次之,川西高原最低;与第二类地脉动相比,第一类地脉动观测到的噪声能量较弱,季节性变化不明显,地理空间分布的噪声水平差异明显减小;在20 s以上的长周期部分,台站噪声未呈明显的季节性和地理空间分布差异。此外,将地震计安置在山洞和井下,可以有效地降低台站周围干扰源、温度和压强对高频段和长周期观测的影响,噪声水平低于地表安装方式。      

新冠肺炎疫情对佘山与大洋山地震台背景噪声的影响

人类的生产生活产生的振动会以高频地震波的形式被地震台站所记录。2020年1月,新冠肺炎疫情爆发,为了应对此次疫情,各地政府分别启动应急响应,国内地震记录出现最长、最突出的人为地震降噪期。各地震台站背景噪声显著下降,在人口稠密及工业发达地区尤为明显。同时,静噪期为探测地下地震源的微弱信号提供了可能。静噪期内,佘山地震台2 Hz频点背景噪声功率谱密度值比平时降低10 dB,而大洋山地震台10 Hz频点背景噪声功率谱密度值较平时降低约5 dB;佘山地震台2—10 Hz以及大洋山地震台10 Hz以上频率的背景噪声受静噪期影响明显。结合地震台站所处位置分析,疫情期间佘山地震台附近人口流出较多,2—10 Hz频率的背景噪声变化明显;大洋山地震台背景噪声则受工厂、轮渡、高速汽车等影响较大,f ≥10 Hz的背景噪声变化显著,而频率在2—10 Hz则无明显变化,表明该台人口总数趋于平稳。地震噪声和人类活动之间的相关性表明,地震学研究可以提供实时人口动态估计。     

不同深度布设台站的噪声及信号幅值特征分析

为探究地震观测中地震计检测到的噪声和信号强度均受其布设深度的影响,本文首先对CPUP和LPAZ两台站布设的不同深度地震计所得到的数据进行噪声水平和地震信号对比;其次采用对比功率谱密度的方法对两台站不同通道采集到的不同时段的噪声数据进行分析;最后比较两台站不同通道采集到的整月数据的噪声幅值、信号幅值、信噪比特征。结果显示:深度较大的通道,其噪声功率均值较小;当事件信号到来时,较深通道的地震计检测到的信号和噪声幅值比较浅通道均有所减小,在信号和噪声幅值均减小的共同影响下,信噪比有一定程度的变化,其中LPAZ台站的信噪比提高较为明显。      

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.     

The IDC Seismic,Hydroacoustic and Infrasound Global Low and High Noise Models

The International Data Centre (IDC) in Vienna, Austria, is determining, as part of automatic processing, sensor noise levels for all seismic, hydroacoustic, and infrasound (SHI) stations in the International Monitoring System (IMS) operated by the Provisional Technical Secretariat of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). Sensor noise is being determined several times per day as a power spectral density (PSD) using the Welch overlapping method. Based on accumulated PSD statistics a probability density function (PDF) is also determined, from which low and high noise curves for each sensor are extracted. Global low and high noise curves as a function of frequency for each of the SHI technologies are determined as the minimum and maximum of the individual station low and high noise curves, respectively, taken over the entire network of contributing stations. An attempt is made to ensure that only correctly calibrated station data contributes to the global noise models by additionally considering various automatic detection statistics. In this paper global low and high noise curves for 2010 are presented for each of the SHI monitoring technologies. Except for a very slight deviation at the microseism peak, the seismic global low noise model returns identically the Peterson (1993) NLNM low noise curve. The global infrasonic low noise model is found to agree with that of Bowman et al. (2005, 2007) but disagrees with the revised results presented in Bowman et al. (2009) by a factor of 2 in the calculation of the PSD. The global hydroacoustic low and high noise curves are found to be in quantitative agreement with Urick’s oceanic ambient noise curves for light to heavy shipping. Whale noise is found to be a feature of the hydroacoustic high noise curves at around 15 and 25 Hz.