Ground Penetrating Radar (Gpr) for Subsurface

Geoinformation Science Journal, Vol. 9, No. 2, 2009, pp: 45-62

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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


1.0 m deep

0.5 m deep

1.5 m deep

2.0 m deep


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


56


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.


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


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%)


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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62

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.


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.


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.


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.


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


Actual Depth, m



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%)


Actual Depth, m



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

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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

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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.

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Documents

Ground Penetrating Radar (Gpr) for Subsurface