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Imaging karstic cavities in transparent3D volume of the GPR data set inAkkopru dam, Mugla, TurkeySelma Kadioglu a b & Emin U. Ulugergerli ca Department of Geophysical Engineering, Faculty of Engineering,Ankara University, 06100, Tandogan, Ankara, Turkeyb Earth Sciences Application and Research Center (YEBIM), AnkaraUniversity, 06100, Tandogan, Ankara, Turkeyc Department of Geophysical Engineering, Faculty of Engineeringand Architecture, Canakkale Onsekiz Mart University, 17100,Canakkale, Turkey

Version of record first published: 02 Aug 2012

To cite this article: Selma Kadioglu & Emin U. Ulugergerli (2012): Imaging karstic cavities intransparent 3D volume of the GPR data set in Akkopru dam, Mugla, Turkey, Nondestructive Testingand Evaluation, 27:3, 263-271

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Imaging karstic cavities in transparent 3D volume of the GPR data setin Akkopru dam, Mugla, Turkey

Selma Kadioglua,b* and Emin U. Ulugergerlic

aDepartment of Geophysical Engineering, Faculty of Engineering, Ankara University, 06100Tandogan, Ankara, Turkey; bEarth Sciences Application and Research Center (YEBIM), AnkaraUniversity, 06100 Tandogan, Ankara, Turkey; cDepartment of Geophysical Engineering, Faculty ofEngineering and Architecture, Canakkale Onsekiz Mart University, 17100 Canakkale, Turkey

(Received 1 December 2011; final version received 18 April 2012)

The use of geophysical methods, particularly ground-penetrating radar (GPR),in the environmental sites has taken very important approximation in locating potentialenvironmental hazards such as landslides and cavities. Generally, slices of athree-dimensional (3D) GPR data set can be used to monitor changes in the surfacelocations and depth of the environmental hazards through the iterative slices. The aimof this study is to introduce volume imaging of subsurface cavities with a transparenthalf bird’s-eye view within a 3D data set. Transparency is obtained by a constructedopaque function of the amplitude–colour range. The half bird’s-eye view is achievedby observing angles of the x-, y- and z-directions to the 3D block. A GPR study wasconducted at a reservoir area (696 m £ 100 m) in Akkopru dam and hydroelectricpower station in Mugla (Turkey) to determine the existence and abundance of karsticcavities. The study revealed the presence of 283 cavities of various size and depth.Borehole data encouraged the results with given depths.

Keywords: GPR; karstic cavity; opaque; transparent 3D visualisation; half bird’s-eye view

1. Introduction

The presence of cavities or cracks beneath sites of urban or industrial construction can

influence the location, design and stability of buildings and other structures. Therefore,

the priority of engineering geologists and geophysicists is to locate these geological

features and specify their exact forms. Many researches have demonstrated that

ground-penetrating radar (GPR) is one of the most appropriate techniques to detect karstic

cavities and sinkholes [1–4,6,10–12]. They have also emphasised the characteristic

features of cavities on a GPR section. Air-filled cavities are strong reflectors and have

maximum reflection coefficients at the top of the cavity. Diffraction hyperbolas are

common patterns because of their small dimensions compared to profile length.

Accurate interpretation of subsurface cavities is often difficult when using

two-dimensional (2D) data sets because they are typically three-dimensional (3D)

complex systems; many studies have examined methods for determining the size,

continuity or extensions of such features. In addition, 3D data visualisation has been a

helpful presentation with iterative depth and profile slices to determine the locations and

depths of cavities and fractures in the study area [5–7].

ISSN 1058-9759 print/ISSN 1477-2671 online

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*Corresponding author. Email: kadioglu@ankara.edu.tr

Nondestructive Testing and Evaluation,Vol. 27, No. 3, September 2012, 263–271

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The aim of this study is to image the karstic cavities in a dam reservoir area, accurately

representing their locations, sizes and depths. The proposed method develops a transparent

3D data volume by displaying a limited amplitude range and appointing the view angle to

the x-, y- and z-directions of the 3D data volume.

2. Study area

The study area is a part of the reservoir area of Dalaman Akkopru dam and hydroelectric

power station (HEPS) on Dalaman Creek, which lies 24 km east of Koycegiz village,

Dalaman, Mugla, on the Mediterranean seacoast of Turkey (Figure 1). The area has a

relatively heterogeneous geological structure, including alluvial and karstic regions and

geomorphology. The watershed itself is considered as a tectonic depression zone [13].

The dam is the sixth largest dam in Turkey. The geological survey of the dam basin

revealed that the area featured porous limestone.

3. GPR data acquisition and processing

We conducted a large GPR survey to locate karstic cavities in the reservoir area. The area

was divided into nine segments, namely A, B, C, D, E, F, G, GH and H (Figure 1). Each

segment was 100 m wide. We used PulseEKKO100A GPR equipment with a 25-MHz

unshielded antenna. Transmitter–receiver antennas were placed perpendicular to the

profile direction. Antenna separation was 4 m, which was equal to the antenna length;

profile interval was 2 m. Each profile had 201 traces (trace spacing 0.5 m) and 640 time

samples, with 1.6 ns sampling interval per trace.

The ReflexW program (Version 5.0) [14] was used for data processing as well as

obtaining transparent GPR data blocks. Zero-time correction and dewow were applied to

GPR data of all the profiles. After background removals and linear amplitude gain, a

second-order band-pass Butterworth filter was used to remove low- and high-frequency

noises on each profile section (radargram). Velocity analysis used different diffraction

hyperbolas on the different profiles (Figure 2). The analysis yielded mean velocity of

0.12 m/ns for limestone. Therefore, the recording time axis was transformed to the depth

axis using the mean velocity. Finally, Kirchhoff migration was applied to the 2D profile

GPR data using the mean velocity [6,15].

Figure 1. Study area in Dalaman Akkopru dam and HEPS, showing nine segments.

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4. Visualisation of interactive 3D sub-volumes of GPR data

The visualisations in this paper were carried out on a part of segment D of the study area,

shown by a black rectangle in Figure 1. A solid 3D data volume was constructed from the

parallel-lined 2D GPR profile sections, a part of the segment D on the study area,

respectively, between 0 and 84 m (profile 17 to profile 60) on the profile step direction

(y-direction) and 100 m on the profile direction (x-direction) at depths between 2 and 22 m.

However, it was decided to restrict the 3D data set between 20 and 76 m in the y-direction

(profile 27 to profile 55) to show a more complex site that included many cavities and large

fractures. The 3D data volume could provide an interactive interpretation phase, enhanced

with an ability to generate cross-section slices and 3D sub-blocks that highlight individual

anomalies. Some unmigrated and migrated profile sections in the 3D blocks are shown in

Figures 2 and 3, respectively. Large cavities were seen as large hyperbolic anomalies in

Figure 2. However, these hyperbolic anomalies were combined at diffraction points of the

top of the cavities in Figure 3. A sloped anomaly, appearing as an interface between 20 and

22 m depths, identified reflections from the hillside next to the start point of the study

segments, due to the use of an unshielded antenna.

Our aim was to construct a transparent 3D representation of the subsurface cavities,

allowing a view of the buried large fractures and cavities in the transparent 3D volume.

Therefore, we applied two new approximations. According to the recording time or the

depth, the first approximation was related to amplitude balance (Figure 4(a)). The second

was an opaque approximation (Figure 4(b),(c)).

The anomaly of the target object could be enclosed in the exaggerated amplitude gain.

Therefore, we reduced the maximum amplitude of the selected colour scale rather than the

gain, to balance the amplitude-decay effect and protect the colour range of the anomaly of

the target object according to the time or depth range of the 3D sub-volumes (Figure 4(a)).

Re-scaled amplitude–colour ranges can be seen on the right side of the different depth

slices (Figure 5). Figure 5 indicates depth slices of 8, 10 and 14 m of the 3D GPR data

volume, including migrated profile sections. It was seen that the cavity anomalies were

Figure 2. Unmigrated profile sections in a 3D GPR data block of part of segment D.

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related to each other on the y-axis. The locations of the cavities could be successfully

identified on the interactive depth slices of the migrated 3D GPR data set.

Transparency was achieved by constructing an opaque function of the amplitude scale

instead of linear opaque function (Figure 4(b),(c)). The horizontal x-axis of the opaque

function was the amplitude–colour range scaled between 0 and 255; the vertical axis was

appointed opacity coefficients of the determined amplitude range [8,9]. The maximum

amplitude range was protected with a value of 1, which was the maximum opacity value.

Other amplitudes were eliminated with a value of 0, which was the minimum opacity

value (Figure 4(c)), and a transparent 3D image was obtained (Figure 6).

Figure 6 indicates transparent 3D sub-data volume imaging of the migrated profile

sections for part of segment D of the study area, for depths of 6–16 m and 8–16 m,

respectively, with the same viewing angle. The opaque range determined on the horizontal

axis depended on the high-amplitude intensity of the 3D sub-volumes. Selection of the

depth range of the 3D data block depended on the interpreter. The depth ranges could be

changed by controlling the maximum amplitude value with its opaque range, to achieve

transparency and reveal buried cavities. The view angle into the 3D block was important

Figure 4. (a) Re-scaling maximum amplitudes according to selected time or depth range of 3D sub-volumes; (b) linear opaque function; (c) new opaque function approximation.

Figure 3. Migrated profile sections in a 3D GPR data block of part of segment D.

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Figure 5. Depth slices of 8, 10 and 14 m for the data volume of migrated profile sections.

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because this affected vision capability. Figure 7 shows the same transparent 3D data

imaging as shown in Figure 6(b), but with a different viewing angle, greater amplitude

scale and more restricted amplitude–opaque range.

5. Results and discussion

We aimed to identify cavity anomalies on unmigrated and migrated profiles in the 3D GPR

data volume. Large cavities were seen as large hyperbolic anomalies on the unmigrated

profiles, as shown in Figure 2. The hyperbolas were gathered on the migrated profile

sections of Figure 3, and the directions of the branches of the cavities could be determined

according to depth. The depth slices near the surface indicated decomposed limestone

including complex fractures and cavities, as seen on the depth slices obtained at 6 m.

Below 10 m depth, the large fractures and cavities could be seen more effectively

(Figure 5). The depth slices were important to determine the location of the cavity. The

cavity could be determined on the profile sections and by their depth slices. However,

Figure 6. Transparent 3D sub-data volume imaging of migrated profile sections on part of segmentD: (a) 6–16 m depths and (b) 8–16 m depths.

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Figure 7. 3D data volume of Figure 6(b) with different view angle, amplitude scale and opaqueranges.

Figure 8. Two boreholes (D1 and D2) on segment D and results from profile 28.

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transparent 3D GPR volume imaging approximates closely a 3D image of the subsurface

to reveal the cavities and helped to show and easily explain the complexity of the

subsurface and facilitated an appropriate engineering solution (Figures 6 and 7).

The figures show only a small part of segment D in the study area. In total, 356 profiles

were studied on the nine segments, and 283 cavities were identified. Finally, 10 drilling

points were selected from the 283 cavity locations, which encouraged the cavities with

given start depths; two were located on segment D. Figure 8 indicates the results of the

two drilling points on segment D, termed D1 and D2. D1 was on Y: 22 m (profile 28) at

X: 3.5 m distance; D2 was on Y: 20 m (profile 27) at X: 25 m distance. Reflection

appearing as an interface in Figure 8 represents reflection from hillside because of the use

of unshielded antenna, as shown in Figures 2 and 3.

Knowledge of the distribution of subsurface cavities was fundamental for this study.

Defining and mapping the location of the cavities could indicate the best way to advance

the dam reservoir area to determine potential hazards and minimise construction costs.

Some chains of cavities could be very dangerous for the proposed dam and for

Akkopru residents, as shown in the figures of segment D. In addition, water could seep

away after any mid-size earthquake. Following this study, the area was sealed using

concrete slabs. The result of the sealing operation and outcome of our survey will be

determined in future studies.

Acknowledgements

The study of 3D visualisation of GPR data was partially supported by Ankara University underProject BAP 11B6055002, by the Ministry of Development of Turkey under Project DPT2012K120440 and by Ankara University Earth Sciences Application and Research Center (YEBIM).The authors would like to thank Ankara University and YEBIM for supporting the project, NTFConstruction Co. and General Directorate of State Hydraulic Works (DSI) of Turkey for thepermission to publish the result.

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