Guidelines and pitfalls of refraction microtremor surveys

Journal of Seismology - The geotechnical industry has widely adopted the refraction microtremor shear-wave velocity measurement technique, which is accepted by building authorities for evaluation...     

Correction: Multidisciplinary site investigations: refraction microtremor surveys

Journal of Seismology -     

Multidisciplinary site investigations: refraction microtremor surveys

Journal of Seismology - We present three case studies from recent site investigations that have utilised geophysical data to supplement traditional geotechnical investigations. The refraction...     

Preliminary seismic microzonation of Sivas city (Turkey) using microtremor and refraction microtremor (ReMi) measurements

Sivas city, located in the inner east part of Anatolia (Turkey), is far from seismic sources. However, the city is under risk owing to strong earthquakes occurring around the area, and different soil conditions that can produce variation in the ground motion amplification. Microzonation of cities provides a basis for site-specific hazard analysis in urban settlements. In particular, seismic microzonation can be achieved by means of detailed seismic assessment of the area, including earthquake recordings and geological studies. In this paper, we propose a preliminary microzonation map for the city of Sivas, based on the variation in the dominant periods of the sediments covering the area. The periods are retrieved from microtremor measurements conducted at 114 sites, using the horizontal-to-vertical spectral ratio technique. The results of microtremor analysis were compared with those obtained from refraction microtremor measurements at two profiles crossing the studied area. According to the classification of dominant periods, Sivas area can be divided into four zones, probably prone to different levels of seismic hazard. However, specific studies including analysis of weak earthquakes are required in the future to validate our microzonation map.     

Detecting underground cavities using microtremor data: physical modelling and field experiment

In this paper, we study the possibilities of the use of microtremor records in the detection and delineation of near‐surface underground cavities. Three‐dimensional physical modelling data showed that the averaging amplitude spectra of a large number of microtremor records makes it possible to evaluate the frequencies and amplitudes of compressional standing waves generated by microtremor in the space between the ground surface and underground cavities. We illustrate how these parameters can be used to estimate the shape of the underground cavity horizontal projection. If the compressional wave velocity in the enclosing rock is known, it is possible to evaluate the depth to the cavity top using the frequencies of the standing waves. The results of the field experiment confirmed the possibility of underground cavities detection using microtremor data.     

微动探测方法研究进展与展望

微动探测方法是在背景噪声成像理论的基础上发展起来的一种新型地球物理探测手段,其根据获取野外数据的观测台站方式分为微动阵列方法和微动单台方法.本文综述了前人对微动探测方法的研究进展,并对目前该方法存在的问题及研究的发展趋势进行了展望.微动阵列方法采用频率波数法或空间自相关法从微动信号中提取频散曲线,进而反演获得地下横波速度结构.微动单台方法即H/V谱比法以三分量台站为观测基础,对水平分量信号与垂直分量信号进行傅里叶谱比,以此估算沉积层厚度、获得共振频率以及场地放大因子等.目前微动探测发展还有诸多问题需要进行深入研究,例如瑞雷波与勒夫波浅层探测理论方法、同时考虑基阶和高阶面波的影响进行多阶面波反演方法研究以及H/V谱比曲线与频散曲线联合反演方法研究等,从而提高微动探测方法浅层探测精度以满足城市地下空间精细探测的需求,加深探测深度为透明地壳、地下第三空间的精细探测提供一种新型地球物理手段.     

工程场地动力特性分析的地脉动法

简要地回顾了地脉动研究的历史,综述了地脉动在场地动力分析中应用的四种主要分析方法：直接富氏谱法、相对参考点谱比法、单点地脉动谱比法和利用地脉动反演场地剪切波速法。介绍各种分析方法的理论依据和适用条件,侧重分析了各种方法中存在的问题,论述了地脉动的应用前景和进一步研究的内容。     

Estimation of damping ratio of soil sites using microtremor

It is widely known that the seismic response characteristics of a soil site depends heavily on several key dynamic properties of the soil stratum, such as predominant frequency and damping ratio. A widely used method for estimating the predominant frequency of a soil site by using microtremor records, proposed by Nakamura, is investigated to determine its effectiveness in estimating the damping ratio. The authors conducted some microtremor measurements of soil sites in Hong Kong and found that Nakamura’s method might also be used to estimate the damping ratio of a soil site. Damping ratio data from several typical soil sites were obtained from both Nakamura’s ratio curves using the half power point method and resonant column tests. Regression analysis indicates that there is a strong correlation between the damping ratios derived from these two different approaches. Supported by: Research project (PolyU 5076/97E), of the Department of Civil and Structural Engineering, The Hong Kong Polytechnic University     

地脉动测试及其在场地评价中的应用

介绍了地脉动的原理、测试方法及其影响因素,通过工程实例,阐述了场地卓越周期的测定及其在场地及场地土判定与评价中的应用,分析了不同场地条件对地脉动频谱特性的影响.结果表明通过地脉动测试确定场地卓越周期的效果较为理想,其在场地及场地土判定与评价中的结果与利用剪切波速和覆盖层厚度划分结果基本一致;场地岩土层结构直接影响地脉动的频谱特性,二者的变化相对应.     

A new direction for shallow refraction seismology: integrating amplitudes and traveltimes with the refraction convolution section

The refraction convolution section (RCS) is a new method for imaging shallow seismic refraction data. It is a simple and efficient approach to full‐trace processing which generates a time cross‐section similar to the familiar reflection cross‐section. The RCS advances the interpretation of shallow seismic refraction data through the inclusion of time structure and amplitudes within a single presentation. The RCS is generated by the convolution of forward and reverse shot records. The convolution operation effectively adds the first‐arrival traveltimes of each pair of forward and reverse traces and produces a measure of the depth to the refracting interface in units of time which is equivalent to the time‐depth function of the generalized reciprocal method (GRM). Convolution also multiplies the amplitudes of first‐arrival signals. To a good approximation, this operation compensates for the large effects of geometrical spreading, with the result that the convolved amplitude is essentially proportional to the square of the head coefficient. The signal‐to‐noise (S/N) ratios of the RCS show much less variation than those on the original shot records. The head coefficient is approximately proportional to the ratio of the specific acoustic impedances in the upper layer and in the refractor. The convolved amplitudes or the equivalent shot amplitude products can be useful in resolving ambiguities in the determination of wave speeds. The RCS can also include a separation between each pair of forward and reverse traces in order to accommodate the offset distance in a manner similar to the XY spacing of the GRM. The use of finite XY values improves the resolution of lateral variations in both amplitudes and time‐depths. The use of amplitudes with 3D data effectively improves the spatial resolution of wave speeds by almost an order of magnitude. Amplitudes provide a measure of refractor wave speeds at each detector, whereas the analysis of traveltimes provides a measure over several detectors, commonly a minimum of six. The ratio of amplitudes obtained with different shot azimuths provides a detailed qualitative measure of azimuthal anisotropy and, in turn, of rock fabric. The RCS facilitates the stacking of refraction data in a manner similar to the common‐midpoint methods of reflection seismology. It can significantly improve S/N ratios.Most of the data processing with the RCS, as with the GRM, is carried out in the time domain, rather than in the depth domain. This is a significant advantage because the realities of undetected layers, incomplete sampling of the detected layers and inappropriate sampling in the horizontal rather than the vertical direction result in traveltime data that are neither a complete, an accurate nor a representative portrayal of the wave‐speed stratification. The RCS facilitates the advancement of shallow refraction seismology through the application of current seismic reflection acquisition, processing and interpretation technology.     

Empirical evaluation of microtremor H/V spectral ratio

The objective of this work is to perform a purely empirical assessment of the actual capabilities of the horizontal-to-vertical (H/V) spectral ratio technique to provide reliable and relevant information concerning site conditions and/or site amplification. This objective has been tackled through the homogeneous (re)processing of a large volume of earthquakes and ambient noise data recorded by different research teams in more than 200 sites located mainly in Europe, but also in the Caribbean and in Tehran. The original recordings were first gathered in a specific database with information on both the sites and recorded events. Then, for all sites close to an instrumented reference, average site-to-reference spectral ratios (“spectral ratio method” (SSR)) were derived in a homogeneous way (window selection, smoothing, signal-to-noise ratio threshold, averaging), as well as H/V ratios (“HVSRE–RF”) on earthquake recordings. H/V ratios were also obtained from noise recordings at each site (either specific measurements, or extracted from pre- or post-event noise windows). The spectral curves resulting from these three techniques were estimated reliable for a subset of 104 sites, and were thus compared in terms of fundamental frequency, amplitude and amplification bandwidth, exhibiting agreements and disagreements, for which interpretations are looked for in relation with characteristics of site conditions. The first important result consists in the very good agreement between fundamental frequencies obtained with either technique, observed for 81% of the analyzed sites. A significant part of the disagreements correspond to thick, low frequency, continental sites where natural noise level is often very low and H/V noise ratios do not exhibit any clear peak. The second important result is the absence of correlation between H/V peak amplitude and the actual site amplification measured on site-to-reference spectral ratios. There are, however, two statistically significant results about the amplitude of the H/V curve: the peak amplitude may be considered as a lower bound estimate of the actual amplification indicated by SSR (it is smaller for 79% of the 104 investigated sites), and, from another point of view, the difference in amplitude exhibits a questioning correlation with the geometrical characteristics of the sediment/basement interface: large SSR/HV differences might thus help to detect the existence of significant 2D or 3D effects.     

Seismic microzonation of the greater Bangkok area using microtremor observations

Bangkok, the capital city of Thailand, is located at a remote distance from seismic sources. However, it has a substantial risk from these distant earthquakes due to the ability of the underlying soft clay to amplify ground motions. It is therefore imperative to conduct a detailed seismic hazard assessment of the area. Seismic microzonation of big cities, like Bangkok, provides a basis for site‐specific hazard analysis, which can assist in systematic earthquake mitigation programs. In this study, a seismic microzonation map for the greater Bangkok area is constructed using microtremor observations. Microtremor observations were carried out at more than 150 sites in the greater Bangkok area. The predominant periods of the ground were determined from the horizontal‐to‐vertical (H/V) spectral ratio technique. A microzonation map was then developed for the greater Bangkok area based on the observations. Moreover, the transfer functions were calculated for the soil profile at eight sites, using the computer program SHAKE91, to validate the results from the microtremor analysis. The areas near the Gulf of Thailand, underlaid by a thick soft clay layer, were found to have long natural periods ranging from 0.8s to 1.2s. However, the areas outside the lower central plain have shorter predominant periods of less than 0.4s. The study shows that there is a great possibility of long‐period ground vibration in Bangkok, especially in the areas near the Gulf of Thailand. This may have severe effects on long‐period structures, such as high‐rise buildings and long‐span bridges. Copyright © 2003 John Wiley & Sons, Ltd.     

Hilbert-Huang变换在地脉动卓越周期计算中的应用

研究如何在白天观测地脉动存在人为振动干扰情况下准确计算卓越周期.选择一个场地进行连续24 h地脉动观测.应用快速傅里叶变换计算地脉动数据的功率谱,应用Hilbert-Huang变换对地脉动数据进行经验模态分解并计算边际谱.通过实验发现,夜间和白天观测地脉动功率谱具有明显差异.在夜间观测的地脉动主峰明显,容易识别卓越周期,在白天观测的地脉动功率谱存在多个峰值,难以识别卓越周期.边际谱与功率谱基本一致,存在多个峰值,难以由边际谱判别卓越周期.通过实验还发现,可以由某一IMF分量的频谱判别卓越周期,但具体为哪个IMF分量需结合场地条件判断.对原始数据减去IMF分量后差值的频谱分析发现主峰明显,频率和夜间观测的非常接近,可以判别卓越周期.实验结果表明,对于在白天观测存在人为振动干扰的场地,难以通过功率谱和边际谱判别卓越周期,可以应用Hilbert-Huang变换进行经验模态分解,由原始数据减去IMF分量后差值的频谱峰值判别卓越周期.     

9度设防区场地脉动测试分析

为了了解9度设防区场地的动力特征,应用脉动法对四川省西昌市(9度设防)25处典型场地进行了脉动测试,并采用Nakamura方法对实测数据进行了处理,获得了该地区场地卓越周期的分布规律以及地基处理、断裂带、湖泊对场地动力特性的影响,为反应谱理论在9度设防区的应用提供了试验依据.测试结果表明:西昌市场地的卓越周期主要分布在...     

工程场地条件评定中的地脉动研究

为了研究开发更经济，更简便的场地动务特性探测方法，本文简要地回顾了工程场地条件评定中地脉动研究的演进，概述了地脉动观测技术的发展，循着地脉动分析方法的演进历程，叙述了谱和谱比分析方法，利用地脉动频散曲线反演场地速度构造的研究，讨论了各种分析方法的依据和适用条件，指出了目前存在的问题，并探讨了可能的改进途径。     

Interpretation of shallow refraction seismic data by reflection/refraction tomography

A new algorithm for tomographic inversion of traveltimes of reflected and refracted seismic waves is developed. The inversion gives interface configurations and velocity distributions in layers. The important features of the algorithm are: (a) the inclusion of shot time delays in the list of unknown parameters; (b) the regularization is applied in such a way that the most probable model is characterized by the similarity of neighbouring interfaces. As the problem under consideration is non-linear, several iterations are necessary in order to obtain the final model. In the case of a very inexact initial model, a 'layer-by-layer' inversion strategy is recommended as a first inversion step. The inversion program is supplied with a user interface, thanks to which one can: (a) pick interactively and identify seismic traveltimes; (b) build and edit depth/velocity models; and (c) display calculated traveltime curves and compare them with picked traveltimes as well as with the original seismic sections. The efficiency of the inversion software developed is illustrated by a numerical example and a field example in which shallow seismic data are considered. Application to wide-aperture reflection/refraction profiling (WARRP) data is also possible.     

On the use of microtremor recordings in seismic microzonation

Experimental methods involving microtremor recordings are useful for determining site effects in regions of moderate seismic activity where ground motion records are few, and in urban or industrial contexts where the noise level is high. The aim of this study is to establish a microzonation by using the Nogoshi–Nakamura method,[1, 2] a simple experimental technique based on microtremor recordings. Since the physical phenomena underlying the method are only partially understood, the spectral responses obtained cannot be used alone. We, therefore, complete our experimental results by comparing them with the solutions of a one-dimensional numerical simulation (SHAKE91).[3, 4] The experimental programme was carried out on a plain near the Rhone Delta (South of France). H/V spectral ratios were calculated at 137 noise measurement points. In addition, we were able to compute the numerical transfer functions from soil columns defined by geotechnical characteristics inside the studied region. A comparison of the results obtained by the experimental and numerical methods showed that the fundamental frequencies are in good agreement, but that the amplitudes obtained by the two techniques are sometimes different. The analysis of H/V spectral ratios enabled us to establish maps to characterize the region: a resonance frequency map and maps of amplification levels as a function of frequency range, leading to a seismic microzonation for the whole of the region.     

Site effect evaluation at Mexico City: Dominant period and relative amplification from strong motion and microtremor records

Site effects in Mexico City are discussed in terms of simple 1D, one-layer, linear models. The analysis is focussed on two parameters: dominant period and maximum amplification relative to a firm site within the city. The data used is a compilation of strong motion data and microtremor measurements. Strong motion data consist of digital acceleration records for nine events recorded by the Accelerographic Network of Mexico City. The authors analyzed spectral ratios of horizontal components of soft soil sites relative to an average of firm site observations for this data set. Dominant period, maximum relative amplification and an estimate of material damping were computed from the empirical transfer functions thus obtained. Microtremor data were compiled from measurement of different groups during the period 1985–1992. In all, 409 measurement points were analyzed. Values of dominant period obtained from microtremor measurements are in excellent agreement with those obtained from empirical transfer functions for strong motion data. The synthesis of results allows us to draw a detailed and robust map of dominant period for Mexico City. Based on this map, the authors propose some modifications to the current microzonation of Mexico City and evaluate a proposed model to account for site effects in this city.     

Application of the microtremor measurements to a site effect study

Earthquake has left much life and property damages. The occurrence of such events necessitates the execution of plans for combating the earthquakes. One of the most important methods for combating earthquakes includes assessing dynamic characteristics of soil and site effect. One of the methods by which one can state dynamic characteristics of the soil of an area is the measurement of microtremors. Microtremors are small-scale vibrations that occur in the ground and have an amplitude range of about0.1–1 microns. Microtremor measurement is fast, applicable, cost-effective. Microtremor measurements were taken at 15 stations in the Babol, north of Iran. Regarding H/V spectral ratio method, peak frequency and amplification factor were calculated for all microtremor stations.According to the analysis, the peak frequency varies from0.67 to 8.10 Hz within the study area. Also, the authors investigated the validity of the results by comparing them with SESAME guidelines and geotechnical conditions of study area. The microtremor analysis results are consistent with SESAME guidelines and geotechnical condition of study area. The results show that the microtremor observations are acceptable methods for assessing dynamic characteristics of soil and site effect in the Babol City.