3d digital documentation, assessment, and damage quantification of the al-deir monument in the...

© W. S. Maney & Son Ltd 2010 DOI 10.1179/175355210X12792909186412

conservation and mgmt of arch. sites, Vol. 12 No. 2, May, 2010, 124–45

3D Digital Documentation, Assessment, and Damage Quantifi cation of the Al-Deir Monument in the Ancient City of Petra, JordanYahya Alshawabkeh and Fadi Bal’awi The Hashemite University, Jordan

Norbert HaalaInstitute for Photogrammetry (ifp), University of Stuttgart, Germany

The rock-cut monuments in the ancient city of Petra in Jordan form an out-standing tangible heritage site. Unfortunately, this important part of the world’s cultural heritage is gradually being diminished due to weathering and erosion problems. In this research, an approach combining in situ surveying and laboratory analysis is applied in order to provide suffi cient and comprehensive data regarding the documentation and evaluation of the status of the Al-Deir monument in Petra. The purpose was not only to quantify the damage, but also to make a fi rst step towards creating a 3D monitoring programme of the deterioration rate. The approach presents a correlation study between the environmental condition and the surface weathering damage, using 2D mapping of the weathering form and accurate 3D realistic modelling from laser scanning and digital photogrammetry. The 2D mapping provides detailed weathering damage information for the entire stone surface of the monument, whereas the 3D modelling provides infor-mation on the spatial distribution and texture of the damage. Additionally, the 3D digital model can provide reference data as an exact guide to the restoration needed. In order to support the visual presentation of 3D surface details, a hybrid approach combining data from laser scanning and digital imagery was developed. Studies of stone texture and spatial distribution of soluble salts were carried out at the monument in order to explain the mechanism of the weathering problem. Relative humidity, temperature, and wind are the main factors in the salt damage process. In order to study the effect of these conditions in the salt crystallization process a series of fi eldwork

1253D DIGITAL DOCUMENTATION IN PETRA

investigations and laboratory work were undertaken. The results show that visible zoning of weathering damage is correlated to different salt concentrations.

keywords damage assessment, 2D weathering form, 3D modelling, laser scanning, photogrammetry, salt content

Introduction

In 2007 the ancient city of Petra, the capital of the Nabataean empire from 400 bc

to ad 106, was selected to represent one of the ‘New Seven Wonders of the World’.

Petra’s temples, tombs, theatres, and other buildings, which are carved into rose-

coloured sandstone cliffs are scattered over 400 square miles. In 1985 Petra was

inscribed into the UNESCO list of world cultural heritage. However, the majority of

its monuments have deteriorated at a fast pace over recent years (Bumbaru et al.,

2000: 123). Due to this fact, in 1998 the World Monument Fund inscribed Petra into

the list of the one hundred most endangered monument assemblies in the world. The

city, located in south-western Jordan, suffers from weathering and erosion problems.

Problems result from the mainly porous inorganic materials of the monuments and

the uncontrolled environmental conditions around them. This weathering results in

potential salt damage as the main force on the monument’s stone structure (Fitzner

& Heinrichs, 2001: 12; Goudie & Viles, 1997; Eklund, 2008).

Reliable monitoring on weathering did not exist for most of the monuments in

ancient Petra. This is due to the lack of referential archive documents on the state of

weathering damage in the past (Heinrichs, 2008: 643; Kühlenthal & Fisher, 2000).

Recently, a large number of digital investigation methods have been newly developed

for reliable, easy, and fast surveying of large-scale monuments, including those that

are diffi cult to access. The integration of these techniques with the laboratory analy-

sis allows for a quantitative registration, documentation, and evaluation of complete

monuments.

This study aims to present a combined documentation and evaluation approach

that can be applied, in order to provide reliable information on the measurement and

characteristics of the stone decay problem at Al-Deir monument in Petra. The work

included a correlation study between the environmental conditions and surface wea-

thering using 2D mapping of the weathering forms and 3D realistic modelling of both

the geometry and radiometric properties of the surface details. The data could be used

in future studies of conservation action at the site.

The fi rst part of this paper presents a literature review for the main environmental

conditions that affect the architecture of Petra. The second part discusses the different

mapping methods used in the in situ surveying, including 2D mapping of the weath-

ering forms and digital photogrammetric techniques. Section 3 presents the weather-

ing mapping forms for Al-Deir. The methodology for the generation of accurate

and realistic 3D models of the Al-Deir monument, combining photogrammetry and

laser scanning, is presented in section 4. The environmental monitoring data and

chemical analysis of the salt content of the samples collected from the monument are

presented in section 5. In section 6 the results are discussed and concluded.

126 YAHYA ALSHAWABKEH et al.

Literature review

Petra: weathering agentsNatural processes and human activities, as well as a lack of maintenance in the

ancient city of Petra, are all involved in the weathering processes. Petra is located in

a tectonically active region: Barjous and Jaser (1992) traced three major faults in the

area. Due to the high seismic slip between these faults, the area has suffered from a

series of serious destructive earthquakes since ancient times. Most of the Theatre

was destroyed in ad 363, and another earthquake in ad 747 destroyed most of the

monuments in the centre of the city. Generally, statistics showed that an earthquake

with a magnitude above 6 on the Richter scale occurs every hundred years in the

Petra area (UNESCO, 1992).

The annual rainfall in Petra is very low, but it falls in a very short period of time.

Water erosion is, therefore, a very active agent in this environment. The Nabatean

hydrological systems show that the Nabateans were very aware of water erosion as

a problem in their area; they constructed ceramic pipes along the bedrock and the

face of the monuments to protect them from fl ood water. Moreover, horizontal sur-

faces were covered with multiple mortar layers to minimize the effect of running

water on these features (Shaer & Aslan, 2000: 89). Unfortunately, nowadays, the

Nabatean water system is the main cause of water erosion at the site. Joints and

cracks that were created by earthquakes, as well as the clogging of the Nabatean

water channels, allow the Petra monuments to be eroded from within and without.

In brief, the monuments of Petra had been affected by groundwater, running water,

and rainwater through different mechanisms.

The American Centre of Oriental Research in Amman (ACOR, 1999) reported that

the water table in Petra is actually much higher at the dawn of the twenty-fi rst

century than it was at the dawn of the last millennium. The rising of the water table

in the area has had a major impact on many other deterioration mechanisms such as

salt crystallization and the dissolution or leaching of materials (clay minerals, for

example, since most of the Petra monuments were carved from sandstone with a high

clay content). In short, water is a major factor in the deterioration process in the city

of Petra through fl ood damage, rainwater, runoff water, and capillary action and

their subsequent effects such as salt damage. Wind is another important weathering

agent for the Petra monuments. Not only does it cause the destruction of monuments

due to wind-blown sand (UNESCO, 1992), but it also enhances the effects of other

weathering agents, such as salt crystallization. The effect of the wind-blown sand is

mainly restricted to the lower parts of the monuments (1–2 m height), as these parts

come into contact with sand particles (UNESCO, 1992).

Salt crystallization is another, if not the major, weathering agent for the Petra

monuments (Eklund, 2008). Previous studies, such as Al Naddaf’s (2002), showed that

drilled samples from the Petra monuments are rich in sodium chloride and calcium

sulphate, while the scraped samples from the monuments’ surfaces were dominated

by calcium sulphate.

Due to wide variations in temperature, both daily and seasonally, the monuments

in Petra suffer from what is known as ‘thermal shock’. This kind of weathering is

related to the fact that some minerals expand more than others at high temperatures


and contract at low temperatures. A study of the temperature variations within a

24-hour cycle in Petra carried out by Fitzner & Heinrichs (1991: 908) showed a

difference of 20ºC in the stone temperature and a difference of 21.1ºC in the air

temperature. In another study of the effect of thermal shock, Paradise (1999: 353)

concluded that, as a weathering agent, thermal shock was more destructive in calcite-

cemented sandstones due to the fact that calcite expands 25x10−6 µm/°C m3 calcite

parallel to the C-axis and contracts 4.9x10−6 µm/°C normal to the same axis in

temperatures between 18–50°C. The current paper does not discuss the thermal shock

decay mechanism; however, it does present a detailed petrography of the case study

monument, the Al-Deir tomb (below).

Weathering form mapping and 3D modelling using photogrammetric

techniques

Precise classifi cation and mapping of the damage phenomena are required for charac-

terization, interpretation, and monitoring of the rate and types of weathering damage,

which affects the stone monuments at Petra. Mapping of the weathering forms allows

for a visual inspection of the surface damage, which is necessary for the recording of

the type, intensity, and distribution of the weathering factors at the stone monuments

(Fitzner et al., 2002: 315; Gillhuber et al., 2009; Siedel, 2008). Although the 2D map-

ping of weathering forms give us an idea about the distributions of the weathering

processes, condition assessment of the structure using an accurate 3D modelling of

the weathering damage is an important component of the remedial programme.

The 3D modelling of the monuments by complete surveying and assessment is real-

ized by the integration of 3D laser scanning and photogrammetry. By these means

spatial data, as well as detailed structural data, can be obtained. The availability of

a 3D realistic model easily allows multiple views and a virtual manipulation. The

purpose of such visual information may serve as a tool for the identifi cation of the

nature, extent and severity of the deterioration (Gaiani et al., 2001: 86; Fuentes et al.,

2007: 543). Mapping is required for characterization, interpretation, and monitoring

of the rate and types of weathering damage affecting the stone monuments at Petra.

The mapping of weathering forms presents a visual inspection of the surface damage,

which is necessary for recording the type, intensity, and distribution of the weathering

factors at the stone monuments. As an archive gathered over time, the 3D models

provide valuable information when restoration is required. In the case of damaged

monuments, the 3D digital model can provide reference data as an exact guide to the

restoration needed and with detailed high-resolution 2D images, the texturing of 3D

models can provide both stone texture and spatial distribution of weathering features

on the monument.

Different contact-free techniques have been used for generating realistic 3D models

of an object or building. Photogrammetry has a long history as a tool for effi cient and

accurate data acquisition, including cultural heritage applications (Grün et al., 2002:

363; El-Hakim et al., 2002: 58; Debevec, 1996). Photogrammetric measurement allows

3D data collection from multiple images (Figure 1). If a point is depicted in at least

two images at different viewpoints, its corresponding 3D object coordinates can be

determined.


In order to determine 3D point coordinates from measurements in overlapping

images, information on the position and orientation of the camera at the time of

exposure are required. In addition to what is termed the exterior orientation, the

parameters of the interior orientation have to be provided: this describes the position

of the image plane with respect to the centre of projection of the camera. Both

exterior and interior orientations can be reconstructed if the object coordinates are

available for a number of the control points. For the orientation of multiple overlap-

ping images, tie points, i.e. corresponding image points are additionally used. If the

parameters of the exterior and interior orientations are available, the object and

image coordinates can be linked by the co-linearity equation. This equation mathe-

matically formulates the theory that a point in the object space, the projective centre

of the optics, and the corresponding point in image space, forms one straight line.

Applying this principle, the 3D coordinates of all points can be determined by a

spatial intersection of straight lines, if they are observed in at least two images. One

of the main advantages of photogrammetry is the potential to simultaneously record

both the geometry and the surface texture of the depicted objects. This is especially

important when attempting to generate 3D virtual models of historical monuments

or artefacts from archaeological excavations (Schindler et al., 2003: 463). On the

other hand, image-based modelling alone is diffi cult or even impractical for the parts

of surfaces where homogenous texture prevents the reliable measurement of point

correspondences. Additionally, the identifi cation, and manual or semi-automatic

measurement of points, can be very labour intensive, especially if dealing with a

complex structure that requires a considerable number of points to be captured.

Due to the disadvantages of close-range photogrammetry, laser scanning has

become a standard tool for 3D data collection for cultural heritage sites and historic

buildings (Boehler et al., 2001: 480). For 3D data collection almost all commercially

available terrestrial laser scanners apply the so-called time-of-fl ight measurement

principle. Within this approach distances to the respective object surface are derived

from run-time measurements of refl ected light pulses. Point clouds covering the visible

fi gure 1 Principle of photogrammetry for 3D object modelling.


object surface can then be collected by scanning the respective area of interest. This

results in a very effective and dense measurement of surface geometry. This is espe-

cially useful for the 3D reconstruction of geometrically complex objects that are

typically encountered in cultural heritage applications. However, the achievable

accuracy of time for a fl ight scanner is relatively low and commonly results in

centimetre range. Another type of scanner is triangulation-based and consists of a

transmitting device, sending a laser beam at a defi ned, incrementally changed angle

from one end of a mechanical base. A CCD camera at the other end of the base

detects the returned laser spot. They have better accuracy in terms of measurements,

which depend on the triangle base relative to its height. Due to this it can only be

used for relatively small objects and short distances (Boehler et al., 2001), an example

is the Arius scanner.

In addition to geometric data collection produced by a laser scanner, texture

mapping is particularly important in the area of cultural heritage to help create more

complete documentation. Photo-realistic texturing can, for example, add information

about the structure’s condition, such as decay of the material, which is not present in

the 3D model. The interpretation of 3D point clouds from laser scanning can be

simplifi ed considerably, if high-resolution imagery from digital cameras is available

(Balis et al., 2004; Alshawabkeh et al., 2004: 424; Abdelhafi z, 2009). Some commercial

3D systems already provide model-registered colour texture by capturing the RGB

values of each LIDAR point using a camera already integrated in the system. These

images are frequently used for scan registration but are not suffi cient for high quality

texturing. High image quality is especially required to provide suffi cient visual qua lity

of 3D surface details such as decay forms and cracks as required for documentation.

The collection of such images presumes suitable lighting, which frequently require

camera stations that are different from the actual scanner position. Thus the use of

independent cameras that are aligned with a suitable mathematical approach is an

appropriate solution.

Mapping of weathering forms for the Al-Deir tomb (the monastery)

Al-Deir, meaning monastery in Arabic, received its name from the cave that is known

as the Hermit’s Cell. The journey to the Al-Deir Tomb, depicted in Figure 2, requires

climbing more than 2000 steps carved into the mountain. It is the largest and most

impressive façade in Petra. The façade is about 50 m wide and 45 m high (Khouri,

1986). It is divided into two storeys; the lower one has a simple doorway (8 m high)

with six columns topped by Nabatean capitals, the upper storey, which is better

preserved, has eight columns with a conical central roof crowned with an urn. The

main chamber in Al-Deir is huge (11.5 m by 10 m). A small part of it was used as a

meeting room (symposium), whereas the main part was a mausoleum for the king.

A huge area in front of the monument was levelled, and seems to have been used for

great congregations of people. There is no actual dating for Al-Deir, however, many

writers such as Bourbon (1999), Taylor (2001), and Khouri (1986) suggest the middle

of the fi rst century (ad 44–70). The monument’s location on the edge of a high

mountain and the presence of different levels of stone decay are the main reasons

for selecting this monument for sampling. The main weathering forms in Al-Deir

tomb are shown in Table 1, while the main weathering groups, weathering forms,


and individual weathering forms at the Al-Deir Tomb, and their distribution across

the surface of the monument, are shown in Figure 3.

3D modelling using photogrammetry and laser scanning techniques

Sensors applied and 3D surface modellingA laser scanning system GS100, manufactured by Mensi S.A., France, was used to

collect 3D point data. It has a 360x60 degree fi eld of view. The maximum captured

range for this scanner is 100 m with a linear error of less than 6 mm. The system is

fi gure 2 Al-Deir Tomb, Petra, Jordan.

TABLE 1

THE MAIN WEATHERING GROUPS AND WEATHERING FORMS AT AL-DEIR TOMB

Main weathering group Main weathering forms

1. Loss of stone materials 1.1. Break out1.2. Back weathering 1.3. Relief1.4. Pitting1.5. Washout

2. Deposits on stones 2.1. Loose salt deposits (Efflorescence)2.2. Biological colonization (Colonization of high plants)

3. Detachments 3.1. Flaking 3.2. Scaling 3.3. Cantour scaling

4. Fractures 4.1. Faults4.2. Cracks


able to capture 5000 points per second. During data collection a calibrated video

snapshot of 768x576 pixel resolution is additionally captured, which is automati-

cally mapped on to the corresponding point measurements. An example of the

coloured collected point clouds is presented in Figure 4. In addition to the laser data,

digital images were captured for photogrammetric processing using a calibrated

Nikon D2x camera, which provides a resolution of 4288x2848 pixels with a focal

length of 20 mm. These images were collected at almost the same time in order to

have the same lighting conditions, which results in similar radiometric properties as

needed for high quality colouring of the laser data. A single laser scan is usually not

suffi cient to cover a full structure. The number of scans necessary depends on the

shape of the object, the amount of self-occlusion and obstacles hindering the object’s

visibility from respective sensor stations, and the object size compared to the sensor

range. During the data collection at the Al-Deir tomb, scans from fi ve different

viewpoints were done to resolve the occlusions. The mountainous environment

surrounding Al-Deir restricted the choice of suitable viewpoint positions. In total, the

fi ve scans resulted in almost 3 million collected points.

All the 3D models have been processed using Innovmetric Software, PolyWorks,

and Mensi Real Work Survey Software. In general, the whole process for generating

the 3D model can be subdivided into three main steps: registration, meshing or trian-

gulation, and merging. For objects with a complex shape, a series of scans must be

undertaken. In this case each scan has its own local coordinate system. The recon-

struction of the 3D model of the surveyed object requires the registration of the scans

in a single local reference system. This phase can be performed in an interactive

fi gure 3 2D mapping of the main weathering forms of Al-Deir Tomb.


fi gure 4 Al-Deir façade recorded by terrestrial laser scanning: a) Overview b) Detailed view of collected point cloud.


environment through the identifi cation of the homologous points in adjacent and

overlapping scans. Once the corresponding points are collected (of which there must

be at least three), the spatial transformation between the scans’ six-parameter trans-

formation can be estimated and all the corresponding points can be transferred into

the coordinate system that has been selected as the reference system. Figure 5 shows

the registration of two different scans used to cover the complete geometry of the

Monastery. The main aim of the triangulation (meshing) process is to convert the

point-based data into a visually more intuitive representation. Once the points are

collected in a single coordinate system, a triangulation algorithm is used to connect

the points to the meshes which represent the respective patches of the object surface.

Finally, in areas of overlapping scans redundant point measurements are eliminated

to reduce the size of the model, for the purposes of visualization or for further

post-processing. Figure 6 shows the 3D meshed model of the Al-Deir tomb.

A combined approach for realistic colouring of the laser dataAlthough the 3D model produced by the laser scanner contains a large number of 3D

data, which represent the object’s surface, it can still be diffi cult to recognize and

localize the outlines of the surface features. This is illustrated by comparison of

Figures 2 and 6: surface features such as structural cracks and edge outlines which

are clearly visible in Figure 2 are not discernible in the corresponding 3D meshed

model shown in Figure 6, as they are beyond the resolution of the available laser data.

In order to support the visual quality of such details, a hybrid approach combining

data from the laser scanner and the digital imagery was developed. Images from a

calibrated camera, and the high quality of the registration process of both data sets,

are crucial factors for achieving a realistic-looking model. This requires accurate

fi gure 5 Registration of two different scans in one coordinate system.


determination of the camera’s interior and exterior orientation parameters. For our

investigations, the camera calibration parameters were computed using Australis

software. These parameters were used to eliminate geometric image distortions.

Corresponding coordinates between image and laser scanner were then measured

using the Photomodeler software, which was also applied to calculate the camera

position and orientation in the coordinate system of the geometric model. After the

position and orientation parameters are computed for the sensor stations, the co-

linearity equations, shown below, are then applied to calculate the image coordinate

for each point of the laser scanner point cloud. The implementation was realized

on a standard PC with an Intel 4, 3GHz Processor, and 1GB RAM using the C++

language.

xc r X X r Y Y r Z Z

r X X r Ya

A A A

A

=- - -

- -

− ( ) + ( ) + ( ) ( ) +

11 0 12 0 13 0

31 0 32 0 YY r Z Z

yc r X X r Y Y r Z Z

A A

a

A A A

( ) + ( ) − ( ) + ( ) + ( )

33 0

21 0 22 0 23 0

-

=- - -

( ) + ( ) + ( ) r X X r Y Y r Z ZA A A31 0 32 0 33 0- - -

In these equations:

xa, ya: an object A’s image coordinates.

XA, YA, ZA: the object’s coordinates in object space.

Xo, Yo, Zo: the object space coordinates of the camera position.

r11–r33: the coeffi cients of the orthogonal transformation between the image

plane orientation and the object space orientation, and are functions of the

rotation angles.

c: camera focal length.

fi gure 6 3D meshed Model for Al-Deir Tomb.


As it can be seen in Figure 7, the different acquisition time of the images collected

from the laser scanner camera results in considerable differences in brightness and

colour, which changes the appearance of the textured 3D model. By contrast, the

proposed method for mapping high resolution digital images onto the geometry gives

a highly realistic appearance to the model and offers a descriptive view of the scene

which can be used for measuring and monitoring purposes, as depicted in Figure 8.

The measured point density for describing the surface of the monument was 2 cm

at the object; this is due to the nature of the used laser scanner which features 6 mm

accuracy. An example of a 3D model of a representative area of damage is given in

Figures 9 and 10. As can be seen, the mapping of close and detailed high resolution

images to the 3D model allows a good visual inspection of the surface damage, which

is necessary for recording of the weathering factors at the stone monument according

to type, intensity and spatial distribution.

Environmental monitoring and salt analysis

The activation of salt damage is highly controlled by the surrounding environmental

conditions. Relative humidity, air temperature, solar radiation, wind direction,

and wind speed are known as important factors that directly infl uence the salt

damage process (Lubelli, 2006; Price, 2000; Steiger & Zeunert, 1996: 353; Price &

Brimblecombe, 1994: 90; Arnold & Zehnder, 1991: 103). Therefore, the collection of

climatic data from the case study site was an essential part of the current research.

The methodology for evaluating the role of environmental conditions on salt damage

behaviour in cultural and historical heritage sites varies signifi cantly. Some scholars,

fi gure 7 3D textured model of Al-Deir Tomb using images collected from the laser scanner camera (768x576 pixels).


fi gure 8 3D textured model of Al-Deir Tomb using images of Nikon D2x camera (4288x2848 pixels).

fi gure 9 3D textured model (using close detailed high-resolution images).

fi gure 10 3D model corresponding to Figure 9.


such as Tricio and Viloria (2002: 67) and Camuffo and Bernardi (1995: 7) have under-

taken a very detailed microclimate investigation in evaluating the effects of environ-

mental parameters on historic buildings, while others, such as Al Naddaf (2002),

prefer a basic monitoring programme to evaluate stone-weathering behaviour. A

series of fi eldwork investigations and laboratory work was undertaken in order to

study the effect of these parameters on the salt crystallization process. The detailed

monitoring approach was considered more appropriate for the current research

because it would provide a more systematic way of evaluating the salt damage

processes at different locations by comparing the microclimate data of each location

with its salt content. This included an eighteen-month programme of microclimate

monitoring for relative humidity, air temperature, and wind speed at the case study

location, accompanied by four sampling periods for the determination of the salt

content at the case study location. For the temperature and relative humidity

measurements, Gemini Tinytag Plus (TGP-1500) loggers were used, and were placed

near the gateway of the tomb. In addition, a Lutron hand anemometer (Am-4201)

was used to measure the wind speed at the case study monument.

Recorded environmental conditions The temperature readings were much more stable than the relative humidity readings

(Figure 11). For example, in January the temperature readings ranged between 9.6

fi gure 11 Diagram combining relative humidity and temperature readings from the data logger. Location: Al-Deir Tomb (Recorded period: September 2003–January 2004).


and 25.1ºC with an average of 12.5ºC. The daily fl uctuation range was less than 2ºC

during this month. Even though the relative humidity overall monthly averages did

not differ greatly, the individual relative humidity readings varied signifi cantly. For

instance, during January the relative humidity reached its maximum (82.1%) and

dropped to 54% toward the end of the same month. May, June, July, September, and

October were moderately dry and hot, while November and December had similar

humidity averages but were colder. January and February were slightly humid and

quite cold months. In March and April both temperature and relative humidity were

unstable with overall humid and rather cold conditions.

Generally, the Petra microclimate data is typifi ed by the domination of dry, hot

conditions and fl uctuating wind speeds throughout the majority of the year. In addi-

tion, the high rate of fl uctuation of the relative humidity and wind speed around the

studied monuments was very obvious. A considerable variation was also noted

between readings that came from the same monument and at the same time, but from

different sampling points.

Petrography of the AL-Deir stoneThe Al-Deir Tomb is carved out of Upper Umm Ishrin sandstone, which is multicol-

oured sandstone from the subarkosic Umm Ishrin formation of the middle to late

Cambrian age. The thin section of this stone (depicted in Figure 12) shows multicol-

oured, fi ne to medium, sub-angular to sub-rounded grained sandstone. Iron oxides

and kaolinite are the main matrix. Quartz grains make up over 80% of the stone

content. Porosity is moderate, with total porosity around 15% and mainly medium

(1–10 µm) to coarse pores (10–100 µm radii).

Salt types and distributionsThe determination of the salt types and their distribution in the Al-Deir tomb

at Petra has great importance for understanding and evaluating the weathering

processes at the monument. The types of salts, their depth of accumulation, the pore

structure and moisture regimes, as well as the surrounding microclimate conditions,

are key features controlling the decay of stone materials (Nicholson, 2001: 819;

Rodriguez-Navarro et al., 1999: 1250; Winkler, 1994; Doehne, 1994: 143; Rossi-

Manaresi & Tucci, 1991: 53).

fi gure 12 Photomicrograph of the petrological thin section of the Upper Umm Ishrin sandstone specimen. Field of view 2.5 mm. Magnifi cation: 40x(ppl).


The Al-Deir Tomb is located on the edge of a high mountain and it has two

different levels of stone decay between its two storeys. The lower part of the Al-Dier

monument, as seen in Figure 8, had a wide range of stone decay features that are

mainly linked to salt damage, such as scaling. The latter feature was the main reason

for evaluating the salt distribution in order to match the weathering form on this

monument and the salt distribution.

Sampling profi les Two sampling profi les were taken from the Al-Deir Tomb, D1 and D2, as depicted

in Figure 13; to minimize the level of intervention neither was from the carved façade.

These two profi les were selected after a geological correlation with the tomb rocks,

and the heights of the profi les where chosen at levels where a high salt content

was expected due to the stone decay features at the tomb. Thirty-two samples were

collected from this area during the fi rst fi eldwork visit using a manual drill. At profi le

D1, samples were collected from three different depths (0–1, 1–3, and 3–5 cm), whilst

at D2 samples were collected from only two different depths (0–1 and 1–3 cm). The

current study applied sampling techniques that could not get further into the rocks

due to their hardness, especially in profi le D2. For the rest of the fi eldwork visits,

between 22–26 samples were collected and the sampling depths were reduced to two

intervals only (0–1 and 1–3 cm), due to the low content of the salt damage at the

3–5 cm interval.

Cation and anion content analysisIn order to identify the total salt content in each of the samples collected from each

monument at the four sampling fi eldwork visits, the cation and anion content of these

fi gure 13 The sampling profi les at Al-Deir Tomb D1 and D2.


samples was measured. Before carrying out the analysis, the samples were diluted

with distilled water. A sample of 0.2P0.0005 g was diluted with 10P0.05 ml of

distilled water. The samples were fi ltered in order to avoid any metal debris from the

drills affecting the results. The cation analysis was carried out using an Inductively

Coupled Plasma Atomic Emission Spectrometer (ICP-AES). This machine was capa-

ble of measuring the following cations: calcium, sodium, iron, aluminium, magne-

sium, potassium, titanium, and zinc. The machine was calibrated automatically every

tenth sample. The anion content was measured using Ion Chromatography (IC). The

experiment was carried out with the same diluted samples that were used for the

ICP-AES in order to maintain homogeneous results. The samples were diluted further

when the anion concentration exceeded the capacity of the machine reading. The

cation content was measured using an Inductively Coupled Plasma Atomic Emission

Spectrometer (ICP-AES), while the anion content was measured using Ion chroma-

tography (IC). The total soluble content in all analyses was expressed as the weight

% of salt per weight unit of dried stone powder sample (0.2 g).

Results and discussionThe total soluble salts content at the Al-Deir Tomb was generally low (Table 2).

During the fi rst sampling season (August 2003), calcium and sodium were the main

cations, while sulphate, chloride, and nitrate were the main anions. Potassium and

phosphate were minor components. There was an obvious general trend of the total

content of soluble salts in the three sampling depth intervals. The salt content started

very low at 5 cm and increased gradually until it reached its maximum at 105 cm,

after which it started to decline again at 155 cm. Thereafter, the salt content increased

gradually with height. The signifi cant increase of the total soluble salt content at

105 cm is mainly related to the presence of a small terrace (40 cm widex250 cm long)

at the height of about 95 cm. This terrace could be the main source of water

accumulation and, therefore, an additional source of soluble salts. The increase of

the total soluble salt content was accompanied by a noticeable increase of bromide

content in the samples. The origin of the bromide is unknown. It should be noticed

here that these readings are from one fi eldwork session, and not for the whole

monitoring period.

During the winter season, sodium and calcium were the main cations, while potas-

sium and magnesium were present in low concentrations. Sulphate and chloride were

the main anions. A high concentration of nitrate was found in a few samples that

came mainly from the lowest level of the sampling profi le (5 cm height). During the

early summer fi eldwork visit (June 2004), the total soluble salt content at the Al-Deir

Tomb was slightly higher than in the previous two visits, with a more uniform

distribution of the salts throughout the profi le. Generally, the higher evaporation

rates during this period resulted in higher soluble salt content in the deeper intervals,

creating a more uniform distribution of the soluble salts between the surface and the

deeper intervals (0–1 and 1–3 cm).

The cation and anion analysis of the samples from the fi rst profi le (D1) at the

Al-Deir Tomb during the spring sampling fi eldwork visit showed the lowest overall

soluble salt content from all fi eldwork visits (Table 3). The excess of cation charges

compared to anion charges in these samples was one of the main features in these


results. This excess was noticed in most of the samples from the previous fi eldwork

visits, but was higher in the samples from the spring and winter fi eldwork visits. The

winter and spring seasons were the most humid ones compared to the early and late

summer periods, causing higher groundwater levels inside the monuments. This situ-

ation strongly supports the authors’ arguments regarding the presence of carbonate

or bicarbonate anions in the samples from the Al-Deir Tomb that originated mainly

from groundwater inside the monument. However, despite the non-proportional

distribution of the cations and anions, a correspondence of calcium and sulphate and

of sodium and chloride ions existed throughout the profi le.

It is worth mentioning that the current research had tested the moisture content

in few samples from the Al-Deir area and the results showed a clear indication

that groundwater is the main source of moisture in the tested section. The evaluation

of salt distribution has shown that it is noticeably higher at the lower levels of

this monument compared to the higher ones. The groundwater may have a direct

infl uence on the salt distribution since it is the main source of these salts. Thus, the

TABLE 2

THE SOLUBLE SALT CONTENT FROM DRILLED SAMPLES, AL-DEIR TOMB, LOCATION (D1). FIRST FIELDWORK VISIT: AUGUST 2003

Sample number Location Height (cm) Depth (cm) Soluble salt content in the sample (%) of dry weight

79 D1 5 surface 0.25

80 D1 5 1 0.12

81 D1 5 1–3 0.13

82 D1 5 3–5 0.17

83 D1 55 0–1 0.22

84 D1 55 1–3 0.13

85 D1 55 3–5 0.11

86 D1 105 0–1 0.41

87 D1 105 1–3 0.24

88 D1 105 3–5 0.44

89 D1 155 Surface 0.36

90 D1 155 0–1 0.40

91 D1 155 1–3 0.15

92 D1 155 3–5 0.10

93 D1 205 Surface 0.21

94 D1 205 0–1 0.35

95 D1 205 1–3 0.09

96 D1 205 3–5 0.18

97 D1 255 0–1 0.44

98 D1 255 1–3 0.16

99 D1 255 3–5 0.19


TABLE 3

THE MAIN ANION CONTENT OF DRILLED SAMPLES FROM AL-DEIR TOMB, LOCATION (D1) (DEPTH INTERVALS: 0–1 AND 1–3 CM). THIRD FIELDWORK VISIT: JUNE 2004

Location Height (cm)

Depth (cm)

Ca (ppm)

Na (ppm)

Mg (ppm)

K (ppm)

F (ppm)

Br (ppm)

Cl (ppm)

NO3 (ppm)

SO4 (ppm)

D1 5 0–1 15.90 24.40 1.90 0.00 2.24 0.00 22.81 7.27 22.70

D1 5 1–3 8.90 12.40 0.00 0.00 0.00 0.00 16.75 5.13 10.31

D1 55 0–1 20.30 18.50 0.00 0.00 2.21 0.00 16.29 5.79 19.50

D1 55 1–3 11.10 11.60 0.00 0.00 1.00 0.00 15.21 5.01 13.37

D1 105 0–1 12.70 9.20 1.50 0.00 4.44 0.00 10.12 10.98 21.01

D1 105 1–3 9.00 11.70 0.00 0.00 5.63 0.00 12.31 10.52 8.63

D1 155 0–1 11.30 13.00 0.00 0.00 1.85 0.00 12.28 11.32 12.47

D1 155 1–3 6.60 8.90 0.00 0.00 5.05 0.00 8.98 0.00 3.34

D1 205 0–1 21.60 17.90 1.80 0.00 1.87 0.00 17.91 11.48 57.37

D1 205 1–3 6.50 10.40 0.00 0.00 0.01 0.00 12.67 8.96 6.84

D1 255 0–1 13.60 10.50 0.00 0.00 0.01 0.00 11.83 10.94 21.14

D1 255 1–3 7.70 17.40 0.00 0.00 1.91 0.00 15.46 11.79 11.78

main decay line at this monument is directly linked to the salt damage problem, since

the highest salt accumulations were along this line.

Conclusions

Uncontrolled environmental conditions and salt content represent the main damage

processes for the Al-Deir monument in the ancient city of Petra in Jordan. This gave

rise to the need to have a comprehensive study and full documentation of the monu-

ment in order to evaluate its status. This research applied a comprehensive approach

utilizing 2D weathering form mapping and 3D modelling using laser scanning and

photogrammetry. 3D laser scanner technology was one of the important technical

methods used to acquire spatial data of the monument. The data collected can be

used to evaluate what restoration is required. In the case of damaged areas, the

detailed high-resolution 2D images and the texturing 3D models of the monument

will give both stone texture and spatial distribution of the weathering features and

will help to explain the mechanism and rate of the weathering problem.

The present study also included laboratory analysis and a correlation study of

the salt content and the surface weathering damage, which are the main damage

processes at work on the stone at the monument, allowing an understanding of the

deterioration mechanisms and causes. The evaluation of the salt distribution has

shown that the visible zoning of weathering damage correlates to different salt

concentrations.

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Notes on contributors

Yahya Alshawabkeh is Head of the Department of Conservation Science at the Queen

Rania Institute of Tourism & Heritage in Jordan.

Correspondence to: Yahya Alshawabkeh, The Hashemite University, PO Box

150459, Zarqa 13115, Jordan. Email: [email protected]

Fadi Bal’awi is Assistant Professor in Conservation Science in the Department of

Conservation Science at Queen Rania’s Institute of Tourism and Heritage.

Correspondence to: Fadi Bal’awi, The Hashemite University, PO Box 150459,

Zarqa 13115, Jordan. Email: [email protected]

Professor Norbert Haala is Deputy Director of the Institute for Photogrammetry, at

the University of Stuttgart.

Correspondence to: Norbert Haala, Institute for Photogrammetry, University of

Stuttgart, Geschwister-Scholl-Strasse 24D, D-70174 Stuttgart, Germany.

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