二维微动剖面探测"孤石":以深圳地铁7号线为例

"孤石"是花岗岩不均匀风化所残留的风化核,在我国南方沿海地区普遍发育."孤石"埋藏分布随机,形状大小各异,给地铁盾构施工带来重大安全隐患,探测"孤石"一直是地铁工程勘察面临的难题.我们首次尝试利用二维微动剖面技术探测"孤石",在深圳地铁7号线车公庙-上沙段区间实测二条剖面,结合少量钻孔资料进行岩性层划分和"孤石"解释.实测结果显示,在二维微动视S波速度剖面上,素填土、粉质粘土、砾质粘性土等岩土层、全风化、强-中风化、微风化和未风化的花岗岩层,视S波速度值各不相同,剖面特征也存在较大差异,利用少量钻孔结果标定,易于划分;在强-中风化花岗岩层中,视S波速度(岩性)横向变化剧烈,局部发育"团块状"高速体,本文将其解释为未风化的花岗岩"孤石".本文结果表明,二维微动剖面技术探测"孤石"是有效的微动视S波速度剖面除能直观显示岩性的纵、横向变化,提供工程基岩面的埋深及起伏形态信息外,还可给出岩土层风化程度的判断信息,为高层建筑的桩基设计提供地球物理依据.作为一种全新的"孤石"探测手段,二维微动剖面技术尤其适用于交通繁忙、建筑物密集的、各种场源干扰严重的闹市区.     

地震电磁辐射观测是短临预测重要手段之一

地震和火山活动是具有相同属性的自然现象,观测表明,在大地震及火山喷发之前的几天或几小时,均呈现有非常宽频谱的电磁辐射和电离层异常现象。本文重点描述了地震前后存在从零开始相当宽频谱的地震电磁辐射以及电离层异常等现象,并提出可利用电离层异常的观测和采用不同频段上地震电磁辐射的立体监测网,作为地震的短临预测、预报手段是现实、可行的。同时还提出了加强我国地震电磁辐射观测研究的建议。     

The Effect of Velocity Inversions on H/V

We analyzed the phenomenology of microtremor H/V curves under inversions in the shear-wave velocity (V_s) profile in the subsoil. Under no V_s inversion the spectral signature of the H/V peaks is found to be ‘eye-shaped’ with the horizontal components higher than the vertical. Conversely, under negative velocity gradients, numerous of differences emerge. I) A H/V ratio below 1 is observed for a wide range of frequencies, due to the decrease of the horizontal components below the vertical one. II) In the presence of persistent H/V < 1, small bumps in the H/V ratio given by local minima in the vertical spectral component may represent the relics of the peaks indicating resonances and stratigraphic discontinuities. As a consequence, in the presence of velocity inversions the H/V > 2 SESAME (2004) criterion fails but a stratigraphic interpretation may still be possible. III) The H/V curves should always be interpreted together with the single component spectra. IV) Microtremor H/V measurements for stratigraphic/microzonation purposes on stiff artificial soils, (asphalt, concrete, cement, pavements) should always be avoided since the latter often produce velocity inversions. This may have consequences in the intermediate to high frequency domain ( > 1 Hz) also in the application of reference site methods, like H_site/H_bedrock, to microtremor. Theoretical modeling confirms these experimental findings.     

泰安重力观测高频波动信号分析

系统分析了泰安重力观测资料正常和异常时信号的周期特征,研究了高频异常信号与台风、气压和强震之间的关系.结果表明:泰安重力观测正常时信号周期为2～4分钟,异常时信号周期只有2～3分钟,异常信号幅值约为正常信号的5倍;高频异常信号主要是由西太平洋洋面上生成的进入中国大陆或近海的台风引起的,占台风引起泰安重力出现高频异常信号的90%;重力高频信号与台风中心速度正相关,与台风距离负相关,并且随台风的结束高频信号也随之消失;少数高频异常信号与强震也有一定关系,占地震总数的17.24%;高频异常信号与气压变化无关.     

利用微动勘察方法探测煤矿陷落柱

陷落柱是危害煤矿安全生产的主要因素之一,如何有效探测陷落柱是实现煤矿安全、高效生产的当务之急.本文首次将微动勘察方法应用于煤矿采区勘探,结果表明,该方法对陷落柱异常区反映敏感.在山西潞安漳村煤矿2002工作面测区已知陷落柱上,无论是单点反演获得的S波速度结构,还是微动视S波速度剖面,均能清晰显示陷落柱.微动剖面确定的陷落柱位置与巷道揭露位置一致,边界误差在10 m左右.微动勘察方法是探测陷落柱的一种行之有效的物探方法,由于其分辨率高、无需人工源、野外施工方法灵活便捷、不受施工场地限制等特点,对探测村庄覆盖区之下的煤层构造、圈定陷落柱等,具有得天独厚的技术优势和应用前景.     

延时参数对地震信号信噪比的改善

当测区噪声很强,采用组合震源工作仍然不能得到高质量地震数据,为进一步提高地震信号的信噪比,引入能形成定向地震波的相控震源.相控震源产生的地震波方向可由延时参数控制,因此延时参数是影响相控地震效果的重要参数.理论分析表明,定向地震波技术可提高反射波信号信噪比,以含3个激震器的相控震源为例,信噪比可提高6.53~9.54 dB.针对延时参数分别为0,0.802,0.384,0.241,0.174,0.123 ms的情况,完成了野外地震实验.根据原始地震记录,得到了反射波信号功率谱,在此基础上定量地计算了不同延时参数下目标层反射信号的信噪比.结果表明,与单震源情况相比,在6种延时参数下,反射波信噪比分别提高了3.3150,8.0150,6.0547,5.0278,4.5383,4.0364 dB;与组合震源情况相比,信噪比分别提高了0,4.7000,2.7397,1.7128,1.2233,0.7214 dB.综合理论分析及野外实验结果,我们可以得到这样的结论：定向地震波技术能够提高反射波信号信噪比,延时参数变化对信噪比有很大影响,采用合理的延时参数能得到更高的信噪比.     

Site characterizations for the Tehran network (TDMMO) in Tehran Region Using Micro-Earthquake,Microtremor and Quarry Blast Data

We did a Study of Horizontal-to-Vertical Component Spectral Ratio in the Tehran seismic zone. Micro-earthquakes, microtremors and quarry blasts data were used as an estimation of the site response in the Tehran zone. Site effects were studied based on horizontal to vertical ratios by the Nakamura׳s technique. Also, we used the spectra of signals for three components with the lowest noise levels for spectral slope studies. The analysis used seismic events from a network of 13 seismic stations by the permanent local seismological network of the Tehran Disaster Mitigation and Management Organization (TDMMO) from 2004 to 2007. The number of events used were different for each station. Quarry blast events were with 1.2≤M_L≤2.2 and micro-earthquakes were with 1.1≤M_L≤4.1.By comparing results for earthquake, microtremor and quarry blast, we could see that there is a significant difference between them. The data showed clear observations, especially in high-frequencies. The H/V spectral ratios indicate dominant frequency for rock/soft site with a higher ratio level for quarry blast ratios, which are comparable to the earthquake results due to their difference sources. The results derived by spectral H/V ratios and spectral analysis may be used to distinguish between local earthquakes and quarry blasts.     

Use of microtremor in liquefaction hazard mapping

This study shows how microtremor measurements can be used as an aid to liquefaction hazard mapping and zonation, as demonstrated in Laoag City, Northern Philippines. From microtremor measurements, qualitative information on subsoil conditions was obtained and a site classification map was generated. The map was combined with the geomorphology-based liquefaction susceptibility map to produce an integrated liquefaction hazard zonation map. This integrated map is deemed to be more accurate in depicting relative liquefaction susceptibility since it combines information on the distribution of potentially liquefiable soils in terms of geology and grain characteristics with information on the stiffness and thickness of these soils. With information about the thickness of the deposits, an idea of the severity of liquefaction-related damage can also be gathered since thicker deposits relate to more serious damage. Plots of historical liquefaction cases, as well as borehole data and resistivity profiles in the study area, support the validity of the integrated map. The use of microtremor, therefore, constitutes an effective and inexpensive approach to liquefaction hazard zonation, and as such is very useful in less-developed countries like the Philippines and other areas where funds for more rigorous investigations are not always available.     

Microtremor survey in Talchir, India to ascertain its basin characteristics in terms of predominant frequency by Nakamura's ratio technique

The Talchir Basin, one of India's oldest basins, has been a subject of interest because of its rich coal deposits. The maximum thickness of the basin is about 1500 m. Beyond the basin is the hard metamorphic rocks of Precambrian age. The ambient noise survey data have been analyzed for the Talchir Basin using Nakamura's technique of horizontal–vertical-spectral-ratio (HVSR) to ascertain the basin structure in terms of the predominant frequency. The predominant frequency varies from 0.25 Hz to 7.8 Hz but a major portion of the basin comes under the range of 0.3 Hz–2.4 Hz while on the metamorphic rocks it is as high as 7.8 Hz. The variation in predominant frequency shows a good correlation with the sediment thickness of the basin. The results have been compared with the previous studies by other researchers and it shows consistency with the northerly dip of the basin. The present study has also been compared with the results of the synthetic seismogram that was performed for the Talchir Basin. The predominant frequency obtained from HVSR technique complements well with the frequency at which the peak response spectra ratio is observed. The present study of the predominant frequency identifies quite well the characteristics of Talchir Basin and is in good agreement with the synthetic ground motion modeling of the region.     

Identifying the occurrence time of an impending mainshock:a very recent case

The procedure by means of which the occurrence time of an impending mainshock can be identified by analyzing in natural time the seismicity in the candidate area subsequent to the recording of a precursory seismic electric signals(SES) activity is reviewed. Here, we report the application of this procedure to an MW5.4 mainshock that occurred in Greece on 17 November 2014. This mainshock(which is pretty rare since it is the strongest in that area for more than half a century) was preceded by an SES activity recorded on 27 July 2014, and the results of the natural time analysis reveal that the system approached the critical point(mainshock occurrence) early in the morning on 15 November 2014.