Validity of the Refraction Microtremors (ReMi) Method for Determining Shear Wave Velocities for Different Soil Types in Egypt

The Refraction Microtremor (ReMi) method is being used around the world by the geotechnical and geophysical community to determine shear-wave velocities. This is due to its faster, less expensive and accurate determination of shear wave velocities, when compared to other methods used. Unlike standard crosshole and downhole techniques, ReMi does not require any drilling. It eliminates the problem of shear-wave source and quiet site that are pre-requisites for good seismic refraction surveys. In this paper we present refraction microtremors (ReMi) measurements done at sites underlain by different soil types in Egypt. The ReMi data were collected using standard refraction equipment employing 12, 24 or 48 channels. We used deep oceanographic noise and ambient noise including energy from power generators, pile drivers and traffic. The data were processed using the SeisOpt? ReMi? (? Optim, Inc.) software to reveal one-dimensional shear-wave velocity structures beneath the arrays. To access the validity of the method for the Egyptian soils, the shear-wave profiles obtained from the ReMi measurements were compared to downhole and crosshole data for different soils. Comparisons demonstrate the robustness of the ReMi technique for obtaining shear-wave velocities for different soil types in Egypt.

Single-station microtremor surveys for site characterization:A case study in Erzurum city,eastern Turkey

The single-station microtremor method is one of the fastest,most reliable,and cheapest methods used to identify dynamic soil properties.This study utilizes 49 single-station microtremor measurements to identify the dynamic soil properties of the Hilalkent quarter of the Yakutiye district in Erzurum.Soil dominant frequency and the amplification factor were calculated by using the Nakamura horizontal/vertical spectral ratio(H/V)method.While the soil dominant frequency values varied between 0.4 Hz and 10 Hz,the soil amplification factor changed between 1 and 10.Higher H/V values were acquired with lower frequency values.The vulnerability index(K_(g))and shear strain parameters that are utilized to estimate the damage that may be caused by an earthquake were mapped.Especially in the west side of the study area,higher K_(g) values were observed.The shear strain map was created with 0.25 g,0.50 g and 0.75 g bedrock accelerations,and soil types that lost elasticity during an earthquake were identified.The average shear wave velocity for the first 30 m(V_(s30))was calculated.Finally,it was observed that the western part of the study area,which resulted in a higher period and higher H/V,higher K_(g) and lower V_(s30) values,presents a higher risk of damage during an earthquake.

Spatial structural characteristics of the Deda ancient landslide in the eastern Tibetan Plateau:Insights from Audio-frequency Magnetotellurics and the Microtremor Survey Method

It is of crucial importance to investigate the spatial structures of ancient landslides in the eastern Tibetan Plateau’s alpine canyons as they could provide valuable insights into the evolutionary history of the landslides and indicate the potential for future reactivation.This study examines the Deda ancient landslide,situated in the Chalong-ranbu fault zone,where creep deformation suggests a complex underground structure.By integrating remote sensing,field surveys,Audio-frequency Magnetotellurics(AMT),and Microtremor Survey Method(MSM)techniques,along with engineering geological drilling for validation,to uncover the landslide’s spatial feature s.The research indicates that a fault is developed in the upper part of the Deda ancient landslide,and the gully divides it into Deda landslide accumulation zoneⅠand Deda landslide accumulation zoneⅡin space.The distinctive geological characteristics detectable by MSM in the shallow subsurface and by AMT in deeper layers.The findings include the identification of two sliding zones in the Deda I landslide,the shallow sliding zone(DD-I-S1)depth is approximately 20 m,and the deep sliding zone(DD-I-S2)depth is 36.2-49.9 m.The sliding zone(DD-Ⅱ-S1)depth of the DedaⅡlandslide is 37.6-43.1 m.A novel MSM-based method for sliding zone identification is proposed,achieving less than 5%discrepancy in depth determination when compared with drilling data.These results provide a valuable reference for the spatial structural analysis of large-deepseated landslides in geologically complex regions like the eastern Tibetan Plateau.

Application of geophysical methods in fine detection of urban concealed karst:A case study of Wuhan City,China

The construction of modern livable cities faces challenges in karst areas,including ground collapse and engineering problems.Wuhan,with a population of 13.74×10^(6) and approximately 1161 km^(2)of soluble rocks in the urban area of 8569.15 km^(2),predominantly consists of concealed karst areas where occasional ground collapse events occur,posing significant threats to underground engineering projects.To address these challenges,a comprehensive geological survey was conducted in Wuhan,focusing on major karstrelated issues.Geophysical methods offer advantages over drilling in detecting concealed karst areas due to their efficiency,non-destructiveness,and flexibility.This paper reviewed the karst geological characteristics in Wuhan and the geophysical exploration methods for karst,selected eight effective geophysical methods for field experimentation,evaluated their suitability,and proposed method combinations for different karst scenarios.The results show that different geophysical methods have varying applicability for karst detection in Wuhan,and combining multiple methods enhances detection effectiveness.The specific recommendations for method combinations provided in this study serve as a valuable reference for karst detection in Wuhan.

Horizontal-to-vertical spectral ratio inversion method based on multimodal forest optimization algorithm

The exploration of urban underground spaces is of great significance to urban planning,geological disaster prevention,resource exploration and environmental monitoring.However,due to the existing of severe interferences,conventional seismic methods cannot adapt to the complex urban environment well.Since adopting the single-node data acquisition method and taking the seismic ambient noise as the signal,the microtremor horizontal-to-vertical spectral ratio(HVSR)method can effectively avoid the strong interference problems caused by the complex urban environment,which could obtain information such as S-wave velocity and thickness of underground formations by fitting the microtremor HVSR curve.Nevertheless,HVSR curve inversion is a multi-parameter curve fitting process.And conventional inversion methods can easily converge to the local minimum,which will directly affect the reliability of the inversion results.Thus,the authors propose a HVSR inversion method based on the multimodal forest optimization algorithm,which uses the efficient clustering technique and locates the global optimum quickly.Tests on synthetic data show that the inversion results of the proposed method are consistent with the forward model.Both the adaption and stability to the abnormal layer velocity model are demonstrated.The results of the real field data are also verified by the drilling information.

Microtremor-based analysis of the dynamic response characteristics of a site containing grouped earth fissures

In this study,the Beibu earth fissure site in the northeastern part of Weihe Basin,which contains four nearly parallel earth fissures,was studied.A long straight microtremor measuring line,containing 49 measuring points across four earth fissures,was established to investigate the dynamic response of this site using Fourier spectrum,response acceleration spectrum,Arias intensity,and HVSR analyses.The main results are as follows:(1)The fundamental frequencies of 44 measuring points obtained from HVSR analysis are concentrated within 1.67 Hz-2.25 Hz,and the existence of the earth fissures has little effect on the fundamental frequency changes.(2)There is an amplification effect near a single earth fissure.The dynamic responses are large at the measuring points near the earth fissure,and the values decrease with increasing distance from the earth fissure.In areas between two adjacent earth fissures,these values decrease and are even lower than those in sites without amplification effects.(3)In this earth fissure site,the general area(or less affected area)and affected areas were delineated based on the amplification effect.In engineering applications,construction design should avoid these affected areas and existing structures should be reinforced to satisfy the seismic fortification requirements.

A study of local amplification effect of soil layers on ground motion in the Kathmandu Valley using microtremor analysis

Past researchers have anticipated the occurrence of a great earthquake in the central Himalayas in the near future.This may cause serious damage in the Kathmandu Valley,which sits on an ancient lake bed zone,with lacustrine sediments of more than 500 m depth.In this study,the predominant frequency of ground motion is evaluated using the Horizontal-to-Vertical (H/V) spectral ratio technique and recordings of ambient noise.The results of the H/V ratio show two peaks in about 20 percent of the locations,which are distributed mainly in and around the center and northern part of the Kathmandu Valley.The predominant frequencies vary from 0.5 Hz to 8.9 Hz in the study area,whereas the second resonance fiequency varies from 4 Hz to 6 Hz in the center and northern part of the valley.This indicates that the center and northern part of the valley have a wide range of resonance frequency due to two levels of impedance contrast- one may be from the surface layer and the other may be from the layer undemeath.These two levels of resonance indicate the importance of considering the effects of surface and lower layers during the planning and designing of infrastructures in the Kathmandu Valley.

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.

Local site effects evaluation by surface wave and H/V survey methods in Senirkent(Isparta)region,southwestern Turkey

This study aimed to investigate the site characterization of the Senirkent Basin(Isparta)by using the microtremor(H/V)and surface wave analysis(ReMi)methods.Senirkent district,which is located on the foothill of the Kap?Mountain,are prone to seismic and landslide hazards.After the last landslip disaster(1995),the city has continued to develop on the alluvial basin.The microtremor data collected at 41 stations were analyzed.The predominant frequency and the amplification of the soil were found to be in the range of 1.8–6.6 Hz and 0.8–6.8,respectively.These are 4.0-6.8 Hz and 1-2.5 around the slope of the Kap?Mountain and 0.8-3.6 Hz and 3.0-6.6 at alluvium deposits of the valley.The ReMi data collected along 41 profiles were analyzed to obtain a Vs30 map.The soil classifications were done by using Vs30 values according to NEHRP soil classification criteria.The results showed that the southern area of the study had better soil conditions with Vs30 between 360-1160 m/s and soil class between C and B.The alluvium deposits on the north are relatively low with Vs30 in the range of 260-360 m/s and soil class D.

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 about 0.1-1 microns. Microtremor measurement is fast, applica- ble, cost-effective. Microtremor measurements were taken at 15 stations in the Babol, north of Iran. Regarding H/V spectral ratio method, peak frequency and amplifica- tion factor were calculated for all microtremor stations. According to the analysis, the peak frequency varies from 0.67 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 obser- vations are acceptable methods for assessing dynamic characteristics of soil and site effect in the Babol City.

Microtremor-based analysis of the dynamic response characteristics of earth-fissured sites in the Datong basin, China

This study conducted microtremor testing along six survey lines that cross three typical earth fissures in the Datong basin to determine the dynamic response characteristics of earth fissure sites with regard to the Fourier amplitude spectrum,response spectrum,and Arias intensity.The results show the following.(1)The predominant frequency of an earth fissure site is mainly affected by the thickness and the shear wave velocity of the soil layer and is minimally effected by the presence of an earth fissure.(2)Earth fissures have a pronounced amplification effect on dynamic response.Fourier amplitude,response acceleration,and Arias intensity are high near an earth fissure and decrease with an increase in distance from the earth fissure,tending toward stability at a distance of 20 m.(3)The area that is seriously affected by this amplification is within 6-8 m of an earth fissure,and the general affected area is farther out than this,to a distance of 25 m.(4)New construction should be avoided in an area affected by the amplification,and existing buildings in general and seriously affected areas need to be reinforced to increase their seismic fortification intensity.

Characteristics of long period microtremor and validation of microtremor array measurements in inland areas of China

To study the characteristics of long period microtremor and applicability of microtremor survey,we have made microtremor observations using long period seismometers of the China's mainland from coastal cities like Shanghai and Tianjin through Beijing,Xi'an,to the far inland cities of Lanzhou and Tianshui.The observation shows that the level of power spectrum of long period microtremors reduced rapidly from the coast to the inland area.However,the energy of long period microtremors in Beijing,Xi'an,Lanzhou and Tianshui area are only approximately ten-thousandth to thousandth of that in Shanghai.Aiming at the complexity of the underground structure in the far inland,a series of underground structure models with different distributions were assessed using three-dimensional,dynamic finite element method(FEM) analyses.The results were used to evaluate microtremor survey methods and their limitations with regard to aggregate variability and thickness determinations.Multiple-wave reflections between layers with the change of underground structure distribution occurred,which have significant effect on the performance of the different field approaches.Information over a broad spectrum from which velocity-depth profiles were produced via inversion approaches.Neither the thickness nor the shear waveVelocity V of the subsurface layer inversion results appeared over a large evaluation with increasing slope angle.In particular,when the angle of the model reached 45°,it could not be accurately evaluated using one-dimensional inversion methods.Finally,the array microtremor survey(AMS) was carried out in Shanghai City,China.Although AMS techniques do not have the layer sensitivity or accuracy(velocity and layer thickness) of borehole techniques,the obtained shear wave velocity structure is especially useful for earthquake disaster prevention and seismic analysis.

Fault detection using microtremor data (HVSR-based approach) and electrical resistivity survey

The faults and fractures are known as two of the most important parameters in earthquake occurrence.During the construction in urban areas, faults and fractures may be covered in depth and thus are not visible at the ground surface. In this context, non-invasive geophysical prospecting methods(microtremor and geoelectrical methods) and borehole data were used to detect subsurface geological structures(hidden faults) in a suburb of Shiraz in Iran. The horizontal to vertical spectral ratio(HVSR) method was used to obtain the dynamic parameters(predominant frequency and resonance amplitude) of the soil, to detect hidden faults. The results show that the abrupt changes in the sediment thickness and predominant frequencies at a specific direction(NW-SE) can be related to the displacement of a nearly vertical fault with NW-SE trend. In addition, the electrical resistivity method using continuous resistivity profiling(CRP) and Schlumberger arrays was employed to detect a hidden fault and the results were compared with previous data. The obtained results of both arrays illustrate the presence of a nearly vertical fault with NW-SE trend in the region. Comparison of all results shows that the detected faults by both methods are consistent with each other. Therefore, it can be conclusive that combination of the two methods is a useful and reliable approach to study and detect hidden faults.

Identification of hidden faults using determining velocity structure profile by spatial autocorrelation method in the west of Mashhad plain(Northeast of Iran)

Characterizing the subsurface structure is an important parameter for the improvement of seismic hazard assessment.Due to the tectonic complexity of the earth,some deep fractures do not reach the earth's surface and are not detectable with visual analysis.Therefore,the lack of knowledge of faults and fractures can result in disasters,especially in urban planning.Many geophysical methods can be used to estimate subsurface structure characterization.However,a more reliable method is required to assess seismic hazards and reduce potential damage in metropolitan areas without destroying buildings and structures.This paper aims to identify hidden faults and structures using shear wave velocity sections.To do this,surface wave dispersion curve was extracted from the vertical component of microtremor array recording using the spatial autocorrelation(SPAC)method in two profiles and 13 array stations(perpendicular to the altitudes)to obtain shear wave velocity structure(Vs)in the west of Mashhad,northeast of Iran.The results of shear wave velocity profiles(Vs)indicate sudden changes in the thickness of sediments.This can be related to the displacement of a normal fault in this area causing the bottom rock to fall and an increase in the alluvial thickness in the central part of the plain.The velocity in the floor rock is 2000 meters per second in this area.According to the surface outcrops and water wells data,its material is slate and Phyllite metamorphic rocks that are exposed in the adjacent heights.Besides,the seismic profile results were well consistent with electrical resistance data and well logs indicating that the tool array method is flexible,non-invasive,relatively fast,and effective for urban areas with satisfactory accuracy.

Site amplification in the Lijiang Valley,Yunnan Province

The site amplifications for three stations, Libin (LIBI), Baisha (BASH) and Yulong (YULG) situated respectivelyin the southern, middle and northern parts of Lijiang Valley, are obtained by analyzing the S-wave soil/bedrockand microtremor horizontal/vertical spectral ratios. The data are digital recordings for the aftershocks of the Lijiang Ms=7.0 earthquake on February 3, 1996. In the frequency range of 1-4 Hi, the S-wave soil/bedrock spectralratio of E-W component for LIBI is the largest and amounts to 4.5. The microtremor soil N-S/vertical (V) spectralratio is approximate to l, E-W/V is about 4.5 and the same with above soil/bedrock spectral ratio. It is shown thatthe vertical and N-S components of microtremor have not been amplified by the soil and the Nakamura's method isapplicable. With this method, the microtremor spectral ratios for BASH and YULG are further evaluated. Theyhave similar characteristics with that of LIBI. In above frequency range, both N-S/V ratios are approximate to 1,while the E-W/V ratio is about 6 for BASH, 4.5 for YULG. Ljiang Valley is characterized by the trans-valleydirectional site response.

Estimation of Shear Wave Structure and Horizontal to Vertical Spectral Ratio at Different Sites in Kathmandu Valley

The present study was carried out to evaluate resonant frequency of the ground and to characterize subsurface ground based on shear wave velocity structure. For this, five sites were selected such as Pulchowk, Chhauni, Gaushala, Buddhanagar and Bhainsepati. About 20 data were recorded in each site and then shear wave velocity structure and graph of amplification ratio with their spatial distribution has been established with the help of software i.e. Seisimager/Seismodule Controller. The results of both analysis methods were then compared to the amplitude of the Gorkha Earthquake and borehole data. All these data and study indicates that the Kathmandu Valley sediments are dependent on the frequency of the seismic waves and the wave velocity is greater in the peripheral region than in the central part of the Valley. The result had also shown that the presence of silty-sand, clay and loose gravel soil with low bearing capacity and elastic modulus in most of the sites are responsible for devastation. It was also noted that apart from few limitations, a non-intrusive microtremor analysis can be adopted for earthquake site characterization in the Kathmandu Valley which can be readily applied and expanded upon in future seismic hazard and microzonation efforts for Kathmandu.

A Study on Health Monitoring of Structural Damages for Two Stories Model by Using Vibration Test

Many structures in Japan were built after the war at a revival term or rapid economic growth. These structures have been reached a life in recent years and it is economically not affordable to conduct repair and reconstruct these structures only with a possibility of being damaged. This paper presents an approach to detect the structural damages for two degrees of freedom (2DOF) model. In this study, we conducted Microtremor measurement, free vibration test and vibration test. The 2DOF model was demonstrated the feasibility of using the proposed approach to damage detection of structural member.

Site Effect Estimation Using Microtremor Measurements at New Luxor City Proposed Site, South Egypt*

The present study aims mainly at investigating the site effect at the new Luxor city, Egypt, using microtremor measurements. Accordingly, 41 ambient noise recordings were implemented covering the investigated area in a reasonable manner with spacing nearly 1 km between each measurement station. The data were analyzed using Nakamura’s technique. The results show that;the investigated area exhibits low fundamental frequency values ranging between 0.28 - 0.47 Hz and its corresponding amplification factor values vary between 2.3 and 6.8. Consequently, the studied site is characterized by the presence of thick soft sediments overlying a hard bed rock. The seismic vulnerability index (Kg) was estimated, relying on the estimated fundamental frequency (F0) and amplification factor (A0), through the range between 11 and 95 across the mapped area. The obtained Kg values were noticed to be compatible with amplification level results. The central part of the investigated area reflects the higher Kg and A0 values.

Applications of the Hilbert-Huang Transform for Microtremor Data Analysis Enhancement

In this paper we discuss the use of the Hilbert-Huang transform（HHT） to enhance the time-frequency analysis of microtremor measurements. HHT is a powerful algorithm that combines the process of empirical mode decomposition（EMD） and the Hilbert transform to compose the HilbertHuang spectrum that contains the time-frequency-energy information of the recorded signals. HHT is an adaptive algorithm and does not require the signals to be linear or stationary. HHT is advantageous for analyzing microtremor data, since observed microtremors are commonly contaminated by nonstationary transient noises close to the recording instruments. This is especially true when microtremors are measured in an urban environment. In our data processing HHT was used to（1） eliminate the unwanted short-duration transient constituents from microtremor data and use only the coherent portion of the data to carry out the widely used horizontal to vertical spectral ratio（H/V） method;（2） identify and eliminate the continuous industrial noise in certain frequency band; and（3） enhance the H/V analysis by using the Hilbert-Huang spectrum（HHS）. The efficacy of this proposed approach is demonstrated by the examples of applying it to microtremor data acquired in the metropolitan Beijing area.

Liquefaction assessment using microtremor measurement, conventional method and artificial neural network （Case study： Babol, Iran）

Recent researchers have discovered microtremor applications for evaluating the liquefaction potential. Microtremor measurement is a fast, applicable and cost-effective method with extensive applications. In the present research the liquefaction potential has been reviewed by utilization of microtremor measurement results in Babol city. For this purpose microtremor measurements were performed at 60 measurement stations and the data were analyzed by suing Nakmaura＇s method. By using the fundamental frequency and amplification factor, the value of vulnerability index （Kg） was calculated and the liquefaction potential has been evaluated. To control the accuracy of this method, its output has been compared with the results of Seed and Idriss [1] method in 30 excavated boreholes within the study area. Also, the results obtained by the artificial neural network （ANN） were compared with microtremor measurement. Regarding the results of these three methods, it was concluded that the threshold value of liquefaction potential is Kg = 5. On the basis of the analysis performed in this research it is concluded that microtremors have the capability of assessing the liquefaction potential with desirable accuracy.