Introduction
In the seismic method, what is observed is the propagation
time of sound waves, which are elastic and generated by specific wave sources
(dynamite, hammer, or weight drop and piles). This study wants to take
advantage of the momentum of installing pile foundations at the Geomatics
Engineering campus of ITS Surabaya (Broto & Afifah, 2008).
This seismic refraction method is one of the most
important and widely used methods in geophysical engineering. The seismic
method has high accuracy and resolution in modeling the geological structure
below the earth's surface. In determining geological structures, seismic
methods can be divided into two
main
categories, namely invasive tests and non-invasive tests. The invasive test
method requires a drill hole (Cross-Hole, Downhole, P-S suspension logging),
while the non-invasive method is carried out at ground level (Reflection,
Refraction and SASW test) (Hudha et al., 2014).
Surface waves are divided into two, namely Rayleigh waves
and Love waves. The surface waves used in this study are Rayleigh waves. The
effect of Rayleigh waves is enormous. When the exciting force on the earth's
surface and thick layers, Rayleigh waves account for 67% of the total energy, S
waves by 26% and P waves by 7%. Rayleigh waves are very well used to identify
soil profile problems because the energy reduction in their propagation is
lower than other types of seismic waves. Therefore, the focus of this research
is the analysis of Rayleigh waves (Fitriyani, 2016).
SASW (Spectral Analysis of Surface Wave) data has played a
significant role in determining the soil profile. The processing is then
carried out from this data to obtain a dispersion curve, namely the curve of the
relationship between phase velocity or frequency to wavelength. After obtaining
this dispersion curve, an inversion was then performed to obtain the profile
character and the parameters of the elasticity of the soil
(Joh, 1996).
Problems encountered in conducting
this research are:
How
to know the characteristics of the structure of the subsurface of the earth
Method
A.
Data Collection Stage
In this study, the distance on each track is 75 meters.
Twelve geophones carry out the recording system straight with a vibrating
source. Geophone pairs are placed with a predetermined spacing. Namely, the
first point is 20 m from the vibration source, the second and third points are
each 5 meters apart from eleven geophone points with eight trajectories.
B.
Seismic Data Processing Stage
Two data signals in the time domain (y1(t) and y2(t) are
first converted in the frequency domain using the Fourier transform so that
Y1(f) and Y2(f) are obtained. Using spectrum analysis makes it possible to
obtain information about data quality. And also the phase velocity as a
function of frequency.
Signal quality is obtained using the "coherence
function," which compares the results recorded by two geophones. A value
close to one indicates a good level of correlation to be seen whether the data
is affected by noise. This inversion process has required the value of the
earth parameter as an initial guess which is then used to determine the theoretical
wave speed. The final wave speed is obtained by iterating between the observed
wave speed and the theoretical wave speed so that the error is smaller than the
tolerance error (Hutahean, 2007). The
phase velocity as a function of frequency is obtained from the phase Cross
Power Spectrum, whose value is used to create the dispersion curve of the
observation.
The inversion process is carried out to obtain a wave
velocity profiling from the depth of the soil. After profiling, the shear wave
velocity from the soil depth for each common midpoint is obtained. The shear
wave velocity data to depth is processed using Surfer8 software to obtain a
subsurface model based on shear wave velocity.
Results
and Discussion
Data were obtained from each configuration of geophone
pairs and impulse sources. The vibrations generated by the vibration source
will be received by the geophone and recorded by the McSEIS-SX Model 1125 A in
the time series format *.org (SEG-D). Furthermore, the data is filtered using
an ombybandpass filter. Prior to filtering, analysis of the dominant frequency
of a signal is first performed. This analysis is carried out with the Fourier
transform. The results of the analysis of all signals show that the signal's
dominant frequency is 16 Hz to 31 Hz. Next, filtering is carried out to pass a
frequency signal of 16 Hz to 31 Hz. After that, the data is digitized into
*.txt format. Filtering and digitizing data is done using Vista 7.0 Software.
This digitized data creates a dispersion curve using
Rayleigh wave spectrum analysis. Spectrum analysis changes the time function
data into a frequency function using the Fast Fourier Transform (FFT)
algorithm. This algorithm can quickly change the time domain to the frequency
domain by requiring the number of data to be 2n.
Spectrum analysis was conducted sequentially from equation
(2.56) to (2.62). The distance used in the above formulation is the distance
from the geophone pair. The results of this analysis are angular velocity as a
function of frequency and coherence of the two signals analyzed. According to
(Foti, 2000),
coherence determines the signal quality to noise. The signal quality is good if
the coherence value is close to one. Figure 1 shows that the coherence value is
one, so it can be said that the quality of the data used in this study is free
from noise.
Figure 1
Rayleigh. wave
dispersion curve results
The resulting dispersion curve is then inverted to obtain
a shear wave velocity profile with depth. In this inversion process, initial
parameters are needed in the form of shear wave velocity, density, layer
thickness and Poison Ratio. These initial parameters are the initial values of
the iterations performed. This parameter is determined based on the table of
seismic wave velocity in various rock types attached in Appendix 2. Based on
the local geological conditions at the research site, it is known that the soil
type is clay, so the initial parameter used is the clay parameter.
The initial parameters are estimated using the Rayleigh
wave equation to get the value of the phase velocity of the Rayleigh wave model
as a function of frequency. The speed of the Rayleigh wave of this model is
then compared with the speed of the Rayleigh wave of the research results so
that an error is obtained. The error value will determine whether or not to
iterate, and the inversion is complete. If the error value is greater than the
desired one (error = 3.0), a new Rayleigh wave velocity is calculated to
produce a lower error until the desired error is obtained. Figure 2 is an
example of a Rayleigh wave inversion result.
Figure 2
Inversion Results,
a)
RMS Error relationship with iteration,
b)
The phase velocity of theoretical and
experimental results.
c)
Shear wave velocity profile with depth
A.
Data Interpretation
Detailed mapping of the subsurface structure is carried
out by analyzing all adjacent geophone pairs in the measurement. Each pair of
geophones produces a profile of shear velocity against depth. This velocity
profile reflects the shear wave velocity between geophone one and geophone 2.
Estimation of shear velocity as a function of depth is
done by inverting the dispersion curve of all closest geophone pairs. The
results of all these shear wave velocity estimates reflect the subsurface
structure between the two geophones. Next, two-dimensional modeling was carried
out using the Surfer 8. software.
Figure 3
Two-dimensional
modeling
The image of the shear wave velocity as a function of the
position of the modeling results has a range between 109 m/s to 1252 m/s.
According to (Sukardi, 1992), the
geology of the research area consists of alluvium composed of clay, sand,
gravel silt and gravel formed by coastal deposits. This geological information
is compared with the sheer velocity values of various rocks in appendix 1,
table
Description:
�Sand loam saturated
Sand
�Clay p Figure
Figure 4
�Suspected subsurface
structure
Besides being used for geological purposes, the shear wave
velocity (Vs) can also be used for geotechnical purposes
(Boominathan, 2006). Vs.
is used as an indicator of rock hardness. Usually, this hardness is measured
using the Standard Penetration Test (SPT). The harder it is, the higher the SPT
value (>50). According to (Bang & Kim, 2007), (Boominathan, 2006) and
(Chen, Evans, & Feldlaufer, 2006)
that the value of the SPT is always positively correlated
with Vs. An increase will always follow the increase in the value of the SPT in
the value of Vs. It means the large Vs. Value of hard rock and small Vs. Value
of soft rock.
The relationship between the SPT hardness values depends
on the type of rock (Boominathan, 2006).
Based on JRA (1980) in (Boominathan, 2006), the
relationship between SPT (expressed in N) and Vs. Clay and sand areas in
equations 1 and 2. Based on this equation, it is possible to estimate the SPT
value of rock layers. In this study, SPT estimation was only carried out at a
depth of 1 meter at Vs. for each geophone pair. Furthermore, the value of SPT
and VS is used as an indicator of rock hardness as a geotechnical basement.
�(for
clay rock)� (1)
�(untuk
batuan pasir)� (2)
Based on the results of the geological interpretation
above, the depth of 1 meter at the point of 2.5 meters to the point of 45
meters and at the point of 52.5 meters in the form of sandy clay rock and 47.5
meters in the form of sandstone. Thus, the SPT estimation is carried out using
both equations (4.1 and 4.2), except for the 47.5-meter point, which only uses
equation 4.2. The estimation results can be seen in Figure 4.5. The estimate is
not appropriate because the SPT value exceeds 60 at the point 47.5 meters from
the first geophone. This means that the rock is too hard, so SPT measurements
cannot be carried out.
Figure 5
SPT graph at each
position at a depth of 1 m
The estimated value of the SPT with the clay approach
always has more minor results. This event is natural because, with the same
hardness (clay and sand), the magnitude of Vs. in the sand is always smaller.
This is caused by the porosity of the sandstone, which is greater than the
porosity of the clay rock. The magnitude of the wave propagation velocity is
always inversely proportional to the rock's porosity (Schoon, 1998). On the
other hand, with the same Vs. Value: It is clear that clay rock's hardness value
is smaller than that of sandstone.
SPT with the sand approach in Figure 5 and attachment 1
table L2 it is known that the soil layer located between the position of 2.5
meters to 37.5 meters from the first geophone is soft soil (SPT value <15 )
and the soil layer between position 37, 5 meters to 52.5 meters from the first
geophone in the form of medium soil (SPT value 15-50) to hard soil (SPT value
> 50). At the same time, the SPT with the clay approach is known that the soil
layer at all points (except around point 47.5) is included in the soft soil.
From these two estimates, it was found that the criteria were similar at points
2.5 to 37.5, namely rocks, including soft rocks. Furthermore, for geotechnical
purposes in building construction, geotechnical engineering is required for the
soft soil layer and the medium layer. Engineering is carried out to hard rock
as a building base. Hardening the rock can be done by compacting the soil layer
and mixed with limestone.
The initial parameters are estimated using the Rayleigh
wave equation to get the value of the phase velocity of the Rayleigh wave model
as a function of frequency. The image of the shear wave velocity as a function
of the position of the modeling results has a range between 109 m/s to 1252
m/s. Next, two-dimensional modeling was carried out using the Surfer 8
software. The estimated value of the SPT with the clay approach always has more
minor results. The SPT value exceeds 60 at a point of 47.5 meters from the first
geophone, and the estimate is not appropriate. This means that the rock is too
hard, so the SPT measurements cannot be carried out. SPT with clay approach is
known that the soil layer (except around point 47.5) is included in soft soil.
The soil layer, which is located between 2.5 meters to 37.5 meters from the
first geophone, is soft soil (SPT value <15 ), and the soil layer between
37.5 meters to 52.5 meters from the first geophone is medium soil (value tax
return 15-50)
Conclusion
After we
studied it in-depth, research on soil profile characterization at the FTSP ITS
geomatics campus using Rayleigh wave dispersion curve analysis can be concluded
that:
Based on
the local geological conditions at the research site, it is known that the type
of characterization of the soil at the research site is clay, so the initial
parameter used is the clay parameter.
The results
of all these shear wave velocity estimates reflect the subsurface structure
between the two geophones. The geological interpretation states that the depth
of 1 meter at a point of 2.5 meters to the point of 45 meters and at a point of
52.5 meters in the form of sandy clay rocks and 47.5 meters in the form of
sandstone. To hard soil (SPT value > 50). At the same time, the SPT with the
clay approach is known that the soil layer at all points (except around the
47.5 m point) is included in the soft soil. From these two estimates, it was
found that the criteria were similar at the point of 2.5 m to 37.5 m, namely
rocks, including soft rocks.
Based on
the results of this study in the context of building development, it is
recommended:
1) For
geotechnical purposes in the construction of buildings, geotechnical
engineering is required for the soft soil and medium layers. Engineering is
carried out to the hard rock as a building base.
2) Hardening
the rock can be done by compacting the soil layer and mixed with limestone.
3) This
research is used to determine government policies in implementing development.
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