World Academy of Science, Engineering and Technology 56 2009
Seismic Refraction Tomography of Mae-Hia
Landfill Sites, Mueang District, Chiang Mai
Pisanu Wongpornchai, Ronnachai Phatchaiyo, and Napakom Srikoch
control systems, but also the integration of the natural subsoil
of a waste site as a potential and additional safety component
against pollutant spreading.
In planning the remediation of contamination by landfills,
precise knowledge of their lateral boundaries and depth is
required. Seismic refraction method has been extensively used
for imaging the subsurface structure. It is appealing because of
its low-cost field operation and easy data interpretation
(Zhang and Toksoz, 1998).
Usually, waste has been dumped in former gravel pits and
covered with thin layers of soil. Geometry and depth of
landfill sites are not easily determined by seismic refraction
traveltime data. At some landfills, seismic refraction
tomography has offered the necessary resolution and
penetration to tackle the depth determination problem.
The principle of tomography divides the exploration area
into cells. Each cell is assumed as a cubic shape of constant
velocity and located between shots and receivers. Seismic
refraction tomography algorithms use two-point ray–tracing
techniques to compute raypaths and traveltimes. The slowness
can be used to calculate velocities of wave in each cell.
Comparing the seismic refraction tomography with seismic
reflection method, the data of the latter are usually disturbed
by noise in the time range of interest (< 50 millisecond);
costly, time-consuming with complicated data processing and
interpretation. Seismic refraction tomography can cover the
entire extent of the rock volume in study area. Therefore the
seismic refraction tomography is appropriate for the
exploration area of the landfill site.
Abstract—The groundwater contamination from the unmanaged
abandoned landfill site creates many problems to the community
surrounding the landfill site. An abandoned landfill site in Mae-Hia
Sub-district, Mueang District, Chiang Mai Province is an example of
the unmanaged landfill site. Remedial work of groundwater quality
needs to understand the geologic conditions. The seismic refraction
survey and tomography provide the solution to delineate the geologic
conditions.
Seven lines of seismic refraction survey were performed using
broadside configuration over an area of approximately 0.12 square
kilometer. The refraction data was process as travel-time curves to
display the two-dimensional velocity model. The refraction
tomography shows both two-dimensional velocity model at a
particular depth and three-dimensional velocity model of the shallow
subsurface structure.
The result demonstrates that the Mae-Hia landfill site has three
layers. The upper layer was interpreted as topsoil having a wave
velocity range from 124 to 849 meters per second and a thickness of
1.2 to 9 meters. The middle layer was interpreted as waste material
and clayey sand having a wave velocity range from 600 to 1,719
meters per second and a thickness 2 to 13.9 meters. The lower layer
was interpreted as saturated sand and saturated sand and gravel at the
base of the landfill site. It has a wave velocity range from 1,351 to
2,000 meters per second and a thickness of 1.5 to more than 6.5
meters.
Keywords—Velocity model, seismic refraction tomography,
Mae-Hia landfill site.
I. INTRODUCTION
B
ECAUSE of fast growing industrial and rural
development, the cities in Thailand face, like many cities
in European states, the problem of rapidly increasing amounts
of municipal and industrial waste. Landfills and waste dumps
can be a serious threat to the environment and to human
health. It is the rural surroundings of the cities which have to
take the burden to host the necessary municipal waste disposal
sites. A powerful approach to enhance the safety of waste
disposal sites is the multi-barrier concept, which requires not
only the installation of technical, man-made barriers and
II. METHOD
A. Refraction Tomography
The seismic refraction tomography can be used to increase
the resolution of the subsurface structure. The broadside
configuration is used to increase ray-path coverage of the
study area. The seismic refraction tomography can increase
the resolution of shallow velocity structure that it provides
both two- and three-dimensional images. The principle of
tomography divides the object into cells, which are called
pixels in two-dimension and voxels in three-dimension.
Tomographic imaging of the velocity distribution in the object
can be determined. The velocity distribution depends on path
length and velocity along the path. Tomographic imaging of
the velocity distribution in the object can be determined.
P. Wongpornchai is with the Department of Geological Sciences, Faculty
of Science, Chiang Mai University, 239 Huaykaew Road, Suthep Sub- district,
Mueang District, Chiang Mai 50200, Thailand (phone: +66-5394-3417; fax
+66-5394-3444; e-mail: scipwngp@chiangmai.ac.th).
R. Phatchaiyo and N. Srikoch is with the Engineering Basics Department,
Faculty of Engineering, North-Chiang Mai University, Chiang Mai 50230,
Thailand
(phone: +66-5381-9999;
fax:
+66-5381-9998; e-mail:
ronnachai@northcm.ac.th).
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World Academy of Science, Engineering and Technology 56 2009
IV. GEOPHYSICAL INVESTIGATION
B. Solution of the Tomography
Tomographic imaging techniques are applied to determine
the velocity structures and depth to a refracting interface. The
method of processing is based on the relation between
propagation velocity and the total traveltime or between
attenuation characteristics and received amplitude.
Tomography can be defined as the reconstruction of a field
model from the knowledge of linear path integrals through the
field. In tomography, traveltimes are a function of the
slowness (inverse of velocity) along ray-paths as in (1).
ti =
The study area covers an area of approximately 0.12 square
kilometer of Mae-Hia landfill site, Mueang District, Chiang
Mai. Seven survey lines of seismic refraction are performed
over the landfill site (Fig. 1).
ri
∫s i u( r )dl
(I = 1, 2, 3…)
(1)
where uI is the slowness, r is the position vector, and l is the
ray-path length. The traveltime (ti) of a seismic wave is
measured between shot (si) and receiver (ri).
In equation (1), the total traveltime can be written as a
summation as in (2).
m
t i = ∑ l ik u k
k= 1
(I = 1, 2, 3…n)
(2)
where uk is the slowness of kth cell, (k=1, 2…. m, and m is
the number of cells)
lik is the ray-path length of ith ray and kth cell
n is the number of rays that passes through each
cell.
The equation (2) can be expressed in a simple form as in (3)
T = LU
Fig. 1 Location of waste disposal sites Mae Hia in the surroundings
of Chiang Mai City
The seismic refraction data were collected by using the
Bison Geopro 8012A. The length of each survey line was set
to 52 meters. The survey line was not passed through the
middle of landfill because of the waste material on the surface.
The waste material is likely to absorb the energy of the source.
Each seismic line consisted of twenty-six shot-points. The
sledgehammer was used as the seismic source. To reduce
noise and improve the data quality in each shot, a sum of
common shot records was used and recorded as the stacked
record. The number of stacks per shot point ranged between 3
and 10 depending on the quality of recorded signal. The
spacing between geophone and source lines was set at four
meters. Each line, geophones spacing and shot-point spacing
was also four meters. Number of geophone is twelve. The data
set comprise 2,184 traces. To determine the accurate velocity
for each layer using refracted waves, the accurate
determination of first-break arrival times is required.
(3)
where T is the traveltime matrix
L is the ray-path length matrix
U is the slowness matrix
The traveltime matrix equation (3) can be used to determine
the slowness as in (4)
U = L-1T
(4)
III. TARGET AREAS
The Mae-Hia waste disposal site is situated at the western
foothills of the Chiang Mai-Lamphun basin, which is
interpreted as a fault-controlled half-graben system filled by
alluvial and colluvial sediments of Quaternary to, probably,
Late Tertiary age. The location of the landfill is in the
transition zone between clay-rich colluvial deposits and the
Ping River alluvial complex. Operations at the site started in
1958 as a sanitary landfill on the existing surface without any
technical safety measures, such as a drainage or technical liner
system. The site, therefore, is considered to demonstrate the
importance of a suitable geological barrier consisting
predominantly of low permeability and contaminant-retarding
material. Operation of the waste disposal closed in 1989.
V. PRESENTATION OF RESULT
A. Determination of Two-Dimensional Velocity Model of
Seismic Refraction Method
A two-dimensional velocity model shows the subsurface
velocity structure as the initial input model for tomography. It
was processed and presented using the SIPT® program, a
registered trademark of Rimrock Geophysics Inc. An initial
model is created based on the time-distance graph. The
program calculates velocity of each layer. The depth of each
layer beneath each geophone was determined. These depths
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World Academy of Science, Engineering and Technology 56 2009
area, Fig. 7 shows three-dimensional tomographic image.
were then interpolated between adjacent geophone positions.
The program assumes that each layer encountered is
horizontally continuous, and there are no lateral changes in
velocity within any one layer (Kutrubes and others, 2002).
Fig. 2 shows a two-dimensional velocity model of Line L1.
Fig. 3 The surface plot shows the thickness of each layer of the earth
model from Lines L1 to L7; A is the surface plot of the upper layer,
B is the surface plot of the middle layer
Fig. 2 Two-dimensional velocity model of line L1
The upper layer has a wave velocity range from 285 to 457
meters per second and a thickness between 1.8 and 7 meters.
It was interpreted to be topsoil. The thickness of upper layer in
the northern and eastern parts of the study area is greater than
that in the western and southern parts of the study area, as
shown in Fig. 3. The middle layer has a wave velocity range
from 909 to 1,746 meters per second and a thickness of 1.3 to
more than 2.5 meters. This layer was interpreted to be clayey
sand and waste material in the landfill site. The thickness this
layer gradually decreases from east to the west, as shown in
Fig. 3. The lower layer has a wave velocity range from 1,354
to 2,667 meters per second at a depth of more than 5.7 meters.
This layer was interpreted to be saturated sand.
B. Determination of Two- and Three-Dimensional Velocity
Model of Tomography
Seismic refraction tomography was used to calculate
traveltimes and slowness of each pixel. The source and
receiver coordinates and traveltimes can be used to calculate
velocities. The tomography calculates velocity within each
pixel. Because the Earth in reality is three dimensional, a
three–dimensional model is created to enhance interpretation.
The GEOTOMCG® program, a registered trademark of
GeoTomCG commercial software (Geotom LLC, Apple
Valley, Minnesota), provides three-dimensional numerical
results and two-dimensional slices of the three-dimensional
grid as ASCII files that read by Tecplot® program.
Two-dimensional tomographic images in XY plane of the
Line L1 in the Mae–Hia landfill site is displayed in Fig. 4.
Fig. 5 shows the final two-dimensional tomographics image in
XZ plane of Line L1. Fig. 6 shows plane section of twodimensional tomographics image in XY plane of Line L1. The
advantage of this presentation can show the velocity
characteristics of earth layers of the study area. In this study
Fig. 4 Final two-dimensional tomographics image in XY plane of
Line L1 (Z = -1.6667, Z = -4.3333 and Z = -7). Color scale
represents values between 0.228 and 1.827 m/ms
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World Academy of Science, Engineering and Technology 56 2009
VI. DATA INTERPRETATION
Fig. 5 shows the final two-dimensional tomographic image
in plane XZ of Line L1. Fig. 6 shows plane section of twodimensional tomographic image in plane XY of Line L1. The
three-dimensional velocity model of seismic refraction data is
shown in Fig. 7. The result of this study demonstrates that the
Mae-Hia landfill site has three layers. The upper layer was
interpreted as topsoil having a wave velocity range from 124
to 849 meters per second and a thickness of 1.2 to 9 meters.
The middle layer was interpreted as waste material and clayey
sand having a wave velocity range from 600 to 1,719 meters
per second and a thickness 2 to 13.9 meters. The lower layer
was interpreted as saturated sand and saturated sand and
gravel at the base of the landfill site. This layer has a wave
velocity range from 1,351 to 2,000 meters per second and a
thickness of 1.5 to more than 6.5 meters. The velocity
anomaly is shown by green color. The base of the gravel pit is
located beneath the green area.
ACKNOWLEDGMENT
First of all, we would like to acknowledge the Department
of Geological Sciences, Faculty of Science, Chiang Mai
University for support 12-channel seismograph (Bison
GeoPro 8012A).
We would like to grateful acknowledge to Dr. Daryl
Tweenton who has given the SIPT® program, the
GEOTOMCG® program and his valuable advice.
Fig. 5 Final two-dimensional tomographics image in XZ plane of
Line L1 (Y = -6, Y = 0, and Y = 6). Color scale represents values
between 0.228 and 1.827 m/ms
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Fig. 6 Final two-dimensional tomographics image in XY plane of
Line L1. Color scale represents values between 0.20 and 1.80 m/ms
Fig. 7 Final three-dimensional tomographics image derives from Line
L1. Color scale represents values between 0.260 and 1.56388 m/ms
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