10th Asian Regional Conference of IAEG (2015)

The S-wave velocity structure and the phase velocity distribution
by the Microtremor array survey
(1)

(2)

(1)

Takao SASAKI , Hisao HAYASHI , Atsushi TANASE
(3)
(4)
(4)
Satoshi HARAYAMA , Yukihiro MIZUOCHI
Yuichiro MINAMI(1) Chiken Sogo Consultants, Japan
E-mail:sasaki.takao@chiso-con.co.jp(2) Geo-X consultants, Japan, (3) Faculty of Science, Shinshu University, Japan(4) Sumiko Resources Exploration & Development, Japan
Abstract
To conduct the geological survey in limited cost and period is one of the criticalissues for civil engineering projects. The microtremor array survey(MAS) enables toobtain the S-wave velocity structure and the phase velocity distribution more quickly
and economically than any other geophysical exploration method. This study presents
some examples to clarify the usefulness and the preciseness of these parameters
obtained by MAS. At first we introduce the chain array method as a new techniquefor geological survey. To our experiences , the phase velocity distribution
elaborated by the chain array method is not inferior to the achievements by the othergeophysical methods. And the geological layers in the sedimentary basin have its own
characteristic S-wave velocity which could identify them respectively. Using thisnature, it is possible to assess the geological structure and the engineering properties
even to the depth of basement formation with no sufficient existing geological databy the simple microtremor array survey. We would like to discuss these advantages of
MAS through some case studies.
Keywords: microtremor array survey, chain array method, SPAC, phase velocity, metropolitan
1. Introduction
Earth surface is vibrating continuously with veryslight amplitude anywhere. This vibration comesfrom human activities such as road traffic or natural
phenomenon such as ocean waves, is called themicrotremors generically. This microtremors is easy
to observe and there is a possibility that the motion isreflecting subsurface geological structure. For thisreason, various researches of the motion have alreadybeen done since more than 100 years ago mainly inJapan. A number of researches tended to presume thesubsurface geological structure from the observationof motion without the controlled seismic source (theman made one such as "vibroseis").
The microtremors array survey , especially SPAC
methodology, had been put into practical use in 1980s,
the basic practical use method was established in
1990s (for example Okada et al.1990 and Matsuoka
et al.,1996), based on a breakthrough theory(Aki,1957) which came out more than 50 years ago.
This method can be used mainly for the deep S-wave
velocity structure researches in the sedimentarybasin targeted the earthquake disaster prevention.
Moreover some application cases detected acontinuous 2D section directly using the chain array
method (Mizuochi et al.,2010, Hayashi et al.,2010,est.).

2.Chain array method
2.1 Basic principle and measurement method
As a new microtremor array survey technique thatevolved conventional techniques, the authors have
utilized a technique called the chain array survey
method that enables to show the apparent phasevelocity distribution in two-dimensional section.
Those images could help to understand the geologicalcondition of the targeting area. This methodology had
been introduced in details by Hayashi et al.( 2010).
The summary of this methodology is shown as
follows.
Surface waves observed by seismometers areanalyzed by a function of phase velocity andfrequency, which is called the dispersion curve using
SPAC or F-K methodology. Using analyzed
correlation of the phase velocity and frequency in thedispersion curve, the wave length could be
calculated , then apparent distribution of the phasevelocity in depth is developed (Fig.1). The
conversion rate between the wave length and thedepth is usually presumed by the one-dimensional
survey result (analyzed S wave velocity structure)
conducted simultaneously and those rates are in therange of 1/2 to1/3 on our experiences.

1

10th Asian Regional Conference of IAEG (2015)

Basically the chain array method uses themeasurement technique of SPAC method which
methodology has already been established (Okada
2003). Seismometer arrangement of the SPAC incircular arrays is fundamental in principle, and thatcan be replaced with a semi-circular array (Okada etal. 2003). According to this theory, the chain arraysurvey method uses linear sequence of equilateraltriangle arrays to detect the dispersion curves of f theevery equilateral triangles. Then the dispersion
curves of some continuous points positioned at every
half pitch of the triangle side length, which is theintervals of seismometer, are elaborated, and theapparent distribution of the phase velocity could becreated by some kriging software , which is deemed
to be a reflection of the two-dimensional
underground geological structure .
Since this method is characterized to express theunanalyzed original phase velocity data, it isnecessary to consider that the phase velocity
distribution is apparent. Therefore, the comparison
with the S wave structure by the one-dimensional
survey results or the existing geological data isimportant in quantitative interpretation.
2.3 Survey depth and Measurement time
Because of securing the homogeneity of thesurvey ground and the work efficiency, microtremor
array survey become accurate if a small size array
could detect deeper survey depth. According toprevious studies, the proportion of array size and thesurvey depth is five times (Miyakoshi et al. 1996) and10 times (Matsuoka et al. 1996). According to ourexperience shown in the table 1, more than 13 times
in the chain array method, more than 11 times in
one-dimensional survey is obtained. The depth intable 1 is the converted depth from the wave length,verified by the usual SPAC survey result and/or
existing borehole data.
Matsuoka et al.(1996) studied about themeasurement time of SPAC method and showed thatthe sufficient measurement time is 10 minutes when
the array size is 3 to 30m , 20 minutes when thearray size is more than 60 m. In our studies of thechain array measurement, the sufficient data had beenobtained in about 20 minutes.
As shown in above, the chain array survey method
is a technique which has a potential to obtain deeper
information more simply than the other geophysicaltechniques. For example, if you want to survey
100m length and 30m depth, we can measure it in
one or two days field work using chain array method.
That survey consists of 8 times of the measurement
which is composed of 6 triangles connected side-byside in the 12.5m length using 8 seismometer. As one
or two measurements per hour can be conducted, 8times measured needs only 6 to 12 hours, we canmeasure about 100m length (Δ 5x2.5x8 = 100m) . For
the detectable depth of 3m array size measurement is

30m or more, it shows that chain array method is avery efficient and economical method.
2.3 Case studies of chain array survey
The samples of the phase velocity distribution
by the chain array survey method are shown in Fig. 2.
 Urban area caseThis case was carried out with the 3m side length
on the sidewalk of the national highway in urban
areas (Hayashi et al.2010). The difference of soil
structure was observed in the center part
approximately by the phase velocity that shown inwhite line. This result had been confirmed by S wave
velocity structure analyzed by SPAC method.
 Mountain region caseThis case was carried out at the canyon plane inmountain region with the array size of 30m side
length (Harayama et al. 2012). It was confirmedthat the river deposit thickness was supposed to be
more than 200m and a fault of which verticaldisplacement was about 50m (shown in white line).
This result had also been confirmed by SPAC method
and the layers of green and blue color were Holocene
sediment which had been correlated with boreholecore dating data.
3- Geological structure analysis using the S-wave
velocity structure
3.1 Methodology
Generally geophysical techniques is used as amethod to complement the drilling data. On the other
hand, the accumulation of physical quantitative dataof geological formations by geophysical survey could
be considerably sufficient to determine the geologicalstructure without the drilling. Table 2 shows the S
wave velocity of sedimentary layers detected by bore
hole loggings implemented in the major sedimentarybasin of Japan. The sedimentary rocks younger thanthe Neogene system have the S-wave velocity shownas follows.
Holocene: 0.1-0.4 km/s, Pleistocene: 0.4-0.9 km/s
Pliocene: 0.5-0.9 km/s, Miocene: 1.1-1.6 km/s
Basement: (1.9) 2.4-3.5 km /s
The options for the geological survey methodcoming up to more than several hundred meters of
depth in the dense urbanized area are limited.
Micretoremor array survey could be the one of thosespecific method. This had been demonstrated byseveral studies such as Matsuoka et al.(2002) andMinami et al.(2011) and a case study shown as
follows. Those studies indicate a advantage of SPAC
method in the metropolitan areas .
3.2 Case study in Jakarta, Indonesia
This case had been implemented in Jakarta
Indonesia, was targeted to clarify the geologicalstructure necessary for settlement of the land
subsidence issues. The microtremor array survey
using SPAC method had been conducted within 10
days of fieldwork in highly urbanized area, which2

10th Asian Regional Conference of IAEG (2015)

Table 1 Array size and survey depth of microtremor array survey method, *analyzed depth is converted
from frequency of dispersion curve (see.Fig.1).
R : Maximum Triangle radius (m)D : Analyzed depth (m)*D/RTarget / Method

1.7

25

14.7

Glacial terrain / Chain Array

1.7

30

17.6

River dyke / Chain Array

1.7

35

20.6

Beach sediment / Chain Array

11.5

150

13.0

River sediment / Chain Array

17.3

250

14.5

Mountain canyon sediment / Chain Array

34.6

1,600

46.2

Sedimentary basin / Chain Array

30

350

11.7

Sedimentary basin / SPAC survey

90

2,200

24.4

Sedimentary basin / SPAC survey

Table 2 S-wave velocity by borehole logging in Japan (assembled from Osaka Jiban (1982) and other data)
Geological Series

Fuchu

Kanto
Iwatsuki

Shimoosa

0.4

0.4

Sanin

Chugoku

Shikoku

0.55

0.55

0.88

0.88

Kinki
Osaka

Kyoto

Holocene
Pleistocene
Pliocene
MiocenePaleogene
Basement

Late
Middle
Early
Late
Early
Late
MiddleEarly

0.5-0.7

1.2

2.4

0.7

1.2
1.5
2.5

0.4-0.9
0.6-0.9

0.7

0.88

1.1
1.3-1.6
2.5

3.5

km/s
1.3
3.5

3.5

2.5-3.0

1.9-2.5

Fig. 1 Dispersion curves,
frequency(left) converted todepth(right) by conversion
calculation

Fig. 2 Case studies of Chain array survey,
Urban area : left (Hayashi et al.2010) and Mountain region : right (Harayama et al. 2012)3

10th Asian Regional Conference of IAEG (2015)

topography and concealed active faults in the
Kamikochi Area defined by Microtremor arraysurvey(SPAC method),Proceedings of theGeological Society of Japan 119th Annual
Conference(in Japanese)
Matsuoka, T.,Umezawa, N.,and Makishima, H.,(1996):Experimental studies on the applicability ofthe spatial autocorrelation method for estimation ofgeological structures using microtremors:Bu
tsuri-Tansa (Geophys.Explor.), 49, 2, 6-41 (inJapanese).
Matsuoka T., Shiraishi H.,2002, Application of an
exploration method using microtremor arrayobservations for high resolution survey of deep
geological structures in the Kanto plains-Estimationof 3-D S-wave velocity structure in the southernpart of Saitama prefecture, Butsuri-Tansa,Vol.55
No.2,pp.127-143 (In Japanese)
Miyakoshi,K.,Okada,H.,Suqun,L.(1996):A Rangof wavelengths possible to estimate phasevelocities of surface waves, Proceeding of the
SEGJ Conference 94, 178-182
Minami,Y., Matsuoka,T., Haraguchi,T., and Motoki
K., 2011, Estimation of deep s-wave structure in
Osaka plains urban area by using microtremor
surveymethod,Jour.Japan
Soc.Eng.Geol.,
Vol.52,No.5,pp.192-198 (in Japanese)
Mizuochi Y., Matsuoka T., Hayashi H., Ono M.,(2010): Structure prospecting of Murono mud
volcano in Niigata prefecture using Arraymicrotremor survey, Abstracts, Japan Geoscience
Union Meeting 2010, SCG084-P16 (in Japanese)
Okada H., Matsushima T., Moriya T., Ssasatani T.,(1990): An Exploration Technique Using
Long-Period Microtremors for Determination of
Deep Geological Structures under Urbanized Areas ,Geophysical Exploration 1990, vol. 43, 402-417 (inJapanese)
Okada H.,(2003): The Microtremor Survey Method,
Geophysical Monograph Series No.12, Society of
Exploration Geophysicists
Okada H.,et al (2003):Spatial autocorrelation
algorithm for the microtremor survey method using
semicircular arrays ,Proceedings of the SEGJ
Conference 109, 183-186 (in Japanese)
Osaka Jiban editorial committee;1982, S-wave
velocity by in-situ test, Osaka JIban, 6.3.2,
238-239 (in Japanese)
Sasaki T., Sawano H., Shimizu H., Halim A.,
Matsuoka T., Mizuochi Y., Hayashi H., Tanase
A.(2014): Geological Modeling in the Urban
Subsurface Using the Microtremor array survey,
IAEG XII Congress volumes, Engineering Geology
for Society and Territory Volume 5, 893-898
Yulianto,E.,Hutasoit,L.M.,Pindratno,H.,Sukapti,W.S.
,Hirakawawa,K.,Plio-Pleistocene Boundary inJakarta :Micropaleontological analysis. http: //www. geocities.ws/ekoy001/foram-rev-2-web.htm

Fig. 3 Geological section analyzed by S-wave
velocity distribution, SPAC method, multiple arraysize 1m-3m-10m-30m (Sasaki et al. 2014)
ranges 30 km by east and west and north and south.
The S wave velocity structure of every survey points
ware correlated with the formations of geologicalmaps elaborated by the prior geologicalstudiesconsidering the relationship between S wave
velocity and geological age shown as Table 2. The
distribution of those formations is shown in Fig. 3.
This result had been correlated with the existinggeological dating data (Yulianto et al.), the boundaryof Pliocene and Pleistocene by the microfossil dating
was coincided with our result (shown as the Bluearrow in Fig.3)
4.Conclusion






Chain array method has been developed inpractical use for several areas and purposes.
Chain array survey can provide the image of thesubsurface geological condition in several depth
even with the simpler procedure than the othergeological survey method.
The preciseness of that image by chain array
method were evaluated by several case studies.
S-wave velocity structure analysis for thegeological structure survey using the SPAC
method had been conducted and this was foundto be the one of the useful and economical
method especially in the metropolitan area.

References
Aki K., (1957) : Space and time spectra of stationary
stochastic waves, with special reference tomicrotremors, Bull. Earthq. Res. Inst., 35, 415-456
Hayashi, H.,Matsuoka, T., Mizuochi, Y., Ono, M.,(2010):Two Dimensional Microtremors Survey Chain Array Survey Method: Soils and Foundations,58(8), 10-13 (in Japanese)
Harayama,S.,Matsuoka,T.,Mizuochi,Y.,Hayashi,I.,Tanase,A.(2012):Buried Paleo Azusagawa Valley

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