The rock-cut monuments in the ancient city of Petra in Jordan form an outstanding 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 sufficient 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 first 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 information 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 fieldwork 3D DIGITAL DOCUMENTATION IN PETRA 125 investigations and laboratory work were undertaken. The results show that visible zoning of weathering damage is correlated to different salt concentrations.
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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
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).
136 YAHYA ALSHAWABKEH et al.
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.
1373D DIGITAL DOCUMENTATION IN PETRA
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).
138 YAHYA ALSHAWABKEH et al.
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;
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).
1393D DIGITAL DOCUMENTATION IN PETRA
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.
140 YAHYA ALSHAWABKEH et al.
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
1413D DIGITAL DOCUMENTATION IN PETRA
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
142 YAHYA ALSHAWABKEH et al.
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