Geoelectric and VLF electromagnetic survey on complex

Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/125
Comunicações Geológicas (2012) 99, 2, 95-100
ISSN: 0873-948X; e-ISSN: 1647-581X
Geoelectric and VLF electromagnetic survey on complex
aquifer structures, Central Sudan
Pesquisa geoeléctrica e electromagnetica VLF em estruturas
aquíferas complexas, Sudão Central
N. E. Mohamed1*, H. Brasse2, M.Y. Abdelgalil1, K. M. Kheiralla1
Artigo original
Original article
Recebido em 15/11/2011 / Aceite em 11/05/2012
Disponível online em Maio de 2012 / Publicado em Dezembro de 2012
© 2012 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: The geophysical integration of the Geoelectric and
Electromagnetic methods was applied to map the complex aquifer
structures in northeast Nuba Mountains. The water flow is structurally
controlled by the northwest-southeast extensional faults as one of several
in-situ deformational patterns that are attributed to the collision of the
Pan-African oceanic assemblage of the Nubian shield against the pre-Pan
African continental crust to the west. This study involved the
reconnaissance techniques of the horizontal electric profiling (HEP),
vertical electrical soundings (VES), electrical resistivity tomography
(ERT), in addition to the electromagnetic very low frequency-resistivity
(VLF-R) profiling; to identify the major geological interfaces suspected
to be faults/fractured zones. These VLF-R measurements were designed
to be overlapped by the HEP, VES and ERT data to improve the
reliability of the observed data and to provide better interpretation of the
hydrogeological setting. This correlation is successfully delineated
several fracture zones boundaries and new target aquifers in the study
area.
by the fractures that are attributed to the collision of the PanAfrican oceanic assemblage of the Nubian shield against the prePan African continental crust to the west (Vail, 1983 & 1985).
The objectives of this study are twofold: to assess the integration
of the geoelectric and the very low frequency electromagnetic
data on the complex aquifer structure and to delineate new
aquifer targets in the studied areas. The selected study area is
bounded between latitude 11º - 12ºN, and 31º - 32ºE, and it
consists of four big villages/small town which are: AlTrtr,
AbuGeris, AlBetira, and AbuGebiha.
Keywords: VLF, geoelectric, Nuba Mountains, Sudan.
Resumo: Aplicou-se uma integração geofísica dos métodos geoeléctricos
e electromagnéticos para mapear as complexas estruturas aquíferas no
nordeste das Montanhas Nuba. O fluxo de água é controlado
estruturalmente pelas falhas extensionais noroeste-sudeste, um dos muitos
padrões de deformação in situ que são atribuídos à colisão do conjunto
oceânico pan-africano para oeste. Este trabalho envolveu técnicas de
reconhecimento do perfil eléctrico horizontal (HEP), sondagens elétricas
verticais (VES), tomografia de resistividade eléctrica (ERT) para além de
perfis de electromagnetismo de muito baixa frequência-resistividade
(VLF-R) para identificar as principais interfaces geológicas suspeitas de
serem zonas de falha/fracturação. Estas medições de VLF-R destinaramse a ser sobrepostas pelos dados de HEP, VES e ERT de modo a melhorar
a fiabilidade dos dados observados e de modo a fornecer melhor
interpretação do enquadramento hidrogeológico. Esta correlação delineou
com sucesso várias zonas de fractura e novos alvos para a presença de
aquíferos na área em estudo.
Palavras-chave: VLF, geoeléctrico, Montanhas Nuba, Sudão.
1
Faculty of Petroleum and Minerals, AlNeelain University, Khartoum, Sudan.
Fachrichtung Geophysik, Freie Universitaet Berlin, Germany.
*Corresponding author /Autor correspondente: [email protected]
2
Fig.1. The Nuba Mountains region lies ~550km from Khartoum in south Kordofan
State.
Fig.1. As Montanhas Nuba ficam a ~550km de Khartoum no sul do estado de
Kordofan.
1. Introduction
2. Geology and Tectonic Setting
The Nuba Mountains lies about 550km south west Khartoum
(Fig.1) in South Kordofan State and occupies about 140.000km2.
It is a crystalline basement uplift that is entirely surrounded by
Mesozoic to Cenozoic rocks filling several graben (Browne &
Fairhead, 1983). The groundwater flow is structurally controlled
Studies by Vail (1973), Shaddad et al. (1979) and Sadig & Vail
(1986) have established the basement divisions in the Nuba
Mountains as: the high-grade gneisses in the west and the lowgrade volcano-sedimentary sequence to the east (Fig.2). The
high-grade gneisses which are exposed as low outcrops overlain
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N. E. Mohamed et al. / Comunicações Geológicas (2012) 99, 2, 95-100
in places by thick Quaternary sediments (Abdelsalam &
Dawoud, 1991). The metamorphic and structural history of
northeastern Nuba Mountains supports a pre-Pan-African age
for the high-grade gneissic terrain. The Kabous ophiolitic
melange (~10 km wide) separates the high-grade gneissic
terrain from the low-grade volcano-sedimentary sequence, and
it can be traced for more than 70 km in a NNE direction
(Fig.2.). The low-grade volcano-sedimentary terrain occupied
the eastern part of northeastern Nuba Mountains; where the
four selected study areas are located; and is characterized by a
thick sequence of poly deformed meta-sedimentary and metavolcanic rocks. Thicknesses of these units range from a few
tens to a few hundreds of meters. The metamorphic mineral
assemblages of the rocks indicate green schist facies grade of
metamorphism.
The superficial deposits exist in the Nuba mountains region
as alluvial, which are head water sections of Wadies. The
deposits are mainly composed of sands with some clayey lenses
and the thickness of these alluvial may range from 8 to 25m.
The gradient of the Wadies decreases downstream with increase
in the clayey content, and the alluvial thickness increases and
reach the maximum at 70m and increases towards the White
Nile. The flow directions of these Wadies are related to the
geology mentioned above and they generally follow the
extensional fractures of NW-SE direction.
Abdelsalam & Dawoud, (1991) studied the structural
deformations of the northeastern Nuba Mountains which
recorded three phases of deformation (D1, D2 and D3) that are
almost completely destroyed the primary structures. The early
phase D1 is characterized by overturned isoclinal folds
associated with extensional lineation. D2 is the main phase and
is characterized by tight, slightly overturned easterly verging
folds dominates most of the southwestern part of the volcanosedimentary sequence. D3 is mainly cataclastic and forms E-W
trending wrench faults and shear zones which can be traced
over all northeastern Nuba Mountains.
3. Hydrogeology
The Nuba Mountains region is characterized by a tropical
summer climate (15°C - 35°C). Rainy seasons (April to October)
re relatively longer compare to other parts of Kordofan State with
an average annual rainfall 700 mm in the southern parts and 400
mm in the northern parts. The wadies flow NW towards the
White Nile (Fig.2), which it is apparently structurally controlled
by the NW-SE extensional faults as one of several in-situ
deformational patterns in the study area. Due to the dominant
sand dunes (qoz) west of the White Nile, it was difficult to know
if the ground water flow in the study areas is well developed; i.e.
reach the White Nile. The most pronounced drainage patterns are
linear, angular and dense dendritic. The major seasonal wadies
are: Tandik, AlBatha, Kabous, BanGadid, AbuRakuba, and
AlTrtr. In addition to numerous minor wadies that are irregular
and running parallel to the fault and joint systems of the
basement complexes, which have considerable amounts of
running water but it apparently seeps downs or highly evaporated
during the rainy season.
Potential groundwater can be found in a complex aquifer
system in the study area, where the weathering of the metasedimentary and meta-volcanic rocks, in addition to the main
fault zones mainly the extensional fractures where the water
reservoir is structurally controlled
4. Hydyrogeophysical Methods
As a consequence, both weathered layers and fracture zones play
an important role for groundwater supply as porous layers and
are considered hydraulic conductors. Moreover, fractures can
easily found by low-expense geoelectrical and electromagnetic
methods. Hence, two techniques were applied in this study to
evaluate this complex aquifer system, the geoelectric and the
electromagnetic methods are now routinely used as measuring
anomalous quantities especially with parallel profiling arrays.
Fig.2. Regional geological map of the Nuba Mountains region in Central Sudan.
Fig.2. Mapa geológico regional das Montanhas Nuba na região central do Sudão.
Geoelectric and VLF electromagnetic survey in Central Sudan
Due to open water-filled fissures, the resistivity within a fracture
zone is in general lower than the resistivity of the host rock even
for dry fracture zones due to enhanced weathering at open
fissures.
4.1. Geoelectric Survey
Three geoelectric techniques were applied: horizontal electrical
profiling (HEP) and vertical electrical sounding (VES), in
addition to the electrical resistivity tomography (ERT). A number
of 16 HEP were carried out using Terrameter SAS1000. A
winner array was performed and laid out in a NE-SW orientation.
A graphical plot of the apparent resistivity (ρa) versus AB/2 for
each sounding was plotted simultaneously. The geoelectric data
were compared to the VLF-R data qualitatively and
quantitatively. More than 23 VES were carried out using
Terrameter SAS1000. A shlumberger array was performed and
laid out mainly in a NW-SE direction along prominent
extensional fracture axis. A log-log plot of ρa versus AB/2 for
each sounding was plotted simultaneously and inverted in
IPI2Win software to calculate for the true resistivity values and
their relative thicknesses. Fig.3 shows the inverted results of
sounding TV19 and GrV11 as inversion examples, and the
equivalence problem was considered during the processing of
these VES data. HEP data were constrained by the nearest
inverted VES locations.
For comparative purposes, the ERT technique was chosen to
provide some redundancy to the HEP and VES data and to
improve the quality of the apparent resistivity responses. Based
on hydrogeological considerations, 16 ERT profiles were initially
measured using Wenner configuration crossing the NE-SW wadi
flow. Each ERT profile was inverted smoothly. This inversion
routine is a cell based inversion technique and subdivided the
subsurface into a number of rectangular cells whose positions and
sizes are fixed, which is used to determine the resistivity of cells
that provides a model response that agrees with the observed data
(Loke et al., 2003 and Günther, 2004).
Fig.3. Examples of inverted soundings showing the different curve types in TV10
and GRV11, where the later illustrated the equivalence problem of the 1D
inversion.
Fig.3. Exemplos de sondagens invertidas mostrando os diferentes tipos de curvas
em TV10 e GRV11, onde as últimas ilustram o problema de equivalência da
inversão em 1D.
4.2. VLF-R Survey
The very-low frequency-resistivity (VLF-R) is basically an
electromagnetic method which detects electrical conductors by
97
utilizing radio signals in the 15 to 30 kHz range. VLF
instruments compare the magnetic field of the primary
(transmitted) signal to that of the secondary signal (induced
current flow within the subsurface electrical conductor). When
a conductor is crossed, the magnetic field becomes elliptically
polarized and the major axis of the ellipse tilts with respect to
the horizontal axis (McNeill & Labson, 1991). As with other
frequency domain electromagnetic systems, both the in-phase
(real or tilt-angle) and the out-of-phase (imaginary, ellipticity,
or quadrature) components are measured.
The VLF-R profiles were collected with Omni-plus
system, manufactured by EDA Instruments Inc. (Now Scintrex
Ltd.). The VLF-Resistivity (VLF-R) method was applied as a
preliminary hydrogeological investigation of shallow structures
in NE Nuba Mountains, using the transmitter stations GBZ,
GBR and RHA as described in Table-1. The total magnitude of
the earth's magnetic field and secondary field components of
the primary field associated with these VLF stations were
measured and stored every 10 meters. A total of 16 VLF-R
profiles were carried out and thier responses at each station are
presented separately where each profile is almost parallel to the
regional strike of wadies with profile lengths vary from 300 to
1200 meters.
Table 1. The used VLF transmitter stations GBZ, GBR and RHA of 16.0, 19.6 and
23.4 kHz.
Tabela 1. As estações de transmissão VLF, GBZ, GBR e RHA usadas a 16.0, 19.6 e
23.4 kHz.
5. Integrated VLF and DC profiles
It was observed that AlTrtr wadi changes its direction northeast
wards instead of following the habitual direction of the
extensional fractures as in the other three studied areas, which
could be explained either by basement uplifting near VLF8 and
DC4 or the release fracture deformation of the NE-SW strike
may cause such effect. Both DCTP4 and VLFT8 profiles well
mapped the southern edge fracture of the wadi, while DCTP1,
VLFT9 and DCTP2 well mapped the northern edge fracture,
with an estimated fracture width of ~600m (Fig.4). The VES
data in DCTP1 and DCTP2 mapped the shallow basement at 8
to10m depth. Both VLF-R and DC profiles collected with 16.0
kHz in AlTrtr area showed shallow basement to the south
which is confirmed by the VES data in DC4.
In Albetira area, the drainage follows the NW-SE fractured
quartzite of AlBetira fold. Khor (wadi) BanGadid and Baggara
are well mapped and the high resistivity values in the middle of
the wadies indicate the low lying basement as was observed in
the field work. The interpretation of the VLF/HEP apparent
resistivity readings is qualitative in nature; this was combined
by VES data in which the current electrode spacings were
gradually increased up to 600 m for the delineation of deeper
structures, as shown in figures (4 and 5).
Wadi Baggara was revealed in VLF3 with ~250m width
while the corresponded DC1 was crossing a low line marble
outcrop. DC3 and DC4 profiles delineate the fracture
boundaries of wadi BanGadid while VLF2 mapped the effect of
buried basement in shallow depths observed in the middle of
the wadi as a high resistive dome anomaly.
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N. E. Mohamed et al. / Comunicações Geológicas (2012) 99, 2, 95-100
fracture zones. The apparent resistivity data were inverted
applying both smooth inversion and 2 layer-initial model robust
inversions. The inverted models are shown in the form of
contoured sections that help in visualizing the geological
structures. Generally these models are geoelectrically
subdivided into three zones: less than 20 ohm.m (clayey/clayey
weathered products), ~120 ohm.m (coarse grained materials)
and high resistive layer (hard rocks) up to 4000 ohm.m.
Fig. 4. Combined VLF-R and HEP (DC) profiles across AlTrtr wadies. Ro and D
are the calculated resistivity and depths to basement respectively from the adjacent
VES locations. Shallow basement was recorded in DCT4 and VLF-T8 and explains
the unordinary water flow toward the northeast direction.
Fig. 4. Perfis combinados de VLF-R e HEP (DC) ao longo de AlTrtr wadies. Ro e D
são a resistividade calculada e a profundidade à base respectivamente a partir das
localizações de VES adjacentes. Base pouco profunda foi registada na DCT4 e
VLF-T8 e explica o pouco usual fluxo de água para a direcção nordeste.
Generally, the integrated VLF and HEP (DC) profiling
combined with VES measurements was very efficient to map the
fracture zones in hard rock terrains. Subsequently the location of
these anomalous zones reveal that the fracture zones is AlTrtr
area are wider and shallower while in AlBetira area are narrower
and deeper especially that the VLF anomaly magnitude is larger
in AlBetira. It is likely that fractures will become dry in summer
season. However, fracture zones showing a large anomaly due to
deeper conductive zones in AlBetira will be more suitable for
groundwater exploitation for longer duration and it is unlikely
that they will dry in summer season. The folded mountainous
areas in AlBetira area represent a continuous recharging source
in the hard rock areas, and as the movement of the groundwater
takes place, the subsurface will become more productive due to
an increase in the second porosity.
6. ERT and Laterally constrained 2D-VLF inversion
For ERT data, a cell based inversion technique is commonly
used; it subdivided the subsurface into a number of rectangular
cells whose positions and sizes are mixed (Loke et al., 2003); to
determine the resistivity of cells that provides a model response
that agrees with the observed data. A commonly used inversion
for 2D and 3D resistivity inversion is the regularized leastsquares optimization method (Oldenburg & Li, 1994). Based on
the VES results and hydrogeological considerations, 16 imaging
profiles were initially measured using wenner configuration and
they crossed the NE-SW stream flow, which are prominent
Fig. 5. Combined VLF-R and HEP (DC) profiles across BanGadid and Baggara
wadies in AlBetira area. Ro and D are the calculated resistivity and depths to
basement respectively from adjacent VES locations. The low lying basement was
observed in the mid-wadi, which explains the anomalous high resistive values.
Fig. 5. Perfis combinados de VLF-R e HEP (DC) ao longo de BanGadid e Baggara
wadies na área de AlBetira. Ro e D são a resistividade calculada e a profundidade à base
respectivamente a partir das localizações de VES adjacentes. A base pouco profunda foi
observada no médio-wadi, o que explica os valores anómalos de alta resistividade.
The 2Layinv program, made by Markku Pirttijärvi (2006),
was used to interpret the electromagnetic VLF-R data (apparent
resistivity and phase) along a single profile at a single frequency.
The inversion is made separately for each data point using a onedimensional two-layer earth model which suits well the
interpretation of the thickness and resistivity of layers and the
resistivity variations of the basement rocks. Starting from an
automatic initial model linearized inversion with adaptive
damping was used to optimize the thickness and the resistivity of
the overburden layer and the resistivity of the basement so that
the model minimizes the error between the measured and the
computed VLF-R data (less than 9%). As a result, a laterally
constrained inversion and a smoothly varying model (Occam
inversion) were obtained although it is actually one-dimensional
model and that the interpretation results are shown as a 2-D
resistivity pseudo section as shown in VLFT9 from AlTrtr area
(Fig.6). More about smooth models from electromagnetic data
Geoelectric and VLF electromagnetic survey in Central Sudan
and Occam inversion are found in Constable & Parker (1987) and
deGroot & Constable (1990).
Fig. 6. Illustrates the VLF-R inversion routine as a 2D layer – laterally constrained
model of VLF-T9 in ALTrtr area.
99
The inverted VLF data has data misfit less than 9% while the
model misfit is less than 0.5%, this is a reasonable misfit range
used for different geophysical inversion. VLF-Gr1 (from south to
north) and VLF-Gr2 (from NE to SW) well mapped the southern
fracture edge of AbuGeris wadi (Fig.7) though the abundance of
the trees (Ketr trees) prevented further measurements towards the
northeastern direction.
In AbuGebiha area, the inverted VLF shows depths to
basement, as in VLF-Gb2 and the corresponded ERT-Gb4. Also
small resistive lenses are mapped at about 12m average depth
(Fig.8). The inverted VLF-Gb1, across Tandik wadi, delineate
the basement at ~12-37m depth while in ERT-Gb3 the basement
was mapped at deeper depths in the southwestern part (~20m),
nevertheless both VLF-Gb1 and ERT-Gb3 succeed to map the
fracture zone in relatively same depths. Generally, the combined
2D laterally constrained VLF data and the ERT data revealed the
depth to basement and located the fracture zones, Some VES
measurements are needed in to improve hydro geophysical model
of the fracture zones in this area especially 2km NW and SE from
Alomda outcrop which is south of DC1 profile (Fig.8).
Fig. 6. Ilustra a rotina de inversão VLF-R como uma camada 2D – modelo
lateralmente constrangido da VLF-T9 na área ALTrtr.
This inversion routine was used for all VLF-R profiles from
the different four studied areas, nevertheless no DC profiles were
prompted in AbuGeris and AbuGebiha areas, the 2D laterally
constrained VLF inversion in both areas will be compared to the
observed electric resistivity tomography (ERT) in these areas.
The VLF apparent resistivity ranges from 30 to >1000 ohm.m,
except of VLF Gb2 which crosses AlBatha wadi.
Fig. 8. A combination of VLF and ERT profiles across AlBatha and Tandik wadis in
AbuGebiha area, to detect the fracture zone. As shown, VLF-2 and ERT-4 do not
reach the basement and a moderate resistive layer is observed in both profiles at
~20m depth. The fractured basement is mapped in both VLF-1 and ERT-3.
Fig. 8. Uma combinação dos perfis VLF e ERTao longo de AlBatha e Tandik wadis
na área de AbuGebiha para detectar uma zona de fractura. Tal como mostrado,
VLF-2 e ERT-4 não chegam à base e é observada uma camada moderadamente
resistiva em ambos os perfis aos ~20m de profundidade. A base fracturada está
mapeada em ambos os VLF-1 e ERT-3.
6. Conclusions
Fig. 7. A combination of VLF and ERT profiles across AbuGeris wadi, to detect the
fracture zone and the basement is mapped as dashed lines with an average width of
200m.
Fig. 7. Uma combinação dos perfis VLF e ERTao longo de AbuGeris wadi para
detectar uma zona de fractura e a base está mapeada com linhas a tracejado com
uma largura média de 200m.
The VLF method is a very effective technique to locate zones of
high electrical conductivity, such as mineralized or water-filled
fractures or faults within the bedrock. Structures such as these
often act as conduits along which ground water and contaminants
flow. The combination of the VLF, VES and ERT data provide
qualitative and quantitative information about the lateral and
vertical variation of the basement surface and the fractured zones
varying from tens to few hundred meters width as shown in
100
AlTrtr area. More data are needed to cover the gap areas of
investigation in order to produce a reliable hydrogeological
model.
From the previous integrated geophysical measurements, the
best aquifer targets for potential water supply in each area can be
suggested as: the northern part of AlTrtr area and the wadi
junction where it changes its direction to the northeast. In
AlBetira area both Wadi Baggara and Wadi BanGadid are
promising areas except that the water quality of BanGadid is
better relatively than Baggara (Mohamed, et.al, 2011). In
AbuGebiha the southern parts of the town and along Wadi
AlBatha and Wadi Tandik of the town are considerably good
aquifer targets.
Acknowledgements
Thanks and appreciations are due to: the German Academic
Exchange Services (DAAD) for the financial support all over the
period of this study. Appreciation goes to the Frie University –
Berlin for providing the VLF equipment and to the field joined
team from the Geological Research Authority in Sudan (GRAS)
for providing the Resistivity Imaging equipment. Special
gratefulness is due to Dr. Thomas Gunter for accessing his
DC2DInvRes software.
References
Abdelsalam, M., Dawoud, A. S., 1991. The kabus ophiolitic melange,
sudan, and its bearing on the western boundary of the nubian shield.
Journal of the Geological Society, London, 148, 83-92.
Browne, S. E., Fairhead, S. D., 1983. Gravity study of the central african
rift system: A model for continental disruption, 1. the ngaoundere and
abu gabra rifts. Tectonophysics, 94, 187-203.
Constable, S., Parker, R., 1987. Occam' inversion: A practical alogrithm
for generating smooth models from elctromagnetic data. Geophysics,
52, 289-300.
N. E. Mohamed et al. / Comunicações Geológicas (2012) 99, 2, 95-100
deGroot Hedlin, C., Constable, S., 1990. Occam'inversion to generate
smooth, two dimensional models from magnetotelluric data.
Geophysics, 55, 1613-1624.
Günther, T. 2004. Inversion methods and resolution analysis for the
2D/3D reconstruction of resistivity structures from DC measurements.
Unpublished PhD Thesis, Univ. of Mining and Technology, Freiberg,
Germany.
Loke, M., Acworth, I., Dahkin, T., 2003. A comparison of the smooth and
blocky inversion methods in 2d electrical imaging surveys.
Exploration Geophysics, 34, 182-187.
McNeill, J., Labson, V., 1991. Geological mapping using vlf radio fields electromagnetic methods in applied geophysics ii. In: Nabighian, M.N.
(Ed.) Soc. Exp. Geophys., 521-640.
Mohamed, N.E., Yaramanci, U., Kheiralla, K.M., Abdelgalil, M.Y., 2011.
Assessment of integrated electrical resistivity data on complex aquifer
structures in NE Nuba Mountains, Sudan. Journal of African Earth
Sciences, 60, 337–345
Oldenburg, D., Li, Y., 1994. Inversion of induced polarization data.
Geophysics, 59, 1327-1341.
Pirttijärvi, M, 2006. 2Layinv program. A geophysical software for
laterally constrained two-layer inversion of VLF-R measurements.
Geophysics Division, Department of Geosciences, University of Oulu,
Finland.
Sadig, A., Vail, J. R., 1986. Geology and regional gravity traverses of the
nuba mountains, kordofan province, Sudan. Journal of African Earth
Sciences, 5, 329_338.
Shaddad, M. S., Kropachev, S. M., Khalil, B., 1979. Regional geological
setting of the nuba mountains in sudan. Annals geol. Surv of Egypt,
IX, 446-454.
Vail, J., 1973. Outline of the geology of the nuba mountains and
vicinity, southern Kordofan province, Sudan. Bull. GMRD,
Khartoum, 23, 24.
Vail, J., 1983. Pan-african crustal accretion in northeast african. Journal
of African Earth Sciences, 1, 285-294.
Vail, J., 1985. Pan-african (late precambrian) tectonic terrains and
reconstruction of the arabian nubian shield. Geology, 13, 839-842.