Shallow S-wave Seismic Reflection Survey around the K

Shallow S-wave Seismic Reflection Survey around the K-NET Anamizu
Site
A. Nozomi KOBAYASHI
Engineering Department, Kyoto University
B. Hiroyuki GOTO & D. Sumio SAWADA
Disaster Prevention Research Institution, Kyoto University
C. Koji YAMADA
HANSHIN CONSULTANTS CO., LTD.
ABSTRACT: Around the K-NET Anamizu site, some residences are damaged during the 2007 Noto Hanto
Earthquake, Japan. Hayashi et al. [2007] reported relations between the residences damages and the subsurface structure estimated from surface-wave survey and so on. The subsoil boundary of estimated velocity
structure is expected to be more distinct by applying shallow S-wave seismic reflection survey. We develop
the seismic reflection survey system to reduce the survey costs by devising equipment and materials. The
shallow S-wave seismic reflection survey delineated detailed structures of the surface layers shallower than
20m. In addition, we verified the structures by comparing to the theoretical refracted travel time.
1 INTRODUCTION
Noto Hanto Earthquake (Mj = 6.9) occurred on
March 25, 2007 at Ishikawa prefecture in Japan,.The
hypocenter is located at 37.22° N, 136.69° N and 11
km depth. One person was killed and more than 300
people were injured. There were numerous slope
failures and liquefaction. About 700 residences in
Wazima city, Nanao city, Anamizu town were seriously damaged. Strong ground motion over 100
cm/s of peak ground velocity was observed at KNET Anamizu site, which is organized by NIED.
However the damage distribution in Anamizu town
was not homogeneous. It implies that the ground
motion during the main shock was distributed inhomogeneously.
Iwata et al. [2007] observed the aftershocks in
Anamizu town on one day after the main shock.
They reported that the amplification of S-wave at the
downtown site of Anamizu town was remarkable respect to an outcrop site.
Hayashi et al. [2007] carried out shallow subsurface structure surveys that are a surface-wave survey
and a sounding test in Anamizu downtown. The results of survey suggested that the shallow subsurface
structure with approximately 0-20 m depth varied
significantly in a horizontal direction around K-NET
Anamizu site (Fig. 1) and these complex structures
are also distributed in the Anamizu downtown.
Then, it is important to know the detail subsurface
structure in order to discuss a relationship between
the damages and the subsurface structure in Anamizu town.
Figure 1. Subsurface structure around K-NET Anamizu site estimated from a surface-wave survey andsounding test (Hayashi
et al. 2007)
We perform shallow S-wave seismic reflection
survey in order to estimate the detail boundary of velocity structure. A simple survey system is developed in order to reduce a survey cost.
2 SYSTEM OF SHALLOW S-WAVE SEISMIC
REFLECTION SURVEY
2.1 Survey system
The ground exploration using a seismic reflection
survey has been developed for the sake of a discovery of oil well. In engineering seismology, the technique is applied to investigate subsurface structures,
shapes of seismic and engineering basement.
Table 1. Parameter sets of survey system
S-wave source
Wooden hammer
Receivers
28 Hz horizontal geophones,
24 sets
Data logger
ES-3000(GEOMETRICS);
24-bit, 8 channels
Stack
10
Shot interval (Line L)
1.0 m
(Line S)
0.5 m
Receiver interval (Line L)
1.0 m
(Line S)
0.5 m
Total profile length (Line L) 40 m
(Line S)
15 m
Figure 2. Schematic figure of the survey system
Figure 3. Schematic figure of the First observation
Photo 1. Survey scene
Figure 4. Schematic figure of the second observation fallowing the first (Fig. 3)
Photo 2. Mortar board to set a vertical-type sensor into horizontal direction
Seismic body wave consists of P-wave and Swave. Conventional P-wave reflection surveys are
powerful to detect a deep structure because the
phases are not disturbed by a pure S-wave and translated S-waves. However, for relatively shallow
structure, P-waves with a long wavelength are not
well scattered and trapped. S-wave with a short
wave length increases a resolution of the survey results. S-wave seismic sources had been used Dynamites and Hammers. Now the formers are applied to
Omnipuise and Hydraulic Impacter. The latter are
applied to Marthor by IFP and S-wave vibrator by
Conoco and Large-scale S-wave reflection surveys
are conducted.
We performed S-wave reflection survey in order
to identify a shallow complex structure as expected
by Hayashi et al. [2007]. For the sake of reducing
the survey casts, we develop a small-scale system,
which can be performed only by two persons, as follows.
S-waves are generated by a wooden hammer
stroke on one side of a wooden plank placed perpendicular to the survey line (Photo 1). 10 times stack
on each side enhances a reflected signal. 24 geophones with a natural frequency of 28 Hz observe a
horizontal ground motion. Since the geophones con-
Table 2. Process of the survey data
1. Setting Line
Offset-distance: 1m
2. Pre-Filter
Line L: 6 ~ 90 [Hz]
Line S: 6 ~ 50 [Hz]
3. Amplitude Recovery
4. AGC
Gate length: 180 [msec]
5. Deconvolution
6. Filter
Same as “Pre-Filter”
7. AGC
Gate length: 180 [msec]
8. Velocity Analysis
9. Residual Statics
10. Time Valiant Filter
Figure 5. Survey layout and a location of K-NET site in
Omachi West Children’s Park
Figure 6. Soil condition gif at K-NET Anamizu [NIED KNET ISK005]
tain only one component of vertical-type sensor.
They are fixed on mortar boards to be placed parallel
to the direction of the source plank (Photo 2). Observed analog data is converted to digital data on
ES-3000 (Geometrics), which supports only 8 channel data (Table 1, Fig. 2). To use 24 receivers, three
sets of observations with a common source point are
conducted. To select 8 receivers belonging to one
set, we use a switch box selecting the occupied receivers (Fig. 3). When one observation finished, the
closest receiver is removed, and the source moves to
the point that the removed receiver placed (Fig. 4).
The observation is regarded as full 24 channels observation when we analyze a survey data.
2.2 Field survey
The survey is performed in Omachi West Children’s
Park on 22-23, November 2007. K-NET Anamziu
site is located at the north-western corner in the park
(Fig. 5). According to the boring data of K-NET
Anamizu site, N-values of the surface layer
(0m~2m) are about 7. The deeper layer (3m~15m) is
Figure 7. Time-distance profile of Line S
very soft because the most of N-values are 0. The
bottom layer (15m~) becomes stiff (Fig. 6). Two
survey lines, Line S and Line L, are placed. Line L
is 40 m length from almost south to north. Line S is
15m length from almost west to east. The starting
points of two lines are overlapped.
Figure 8. Velocity Analysis of Line L in 0-20m
Figure 9. Velocity Analysis of Line L in 20-40m
Figure 10. Time-distance profile of Line L
3 DATA ANALYZING OF THE SURVEY
3.1 Data processing
The common-mid-point (CMP) stack is applied to a
data processing of the survey. The data were processed on PC through the steps listed in Table 2. Initial processing steps include line setting, pre-filter,
amplitude recovery, automatic gain control (AGC),
deconvolution and filter to enhance the signal and
suppress coherent and random noise. Pre-filter trims
a target frequency component by band pass filters, 690 Hz for Line L and 6-50 Hz for Line S. Amplitude
recovery process recovers signal amplitudes absorbed in the structure by assuming geometrical attenuation and inelasticity. The gate length of AGC is
set to be 180 msec.
The data are analyzed in velocity analysis, NMO
and residual statics. After CMP stack, the data sets
are filtered by time valiant filter and migration in order to estimate a final profile. Time valiant filter cuts
a high frequency component in the deep region.
Figure 11. Depth-distance profile of Line L
3.2 The result of processing
Hayashi et al. [2007] reported the survey result of
surface waves only along Line L as shown in Fig. 1.
First, we show a time-distance profile of Line S in
Fig. 7. The horizontal axis is distance from the
crossing point with Line L and the vertical axis is
two-way time. Sources and receivers spaces are set
to be 0.5 m for Line S. A river is located on an extension of the line. Emphasized phases are given between 0.0 s and 0.05 s. The image implies the existence of the clear structure boundary in a shallow
region. Emphasized images in deeper region are not
clear rather than the first boundary.
For Line L, sources and receivers spaces were set
to be 1.0 m and the source energy was the same as
Line S. K-NET Anamizu site is located on the extension of the line. It has been already reported that
the subsurface structure of Line L may vary in the
horizontal direction. The characteristics are shown in
velocity analysis (Fig. 8 and 9). Figure 8 shows the
phase peaks for CMP of 0-20m and Figure 9 shows
them for CMP of 20-40m. Layer velocities for 020m increases with depth, while the second layer velocity for 20-40m is slower than the first. Figure 10
shows a time-distance profile of Line L. The subsoil
boundary between 0.0 s to 0.05 s is almost flat from
0-9 m. The boundary from 9 m to 40 m gradually
decline. The declination of two-way time range is
about 0.25 s from 0.15 s to 0.4 s. Figure 11 shows
depth-distance profile of Line L. The depth-distance
profile also shows a boundary change of the layer.
The difference is almost 14 m. Comparing between
images of Fig. 1 and Fig. 11, boundary shapes are
almost identical, in 0-10 m and 30-40m. The shapes
in 10-30 m are different. It may be related to the difference of the survey methods because the surface
wave survey assumes moderate change of layer
boundary. On the other hand, we apply migration
process to estimate the boundary shape in analyzing
the data of reflection survey in order to understand
the detail slopes.
3.3 Comparison of first travel time
The survey also observes waveforms in order to discuss a first travel time of Line L in 0-24 m. The observation was a set of line surveys including two
sources at the ends of the target line that are 0 m and
24 m of Line L. To calculate a theoretical first travel
time of S-waves, we construct a model with two layers based on the reflection survey as shown in Figure
11. The theoretical first travel times are compared to
the observed waveforms. Figure 12 shows a comparison between the theoretical first travel time and
the observed waveforms for the case of 0 m source,
and Figure 13 shows one for the case of 24 m
source. First travel time of theoretical calculation
almost corresponds to the observed time that is recognized as the first phases.
Figure 12. Comparison between theoretical first travel time and the observed waveforms for 0m case
Figure 13. Comparison with theoretical first travel time and the observed waveforms for 24m case
4 CONCLUSION AND DISCUSSION
We estimate a subsurface velocity structure by applying a shallow S-wave reflection survey around
the K-NET Anamizu site. Distinct image with the
complex ground structure is investigated. A simple
survey system of two participants is applied by developing the equipments in order to save steps moving sources and receivers. Comparing with the results of the previous survey, the result of this survey
was almost identical to that in the flat region of
boundaries. The depth of the boundary rapidly
changes. First arrival time of theoretical calculation
related to the estimated structure corresponded to the
observed travel time. We verified the steep structure
change around the K-NET site. The similar structures are distributed in Anamizu town. The relation-
ship between the complex structures and damages
will be discussed in the future works.
5 REFERENCES
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SEISMOLOGICAL SOCIETY OF JAPAN 2007, FALL
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Iwata, T., Asano, K., Kuriyama, M., Iwaki, A. 2008. NONLINEAR SITE RESPONSE CHARACTERISITCS OF KNET ISK005 STATION AND RELATION TO THE
EARTHQUAKE DISASTER DUARING THE 2007
NOTO-HANTO EARTHQUAKE, CENTRAL JAPAN.
The 14th World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China
The Social of Exploration Geophysicists of Japan. 1998. The
Geophysical Exploration Hand Book: 93
National Research Institute for Earth Science and Disaster Prevention K-NET ISK005 Soil condition gif
Vidale, J. 1988. Finite-Difference calculation of travel times,
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