corrosion imaging and thickness determination using … · in direct radiography, the images of...

Research in Nondestructive Evaluation, 26: 43–59, 2015Copyright © American Society for Nondestructive TestingISSN: 0934-9847 print/1432-2110 onlineDOI: 10.1080/09349847.2014.967899

CORROSION IMAGING AND THICKNESS DETERMINATION USINGMICRO-CURIE RADIATION SOURCES BASED ON GAMMA-RAYBACKSCATTERING: EXPERIMENTS AND MCNP SIMULATION

Samir Abdul-Majid, Ahmed Balamesh, Dheya Al Othmany, Ahmed Alassiaa,and Hussein Al-Huraibi

Faculty of Engineering, King Abdulaziz University, Jeddah, Saudi Arabia

Gamma radiography is used to monitor the corrosion of pipelines in remote locations; usu-ally high radioactivity (1011–1012 Bq) is used. The technique is also not useful for imagingpipes with thick walls or large vessel walls. In this work, Compton backscattered radia-tion was used for the wall-thickness determination and corrosion imaging of pipe and flatmaterials using extremely-low-activity sources with radioactivities on the order of 104–105

Bq. A two-dimensional scanning system was designed to scan object surfaces, and the sig-nals from a NaI(Tl) scintillation detector were fed into a computer for image constructionusing the LabView program. Thicknesses greater than 1 cm and 1.5 cm could be measuredfor Fe and Al and for polyvinyl chloride (PVC) and poly methyl methacrylate (PMMA),respectively. It was also possible to detect changes of less than 1 mm in depression depthfor depressions measuring 3 mm in diameter. One- and two-dimensional images artificialdefects on a pipe surface were successfully constructed.

Keywords: corrosion imaging, MCNP, thickness determination

1. INTRODUCTION

Gamma-ray nondestructive industrial radiography is a technique usedworld-wide for the inspection of pipelines that extend for miles in remotelocations where electricity is not available. Pipelines corrosion and erosionrepresent the most prevalent failure mechanism, with pipelines wall thicknessbeing one of the most important parameters to measure. When gamma directradiography is used for imaging large-diameter pipes, especially those usedfor transporting petroleum or liquid gas, sources with high radioactivities of1.85 x 1011–3.7 x 1012 Bq (5–100 Ci) are used.

Radiation exposure from industrial radiography is reviewed by manyinvestigators in several countries [1–5]. Several industrial radiography radi-ation accidents have been reported [6–9]. Many international and nationalreports have been published on radiation safety and accidents with respectto industrial radiography [10–12]. Accidents are caused by the loss or theftof gamma-ray sources or by negligence that leads to high overexposure oreven death.

Address correspondence to Samir Abdul-Majid, Faculty of Engineering, King Abdulaziz University,P. O. Box 80204, Jeddah 21589, Saudi Arabia. E-mail: [email protected]

Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/urnd.

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44 S. ABDUL-MAJID ET AL.

Despite the high radioactivity used in industrial radiography, the tech-nique fails to produce images of large-diameter pipes. Most of the radiationemitted from the source is absorbed by the liquid contained in pipes or thepipe wall, and a small or negligible fraction reaches the other side of the pipe,where the radiographic film or image plate is located. In direct radiography,the images of both sides of a pipe overlap, making image interpretation diffi-cult. Moreover, the technique does not function if no access to the other sideof the pipe is available or if the object is a vessel or tank.

In the last decade, tangential radiography technique was developed [13–15]. In this technique, only part of the pipe wall through which gamma- orX-ray radiation passes tangentially is imaged; radiation that passes throughthe liquid within the pipe is ignored. In large-diameter pipes or pipes withthick walls, the radiation path through the walls is large, and attenuation ishigh. Small defects in pipe walls can be missed. Several rounds of exposureare needed to image same section of a pipe. The risk associated with thistechnique is similar to that experienced with direct radiography because high-radiation-level sources are also used.

In this work, we used Compton backscattered radiation for the wall-thickness determination and corrosion imaging of a large pipe usingextremely-low-activity sources with radioactivities on the order of 104–105

Bq (few micro-Curies). This level of radioactivity is close to the exemptionlevel for sources, and it is 107–108 times lower than the activity used in con-ventional gamma-ray industrial radiography. Accordingly, the radiation riskassociated with this imaging technique is negligible. The detector and radia-tion source are located on the same side of the pipe, not on opposite sides,as in conventional radiography. Thus, the method functions without requiringaccess to the other side of the object of interest. In addition, the method iseffective for imaging corrosion in large vessel walls as well as in areas wheredirect radiography or tangential radiography fail. Little or no attenuation ofradiation occurs in the liquid contained in pipes. Recently, Abdul-Majid andBalamesh [16,17] used a collimated beam to image the corrosion under theinsulation of an industrial pipe by the gamma-ray backscattering methodusing a 1.85 x 108-Bq (5-mCi) radioactive source. The same authors imagedsteel metal plates containing depressions of various diameters and at variousdepths using micro-Curie 137Cs source [17]. In this work, further experi-mental studies are presented on spatial resolution, inspecting materials otherthan steel and use 60Co a higher energy radioactive source in addition to137Cs. Theoretical studied were also presented by using General Monte CarloN-Particle Transport Code Version 5 (MCNP-5) calculations.

A sketch of the system used for pipe-wall imaging and thickness determi-nation in this work is shown in Fig. 1. A 4.8 x 105-Bq (13-μCi) 137Cs sourceor a 1.1 x 104-Bq (0.3-μCi) 60Co source is attached to the base of a 2” × 2”NaI(Tl) scintillation detector. During imaging or thickness measurement, thesource and detector are placed very close to the object to be inspected. Partof the incident radiation scatters back to the detector; the radiation generated

CORROSION IMAGING OF PIPE AND FLAT MATERIALS 45

Scattered

Incident

Incident

Pipe wall

NaI(Tl) detector

Source

FIGURE 1. Sketch of gamma-ray backscattering using a point source.

by the Compton interaction is lower in energy than the primary radiation. Thedetector measures both the direct and scattered radiation. The Compton scat-tered gamma-ray energy E’ following the incidence of a gamma ray of energyE is given by the following equation:

E′ = EEe/(E(1 − cosθ) + Ee),

where Ee is the rest mass of an electron (0.511 MeV) and θ is the scatter-ing angle. For a scattering angle of 180◦, E’ is 0.184 MeV for 137Cs and aprimary gamma ray with an energy of 0.662 MeV and 0.212 MeV for 60Coand primary gamma rays with energies of 1.172 MeV and 1.332 MeV. Themeasured spectrum when 137Cs is used is shown in Fig. 2. Both the primary

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200

Channels

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Counts Window

Primary 662 keV

Backscattered 184 keV

FIGURE 2. Gamma-ray spectrum.


and scattered radiation peaks are clear. As shown, a window can be selectedto measure the backscattered radiation only and most of undesired radia-tion is rejected improving signal-to-noise ratio. The amount of backscatteredradiation is proportional to the pipe-wall thickness. For a given primaryradiation source, the amount of scattered radiation increases with thicknessbefore count saturation is achieved. For Fe, count saturation is observed at athickness of approximately 1–1.5 cm when 137Cs is used [18,19]. Differentsaturation thicknesses have been reported for different sources [20,21]. Formaterials of higher atomic number, high-energy radiation must be appliedand vice versa. Details regarding backscattering interactions and analyticaltreatments can be found in the literature [22–25].

In most applications of gamma backscattering for thickness determinationfound in the literature [19–21], both the primary and scattered radiation arecollimated. This configuration makes the detection device heavy and requiresthe use of higher-activity sources. The smallest activity reported is 30 mCi for203Hg (279 keV) in a system that weighs 7.27 kg (16 lb) [21]. The system has arelatively light weight because of the lower energy used, which requires lessshielding, but a smaller thickness range is covered. In this work, no shieldor collimators were used; the only weight was that of the detector and alaptop computer. In other applications [26,27], X-ray machines are used. X-ray machines are generally heavy, and their emitted radiation energies arebelow those emitted from radioactive sources, which limits their applicationsto thin-wall pipes. Moreover, X-ray machines provide a continuous radiationspectrum and no well-defined backscatter peak. Harding and Harding [23]described a commercially available backscatter imaging device, ComScan,which utilizes an X-ray machine that operates at a maximum voltage of160 kV with a mean X-ray energy of approximately 60 keV. The machinecan be useful for imaging low-atomic-number materials such as aluminumbut is not useful for imaging materials such as iron.

Other applications of Compton backscattered radiation include imagingweapons hidden under clothing [28], landmines [29], historical objects [23],explosives [24], and biomedical and industrial objects [25]. In these variousapplications, different methodologies are used. In point-by-point imaging, anarrow, collimated beam is focused on a point on the surface of the objectof interest, and scattered radiation at a fixed angle is measured by a singledetector. In line-by-line imaging, a slit beam is incident on the surface ofthe object, and scattered radiation is measured by a linear detector array.In these methods, heavy collimators and a large-activity source or a largenumber of detectors are used. In plane-by-plane imaging, a broad beam isincident on the object surface, and the scattered radiation passes through apinhole in an absorber and then falls onto two-dimensional film. For cor-rosion measurements, which usually require high-energy radiation, a thickabsorber is needed to stop the primary radiation [23,24]. Monte Carlo sim-ulation of nondestructive testing has been discussed by Shengli et al. [30],and the implementation of this simulation technique for medical imaging


has been discussed by Driol et al. [31]. The technique used in this work isdifferent from the above-mentioned techniques in that both the primary andscattered radiation are measured by the same detector, which results in a verylight weight and very-low-activity system.

2. MATERIALS AND METHODS

2.1. Detection System

The detector used was a 2” × 2” NaI(Tl) model NAIS-2x2 scintilla-tion detector. The detector was connected to a fully integrated multichannelanalyzer (MCA) tube base containing a high-voltage power supply and pre-amplifier and is commercially known as “Osprey.” The signal was fed intoa computer-based Gene Software Multichannel Analyzer. The detector andnuclear electronics components were all supplied by Canberra Industries,USA. The energy window was selected to only measure the counts of single-scattered radiation, as shown in Fig. 1. All other counts were rejected. Thewell-defined backscattered radiation peak on the multichannel analyzer andthe selection of a well-defined window offer a clear advantage over thewidely distributed backscattered radiation produced when X-ray machinesare used.

2.2. Imaging System

For imaging purposes, counts under the backscattered radiation peak win-dow were recorded. A scanning system, consisting of a platform capable ofindependent movement in both the horizontal and vertical directions, wasdeveloped. The platform carries the detector and source only, and it is drivenby two smart motors controlled by LabView drivers through a serial port. TheMCA (Osprey) is interfaced to a personal computer through an Ethernet con-nection using LabView. A C-language interface was developed to allow forcommunication between the Osprey C Lynx library and LabView. A LabViewprogram (containing several VIs) was developed to acquire the counts fromthe MCA and form an image from the backscatter counts.

2.3. Samples

Step wedges were used for thickness measurements and imaging. Theused iron and aluminum step wedges were each 100 mm long and 50 mmwide and composed of five steps. In addition, five-step polyvinyl chloride(PVC) and poly methyl methacrylate (PMMA) step wedges, each 200 mm longand 70 mm wide, were used. These materials are widely used in industry tofabricate pipes or sheet metals.


TABLE 1 Wall thickness W, depression depth d, and depression diameter D for differentpipe sections; all dimensions are in mm

Section Wall thickness (W) Depression depth (d) Depression diameters (D)

1 11 5.5 10, 5, 32 9 4.5 10, 5, 33 7 3.5 10, 5, 34 5 2.5 10, 5, 35 3 1.5 10, 5, 3

A 400-mm-long iron pipe with a 197-mm internal diameter was used.Each 80 mm of the pipe wall was machined to the same thickness suchthat there were a total of five thicknesses that differed from each other by2 mm. In each thickness W, three depressions of equal depth d and differentdiameter D were made. The thickness and depression dimensions are givenin Table 1.

Imaging was also conducted on a carbon steel pipe (measuring 280 mmin outside diameter, 9 mm in wall thickness, and 1,250 mm in length) withartificial defects created on its outer surface. These pipe dimensions are rep-resentative of those normally used in many industrial plants. The defect wasnearly circular and approximately 70 mm in diameter with a nonconstantdepth that was deepest at the center at approximately 3 mm (Fig. 3). A Perspexcylindrical pipe with a diameter of 260 mm that was filled with water wasinserted inside the steel pipe for water-filled-pipe measurements.

2.4. MCNP calculations

Backscattered radiation and spectrum calculations were performed usingthe multipurpose MCNP-5 code [32].

FIGURE 3. Artificial defect on pipe surface.


3. RESULTS

3.1. Step-Wedge-Thickness Measurements and Imaging

Using the setup shown in Fig. 1, it can be observed that radiationbackscatters from a region in the pipe around the point source. The inten-sity of the backscattered radiation coming from locations far from the sourceshould be small because of the attenuation of both incident as well asscattered radiation within the object. The region of interaction was studiedexperimentally and by MCNP calculations by using square Fe objects ofdifferent areas and thicknesses. Figure 4 shows the counts of backscatteredradiation obtained over a 30-s period using a 137Cs source for four materials.As the square object increases in thickness, more scattered radiation reachesthe detector, resulting in more counts. The increasing trends of the counts aresimilar, but as the object becomes thicker, a greater degree of interaction withthe object materials takes place, and accordingly, more scattering of radia-tion occurs. The counts are expected to approach saturation for larger objectareas. The MCNP calculated spectrum up to 0.7 MeV is shown in Fig. 5. Thesimilarity between the calculated and measured spectra (Fig. 2) is clear.

The results of thickness evaluation by gamma backscattered radiationfor the different materials using 137Cs are presented in Fig. 6 based on

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1 1.5 2 2.5 3

Co

un

t p

er 3

0 s

ec

Square side (cm)

Experiment thickness 5mm Experiment thickness 10 mm Experiment thickness 15 mm

Experiment thickness 20 mm MCNP Thickness 5 mm MCNP Thickness 10 mm

MCNP Thickness 15 mm MCNP Thickness 20 mm

FIGURE 4. Measured and MCNP calculated backscatter counts from plates of different sizes for fourmaterials.


FIGURE 5. MCNP calculated spectrum with backscatter window.

measurements and by MCNP calculations using the step wedges. The fig-ure indicates that there is good agreement between the results yielded thetwo methods. The iron counts were higher because of the higher densityand atomic number of the element. The counts increase almost linearly upto approximately 1 cm and then approach saturation near approximately1.5 cm. As the thickness increases, the incident as well as scattered radia-tion undergo more attenuation in the object, and the backscattered radiationundergoes a greater extent of self-absorption before reaching the detector.The other materials displayed less backscattered radiation. The counts con-tinued to increase almost linearly with thickness for the PVC and PMMAmaterials. Al displayed behavior intermediate to that exhibited by the iron andPVC.

Using the scanner and LabView program, images of step wedges wereobtained using two methods. First, the system response at the midpoint of


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Backscattering of Cs-137 with Iron, Al, PVC, and PMMA

Experimental Iron MCNP Iron

Experimental Aluminum MCNP Aluminum

Experimental PVC MCNP PVC

Experimental PMMA MCNP PMMA

FIGURE 6. Measured and MCNP calculated counts versus thicknesses using step wedges made of fourmaterials.

each thickness of the step wedge was recorded, and an image was con-structed. Second, each step wedge was scanned, starting from the thinner partcontinuously in 1-mm increments. Images of the Fe and PVC step wedgesobtained using 137Cs and of the Fe step wedges obtained using 60Co areshown in Fig. 7 for a counting time of 30 s per step. The darker region inthe grey scale indicates lower counts and a thinner wall. In the left, theimage darkness decreases with the increase in each step thickness, exceptfor the last thickness, where it increases. These patterns occurred becausethe effective interaction region was larger than the width of the step (Fig. 4).As observed in the right images, when the scanner was set to move contin-uously, the system could not differentiate each thickness separately, againbecause the gamma interaction region was wide. Darker lines appearedat the bottom of the image because the detector was nearing the edge of


a) b)

c) d)

e) f)

FIGURE 7. Images of step wedges for a 30-s counting time per step: a) Fe one step per thickness countswith Cs137, b) Fe 1-mm steps with continuous scanning with Cs137, c) PVC one step per thickness countswith Cs137, d) PVC 1-mm steps with continuous scanning with Cs137, e) Fe one step per thickness countswith 60Co, and f) Fe 1-mm steps with continuous scanning with 60Co.

the step wedge. In the last step, there was not enough material to providebackscattering. The PVC image is less clear than the Fe image because ofless interaction with the object material atoms. Visibility could be improvedby extending the collection time or source strength. In the Fe step wedgeimage using 60Co, visibility was not as good as that of 137Cs because ofthe smaller activity used. Again, improvement could be achieved with alonger collection time or stronger source. The main advantage of 60Co over137Cs is the higher penetration of gamma rays makes it more useful forthick wall measurements. Similar images were obtained for Al and PMMAmaterials.


3.2. System Sensitivity to Defect Size

The system sensitivity to defect size was examined by placing the detectorin contact with the pipe surface at the depression depths indicated in Table 1,and counts over a period of 300 s (5 min) were obtained. Figures 8 and 9display the backscattered radiation recorded at different wall thicknesses,depression depths, and diameters. In Fig. 8, depressions of the same diam-eter are grouped, whereas in Fig. 9, those of the same depth are grouped.Clearly, the counts decrease with an increase in defect diameter or depth.For the smallest counts of 106 in Fig. 8, the theoretical statistical standarddeviation is 103. This value is much smaller than the difference in counts of200,000 between one depression depth and another having the same diam-eter but differing by only 1 mm in depth. As shown in Fig. 9, for depths of3.5 mm and above, it was possible to distinguish depressions of the samedepth that differed by approximately 2 mm in diameter. Higher accuracy canbe reached by increasing the collection time.

It can be concluded that the detection limit is less than 1 mm in depthfor depressions measuring at least 3 mm in diameter. Below a diameter ofapproximately 3 mm and for depressions measuring approximately 2.5 mmin depth, it becomes difficult to detect changes in depression size unless thecounting time is significantly increased.

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W = wall thickness, d = depth, D = diameter

(mm)

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FIGURE 8. Counts for 300 s of backscattered radiation using 137Cs for depressions of different diametersand depths; each group of columns has the same diameter.


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(mm)

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FIGURE 9. Counts for 300 s of backscattered radiation using 137Cs for depressions of different diametersand depths; each group of columns has the same depth.

3.3. Imaging Pipe Defects

The defect on the pipe surface shown in Fig. 3 was imaged by scanning170 mm in one dimension across the middle in 1-mm steps when the pipewas empty and when it was filled with water. Images obtained using 137Cswith collection time steps of 1 s, 10 s, 30 s, and 60 s and images obtainedusing 60Co with a 30-s step are shown in Fig. 10. The images generallyimproved significantly between collections times of 1 s and 10 s per stepfor 137Cs. The total scanning time at 1 s per step was less than 3 minutes.Using 60Co, a lower-activity source, the image darkness was reduced, evenwhen compared with an image taken over a collection time of 10 s or 30 swhen using 137Cs. The effect of water inside the pipe on defect imaging wasinsignificant, which represents a major advantage of this technique comparedwith direct radiography, in which most of the radiation is absorbed by theliquid contained in pipes before reaching the image receptor, especially inlarge pipes.

Two-dimensional scanning was performed over the same defect to pro-duce two-dimensional images. The scanning area was 120 x 80 mm, and thedetector moved in 2-mm steps with collection times of 10 s per step. Theimage generated using 137Cs is shown in Fig. 11.

4. DISCUSSION AND CONCLUSIONS

This study clearly demonstrates the feasibility of integrating very-low-activity gamma sources with a backscattering method for pipe-wall-thickness


a) 1 s, no water, 137

Cs b) 1 s, with water, 137

Cs

c) 10 s, no water, 137

Cs d) 10 s, with water, 137

Cs

e) 30 s, no water, 137

Cs f) 30 s, with water, 137

Cs

g) 60 s, no water, 137

Cs h) 60 s, with water, 137

Cs

i) 30 s, no water, 60

Co j) 30 s, with water, 60

Co

FIGURE 10. Linear scanning of 170 mm across a pipe defect in 1-mm steps.

determination and imaging. The system described depends on single-photoncounting. Accordingly, the system is much more sensitive than radiographicfilms or image plates, and it is also much safer to use. The system was testedwith four materials widely used in industry.

As shown in Fig. 6, the theoretical statistical standard deviation for mostcounts was 200–400 for a 30-s counting time. Two or three standard deviationis still much less than the difference in counts obtained from two thicknessesthat differ by 1 mm. Accordingly, changes of less than 1 mm in thicknesscan be detected. The recorded sensitivity was 320 counts/mm/s. There werealways background counts at zero thickness, which is attributed to Comptonscattered radiation within the detector material and casing.

The standard deviation of net counts can be computed from the followingequation [33,34]:

σ = √(σg

2 + σb2),

where σ g and σ b are the standard deviation of gross counts and backgroundcounts, respectively. The Background counts are those the system gives with-out a pipe (thickness equal to zero). From Fig. 6, taking a middle thicknessvalue of 5 mm, the coefficient of variation is about 0.5%.


a) 137Cs without water b) 137Cs with water

c) 60Co without water d) 60Co with water

FIGURE 11. 137Cs two-dimensional image of defect for 120 x 80 mm scanning area for 2 mm and 10-ssteps: a) for 137Cs without water inside, b) for 137Cs with water inside, c) for 60Co without water inside,and d) for 60Co with water inside.

For Fe, it can be concluded that the measurable thickness range of thedetection system is at least 1 cm. The system exhibited a slightly wider thick-ness range for aluminum, whereas the PVC and PMMA counts increasedalmost linearly for thickness greater than 1.5 cm. The thicknesses that can bemeasured depend mainly on the gamma-ray energy and the material atomicnumber and density. In low-atomic-number and low-density materials, thelinear attenuation coefficient [35,36] is smaller, and incident radiation canpenetrate more deeply; the level of self-absorption of backscattered radiationsis also lower. In addition, the attenuation of incident and scattered radiationis reduced when higher energy is used.

The radiation doses from the sources used in this study are negligible. Thespecific gamma-ray dose constants for 137Cs and 60Co are 1.032 x 10−4 and3.703 x 10−4 (mSv/h)/MBq at 1 m, respectively [37]. A 1-μCi (3.7 x 104 Bq)137Cs source and a 0.3-μCi (1.1 x 104 Bq) 60Co source yield approximately


4 μSv/h at 1 m. At 4 hours of exposure per week near the source withoutany shield, a radiation worker will receive 1 mSv/y, which is within the pub-lic radiation exposure limit. This radiation exposure level indicates that nolicense for using the sources is required. Thus, a significant amount of moneyand time can be saved.

Although ultrasonic methods can be used for thickness measurementfrom one side of an object of interest, the object surface has to be well pre-pared and polished. This process is time-consuming and can be expensive.The method presented here can function very well for unprepared surfaces.A single thickness measurement can take few seconds. The method, there-fore, takes about the same or shorter period than the ultrasonic methodif surface preparation is required. In addition, ultrasonic methods cannotbe used for high-temperature objects because the grease used between thesensor and object surface melts down. With the backscattering technique,a distance on the order of a few millimeters between the detector andthe object surface can be used to avoid direct contact without affectingthe collected data and allowing for hot-surface measurements. Althoughthe measurements performed in this study were on a pipe, the tech-nique is expected to operate successfully for imaging the walls of largevessels.

The line imaging technique (Fig. 10) used in this study has clear advan-tages over the widely used tangential radiography method. Both techniquesmeasure only part of a pipe surface, essentially producing a line image, butin this technique, much lower radioactivity is used, and a wider pipe-wallthickness is covered. Scanning of 170 mm in 1-mm, 1-s steps took less than3 min and in 1-mm, and 5-s took less than 15 min. This is close to the imag-ing period in tangential radiography, keeping in mind no image processingor development is required. Two-dimensional imaging is also possible (Fig.11), but a relatively extended measurement period is required to produce animage. But in many applications two dimensional images are not necessary,and it is enough to take a line image or inspect the pipe at few points to havean idea of its condition

ACKNOWLEDGEMENTS

The authors also thank the Deanship of Scientific Research at KingAbdulaziz University for their continuous cooperation and support.

FUNDING

The authors would like to express their gratitude to King Abdulaziz Cityfor Science and Technology for their full financial support of this work (project8-OIL-123-3).


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