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GROUND PENETRATING RADAR (GPR) FOR SUBSURFACE MAPPING:
PRELIMINARY RESULT
Awangku Iswandy Awangku Serma and Halim SetanUTM-Photogrammetry
and Laser Scanning Research Group,
Universiti Teknologi [email protected] &
[email protected]
ABSTRACT
Ground Penetrating Radar (GPR) is a noninvasive geophysical
technique that detects electrical discontinuities in the shallow
subsurface. It does this by generation, transmission, propagation,
reflection and reception of discrete pulses of high frequency
electromagnetic energy. This paper presents preliminary result
using Ramac CUII GPR from Mala Geoscience, and test it
effectiveness to detect object buried at a known depth, location,
spacing and diameter at the test site of Nuclear Agency of Malaysia
(MINT). The data that had been collected were not given the
appropriate processing steps, but just applying data enhancement
technique, including Automatic Control Gain (AGC) function and this
was done upon field test using GroundVision data acquisition
software. No further processing steps were taken as there were no
processing software available from Nuclear Agency of Malaysia
(MINT). The result shows that not all parameters can be detected
successfully via the 250 MHz shielded antenna. The best data
acquired were on a survey profile across the Line Number 2 (L2)
survey line, which consist the same 6 inch metal pipe buried at
different depth. The hyperbola reflection from the radargram is
almost accurate when compared to known depth. Contrary, the 250 MHz
shielded antenna failed to detect the metal pipe buried with close
spacing at about 0.25 0.5 meter at Line Number 1 (L1) survey line,
where the data acquired are blur and did not give a strong
reflection of the object. This also happen to Line Number 3 (L3)
survey line, which consist of different diameter metal pipe but
buried at the same depth and the data shows that the 250 MHz
shielded antenna cannot detect the metal pipe with diameter less
than 4 inch.
Keywords : Ground Penetrating Radar (GPR), geophysical,
subsurface
1.0 INTRODUCTION
On earth, the subsurface is perhaps the most important
geological layer as it contains many of the earth natural resources
(e.g. building aggregates/stones, placer deposits, drinking water
aquifers, soils). Additionally, through the study of rocks and
unconsolidated sediment accumulations at or near the surface by
soil scientist and geologist, scientist have discovered much about
earth history and behavior of its dynamic landforms (Neal, A.,
2004). For soil scientist, the subsurface are typical soil horizons
and layers classified to a depth of 2 meter or to bedrock (if
within depths 2 meter) (Soil Survey Staff, 1999). For geologist and
engineering geologist, the subsurface comes in terms with the
underlying structures, spatial distribution of rock units,
structures such as faults, folds and intrusive rocks and the depth
of investigation may vary (Wikipedia, 1). The study of subsurface
for geologist is an indirect method for assessing the likelihood of
ore deposits or hydrocarbon accumulations, by using exploration
geophysical
ISSN 1511-9491 2009 FKSG
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methods. Exploration geophysics is the practical application of
physical methods or known as geophysical methods (such as seismic,
gravitational, magnetic, electrical and electromagnetic) to measure
the physical properties of rocks, and in particular, to detect the
measurable physical differences between rocks that contain ore
deposits or hydrocarbons and those without. Geophysical methods
have a major role to play in resource assessment and the
determination of engineering parameters, such as to directly detect
the target style of mineralisation, via measuring its physical
properties directly. For example one may measure the density
contrasts between iron ore and silicate wall rocks, or may measure
the conductivity contrast between conductive sulfide minerals and
barren silicate minerals. A wide variety of sensors could be
considered to aid this situation, and generally each will have a
particular niche role but the geophysical electromagnetic method
that is of most universal value is Ground Penetrating Radar, or
Ground Probing Radar (GPR) or also known by Surface Probing Radar
or Surface Penetrate Radar, and had already used widely with
on-going research and publications up to date (Daniels, 1996). This
method is a kind of mobile survey and works by sending of a tiny
pulse of energy into material and recording the strength and the
time required for the return of any reflected signal, and display
it as radargram (Figure 1).
Figure 1. An example of radargram on which shown with depth
section (Wikipedia, 2).
A geologic map or geological map is a special-purpose map made
to show geological features (Figure 2). Rock units or geologic
strata are shown by color or symbols to indicate surface coverage.
Structural features are shown with strike and dip symbols which
consist of (at minimum) a long line, a number, and a short line
which are used to indicate tilted beds. The long line is the strike
line, which shows the true horizontal direction along the bed, the
number is the dip or number of degrees of tilt above horizontal,
and the short line is the dip line, which shows the direction of
tilt. Stratigraphic contour lines may be used to illustrate the
surface of a selected stratum illustrating the subsurface
topographic trends of the strata. Isopach maps detail the
variations in thickness of stratigraphic units. It is not always
possible to properly show this when the strata are extremely
fractured, mixed, in some discontinuities, or where they are
otherwise disturbed. On the contrary, this can also applied to
subsurface geological features, which we cannot see directly as
there would be no exposure of outcrops for observations, and shown
through the subsurface geological map (Figure 3).
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Figure 2. Example of surface geological map of East Johor,
Malaysia (Kamal, 2004).
Figure 3. Example of a subsurface geological map without colour,
(Awni et. al., 2001).
2.0 GROUND PENETRATING RADAR A REVIEW
Geophysical exploration started in the early 1920s following the
successful development of electrical prospecting methods by the
brothers Conrad and Marcel Schlumberger in France, and the seismic
refraction method in the newly discovered oil fields of the
mid-south USA by Karcher, Mintrop and other pioneers. A wide range
of geophysical method used for subsurface investigation could be
found in the report of the Geological Society Engineering Group
Working Party (1988). The word RADAR is an acronym coined in 1934
for Radio Detection and Ranging (Buderi, 1996). Ground-penetrating
radar (GPR) is a geophysical method that uses radarpulses to image
the subsurface. This non-destructive method uses electromagnetic
radiationin the microwave band (UHF/VHF frequencies) of the radio
spectrum, and detects the reflected signals from subsurface
structures (Daniels, 2004). GPR can be used in a variety of media,
including rock, soil, ice, fresh water, pavements and structures.
It can detect objects, changes in material, and voids and cracks.
GPR systems work by sending a tiny pulse of energy into the ground
from an antenna. An integrated computer records the strength and
time required for the return of reflected signals. Any subsurface
variations, metallic or non-metallic, will cause signals to bounce
back. When this occurs, all detected items are revealed on the
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computer screen in real-time as the GPR equipment moves along.
In data processing, detailed examination/interpretation of GPR
sections may be able to identify soils, bedrock, groundwater, etc.
The depth range of GPR is limited by the electrical conductivity of
the ground, the transmitted center frequency and the radiated
power. With respect to radar data interpretation, the degree that
the results is assumed to be true is dependent upon a wide range of
factors such as nature of the sediment body under investigation,
the groundwater regime, the type of terrain immediately adjacent to
the survey line, the nature and appropriateness of any data
processing undertaken, the interpretation techniques employed and
the overall understanding of the researcher with respect to GPR and
their subject background. One of the original and most promising
ground penetrating radars was presented by Moffatt and Puskar
(1976). Their system used an improved antenna that gave a better
target-to-clutter ratio and was able to more accurately detect
important subsurface reflections. The early work using radar was in
glaciology by Plewes and Hubbard (2001) along with civil
engineering, archaeological and geological applications that came
onwards (Daniels, 1996; Conyers and Goodman, 1997; Reynolds, 1997).
Other research using GPR includes fluvial and fluvioglacial (Best
et al., 2003), coastal and aeolian delta (Botha et al., 2003),
peatland (Holden et al., 2002),slopes (Degenhardt and Giardino,
2003), carbonates (Pedley and Hill, 2003), faults, joints and folds
in sediments (Anderson et al., 2003), marble structure (Selma,
2008) and has been successful in delineating gem-bearing zones in
the Himalaya pegmatite mine of the Mesa Grande district of southern
California (Jeffrey et al., 2007). Varied references exist that
cover topics ranging from building GPR units, obtaining GPR data,
processing GPR data, and analyzing GPR data. Some technologies have
emerged in the past ten years that give GPR users better methods of
processing and analyzing the GPR data than were available before.
One of these technologies is the ability to visualize GPR data in
three dimensions, with the ability to add time as a fourth
dimension. Among the first to visualize GPR results in three
dimensions is Birken and Versteeg (2000). More advance and thorough
GPR applications and research is given by Jol, H.M. (2009).
Consider the behavior of a beam of electromagnetic wave (EM) energy
as it strikes an interface, or boundary, between two materials of
different dielectric constants (Figure 4). A portion of the energy
is reflected, and the remainder penetrates through the interface
into the second material. The reflection coefficient at the
interface, 1,2 is given by equation (1),
a) Radar energy traveling outwards from transmitter.
b) Straight ray paths show routes of individual points on the
radar wave Front.
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c) Radar energy is reflected (r) at an angle equal to the angle
of incidence (i) from interfaces with a contrast in electrical
properties.
Figure 4. Geometry of GPR signal path through simplified
subsurface.
)(
)(
21
212,1
(1)
where 1 and 2 are the dielectric constants of materials 1 and 2,
respectively (Davis and Annan, 1989). Equation 1 indicates that
when a beam of microwave energy strikes the interface between two
materials, the amount of reflection, 1,2 is dictated by the values
of the relative dielectric constants of the two materials. If
material 2 has a larger relative dielectric constant than material
1, then 1,2 would have a negative value; i.e., with the absolute
value indicating the relative strength of the reflected energy and
the negative sign indicating that the polarity of the reflected
energy is the opposite of that arbitrarily set for the incident
energy. After penetrating the interface and entering into material
2, the wave propagates through material 2 with a speed, V2, given
by equation (2),
2
2 C
V (2)
where C is the propagation speed of EM waves through air, which
is equivalent to the speed of light, or 0.3 m/ns). As the wave
propagates through material 2, its energy is attenuated as
follows:
= 12.863 x 10-8 f 2/122 1tan1 (3)where = attenuation, in
decibel/meter, f= wave frequency, in Hz, and = the loss tangent (or
dissipation factor) is related to , the electrical conductivity (in
mho/meter) of the material by:
tan = 1.80 x 10' 2
f
(4)
When the remaining microwave energy reaches another interface, a
portion will be reflected back through material 2 as given by
Equation 1. The resulting two-way transit time (t2) of the
microwave energy through material 2 can be expressed as,
t2 = C
D
V
D 22
2
222 (5)
where D2 is the thickness of material 2.
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Common geophysical reflection data are of four main types:
common offset, common mid (or depth) point, common source and
common receiver. Common offset surveys are most frequently used in
GPR studies, with commercial radar systems consisting of either a
single transmitting and receiving antenna, or two, separate,
transmitting and receiving antennae. In the latter systems, a fixed
spacing is employed between the antennae, typically with both
orientated in the same direction (i.e. copolarised). In
conventional surveys, antennae are perpendicular to the survey
line, with their broad sides orientated towards each other. With
such an antenna configuration the survey is said to be copolarised,
perpendicular broadside. However, other potential configurations do
exist and these may provide important additional information (van
Gestel and Stoffa, 2001; Jol et al., 2002; Lutz et al., 2003).
During surveying, antennae are either dragged along the ground
(Figure 5) and horizontal distances recorded on a time-base, which
can be converted to a distance-base through manual marking, or they
are moved in a stepwise manner at fixed horizontal intervals (the
step size). Step-mode operation generates more coherent and higher
amplitude reflections, as antennae are stationary during data
acquisition. This allows more consistent coupling between antennae
and the ground, with the added benefit of better trace stacking
(Annan and Davis, 1992). As data are recorded during surveying,
horizontally sequential reflection traces build up a radar
reflection profile. Each trace results from the GPR system emitting
a short pulse of high-frequency electromagnetic energy, typically
in the MHz range, that is transmitted into the ground. As the
electromagnetic wave propagates downwards it experiences materials
of differing electrical properties, which alter its velocity. If
velocity changes are abrupt with respect to the dominant radar
wavelength, some energy is reflected back to the surface. The
reflected signal is detected by the receiving antenna. In systems
with a single antenna, it switches rapidly from transmission to
reception. The time between transmission, reflection and reception
is referred to as two-way travel time (TWT) and is measured in
nanoseconds (10- 9 s). Reflector TWT is a function of its depth,
the antenna spacing (in systems with two antennae), and the average
radar-wave velocity in the overlying material. Reflections from
subsurface discontinuities are not the only signals recorded on a
radar trace. The first pulse to arrive is the airwave, which
travels from transmit antenna to receive antenna at the speed of
light (0.2998 m ns-1). The second arrival is the ground wave, which
travels directly through the ground between the transmit and
receive antennae. The air and ground waves mask any primary
reflections in the upper part of a radar reflection profile.
Lateral waves can also be present and result from shallow
reflections that approach the surface at the appropriate critical
angle and are subsequently refracted along the airground interface
(Clough, 1976). It should be noted that reflections associated with
lateral waves are not correctly placed in time (depth) with respect
to the interface that generated them. Pseudo-3-D surveys involve
collecting data on regular or irregular survey grids, usually in
two mutually perpendicular directions, and often display results in
fence diagrams (for example, Russell et al., 2001; Holden et al.,
2002; Skelly et al., 2003). In true 3-D surveys, transect lines are
so closely spaced that data for individual traces overlap. 3-D data
cubes can be generated from these surveys (Nitsche et al., 2002;
Heinz and Aigner, 2003). Collecting true 3-D data is particularly
time consuming, largely because of time required to accurately
record the position and elevation of data points. Lehmann and Green
(1999) attempted to overcome this problem by developing a semi
automated system that records coordinates during radar data
collection using a self-tracking laser theodolite. Other
experiments have combined the use of GPR with Global Positioning
Systems (GPS) (e.g. Urbini et al., 2001; Freeland et al., 2002).
Jol and Bristow (2003) consider other practical difficulties in
performing GPR field surveys.
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Figure 5. A GPR cart (A) and hand-towed GPR (B) being used on
research.
3.0 PRELIMINARY TEST
A preliminary study has been carried out with cooperation from
Non Destructive Testing (NDT), Technology Industry Section of
Nuclear Agency of Malaysia (MINT), Bangi, with help from Dr. Mohd.
Azmi Ismail and Amry Amin Abas to used their available RAMAX CUII
GPR unit and test it upon their own test site with the size of 14 m
x 6 m, which include buried metal pipe with 6 inch in diameter
(Figure 6 and Figure 7). The test included detecting 4 metal pipe,
buried 2 meter deep, with same diameter, same depth but different
spacing (Figure 8); same diameter, same spacing but different depth
(Figure 9); same spacing and same depth but different diameter
(Figure 10). The test site was excavated at about 2.5 m depth, and
filled back with sand and gravel, with the metal pipe placed
inside, suitable for a 3 single line survey with GPR.
Figure 6. The drawing of the test site, 14 m x 6 m wide, with 2
m length in between the metal pipe.
3.1 Survey Procedure
RAMAC/GPR made by MALA Geoscience, Sweden with 250 MHz shielded
antenna was used during the survey. MINT also purchases 150 MHz,
400 MHz, 800 MHz and 1 GHz shielded antenna (Figure 11).The survey
was carried out in the MINT test site. The purpose of setting up
the test site is eventually to test out the GPR unit, and trying to
configure the best practice for detecting buried utilities such as
the metal pipe for simple parameters like with differences in their
diameter, their buried depth and their spacing, and also to learn
the operating
A B
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Figure 7. Picture of the test site for GPR testing in MINT,
Figure 8. Testing and detection for metal pipe with same Bangi.
diameter, same depth but different spacing, L1 single line
survey.
Figure 9. Testing and detection for metal pipe with same Figure
10. Testing and detection for metal pipe with same diameter, same
spacing but different depth, L2 single line depth, same spacing but
different diameter, L3 single line survey. survey.
Figure 11. Shown here is the RAMAC 5 shielded antenna Figure 12.
The single line survey being conducted by Dr. with their metal
casing. Note that the lower the frequency, Azmi (red shirt) and
Amry Amin, from NDT Group, MINT.the bigger the size.
250 MHz
800 MHz400 MHz
150 MHz
1 GHz
L1 Single Line Survey
6.0 inch pipe
2.5 inch pipe
1.0 inch pipe
4.0 inch pipe
L3 Single Line Survey
1.0 m deep
0.5 m deep
1.5 m deep
2.0 m deep
L2 Single Line Survey
6 inch metal pipe
0.25 m spacing
0.5 m spacing
1.0 m spacing
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procedures of a GPR survey. There were three (3) areas scanned
namely L1 single line survey, consist of 4 buried metal pipe at
depth of 2 meter, with the same in diameter but different spacing,
starting from 0.25 meter between pipe A and pipe B, 0.5 meter
between pipe B and pipe C and 1.0 meter between pipe C and pipe D
(Figure 8). Secondly is L2 single line survey consist the same size
of metal pipe with the same spacing interval of 1.0 meter, but with
different depth starting with pipe A buried 2.0 meter, pipe B with
1.5 meter, pipe C with 1.0 meter and pipe D with 0.5 meter from the
surface (Figure 9). Lastly is L3 single line survey consist of 4
metal pipe with the same spacing interval of 1 meter and depth of 2
meter from the surface but with different size in diameter,
starting from pipe A with 1 inch, pipe B with 2.5 inch, pipe C with
4.0 inch and pipe D with 6.0 inch (Figure 10). Scanning was done
along a single line survey, on top of the buried metal pipe (Figure
12). The line survey consist of scanning lines that are of the same
length and has parallel starting points. The GPR cart was pushed
along the single line survey, with step size spacing. The radargram
window being adjusted to maximum of 4 meter depth time window and
the distance of 6 meter. Real time data adjustment including the
Automatic Gain Control (AGC) and time gain control is applied.
3.2 Radar Data Processing
For a normal radar data processing is confronted by three main
tasks (Yilmaz, 1987):
(1) selecting an appropriate sequence of processing steps;(2)
choosing an appropriate set of parameters for each processing
step;(3) evaluating output resulting from each processing step and
identifying problems caused
by incorrect parameter selection.
Yilmaz (1987) demonstrates how different processors can produce
significantly different end products from the same initial data
set, because of different decisions made. Fisher et al. (1992) and
Greaves et al. (1996) demonstrate this point very well with respect
to radar, with their different approaches to the processing of the
same multi-offset data. A processors ability to make the right
choices is often as important as effectiveness of the processing
algorithms in determining final image quality. Processing,
therefore, cannot be entirely objective, with some considering it
more of an art than a science (Yilmaz, 1987). A wide range of
options are available and processors is chosen depending upon
algorithms available, objectives of the study, and their experience
and ability, meaning that accurate records of all processing steps
performed should be maintained.
3.3 Data Interpretation
Soon after the realisation that GPR could provide useful data
for various subsurfaces investigation, various authors suggested
that the principles of seismic stratigraphy could be applied to the
interpretation of radar reflection profiles (Baker, 1991; Beres and
Haeni, 1991; Jol and Smith, 1991). Jol and Smith (1991) first used
the term radar stratigraphy for this interpretation technique,
although Gawthorpe et al. (1993) were the first to fully define the
concept and its relationship to seismic stratigraphy Consequently,
it is recommended that radar facies reflection configurations are
described in terms of the: (1) shape of reflections; (2) dip of
reflections; (3) relationship between reflections and (4)
reflection continuity. A diagram (Figure 13) and table (Table 1)
shown to simplify all the basic processes needed immediately when
practicing GPR survey and for various purposes of
investigation.
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Table 1. Basic description of the steps in Figure 6.
Editing Removal and correction of bad/poor data and sorting of
data files.Rubber-banding Correction of data to ensure spatially
uniform increments.Dewow Correction of low-frequency and DC bias in
data.Time-zero correction Correction of start time to match with
surface position.Filtering 1D & 2D filtering to improve signal
to noise ratio and visual quality.Deconvolution Contraction of
signal wavelets to spikes to enhance reflection events.Velocity
analysis Determining GPR wave velocities.Elevation correction
Correcting for the effects of topography.Migration Corrections for
the effect of survey geometry and spatial distribution.Depth
conversion Conversion of two-way travel times into depths.Display
gains Selection of appropriate gains for data display and
interpretation.Image analysis Using pattern or feature recognition
tools.Attribute analysis Attributing signal parameters or functions
to identifiable features.Modelling analysis Simulation of GPR
responces.
Figure 13. GPR data processing flow and basic analysis steps
(Nigel, 2009).
4.0 RESULT AND DISCUSSION
DATA ACQUISITION
At Site(commonly automated)Editing Simple FilteringData Analysis
& Gain
POST COLLECTION Editing Rubber-Banding Dewow Time Zero
Correction Filtering Deconvolution Velocity Analysis Elevation
Correction Migration Depth Conversion Data Display and Gains
Image analysisAttribute analysisModelling analysis
CMP Data
TOPOGRAPHY DATA
INTERPRETATION
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The GPR radargrams is shown in respective figures below. The red
circle indicates the metal pipe buried below the surface, marked
after calculation and estimation from their original placement on
the test site, compared to the radargram that was acquired during
the survey. The steps taken during survey is time gain adjustment
and filtering. Nothing can be done for post-processing for the data
acquired as MINT does not have the processing software. The simple
interpretation step taken is similar to seismic data
interpretation, where the reflection pattern in the form of
hyperbola is targeted and the peak of the hyperbola represent the
centre of the target. For L1 single line survey (Figure 14), shows
a blur hyperbola, and cannot accurately determine the location of
the target. For pipe A and pipe B, there seems to be a merge of its
reflections and if not calculated for its known depth and position,
it is hard to tell their position by just relying on the radargram.
The spacing between Pipe A and Pipe B is 0.25 m, therefore it is
presume that this distance is too close to be detected, although
their diameter is still the same. For pipe D, nothing can be seen
to show that it exist, where it supposed to be no problem in
detecting it apart from Pipe C with spacing of 1.0 m. There seem to
be a large disturbance from the air wave at depth of 0.5 m.
Figure 14. Radargram for L1 single line survey.
For L2 single line survey (Figure 15), the radargram
successfully display the hyperbola of each buried pipe, but still
not strong enough for first glance interpretation. The pipe with
same size of 6 inch can be detected easily, but for pipe A which is
buried at 2 m, and pipe D buried at 0.5 m, the radargram shows
little significant trace, as their hyperbola did not show a strong
contrast with the background. Each pipe is located at the centre of
the peak of the hyperbola shown on the radargram, but still there
is a disturbance from ground wave interference as shown at pipe A
and pipe B at about 0.5 m from the surface. The excellent example
of good radar data display is Pipe C (buried 1.0 m),where the
hyperbola and its peak really represent the real depth of the pipe.
The result is quiet acceptable, and the radargram display a pattern
of all the four (4) metal pipe buried.
2
3
1 2 3 40
1
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Figure 15. Radargram for L2 single line survey.
For L3 single line survey (Figure 16), it seems that the
radargram shows nothing at all except for pipe D, but still it is a
week reflection. This is due to the fact that not all investigation
can be successful using a single frequency antenna, like this
survey where only 250 MHz shielded antenna were used, because of
time constraint. The failure also occurred for detecting the metal
pipe with different spacing, as shown in Figure 15, maybe caused by
the spacing between pipe A and pipe B that was within 0.25 meter in
spacing, so there seem to be some interference between the
reflections from both of it, and causing a blur image of two
hyperbola merging together. Comparison between the three (3) test
parameters and their significant result is shown in Table 2.
Figure 16. Radargram for L3 single line survey.
2
21 3 4
2
1
3
4
21 3 4
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Table 2. Comparison of the metal pipe real depth buried and the
depth from the radargram from the three (3) test survey.
Line Number 1 Survey Actual Depth, m Depth From Radargram, m
Differences, m (Accuracy, %)
Pipe A 2.0 1.8 0.2 (90%)
Pipe B 2.0 1.7 0.3 (85%)
Pipe C 2.0 2.2 -0.2 (90%)
Pipe D 2.0 Not Confirmable Nil
Line Number 2 Survey Actual Depth, m Depth From Radargram, m
Differences, m (Accuracy, %)
Pipe A 2.0 1.8 0.2 (90%)
Pipe B 1.5 1.4 0.1 (95%)
Pipe C 1.0 1.0 0.0 (100%)
Pipe D 0.5 0.4 0.1 (98%)
Line Number 3 Survey Actual Depth, m Depth From Radargram, m
Differences, m (Accuracy, %)
Pipe A 2.0 Not Confirmable Nil
Pipe B 2.0 Not Confirmable Nil
Pipe C 2.0 Not Confirmable Nil
Pipe D 2.0 2.5 -0.5 (75%)
Table 2 shows that by using the 250 MHz antennae, it can detect
6 inch metal pipe with different depth ranging from 0.5 meter to
2.0 meter below ground with much accuracy as in Line Number 2
result column (average 90% - 100% accuracy). For the metal pipe in
Line Number 1 column, clearly stated that the antenna frequency of
250 MHz cannot fully detect the 6 inch metal pipe with different
spacing from each other. The closer the metal pipe is placed with
each other, the more inaccurate the hyperbola became. As for the
Line Number 3 result that included the same depth of 2.0 meter
below ground but different diameter, shows that all sizes smaller
than 6 inch in diameter are failed to be detected by the GPR with
250 MHz antenna.
5.0 CONCLUSION AND FUTURE RECOMMENDATION
Using GPR wisely, it is possible to image the two and three
dimensional structures of a range of subsurface structures whether
be it metal pipe or sedimentary rocks in example. It is considered
that to extract the maximum amount of meaningful information, the
user must understand the scientific principles that underlie the
technique, the effects of the data collection regime employed, the
implications of the techniques finite resolution and depth of
penetration, the nature and causes of reflections unrelated to
primary sedimentary structure, and the appropriateness of each
processing step with respect to the overall aim of the study.
However,
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in order to do this accurately, many of the inherent limitations
to the field data must beacknowledged and where possible overcome
by careful and systematic data processing. More advanced data
processing, such as migration, which is essential to obtain
correctly positioned subsurface reflections, is only just beginning
to be performed on a regular basis by GPR researchers.
Supplementary information, such as geological context, the
relationship between the various radar surfaces and facies, data
from ground from subsequent laboratory analyses, can then be used
in conjunction with the radar data for more accurate
interpretation. Less robust interpretation techniques such as radar
facies analysis, which does not define the radar surfaces that
bound the facies or the resulting radar packages, should be
abandoned as a primary interpretive tool, except in very specific
instances. In order to apply radar data successfully, an
interpreter must have a thorough understanding of a wide and
complex range of factors, including: the scientific principles that
underlie the GPR technique, the effects of the data collection
configuration used, the effects of survey-line topographic
variation, the effects of the techniques finite resolution (both
vertical and horizontal) and depth of penetration, the causes of
reflections unrelated to primary depositional structure, and the
nature and appropriateness of each processing step undertaken. Data
processing should aim to provide, within the limitations of the
field data and processing routines employed, an accurate record of
the subsurface location and orientation of reflections caused by
primary sedimentary structure.
ACKNOWLEDGEMENT
The authors acknowledge the support and assistance from Nuclear
Agency of Malaysia (MINT) for the used of their GPR equipment and
technical advise on the processing of the GPR data for this
research.
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AUTHORS
Awangku Iswandy Bin Awangku Serma is a part-time master student
in Geomatic Engineering supervised by Prof. Dr. Halim Setan.
Currently working as a geologist in Mineral and Geoscience
Department of Malaysia.
Prof. Dr. Halim Setan is a lecturer with a Ph.D from City
University, London. He has received recognition in his work as a
researcher by winning several awards such as The Best Researcher
Award, UTM, 2006 and The Best Article Award from Institution of
Surveyors Malaysia, 2006.
Geoinformation Science Journal, Vol. 9, No. 2, 2009, pp:
45-62
GROUND PENETRATING RADAR (GPR) FOR SUBSURFACE MAPPING:
PRELIMINARY RESULT
Awangku Iswandy Awangku Serma and Halim Setan
UTM-Photogrammetry and Laser Scanning Research Group,
Universiti Teknologi Malaysia
[email protected] & [email protected]
ABSTRACT
Ground Penetrating Radar (GPR) is a noninvasive geophysical
technique that detects electrical discontinuities in the shallow
subsurface. It does this by generation, transmission, propagation,
reflection and reception of discrete pulses of high frequency
electromagnetic energy. This paper presents preliminary result
using Ramac CUII GPR from Mala Geoscience, and test it
effectiveness to detect object buried at a known depth, location,
spacing and diameter at the test site of Nuclear Agency of Malaysia
(MINT). The data that had been collected were not given the
appropriate processing steps, but just applying data enhancement
technique, including Automatic Control Gain (AGC) function and this
was done upon field test using GroundVision data acquisition
software. No further processing steps were taken as there were no
processing software available from Nuclear Agency of Malaysia
(MINT). The result shows that not all parameters can be detected
successfully via the 250 MHz shielded antenna. The best data
acquired were on a survey profile across the Line Number 2 (L2)
survey line, which consist the same 6 inch metal pipe buried at
different depth. The hyperbola reflection from the radargram is
almost accurate when compared to known depth. Contrary, the 250 MHz
shielded antenna failed to detect the metal pipe buried with close
spacing at about 0.25 0.5 meter at Line Number 1 (L1) survey line,
where the data acquired are blur and did not give a strong
reflection of the object. This also happen to Line Number 3 (L3)
survey line, which consist of different diameter metal pipe but
buried at the same depth and the data shows that the 250 MHz
shielded antenna cannot detect the metal pipe with diameter less
than 4 inch.
Keywords : Ground Penetrating Radar (GPR), geophysical,
subsurface
1.0INTRODUCTION
On earth, the subsurface is perhaps the most important
geological layer as it contains many of the earth natural resources
(e.g. building aggregates/stones, placer deposits, drinking water
aquifers, soils). Additionally, through the study of rocks and
unconsolidated sediment accumulations at or near the surface by
soil scientist and geologist, scientist have discovered much about
earth history and behavior of its dynamic landforms (Neal, A.,
2004). For soil scientist, the subsurface are typical soil horizons
and layers classified to a depth of 2 meter or to bedrock (if
within depths 2 meter) (Soil Survey Staff, 1999). For geologist and
engineering geologist, the subsurface comes in terms with the
underlying structures, spatial distribution of rock units,
structures such as faults, folds and intrusive rocks and the depth
of investigation may vary (Wikipedia, 1). The study of subsurface
for geologist is an indirect method for assessing the likelihood of
ore deposits or hydrocarbon accumulations, by using exploration
geophysical methods. Exploration geophysics is the practical
application of physical methods or known as geophysical methods
(such as seismic, gravitational, magnetic, electrical and
electromagnetic) to measure the physical properties of rocks, and
in particular, to detect the measurable physical differences
between rocks that contain ore deposits or hydrocarbons and those
without. Geophysical methods have a major role to play in resource
assessment and the determination of engineering parameters, such as
to directly detect the target style of mineralisation, via
measuring its physical properties directly. For example one may
measure the density contrasts between iron ore and silicate wall
rocks, or may measure the conductivity contrast between conductive
sulfide minerals and barren silicate minerals. A wide variety of
sensors could be considered to aid this situation, and generally
each will have a particular niche role but the geophysical
electromagnetic method that is of most universal value is Ground
Penetrating Radar, or Ground Probing Radar (GPR) or also known by
Surface Probing Radar or Surface Penetrate Radar, and had already
used widely with on-going research and publications up to date
(Daniels, 1996). This method is a kind of mobile survey and works
by sending of a tiny pulse of energy into material and recording
the strength and the time required for the return of any reflected
signal, and display it as radargram (Figure 1).
Figure 1. An example of radargram on which shown with depth
section (Wikipedia, 2).
A geologic map or geological map is a special-purpose map made
to show geological features (Figure 2). Rock units or geologic
strata are shown by color or symbols to indicate surface coverage.
Structural features are shown with strike and dip symbols which
consist of (at minimum) a long line, a number, and a short line
which are used to indicate tilted beds. The long line is the strike
line, which shows the true horizontal direction along the bed, the
number is the dip or number of degrees of tilt above horizontal,
and the short line is the dip line, which shows the direction of
tilt. Stratigraphic contour lines may be used to illustrate the
surface of a selected stratum illustrating the subsurface
topographic trends of the strata. Isopach maps detail the
variations in thickness of stratigraphic units. It is not always
possible to properly show this when the strata are extremely
fractured, mixed, in some discontinuities, or where they are
otherwise disturbed. On the contrary, this can also applied to
subsurface geological features, which we cannot see directly as
there would be no exposure of outcrops for observations, and shown
through the subsurface geological map (Figure 3).
Figure 2. Example of surface geological map of East Johor,
Malaysia (Kamal, 2004).
Figure 3. Example of a subsurface geological map without colour,
(Awni et. al., 2001).
2.0GROUND PENETRATING RADAR A REVIEW
Geophysical exploration started in the early 1920s following the
successful development of electrical prospecting methods by the
brothers Conrad and Marcel Schlumberger in France, and the seismic
refraction method in the newly discovered oil fields of the
mid-south USA by Karcher, Mintrop and other pioneers. A wide range
of geophysical method used for subsurface investigation could be
found in the report of the Geological Society Engineering Group
Working Party (1988). The word RADAR is an acronym coined in 1934
for Radio Detection and Ranging (Buderi, 1996). Ground-penetrating
radar (GPR) is a geophysical method that uses radar pulses to image
the subsurface. This non-destructive method uses electromagnetic
radiation in the microwave band (UHF/VHF frequencies) of the radio
spectrum, and detects the reflected signals from subsurface
structures (Daniels, 2004). GPR can be used in a variety of media,
including rock, soil, ice, fresh water, pavements and structures.
It can detect objects, changes in material, and voids and cracks.
GPR systems work by sending a tiny pulse of energy into the ground
from an antenna.An integrated computer records the strength and
time required for the return of reflected signals. Any subsurface
variations, metallic or non-metallic, will cause signals to bounce
back. When this occurs, all detected items are revealed on the
computer screen in real-time as the GPR equipment moves along. In
data processing, detailed examination/interpretation of GPR
sections may be able to identify soils, bedrock, groundwater, etc.
The depth range of GPR is limited by the electrical conductivity of
the ground, the transmitted center frequency and the radiated
power. With respect to radar data interpretation, the degree that
the results is assumed to be true is dependent upon a wide range of
factors such as nature of the sediment body under investigation,
the groundwater regime, the type of terrain immediately adjacent to
the survey line, the nature and appropriateness of any data
processing undertaken, the interpretation techniques employed and
the overall understanding of the researcher with respect to GPR and
their subject background. One of the original and most promising
ground penetrating radars was presented by Moffatt and Puskar
(1976). Their system used an improved antenna that gave a better
target-to-clutter ratio and was able to more accurately detect
important subsurface reflections. The early work using radar was in
glaciology by Plewes and Hubbard (2001) along with civil
engineering, archaeological and geological applications that came
onwards (Daniels, 1996; Conyers and Goodman, 1997; Reynolds, 1997).
Other research using GPR includes fluvial and fluvioglacial (Best
et al., 2003), coastal and aeolian delta (Botha et al., 2003),
peatland (Holden et al., 2002),slopes (Degenhardt and Giardino,
2003), carbonates (Pedley and Hill, 2003), faults, joints and folds
in sediments (Anderson et al., 2003), marble structure (Selma,
2008) and has been successful in delineating gem-bearing zones in
the Himalaya pegmatite mine of the Mesa Grande district of southern
California (Jeffrey et al., 2007). Varied references exist that
cover topics ranging from building GPR units, obtaining GPR data,
processing GPR data, and analyzing GPR data. Some technologies have
emerged in the past ten years that give GPR users better methods of
processing and analyzing the GPR data than were available before.
One of these technologies is the ability to visualize GPR data in
three dimensions, with the ability to add time as a fourth
dimension. Among the first to visualize GPR results in three
dimensions is Birken and Versteeg (2000). More advance and thorough
GPR applications and research is given by Jol, H.M. (2009).
Consider the behavior of a beam of electromagnetic wave (EM) energy
as it strikes an interface, or boundary, between two materials of
different dielectric constants (Figure 4). A portion of the energy
is reflected, and the remainder penetrates through the interface
into the second material. The reflection coefficient at the
interface, (1,2 is given by equation (1),
a) Radar energy traveling outwards from transmitter.
b) Straight ray paths show routes of individual points on the
radar wave Front.
c) Radar energy is reflected (r) at an angle equal to the angle
of incidence (i)
from interfaces with a contrast in electrical properties.
Figure 4. Geometry of GPR signal path through simplified
subsurface.
(1)
where (1 and (2 are the dielectric constants of materials 1 and
2, respectively (Davis and Annan, 1989). Equation 1 indicates that
when a beam of microwave energy strikes the interface between two
materials, the amount of reflection, (1,2 is dictated by the values
of the relative dielectric constants of the two materials. If
material 2 has a larger relative dielectric constant than material
1, then (1,2 would have a negative value; i.e., with the absolute
value indicating the relative strength of the reflected energy and
the negative sign indicating that the polarity of the reflected
energy is the opposite of that arbitrarily set for the incident
energy.
After penetrating the interface and entering into material 2,
the wave propagates through material 2 with a speed, V2, given by
equation (2),
(2)
where C is the propagation speed of EM waves through air, which
is equivalent to the speed of light, or 0.3 m/ns). As the wave
propagates through material 2, its energy is attenuated as
follows:
( = 12.863 x 10-8 f
(3)
where (= attenuation, in decibel/meter, f= wave frequency, in
Hz, and ( = the loss tangent (or dissipation factor) is related to
(, the electrical conductivity (in mho/meter) of the material
by:
tan ( = 1.80 x 10'
(4)
When the remaining microwave energy reaches another interface, a
portion will be reflected back through material 2 as given by
Equation 1. The resulting two-way transit time (t2) of the
microwave energy through material 2 can be expressed as,
t2 =
(5)
where D2 is the thickness of material 2.
Common geophysical reflection data are of four main types:
common offset, common mid (or depth) point, common source and
common receiver. Common offset surveys are most frequently used in
GPR studies, with commercial radar systems consisting of either a
single transmitting and receiving antenna, or two, separate,
transmitting and receiving antennae. In the latter systems, a fixed
spacing is employed between the antennae, typically with both
orientated in the same direction (i.e. copolarised). In
conventional surveys, antennae are perpendicular to the survey
line, with their broad sides orientated towards each other. With
such an antenna configuration the survey is said to be copolarised,
perpendicular broadside. However, other potential configurations do
exist and these may provide important additional information (van
Gestel and Stoffa, 2001; Jol et al., 2002; Lutz et al., 2003).
During surveying, antennae are either dragged along the ground
(Figure 5) and horizontal distances recorded on a time-base, which
can be converted to a distance-base through manual marking, or they
are moved in a stepwise manner at fixed horizontal intervals (the
step size). Step-mode operation generates more coherent and higher
amplitude reflections, as antennae are stationary during data
acquisition. This allows more consistent coupling between antennae
and the ground, with the added benefit of better trace stacking
(Annan and Davis, 1992). As data are recorded during surveying,
horizontally sequential reflection traces build up a radar
reflection profile. Each trace results from the GPR system emitting
a short pulse of high-frequency electromagnetic energy, typically
in the MHz range, that is transmitted into the ground. As the
electromagnetic wave propagates downwards it experiences materials
of differing electrical properties, which alter its velocity. If
velocity changes are abrupt with respect to the dominant radar
wavelength, some energy is reflected back to the surface. The
reflected signal is detected by the receiving antenna. In systems
with a single antenna, it switches rapidly from transmission to
reception. The time between transmission, reflection and reception
is referred to as two-way travel time (TWT) and is measured in
nanoseconds (10- 9 s). Reflector TWT is a function of its depth,
the antenna spacing (in systems with two antennae), and the average
radar-wave velocity in the overlying material. Reflections from
subsurface discontinuities are not the only signals recorded on a
radar trace. The first pulse to arrive is the airwave, which
travels from transmit antenna to receive antenna at the speed of
light (0.2998 m ns-1). The second arrival is the ground wave, which
travels directly through the ground between the transmit and
receive antennae. The air and ground waves mask any primary
reflections in the upper part of a radar reflection profile.
Lateral waves can also be present and result from shallow
reflections that approach the surface at the appropriate critical
angle and are subsequently refracted along the airground interface
(Clough, 1976). It should be noted that reflections associated with
lateral waves are not correctly placed in time (depth) with respect
to the interface that generated them. Pseudo-3-D surveys involve
collecting data on regular or irregular survey grids, usually in
two mutually perpendicular directions, and often display results in
fence diagrams (for example, Russell et al., 2001; Holden et al.,
2002; Skelly et al., 2003). In true 3-D surveys, transect lines are
so closely spaced that data for individual traces overlap. 3-D data
cubes can be generated from these surveys (Nitsche et al., 2002;
Heinz and Aigner, 2003). Collecting true 3-D data is particularly
time consuming, largely because of time required to accurately
record the position and elevation of data points. Lehmann and Green
(1999) attempted to overcome this problem by developing a semi
automated system that records coordinates during radar data
collection using a self-tracking laser theodolite. Other
experiments have combined the use of GPR with Global Positioning
Systems (GPS) (e.g. Urbini et al., 2001; Freeland et al., 2002).
Jol and Bristow (2003) consider other practical difficulties in
performing GPR field surveys.
Figure 5. A GPR cart (A) and hand-towed GPR (B) being used on
research.
3.0 PRELIMINARY TEST
A preliminary study has been carried out with cooperation from
Non Destructive Testing (NDT), Technology Industry Section of
Nuclear Agency of Malaysia (MINT), Bangi, with help from Dr. Mohd.
Azmi Ismail and Amry Amin Abas to used their available RAMAX CUII
GPR unit and test it upon their own test site with the size of 14 m
x 6 m, which include buried metal pipe with 6 inch in diameter
(Figure 6 and Figure 7). The test included detecting 4 metal pipe,
buried 2 meter deep, with same diameter, same depth but different
spacing (Figure 8); same diameter, same spacing but different depth
(Figure 9); same spacing and same depth but different diameter
(Figure 10). The test site was excavated at about 2.5 m depth, and
filled back with sand and gravel, with the metal pipe placed
inside, suitable for a 3 single line survey with GPR.
Figure 6. The drawing of the test site, 14 m x 6 m wide, with 2
m length in between the metal pipe.
3.1Survey Procedure
RAMAC/GPR made by MALA Geoscience, Sweden with 250 MHz shielded
antenna was used during the survey. MINT also purchases 150 MHz,
400 MHz, 800 MHz and 1 GHz shielded antenna (Figure 11).The survey
was carried out in the MINT test site. The purpose of setting up
the test site is eventually to test out the GPR unit, and trying to
configure the best practice for detecting buried utilities such as
the metal pipe for simple parameters like with differences in their
diameter, their buried depth and their spacing, and also to learn
the operating
procedures of a GPR survey. There were three (3) areas scanned
namely L1 single line survey, consist of 4 buried metal pipe at
depth of 2 meter, with the same in diameter but different spacing,
starting from 0.25 meter between pipe A and pipe B, 0.5 meter
between pipe B and pipe C and 1.0 meter between pipe C and pipe D
(Figure 8). Secondly is L2 single line survey consist the same size
of metal pipe with the same spacing interval of 1.0 meter, but with
different depth starting with pipe A buried 2.0 meter, pipe B with
1.5 meter, pipe C with 1.0 meter and pipe D with 0.5 meter from the
surface (Figure 9). Lastly is L3 single line survey consist of 4
metal pipe with the same spacing interval of 1 meter and depth of 2
meter from the surface but with different size in diameter,
starting from pipe A with 1 inch, pipe B with 2.5 inch, pipe C with
4.0 inch and pipe D with 6.0 inch (Figure 10). Scanning was done
along a single line survey, on top of the buried metal pipe (Figure
12). The line survey consist of scanning lines that are of the same
length and has parallel starting points. The GPR cart was pushed
along the single line survey, with step size spacing. The radargram
window being adjusted to maximum of 4 meter depth time window and
the distance of 6 meter. Real time data adjustment including the
Automatic Gain Control (AGC) and time gain control is applied.
3.2Radar Data Processing
For a normal radar data processing is confronted by three main
tasks (Yilmaz, 1987):
(1) selecting an appropriate sequence of processing steps;
(2) choosing an appropriate set of parameters for each
processing step;
(3) evaluating output resulting from each processing step and
identifying problems caused
by incorrect parameter selection.
Yilmaz (1987) demonstrates how different processors can produce
significantly different end products from the same initial data
set, because of different decisions made. Fisher et al. (1992) and
Greaves et al. (1996) demonstrate this point very well with respect
to radar, with their different approaches to the processing of the
same multi-offset data. A processors ability to make the right
choices is often as important as effectiveness of the processing
algorithms in determining final image quality. Processing,
therefore, cannot be entirely objective, with some considering it
more of an art than a science (Yilmaz, 1987). A wide range of
options are available and processors is chosen depending upon
algorithms available, objectives of the study, and their experience
and ability, meaning that accurate records of all processing steps
performed should be maintained.
3.3Data Interpretation
Soon after the realisation that GPR could provide useful data
for various subsurfaces investigation, various authors suggested
that the principles of seismic stratigraphy could be applied to the
interpretation of radar reflection profiles (Baker, 1991; Beres and
Haeni, 1991; Jol and Smith, 1991). Jol and Smith (1991) first used
the term radar stratigraphy for this interpretation technique,
although Gawthorpe et al. (1993) were the first to fully define the
concept and its relationship to seismic stratigraphy Consequently,
it is recommended that radar facies reflection configurations are
described in terms of the: (1) shape of reflections; (2) dip of
reflections; (3) relationship between reflections and (4)
reflection continuity. A diagram (Figure 13) and table (Table 1)
shown to simplify all the basic processes needed immediately when
practicing GPR survey and for various purposes of
investigation.
Table 1. Basic description of the steps in Figure 6.
Editing
Removal and correction of bad/poor data and sorting of data
files.
Rubber-banding
Correction of data to ensure spatially uniform increments.
Dewow
Correction of low-frequency and DC bias in data.
Time-zero correction
Correction of start time to match with surface position.
Filtering
1D & 2D filtering to improve signal to noise ratio and
visual quality.
Deconvolution
Contraction of signal wavelets to spikes to enhance reflection
events.
Velocity analysis
Determining GPR wave velocities.
Elevation correction
Correcting for the effects of topography.
Migration
Corrections for the effect of survey geometry and spatial
distribution.
Depth conversion
Conversion of two-way travel times into depths.
Display gains
Selection of appropriate gains for data display and
interpretation.
Image analysis
Using pattern or feature recognition tools.
Attribute analysis
Attributing signal parameters or functions to identifiable
features.
Modelling analysis
Simulation of GPR responces.
Figure 13. GPR data processing flow and basic analysis steps
(Nigel, 2009).
4.0RESULT AND DISCUSSION
The GPR radargrams is shown in respective figures below. The red
circle indicates the metal pipe buried below the surface, marked
after calculation and estimation from their original placement on
the test site, compared to the radargram that was acquired during
the survey. The steps taken during survey is time gain adjustment
and filtering. Nothing can be done for post-processing for the data
acquired as MINT does not have the processing software. The simple
interpretation step taken is similar to seismic data
interpretation, where the reflection pattern in the form of
hyperbola is targeted and the peak of the hyperbola represent the
centre of the target. For L1 single line survey (Figure 14), shows
a blur hyperbola, and cannot accurately determine the location of
the target. For pipe A and pipe B, there seems to be a merge of its
reflections and if not calculated for its known depth and position,
it is hard to tell their position by just relying on the radargram.
The spacing between Pipe A and Pipe B is 0.25 m, therefore it is
presume that this distance is too close to be detected, although
their diameter is still the same. For pipe D, nothing can be seen
to show that it exist, where it supposed to be no problem in
detecting it apart from Pipe C with spacing of 1.0 m. There seem to
be a large disturbance from the air wave at depth of 0.5 m.
Figure 14. Radargram for L1 single line survey.
For L2 single line survey (Figure 15), the radargram
successfully display the hyperbola of each buried pipe, but still
not strong enough for first glance interpretation. The pipe with
same size of 6 inch can be detected easily, but for pipe A which is
buried at 2 m, and pipe D buried at 0.5 m, the radargram shows
little significant trace, as their hyperbola did not show a strong
contrast with the background. Each pipe is located at the centre of
the peak of the hyperbola shown on the radargram, but still there
is a disturbance from ground wave interference as shown at pipe A
and pipe B at about 0.5 m from the surface. The excellent example
of good radar data display is Pipe C (buried 1.0 m),where the
hyperbola and its peak really represent the real depth of the pipe.
The result is quiet acceptable, and the radargram display a pattern
of all the four (4) metal pipe buried.
Figure 15. Radargram for L2 single line survey.
For L3 single line survey (Figure 16), it seems that the
radargram shows nothing at all except for pipe D, but still it is a
week reflection. This is due to the fact that not all investigation
can be successful using a single frequency antenna, like this
survey where only 250 MHz shielded antenna were used, because of
time constraint. The failure also occurred for detecting the metal
pipe with different spacing, as shown in Figure 15, maybe caused by
the spacing between pipe A and pipe B that was within 0.25 meter in
spacing, so there seem to be some interference between the
reflections from both of it, and causing a blur image of two
hyperbola merging together. Comparison between the three (3) test
parameters and their significant result is shown in Table 2.
Figure 16. Radargram for L3 single line survey.
Table 2. Comparison of the metal pipe real depth buried and the
depth from the radargram from the three (3) test survey.
Line Number 1 Survey
Actual Depth, m
Depth From Radargram, m
Differences, m (Accuracy, %)
Pipe A
2.0
1.8
0.2 (90%)
Pipe B
2.0
1.7
0.3 (85%)
Pipe C
2.0
2.2
-0.2 (90%)
Pipe D
2.0
Not Confirmable
Nil
Line Number 2 Survey
Actual Depth, m
Depth From Radargram, m
Differences, m (Accuracy, %)
Pipe A
2.0
1.8
0.2 (90%)
Pipe B
1.5
1.4
0.1 (95%)
Pipe C
1.0
1.0
0.0 (100%)
Pipe D
0.5
0.4
0.1 (98%)
Line Number 3 Survey
Actual Depth, m
Depth From Radargram, m
Differences, m (Accuracy, %)
Pipe A
2.0
Not Confirmable
Nil
Pipe B
2.0
Not Confirmable
Nil
Pipe C
2.0
Not Confirmable
Nil
Pipe D
2.0
2.5
-0.5 (75%)
Table 2 shows that by using the 250 MHz antennae, it can detect
6 inch metal pipe with different depth ranging from 0.5 meter to
2.0 meter below ground with much accuracy as in Line Number 2
result column (average 90% - 100% accuracy). For the metal pipe in
Line Number 1 column, clearly stated that the antenna frequency of
250 MHz cannot fully detect the 6 inch metal pipe with different
spacing from each other. The closer the metal pipe is placed with
each other, the more inaccurate the hyperbola became. As for the
Line Number 3 result that included the same depth of 2.0 meter
below ground but different diameter, shows that all sizes smaller
than 6 inch in diameter are failed to be detected by the GPR with
250 MHz antenna.
5.0CONCLUSION AND FUTURE RECOMMENDATION
Using GPR wisely, it is possible to image the two and three
dimensional structures of a range of subsurface structures whether
be it metal pipe or sedimentary rocks in example. It is considered
that to extract the maximum amount of meaningful information, the
user must understand the scientific principles that underlie the
technique, the effects of the data collection regime employed, the
implications of the techniques finite resolution and depth of
penetration, the nature and causes of reflections unrelated to
primary sedimentary structure, and the appropriateness of each
processing step with respect to the overall aim of the study.
However, in order to do this accurately, many of the inherent
limitations to the field data must be acknowledged and where
possible overcome by careful and systematic data processing. More
advanced data processing, such as migration, which is essential to
obtain correctly positioned subsurface reflections, is only just
beginning to be performed on a regular basis by GPR researchers.
Supplementary information, such as geological context, the
relationship between the various radar surfaces and facies, data
from ground from subsequent laboratory analyses, can then be used
in conjunction with the radar data for more accurate
interpretation. Less robust interpretation techniques such as radar
facies analysis, which does not define the radar surfaces that
bound the facies or the resulting radar packages, should be
abandoned as a primary interpretive tool, except in very specific
instances. In order to apply radar data successfully, an
interpreter must have a thorough understanding of a wide and
complex range of factors, including: the scientific principles that
underlie the GPR technique, the effects of the data collection
configuration used, the effects of survey-line topographic
variation, the effects of the techniques finite resolution (both
vertical and horizontal) and depth of penetration, the causes of
reflections unrelated to primary depositional structure, and the
nature and appropriateness of each processing step undertaken. Data
processing should aim to provide, within the limitations of the
field data and processing routines employed, an accurate record of
the subsurface location and orientation of reflections caused by
primary sedimentary structure.
ACKNOWLEDGEMENT
The authors acknowledge the support and assistance from Nuclear
Agency of Malaysia (MINT) for the used of their GPR equipment and
technical advise on the processing of the GPR data for this
research.
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AUTHORS
Awangku Iswandy Bin Awangku Serma is a part-time master student
in Geomatic Engineering supervised by Prof. Dr. Halim Setan.
Currently working as a geologist in Mineral and Geoscience
Department of Malaysia.
Prof. Dr. Halim Setan is a lecturer with a Ph.D from City
University, London. He has received recognition in his work as a
researcher by winning several awards such as The Best Researcher
Award, UTM, 2006 and The Best Article Award from Institution of
Surveyors Malaysia, 2006.
B
A
Data Acquisition
Post Collection
Editing
Rubber-Banding
Dewow
Time Zero Correction
Filtering
Deconvolution
Velocity Analysis
Elevation Correction
Migration
Depth Conversion
Data Display and Gains
Image analysis
Attribute analysis
Modelling analysis
At Site
(commonly automated)
Editing Simple Filtering
Data Analysis & Gain
CMP Data
Topography Data
Interpretation
0
4
3
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3
2
1
2
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ISSN 1511-9491 2009 FKSG
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1.0 m spacing0.5 m spacing0.25 m spacing6 inch metal pipe
Figure 7. Picture of the test site for GPR testing in MINT,
Figure 8. Testing and detection for metal pipe with same
Bangi. diameter, same depth but different spacing, L1 single
line
survey.
L2 Single Line Survey0.5 m deep1.0 m deep1.5 m deep2.0 m deepL3
Single Line Survey1.0 inch pipe2.5 inch pipe4.0 inch pipe6.0 inch
pipe
Figure 9. Testing and detection for metal pipe with same Figure
10. Testing and detection for metal pipe with same
diameter, same spacing but different depth, L2 single line
depth, same spacing but different diameter, L3 single line
survey. survey.
L1 Single Line Survey800 MHz400 MHz150 MHz1 GHz250 MHz
Figure 11. Shown here is the RAMAC 5 shielded antenna Figure 12.
The single line survey being conducted by Dr.
with their metal casing. Note that the lower the frequency, Azmi
(red shirt) and Amry Amin, from NDT Group, MINT.
the bigger the size.
_1315809417.unknown
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