Application of microtremor H/V spectral ratio method in determining characteristic parameters of typical laterite sites

Microtremor signals contain rich geophysical information, and the application of microtremor measurements in determining site characteristic parameters is increasingly valued because of their advantages of simple operation, environmental friendliness, and low cost. In this study, systematic microtremor measurements were conducted at a typical laterite site. The microtremor measurement data were processed with different combinations of horizontal components using the H/V spectral ratio method(H: Horizontal frequency domain signal of the microtremor; V: Vertical frequency domain signal of the microtremor). to take the HVSR curve. The period corresponding to the peak of the HVSR curve was considered to be the predominant period of the site, the site classifications were determined, and geotechnical types were divided. This study indicates that the microtremor H/V spectral ratio method is highly consistent with the results of borehole exploration for determining site classifications and dividing geotechnical types. Moreover, using the combination of vectors sum of East-West direction and North-South direction as horizontal components in the microtremor H/V spectral ratio method can more easily and accurately determine the site-predominant period.


Introduction
The determination of site characteristic parameters using microtremor measurements has become increasingly important in the field of engineering seismology [1].In processing microtremor signal data, Nakamura [2,3] first proposed an evaluation of site characteristics based on the Fourier amplitude spectrum ratio of the horizontal component to the vertical component of the microtremor at 1334 (2024) 012046 IOP Publishing doi:10.1088/1755-1315/1334/1/012046 2 the same surface observation point.This is known as the H/V spectral ratio method (i.e., Nakamura method).
The H/V spectral ratio method has been recognized by most scholars worldwide and has been applied in engineering.In a study on microtremors, Guo [4] found that the single-point H/V spectral ratio method can reliably provide the basic period of the site.Lin [5] proposed a site classification scheme based on resonance frequency through microtremor measurement research.Shi [6] proposed a set of division schemes of microtremor predominant period, which is suitable for site classifications of the current seismic design code, by studying the corresponding relationship between the predominant period of the microtremor H/V spectrum ratio and site classifications.Macau [7] performed single point microtremor measurements in Spain and determined the sites in this area by calculating the equivalent shear wave velocity and resonance frequency using the Spatial Autocorrelation method and H/V spectral ratio method.Sánchez-Sánchez [8] determined the resonance frequency of a site by calculating the microtremor H/V spectral ratio, provided a set of site classification schemes suitable for Mexico City, Mexico, and drew a resonance frequency distribution map of microtremors as a reference for the seismic design of Mexico City, Mexico.Khedr [9] classifies the site types of Kochi City, Japan, and Nile Delta, Egypt according to the resonance frequency and peak value of microtremor H/V spectral ratio.Laouami [10,11] obtains the standard H/V spectral ratio curves of different site types by calculating the theoretical transfer function of the site and synthesizing them with the spectral ratio period to establish a new site classification standard.Finally, the accuracy of the method was verified by calculating the microtremor H/V spectral ratio.
Experts and scholars in the field of engineering seismology have obtained stable spectral curves using the microtremor H/V spectral ratio method, which can provide a more accurate resonance frequency for the site [12].By converting frequencies into periods, they can determine site classifications, divide geotechnical types, and calculate the overburden thickness [13].Compared with the borehole exploration method, the microtremor H/V spectral ratio method has the characteristics of rapid study of the site, and it has the advantages of simple operation, economic efficiency, environmental friendliness, and less limited conditions of the site [14].Although borehole exploration is widely used in geotechnical engineering owing to its high precision and direct and objective data, its cost increases exponentially with increasing drilling depth, and a large number of boreholes need to be arranged at each site.Its cost and time consumption far exceed those of the microtremor measurements.
In this study, microtremor measurements were carried out at a typical laterite site with detailed geological borehole and shear wave velocity test data in Nanchang.The microtremor measurement data were processed using the microtremor H/V spectral ratio method.The H/V spectral ratio curve was obtained by calculating the ratio of the horizontal components (East-West direction and North-3 South direction) to the vertical components in the equation, and the site-predominant period was obtained from the peak value of the HVSR curve.Subsequently, site classifications were determined, geotechnical types were divided, and the results of borehole exploration and microtremor measurements were compared and analyzed.

Principle of microtremor measurement
Microtremors are regular, stable, and frequent vibrations that occur on the Earth's surface at all times.
It is mainly excited by changes in natural phenomena, such as tides, ocean waves, wind, and climate, as well as random sources caused by mechanical vibration, transportation, engineering construction, and other artificial factors, and propagates to the surface after multiple reflexes and refractions from different media layers of the site.It is a stable, nonrepetitive, random wave on the surface.In the process of propagation in the soil layer, the microtremor carries rich information reflecting the characteristics of the soil layer through multiple reflexes and refractions, whereas the inherent characteristics of the soil layer are steady, do not change with time, and save microtremor information [15].Therefore, through microtremor measurements, a mathematical method (e.g., Fast Fourier Transform) is used to extract the spectrum curve from the microtremor signal, analyze the microtremor signal, and estimate the site influence, further inferring the site characteristic parameters [16,17].

Measurement
During the measurement, the position was selected with less ambient noise, and three seismic sensors were arranged in a pairwise orthogonal manner at each measuring point, which were placed on the flat and dense soil layer; the distance between the three seismic sensors was as close as possible.The horizontal sensors were placed in the East-West and North-South directions, and the vertical sensors were placed perpendicular to the surface.Measurement should be avoided in extreme weather, and when the seismic sensors are affected by wind or drizzle owing to placement on the surface, they can be covered with a special transparent shield [18].The measurement time was selected for a period of less environmental impact, and the same point was measured many times; each measurement time was not less than 30 min, and finally, the stable 10 min was selected for analysis.
The measurement instruments used were the DH5922N dynamic signal acquisition and analysis system produced by the Jiangsu Donghua Testing Technology Corporation and the 891-II seismic sensors produced by the Institute of Engineering Mechanics, CEA.The microtremor signal of the target site was collected at a sampling frequency of 100 Hz.

H/V spectral ratio method
The microtremor H/V spectral ratio was used to transform the horizontal component H and vertical component V of the microtremor signal into frequency domain functions H(f) and V(f) by Fourier Transform and make H(f)/V(f) curve (i.e., HVSR curve).According to the relationship between the peak value of the HVSR curve and the fundamental resonance frequency of the site's geotechnical structure, the site-predominant period can be obtained and used to determine site classifications, divide geotechnical types, infer the range of the shear wave velocity, etc.
According to Nakamura et al. [2][3], the spectral ratio of microtremors (Horizontal-to-Vertical Spectral Ratio, e.g., HVSR) can be defined as ( ) HVSR ( ) The microtremor signal consisted of two horizontal components (H EW : East-West direction; H NS : North-South direction) and a vertical component (V).The method for selecting the horizontal component H(f) is not strictly defined in Equation (1).Different scholars have selected different values of H(f) for practical applications [14], which can be summarized in the following 4 ways:  Arithmetic mean of the East-West and North-South horizontal components:  Geometric mean of the East-West and North-South horizontal components:  Vectors sum of the East-West and North-South horizontal components:  Root mean square of the East-West and North-South horizontal components: In this study, the above 4 different combinations of horizontal components were calculated according to Equation (1), and the results were analyzed and compared.

Site geotechnical situation
The target site was a typical laterite site with flat topography.According to a geotechnical investigation report of the site, the stratigraphic structure of the site is composed of an artificial soil layer (Q

Data processing
The equivalent shear wave velocity adopted in the site classification index in China is v s20 , and the predominant period of the wave velocity T s20 can be calculated using Equations ( 2) and (3) [12].Table 1 shows the borehole information of the representative measurement points at the site and the calculation results of the predominant period of the equivalent wave velocity, in which the predominant period of the wave velocity at the measuring point was in the range of 0.2608~0.3150s.
where d 0 is the smaller of the thicknesses of the covering layer and 20m; d i is the thickness of the i layer of soil; v i is the shear wave velocity of the i layer of soil.
The geotechnical type determined from table 4 was medium-hard and thick soil represented by alluvium.The results of the microtremor measurements were consistent with the borehole exploration results.
Based on a comparative analysis of the microtremor HVSR curves of different horizontal component combinations of each measurement point, we obtained the site-predominant period from the peak value of the HVSR curves in the four different horizontal component combinations in figure 2. The results of the four methods were similar, and the site classification and geotechnical type could be accurately identified.However, different combinations of horizontal components, the height of the peak in the HVSR curve, and the width of the frequency band of the peak are different.The higher the peak value and the narrower the corresponding bandwidth of the peak, the simpler and more accurate it is to determine the resonance frequency of the site and then efficiently obtain the site-predominant period.By analyzing the HVSR curve of each measurement point, the horizontal component combination with the highest peak value and narrowest peak bandwidth is the vector sum of East-West direction and North-South direction, followed by the root mean square, geometric mean, and arithmetic average, respectively.
The comparison of the microtremor measurement results was highly consistent with the borehole exploration results in determining the predominant period, site classification, and geotechnical type.A study on the H/V spectrum ratio of different horizontal component combinations showed that the combination of the vector sum of horizontal components can obtain the site-predominant period more easily and accurately.

Conclusion
This article provides a brief introduction to the development and application of microtremor measurements and applies the microtremor H/V spectral ratio method with different horizontal component combinations to practical engineering.The conclusions are as follows:  The predominant period range of the target site obtained through the microtremor H/V spectral ratio method was between 0.2393~0.3729s,and the site classification was determined as Class II.The geotechnical type was medium-hard and thick soil represented by alluvium. Using microtremor measurements to obtain the site-predominant period, determine the site classification, and divide the site geotechnical type was highly consistent with the borehole exploration results.
IOP Publishing doi:10.1088/1755-1315/1334/1/01204610  The combination of the arithmetic mean, geometric mean, vectors sum, and root mean square of East-West direction and North-South direction as horizontal components in the microtremor H/V spectral ratio method can be used to accurately obtain the site-predominant period. In the microtremor H/V spectral ratio method, the HVSR curve with the combination of the vectors sum of East-West direction and North-South direction has the highest peak value and the narrowest peak bandwidth, which can obtain the site-predominant period more efficiently.
Therefore, it is recommended to use a combination of the vectors sum of East-West direction and North-South direction as the horizontal component in the microtremor H/V spectral ratio method.

Figure 1 .
Figure 1.Measured data of borehole and shear wave velocity at representative measurement points.

8 Figure 2 .
Figure 2. HVSR curve of representative points of different horizontal component combinations.

Table 1 .
Borehole information of representative measurement points and calculation results of wave velocity predominant period.

Table 2 .
Calculation results of microtremor H/V spectrum ratio with different horizontal component combinations.

Table 3 .
The historical version of the specification manual and the site classification scheme of different scholars based on the predominant period of microtremor.

Table 4 .
Classification of geotechnical types based on the microtremor predominant period.

Table 3 )
and "Technical Specification for Site