Ambient Vibration Measurements Optimized to Natural Frequency of Soil

ICEANS 2022
2nd International Conference on Engineering and Applied
Natural Sciences
October 15 - 18, 2022: Konya, Turkey
Proceeding Book

2nd International Conference on Engineering and
Applied Natural Sciences
https://www.iceans.org/
October 15-18, 2022, Konya, Turkey
Ambient Vibration Measurements Optimized to Natural Frequency of Soil
Estimating: Case Study
Khalissa Layadi *1 , Fethi Semmane 1 and Sabrina Madadi 2
1 Centre de Recherche en Astronomie Astrophysique et Géophysique, Algeria
2
Ferhat Abbas University – Sétif-Algeria
*(layastar20@yahoo.fr)
Abstract – The natural frequency of soil, f0, identified by horizontal over vertical (H/V) spectral ratio from
ambient vibrations recording is a complementary tool in seismic site characterization. In the present study,
we mapped the f0 variation with the corresponding amplitude, A0, correlated to the topography of Sétif City
(north Algeria) from 47 measurement points. High-frequency peaks are observed between 1 to 5.5 Hz from
clear HV curves with amplitude variation between 2 and14. The H/V technique revealed an independent
geological zone in the northeast region of the study area of the highest elevations with a special HV curve
shape. To explain the local amplification's geological origin, we based it on the available deep boreholes
and geo-electrical profiles performed within the City. The impedance contrast between the silt-sandstone
of Mio-Pliocene deposits and the marl-limestone of Eocene-Cretaceous hard rock is the cause of the
observed local amplification. The first shear wave velocity computation derived from soil resistivity
conversion showed that the Mio-Pliocene sedimentary layer is characterized by 1000 m/s overlaying hard
rock bedrock of 2400 m/s. The sediment layer thickness was estimated using the quarter-wavelength
approach illustrating that the deepest zone corresponds to the south region in good agreement with the geoelectrical
profiles of moderate elevation. The center region is characterized by shallower thicknesses with
complex topography.
Keywords – H/V-technique; Fundamental frequency; Soil resistivity; Shear wave velocity; Sétif City.
I. INTRODUCTION
The use of ambient noise (or vibration) in site
characterization is an excellent tool since it is
available data with a large frequency band (>0.1 Hz)
compared to natural seismic events (Signal-Noise
Ratio constrain) and active sources (e.g. weight drop
hammer). It is employed in fundamental frequency
determination, f0, of soil by the Horizontal over
Vertical Spectral Ratio technique (HVSR; [1]) and
shear wave velocity, Vs, structure assessment ([2]),
essential for microzonation study in seismic risk
mitigation projects. The estimation of the Vsstructure
provides seismological information on the
sediment layer and the overlying bedrock
(thickness, density, body wave velocities, damping,
etc.). Therefore, the coupling between f0 mapping
and Vs-structure is a key step required in spectral
analysis for amplification factors and transfer
functions calculations in site effects investigation
(e.g., [3]; [4]). It is well known that this
phenomenon increases and changes the special
damage distribution caused by an earthquake.
In Algeria, two major microzonation studies were
realized, the first after the El Asnam earthquake,
1980, by Woodward Clyde Consultants ([5]), the
second after the Boumerdès earthquake, 2003, by
JICA and CGS [6] considering 1D theoretical
computation of transfer function. Then, the
experimental site effects evaluation was started just
after this last earthquake, where [7] is the first work
analyzing the relationship between the damage
caused by the Boumerdès earthquake, 2003, and site
effects using ambient noise. A set of studies were
followed and focused on dense, strategic and
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Layadi et al., Ambient Vibration Measurements Optimized to Natural Frequency of Soil Estimating: Case Study, ICEANS 2022, Konya,
Turkey
seismically active urban areas ([8]; [9]; [10]; [11]).
The commonality among the cited works is the
geological situation of the study area in Neogene
basins.
In the present study, we analyze a preliminary site
effects characterization in Sétif City, situated in the
High Plateau surrounded by an important number of
active faults (Figure 1) using the HVSR technique.
The estimation is represented by f0 mapping with the
corresponding amplitude, A0, plotted on
topography. We consider boreholes and geoelectrical
data from previous investigations ([12]) to
constrain the obtained results.
Fig. 2 (a) geological map of Sétif City (purple line is the
City plan) and the surrounding area, modified from
geological map 1/50 000. The red dashed line (AA’) is a 2D
geological cross-section detailed in (b). Bleu-filled circles are
ambient vibration measurement points. Black lines with red
filled circles are geo-electrical profiles. Red inverted triangles
are available boreholes.
Fig. 1 Map depicting the main active or potentially active
geological structures, surrounded by numbers (fault: black
line with triangles; fold: dashed line with double arrows)
([13]; [14]) with seismic activity of events with
magnitudes >= 4.0 and intensities >= VI MM of historical
seismic events located 100 km from Sétif City.
Fig. 3 Detailed CHF borehole in Sétif City located in
Figure 2a showing the geological layer thicknesses with the
corresponding geological ages.
II. GEOLOGICAL SETTING AND PAST EARTHQUAKES
Sétif City is situated in the high plateau of the
main geological domains of Northern Algeria and
has been subject to earthquakes throughout its
history and also to instrumental seismicity caused
by different active geological structures (Figure 1).
From the local geological framework, the subsurface
of the City is cover mainly by Quaternary
deposits and Mio-Pliocene sediments (silt, gravel,
clay, limestone) (Figure 2a). The old formation is
Cretaceous with a thin layer of Eocene (AA'
geological cross section in Figure 2b) of hard
limestone and limestone-marls as illustrated by the
litho-stratigraphic profile of the CHF site (Figure 3)
in the northwest of the study area.
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III. MATERIALS AND METHOD
In this section, we present the data that was used
and how it was processed for the preliminary site
effects investigation in Sétif City.
A. Ambient vibration measurements
We carried out 47 measurement points of ambient
vibration of 20–30 min duration on Mio–Pliocene
and Quaternary deposits (Figure 2a) using a
TROMINO seismograph with a sampling rate of
512 samples per second. More dense recording
points were on the geo-electric P and N
profile directions, in order to compare the f0
variation with the sediment thickness.
In f0 calculation we used the GEOPSY code ([15])
where all ambient vibration data of the measurement
points were processed in the same manner. The Fast

Layadi et al., Ambient Vibration Measurements Optimized to Natural Frequency of Soil Estimating: Case Study, ICEANS 2022, Konya,
Turkey
Fourier Transform was applied to the spectral
amplitude, A, of East-West (EW), North-South
(NS), and vertical (Z) components smoothed by the
Konno-Ohmachi algorithm ([16]) using the autoselection
of individual windows of 30 seconds.
Individual spectral ratios were calculated using the
following equation:
HVSR(f) = 1⁄ 2 2
√2 ∗ √A(f) EW + A(f) NS A(f) z
⁄ (1)
In Figure 6b, c, and d, examples of this curve are
given.
The geometric mean was computed for all
individual HVSRs.
4.3. Geophysical-Drillings Data
To complete our experimental site effects
investigation in the study area, we considered
geophysical data obtained from geo-electrical
technique where the soil resistivity is determined
between different layers by [12]. In the
interpretation of geological formations, correlation
was performed using soil resistivity values or
domains with deep drilling. An example of this
correlation is given in Table 1. In Figure 4, it is
illustrated a detailed geo-electrical profile as an
example (N-profile in Figure 2a).
Table 1 Example of Geological/Geo-Electrical correlation in
Sétif City from [12].
Geological Formation
Soil resistivity
Barremian-limestone 100-200
Turonian-limestone 100-200
Miocene-marls Vindobonian 10
Miocene-sandstone 50-100
Fig. 4 Example of interpreted geo-electrical profile modified
from [12] located in Figure 2a.
IV. RESULTS
The f0 and A0 maps of Sétif City are shown in
Figure 5. A projection of f0 on Sétif City topography
is presented in order to find a relation between
frequency and elevation. From this coupling, the
highest f0 is at the lowest elevation. We observed
another shape concentrated in a zone to the northeast
of the city (the white rectangle in Figure 5)
among the HVSR curves observed in the study area.
Fig. 5 Fundamental soil frequency (f 0) (top) and the
corresponding amplitude (A 0) (bottom) variations obtained
by the HVSR technique with respect to the Sétif City plan.
The figure in the middle is the f 0 projection variation on
topography. The white rectangle is a limitation zone with
HVSR curves given in Fig. 6b, c and d.
V. DISCUSSION
The analysis of the obtained results in Figure 5
shows that the subsurface of the study area is not
homogeneous from the sediment layer and/or the
bedrock, with a different shape of the HVSR curve
observed in the white zone to the north-east (Figure
6b, c, and d). This kind of curve is classified as R2-
type ellipticity of [17]. According to the map of the
shear-wave ratio, rs, and Poisson’s ratio, υ,
dependence of [17], the zone is characterized by low
υ (~0.24) and low rs (~0.05). The parameter rs is
defined as the ratio between the shear wave velocity
of the sediment layer Vs1 and that of the half-space
(or bedrock) Vs2 (rs= Vs1/ Vs2). The ratio rs = 0.05
means that Vs2 is higher than Vs1 by 20 times
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Layadi et al., Ambient Vibration Measurements Optimized to Natural Frequency of Soil Estimating: Case Study, ICEANS 2022, Konya,
Turkey
(Vs1>>Vs2) which means a very important
impedance contrast, but υ1 of the layer for R2-type
of 0.24 indicates that it is a hard rock. The
explanation of the minimum in the HVSR curve
between maximums is well developed
mathematically by [18] where the geological origin
is due to stiff soil overlaying soft one.
layer are estimated at 1000 m/s. Using the quarterwavelength
approach (formula 3) relating the f0,
thickness, H, and Vs of the sediment layer, we found
that the deepest zone corresponds to the south region
and is in good agreement with the geo-electrical
profiles of moderate elevation, which indicates a
deep bedrock (Figure 4). The centre region is
characterized by shallower thicknesses with less
topography and f0 around 5-6 Hz (highest values)
(Figure 5).
f0 = V s
4∗H
(3)
Fig. 6 Example of HVSR curves observed in Sétif City. In (a)
it is shown a clear curve; the red square indicates the
identified f 0 with the corresponding A 0. In (b), (c) and (d) are
curves with a minimum (red arrow) comprised between two
maximums.
In the first evaluation of the near-surface Vsstructure
for Sétif City, we based our analysis on
velocity function, including lithological variation
and soil resistivity of [19], using the following
equation:
V P = γ. (Z. R) 1⁄ 6
(2)
Where:
V P is P-wave velocity is feet/s; R is soil resistivity in
Ωm, Z is the depth in feet, γ is a constant equal to
1948.
After a sequence of computation and conversion
for different points on P and N profiles (Figure 2 and
4), we found that Vs for bedrock varies between
1700 and 2400 m/s in good agreement with previous
studies on hard rock in Algeria ([20]; [3]; [9]; [11]).
The thickness of the layer is variable depending on
its position. The Vs in the Mio-Pliocene sediment
216
VI. CONCLUSION
In the present study, we estimated a first site
effects map for Sétif City by the HVSR technique
using ambient vibration. An inversion proportional
relation is found between the topography and the
frequency peak of soil, where the lowest topography
is coupled with the highest frequency. The HVSR
curves help in understanding sediment/bedrock 2
and 3D underground interfaces and seismological
properties.
ACKNOWLEDGMENT
We thank the community of ICEANS-2022 for
accepting publication of this extended abstract. We
thank CRAAG for funding this study and providing
material for ambient vibration recordings. We
would like to thank Ouahab Issaadi and Oualid
Boulahia from GRAAG for their support during the
acquisition survey. We thank the staff of the ANRH
service documentary for providing geophysical
documents and drillings.
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