汤加火山喷发所产生的次声波

2022年1月15日汤加火山发生了剧烈的喷发。在火山喷发后的8 h 39 min,距离火山喷发位置10151 km的昆明次声台阵记录到很强的次声波信号。次声波波列前几个周期平均为443 s,传播速度约为321 m/s,其波形幅度与频率随时间变化与核爆炸产生的次声波形相似。采用相关检测方法估计次声信号的方位角为119°。在持续监测中,还观测到了火山喷发产生的次声信号沿反方向绕地球到达的信号和沿正方向绕地球一周后再次到达的信号。     

基于薄膜动圈式的次声波监测装置设计

基于薄膜动圈式声-电转换原理,利用次声波产生的空气微压波动带动薄膜上线圈受迫振动,引起线圈内磁通量的变化产生感应电动势,将声信号转换成电信号进行采集,设计便携式次声监测装置.在模拟简谐次声环境中的测量结果表明,该装置能够准确地测量出振动频率,并对其幅度进行监测.     

简易次声振动及监测实验装置设计

本装置利用低频扬声器作为模拟次声振源,并基于光反馈自混合干涉原理,将次声信号转换为自混合干涉信号,利用快速傅里叶变换进行分析,进而得到次声波的频率和强度.实验结果表明,该装置能够准确地测量出次声振动频率,并对其幅度进行监测.装置光路简单,结构紧凑、易实现,可推广应用于次声波检测的实验教学和监测实践.     

基于虚拟仪器的核电站主管道振动监测系统设计

针对主管道振动在线监测的迫切性需要,分析了主管道振动监测的原理;开展了核电站主管道振动监测系统的硬件设计和软件设计;采用LabView为开发平台,集虚拟仪器技术、设备组态图形技术和数据库管理技术于一体,实现了主管道振动的在线实时监测;利用电磁振动台模拟主管道的振动,验证了主管道振动监测系统的性能和基本功能。     

平台转动对SAR成像的影响分析

分析了合成孔径雷达(SAR)平台偏航、俯仰和横滚3个方向转动对成像的影响。建立了平台转动模型,从雷达照射区域的变化、回波的幅度调制和回波的相位调制等3个方面展开分析,定量分析了平台转动对成像幅宽和方位聚焦的影响。分析结果表明,平台转动会产生成像幅宽损失,影响方位聚焦,并会改变目标的方位位置。分析结果可以为SAR系统设计和算法论证提供一定的参考。     

电离层调制加热产生极低频/甚低频波定向辐射的理论分析

通过低电离层调制加热能够产生极低频/甚低频(ELF/VLF)波. 基于调制加热理论, 并引入相控阵天线思想, 建立了通过双波束幅度调制模式(DAM)和圆形几何调制模式(CGM)产生ELF/VLF波的定向辐射模型, 并通过与实验数据的对比验证了模型的正确性. 据此模型, 采用HARRP加热阵参数, 对比分析了上述两种定向辐射模式与常规幅度调制模式(AM)之间的特性差异, 并研究了调制频率(f_ELF/VLF)和加热波束与垂直方向倾角(ψ)对各模式的影响. 结果表明: 相对AM模式, 通过合理设置初始相位、f_ELF/VLF和ψ, DAM 模式和CGM模式在实现ELF/VLF信号定向辐射的同时还可以提高其辐射强度, 相对AM模式, CGM模式信号强度最大提高约11.3 dB. 关键词： 定向辐射 双波束幅度调制 圆形几何调制     

频率调制强吸收光谱中残余幅度调制的理论分析

由电光调制器(EOM)中双折射效应及线偏光不完全沿EOM调制方向诱发的残余幅度调制(RAM)使频率调制(FM)光谱技术在微量气体检测中的应用受到极大的限制。基于光场与晶体相互作用及光学干涉原理推导出存在RAM时FM光谱的线型表达式,确定出输入线偏光角度、EOM中双折射效应、FM系数等是影响线型的主要因素,且当入射EOM光的偏振角度偏离调制方向越大,双折射效应引起的特征偏振方向相位差越大,线型扭曲越严重;同时在FM色散光谱中存在一个受两者影响的直流偏置;最后给出通过伺服控制这两过程可以达到抑制RAM的目的。这些现象及线型的分析将为基于光纤器件的FM光谱提供必要的理论支持。     

基于WiFi和虚拟仪器的噪声监测系统的设计

提出一种基于WiFi和虚拟仪器的嵌入式噪声监测方案,主要由噪声采集单元、主控电路、存储单元、WiFi模块及监控中心组成。噪声提取电路将数据通过单片机控制器处理后输出到与其相连接的存储器中,并通过WiFi模块输出,监控中心通过无线AP点接收噪声数据,利用虚拟仪器软件对噪声数据进行解析和实时显示数据。实验结果表明,该装置功耗低、通信速率高、抗干扰性能好,具有全双工通信的特点。     

基于幅度调制的非线性超声相控阵成像方法*

超声相控阵是超声检测领域常用的材料缺陷定位和成像技术,可便捷快速地对裂纹、孔洞等缺陷进行成像。但是,传统超声相控阵方法对较小缺陷如闭合裂纹不太敏感。非线性超声信号因对材料性能退化以及微小缺陷敏感而广受关注。本文针对疲劳闭合裂纹检测,提出一种基于幅度调制的非线性超声相控阵成像方法,通过测量物理聚焦和虚拟聚焦两种聚焦模式下超声扩散场中的声能差,并将其作为非线性参量,实现疲劳裂纹闭合部分的定位成像和定量表征。将该法应用于7075铝合金试样疲劳裂纹的实验测量,并研究了扩散场信号延迟时间对非线性超声相控阵成像结果的影响。结果表明：相较于传统超声相控阵全聚焦法,基于幅度调制的非线性超声相控阵成像方法能够更准确地定位和成像疲劳裂纹闭合部分;延迟时间的选择对疲劳裂纹长度的表征精度影响较大,本文研究了该延迟时间的选择方法并实现了检测结果的优化。     

国际次声监测研讨会

全面核禁试条约第三届全球次声监测工作研讨会于1997年8月25至28日在美国新墨西哥州西班牙式的古城圣菲举行．会议云集了中国、美国、法国、俄罗斯、阿根廷、澳大利亚等5大洲12个国家61位活跃在次声学科领域的研究人员、政府官员以及联合国临时技术秘书等要员，围绕着全球次声监测中的7个专题进行报告和讨论，它们依次为：欢声阵的设计和信号处理，次声阵性能和减噪设备；法国欢声监测系统，国际次声监测系统60个站网的能力模型，对流层风对长距离次声传播的影响，高空风对次声同性能的影响，渗透管的特性以及对次声监测的减噪作用，欢声减噪器；爆炸检测，声遥感技术对爆炸源能量的估计，小当量地下，地面和近地面爆炸远距离声传     

次声噪声场的测量与分析

本文阐述了大气次声噪声场的测量和分析方法,得到一组风速为4m/s噪声的空间相关半径,它可以作为声阵设计的初步依据;同时,还测量和计算了风噪声的速度——声压关系曲线,得到声阵在接收次声信号时的抗干扰能力。     

Eddy-current non-inertial displacement sensing for underwater infrasound measurements

A non-inertial sensing approach for an Acoustic Vector Sensor (AVS), which utilizes eddy-current displacement sensors and operates well at Ultra-Low Frequencies (ULF), is described here. In the past, most ULF measurements (from mHertz to approximately 10 Hertz) have been conducted using heavy geophones or seismometers that must be installed on the seafloor; these sensors are not suitable for water column measurements. Currently, there are no readily available compact and affordable underwater AVS that operate within this frequency region. Test results have confirmed the validity of the proposed eddy-current AVS design and have demonstrated high acoustic sensitivity.     

Microseism and infrasound generation by cyclones

A two-dimensional cylindrical shear-flow wave theory for the generation of microseisms and infrasound by hurricanes and cyclones is developed as a linearized theory paralleling the seminal work by Longuet-Higgins which was limited to one-dimensional plane waves. Both theories are based on Bernoulli's principle. A little appreciated consequence of the Bernoulli principle is that surface gravity waves induce a time dependent pressure on the sea floor through a vertical column of water. A significant difference exists between microseisms detected at the bottom of each column and seismic signals radiated into the crust through coherence over a region of the sea floor. The dominant measured frequency of radiated microseisms is matched by this new theory for seismic data gathered at the Fordham Seismic Station both for a hurricane and a mid-latitude cyclone in 1998. Implications for Bernoulli's principle and this cylindrical stress flow theory on observations in the literature are also discussed.     

Topographic effects on infrasound propagation

Infrasound data were collected using portable arrays in a region of variable terrain elevation to quantify the effects of topography on observed signal amplitude and waveform features at distances less than 25 km from partially contained explosive sources during the Frozen Rock Experiment (FRE) in 2006. Observed infrasound signals varied in amplitude and waveform complexity, indicating propagation effects that are due in part to repeated local maxima and minima in the topography on the scale of the dominant wavelengths of the observed data. Numerical simulations using an empirically derived pressure source function combining published FRE accelerometer data and historical data from Project ESSEX, a time-domain parabolic equation model that accounted for local terrain elevation through terrain-masking, and local meteorological atmospheric profiles were able to explain some but not all of the observed signal features. Specifically, the simulations matched the timing of the observed infrasound signals but underestimated the waveform amplitude observed behind terrain features, suggesting complex scattering and absorption of energy associated with variable topography influences infrasonic energy more than previously observed.     

Results of measurements and analyses of nuclear fallout up to the beginning of 1965

In consequence of the Moscow Agreement on the cessation of nuclear tests in the atmosphere there was a further decrease in the activity of dry fallout and rainout, as well as in ground level air, in 1964 (Table I). The daily activities decreased in order to tenths of mc/km².From April to August an increase in the activity was again seen, which was caused by the fall of radioactive dust from the stratosphere to the troposphere in consequence of meteorological factors. Seasonal variation was not so pronounced in 1964 as in the previous year, when the activity increased in May and June 6 to 7 times that of February and March. In 1964 the activity increased approximately 3 times. The conception of the existence of a stratospheric reservoir of artificial radioactivity, formed as a result of nuclear tests, was thus confirmed once again and so was the existence of a certain residence time of radioactive dust in the stratosphere because in 1963 nuclear explosions were no longer performed. The mean residence time was calculated from the decrease of strontium 90, separated by radiochemical analysis. The mean value of the residence time is 1.6 of a year, which indicates a reservoir in the higher stratosphere.The author thanks Academician F. Bhounek for his continual interest in our measurements and for providing us with preparations of Sr 90. He also thanks Dr. P. Stránský, a lecturer at our Department of Physics, for his help in the measurements.     

On infrasound radiation of shallow water waves

Sound waves generated by low-frequency gravity modes have been studied. It has been shown that the response function is inversely proportional to the fourth power of the wave number.     

Some effects of infrasound on task performance

Some effects of moderate levels of infrasound on the performance of a complex task have been investigated using two experimental designs. A comparison between these effects and those due to alcohol, audio frequency noise, and combinations of infrasound-alcohol and alcohol-audio frequency noise is also presented. The complex task adopted for these experiments consisted of (a) a centrally located high priority pointer following task which had to be performed continuously and (b) the response to the onset of any one of four lights two of which were situated in front of the subject and two on his periphery of vision. The task was performed over a period of 36 minutes. Our results indicate that although performance in the infrasound condition does not suffer significant decrements in either the primary task or the central and peripheral components of the secondary task, the effects through time, both within the infrasound condition and in relation to the control, produce changes which are of a different nature to those of audible noise. In audible noise performance is maintained through time, whereas with infrasound and alcohol it appears to be degraded.     

电容式次声传感器频率特性研究

研究了次声传感器的频率特性的分布情况,定量分析了次声传感器组成部件参数对幅频特性的影响。通过求解次声传感器前后腔内的次声波动态方程得到频率响应模型和截止周期误差模型,选取一批进气口、均压管和膜片相对误差在5%之内的部件,与体积相对误差控制在0.1%之内的前后腔体部件组装成32个次声传感器并对其进行校准实验,校准结果表明该批传感器的截止周期分布在540 s至610 s区间,根据截止周期误差模型计算的理论变化区间范围为530.6 s至614.9 s区间,二者的一致性验证了模型的有效性,为控制次声传感器截止周期一致性和传感器批量精细设计提供了理论指导。     

火箭发射的次声信号分析

在距离发射点20 km和230 km的两个阵列上,记录了我国一次火箭发射产生的次声波.从信号中识别出点火与声爆事件,观察到火箭在飞行中产生持续的次声波,以及声爆前后明显能量特征上的变化,观测到完整的火箭发射和飞行过程中系列次声波.为了验证采集信号中包含声爆事件,使用Fisher检测估计方位角和视速度,计算结果与火箭飞行...