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GRAVITY AND GROUND MAGNETIC FOLLOW UP OF AEROMAGNETIC ANOMALY WEST Al-GARRAF RIVER, SOUTH IRAQ

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GRAVITY AND GROUND MAGNETIC FOLLOW UP OF AEROMAGNETIC ANOMALY WEST Al-GARRAF RIVER, SOUTH IRAQ

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ABSTRACT A more detailed gravity and ground magnetic surveys have been executed in a mostly agricultural area, which covers (28 × 40) Km and lies between Al-Nasiriya and Al-Kut Governorates, close to Al-Gharraf River and towards west. The surveys aim to determine deep structures within the sedimentary cover, by following-up a deep source anomaly, which appears in aeromagnetic and Unified Bouguer gravity maps. A net of polygons, including 868 gravity and magnetic stations with spacing interval of 0.5 Km, has been measured. Bouguer anomaly and total magnetic intensity (TMI) maps are constructed. Filters for enhancing shallow and deep source anomalies and high gradient areas were applied to Bouguer anomaly and TMI maps. The Bouguer anomaly map shows a prominent gravity high in ENE – WSW direction. In addition, residual anomalies that may reflect antiforms and synforms or faults are pointed out on this map. Gravity profiles across the gravity high and some residual positive anomalies are plotted. On the other hand, the TMI map shows a magnetic high, which has the same direction, extension, length and location of the gravity high. The magnetic high is, essentially, related to a basement intrusion, and it may be a causative source for the gravity high. Two magnetic profiles across and along this high are displayed. The study of depth maps of four seismic reflectors showed no anomalous structures down to the Permian’s surface (~ 6000 m deep), while it shows considerable paleostructures on this surface. Moreover, the magnetic high (the intrusion) is older than the Permian and it is (6000 – 9000) m deep. However, local magnetic anomalies (LMA) related to Quaternary gray sand sediments, can easily be recognized throughout the area. The LMA tend to disturb the earth’s magnetic field. However, upward continuation filter is applied to remove the effect of these anomalies.
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Iraqi Bulletin of Geology and Mining Vol.9, No.3, 2013 p 85 102
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GRAVITY AND GROUND MAGNETIC FOLLOW UP OF
AEROMAGNETIC ANOMALY WEST Al-GARRAF RIVER,
SOUTH IRAQ
Hayder A. Al-Bahadily1and Ahmed S. Musa١
Received: 00/ ٠0/ 2000, Accepted: 00/ ٠0/ 2000
Keywords: Gravity and Magnetic surveys, Aeromagnetic anomaly, Integrated geophysical studies, Iraq
ABSTRACT
A more detailed gravity and ground magnetic surveys have been executed in a mostly
agricultural area, which covers (28 × 40) Km and lies between Al-Nasiriya and Al-Kut
Governorates, close to Al-Gharraf River and towards west. The surveys aim to determine
deep structures within the sedimentary cover, by following-up a deep source anomaly, which
appears in aeromagnetic and Unified Bouguer gravity maps. A net of polygons, including 868
gravity and magnetic stations with spacing interval of 0.5 Km, has been measured. Bouguer
anomaly and total magnetic intensity (TMI) maps are constructed. Filters for enhancing
shallow and deep source anomalies and high gradient areas were applied to Bouguer anomaly
and TMI maps. The Bouguer anomaly map shows a prominent gravity high in ENE – WSW
direction. In addition, residual anomalies that may reflect antiforms and synforms or faults are
pointed out on this map. Gravity profiles across the gravity high and some residual positive
anomalies are plotted. On the other hand, the TMI map shows a magnetic high, which has the
same direction, extension, length and location of the gravity high. The magnetic high is,
essentially, related to a basement intrusion, and it may be a causative source for the gravity
high. Two magnetic profiles across and along this high are displayed. The study of depth
maps of four seismic reflectors showed no anomalous structures down to the Permian’s
surface (~ 6000 m deep), while it shows considerable paleostructures on this surface.
Moreover, the magnetic high (the intrusion) is older than the Permian and it is (6000 9000)
m deep. However, local magnetic anomalies (LMA) related to Quaternary gray sand
sediments, can easily be recognized throughout the area. The LMA tend to disturb the earth’s
magnetic field. However, upward continuation filter is applied to remove the effect of these
anomalies.
ﻲﻓ يوﺟﻟا ﻲﺳﯾطﺎﻧﻐﻣﻟا ﺢﺳﻣﻟا ةذﺎﺷﻟ ﺔﯾﺿرﻷا ﺔﯾﺳﯾطﺎﻧﻐﻣﻟاو ﺔﯾﺑذﺟﻟا ﺔﻌﺑﺎﺗﻣﻟا
ﺔﻘطﻧﻣ بوﻧﺟ ،فاّرﻐﻟا رﮭﻧ برﻏقارﻌﻟا
ﻰﺳوﻣ مﻟﺎﺳ دﻣﺣأ و ﻲﻟدﺎﮭﺑﻟا نﺎﻧدﻋ ردﯾﺣ
صﻠﺧﺗﺳﻣﻟا
نﻣﺿﺗﯾثﺣﺑﻟاﻲﻟﺎﺣﻟاءارﺟإﺢﺳﻣﻲﺳﯾطﺎﻧﻐﻣﻲﺑذﺟ،رﺛﻛأﻼﯾﺻﻔﺗنﻣتﺎﺣوﺳﻣﻟا،ﺔﻘﺑﺎﺳﻟاﻲﻓﺔﻘطﻧﻣتاذﺔﻌﯾﺑط
ﺔﯾﻋارزﻎﻠﺑﺗﺎھدﺎﻌﺑأ)١٨ ×٤٠ (مﻛﻊﻘﺗونﯾﺑﻲﺗظﻓﺎﺣﻣطﺳاويذو،رﺎﻗﻰﻟإبرﻐﻟانرﮭﻧفاّرﻐﻟاكﻟذوضرﻐﻟﺔﻌﺑﺎﺗﻣ
ةذﺎﺷﺢﺳﻣﻠﻲﺳﯾطﺎﻧﻐﻣﻟايوﺟﻟاةرھﺎظﻟاﻲﻓهذھ،ﺔﻘطﻧﻣﻟاةددﺣﻣﻟاوﺎﻘﺑﺳﻣﺎﻘﻓوﺔﺳاردﻟطﺋارﺧﻟاﺔﯾﻣﯾﻠﻗﻹاﺔﯾﺳﯾطﺎﻧﻐﻣﻟا
ﺔﯾﺑذﺟﻟاوةرﻓوﺗﻣﻟا .نإردﺻﻣهذھةذﺎﺷﻟا ًةدﺎﻋوھقﺎﺛﺑﻧايرﯾﮭﺻ)intrusion (يدﻋﺎﻗوأطﺳوﺗﻣﻊﻘﯾبرﻘﻟﺎﺑنﻣةدﻋﺎﻘﻟا
روﻠﺑﻟاﺔﯾيذﻟاونﻣنﻛﻣﻣﻟانأنوﻛﯾ ًاردﺻﻣ ًﺎﺑﺑﺳﻣبﯾﻛارﺗﻟتﺣﺗﺔﯾﺣطﺳةدﯾﻔﻣﻲﻓتﺎﻓﺎﺷﻛﺗﺳﻻاﺔﯾطﻔﻧﻟاﻲھوفادھأ دﺣأ
هذھﺔﺳاردﻟا .لﻣﺷﺢﺳﻣﻟاسﺎﯾﻗ٨٦٨ﺔطﺣﻣ)ﺔﯾﺑذﺟﺔﯾﺳﯾطﺎﻧﻐﻣو(،ﺔﻓﺎﺳﻣﺑوﺔﻠﺻﺎﻓﺎھرادﻘﻣ٥٠٠،مﺔﻋزوﻣﻠﻋﺔﻛﺑﺷ
نﻣتﺎﻌﻠﺿﻣﻟاتطﻏﺔﻘطﻧﻣلﻣﻌﻟا .تدﻋأﻲﺗطرﺎﺧذاوﺷرﯾﺟووةدﺷلﺎﺟﻣﻟاﻲﺳﯾطﺎﻧﻐﻣﻟا.....
_____________________________________________________________________________
1AssistantChief Geophysicist, Iraq Geological Survey, P.O. Box 986, Baghdad, Iraq
Gravity and Ground Magnetic Follow Uup of Aeromagnetic Anomaly West Al-Garraf
River, South Iraq Hayder A. Al-Bahadily and Ahmed S. Musa
٨٦
ﻲﺿرﻷاﻲﻠﻛﻟاﺔﻘطﻧﻣﻠﻟتﻘﺑُطوتﺎﺣﺷرﻣةددﻌﺗﻣﻰﻠﻋنﯾﺗطرﺎﺧﻟازارﺑﻹذاوﺷﺔﻧﯾﻌﻣوأقطﺎﻧﻣﻻاتارادﺣﻧﯾﻟﺎﻌﻟا،
ًﺔﻓﺎﺿإﻰﻟإنﯾﺳﺣﺗةروﺻﻟاﺔﯾﺳﯾطﺎﻧﻐﻣﻟاﺔﯾﺑذﺟﻟاو .تﻧﯾﺑﺔطﯾرﺧرﯾﺟوﺑدوﺟوﻊﻔﺗرﻣﻲﺑذﺟﻲﯾﻣﯾﻠﻗإزرﺎﺑدﺗﻣﯾﺔﻓﺎﺳﻣﻟ١٧
مﻛﺎﺑﯾرﻘﺗنﻣﺿﺔﻘطﻧﻣﻟاهﺎﺟﺗﺎﺑوقرﺷلﺎﻣﺷقرﺷ، ًﺔﻓﺎﺿإﻰﻟإرﯾﺷﺄﺗددﻋنﻣمﻟﺎﻌﻣﻟاﺔﯾﺑذﺟﻟاﺔﯾﻘﺑﺗﻣﻟاﺔﺑﺟوﻣﻟاﻲﺗﻟاونﻣ
نﻛﻣﻣﻟانأدوﻌﺗﺎھردﺎﺻﻣﻰﻟإتﺎﻌﻔﺗرﻣﺑﯾﻛارﺗﺔﯾنﻣﺿءﺎطﻐﻟاﻲﺑوﺳرﻟا .ﺎﻣﻛمﺳرﻊطﻘﻣﻲﺑذﺟيدوﻣﻋﻰﻠﻋﻊﻔﺗرﻣﻟا
ﻲﺑذﺟﻟارﺧآوﻰﻠﻋذاوﺷﻟاﺔﯾﺑذﺟﻟاﺔﯾﻘﺑﺗﻣﻟاﺔﺑﺟوﻣﻟاﺔﺳاردﻠﻟﺔﯾﻔﺻوﻟا .نﻣﺔﯾﺣﺎﻧ،ىرﺧأترﮭظأرﺔطيوﺎﺳﺗةدﺷﻟا
ﺔﯾﺳﯾطﺎﻧﻐﻣﻟاﺔﯾﻠﻛﻟادوﺟوﻊﻔﺗرﻣﻲﺳﯾطﺎﻧﻐﻣﮫﻟصاوﺧرﺎﻘﻣﻊﻔﺗرﻣﻠﻟﻲﺑذﺟﻟانﻣﺔﯾﺣﺎﻧ،ﻊﻗوﻣﻟاﻻادادﺗﻣ،ﻻاهﺎﺟﺗ،قﻣﻌﻟاو
نﻣوانﻛﻣﻣﻟنوﻛﯾردﺻﻣنﯾذھنﯾﻌﻔﺗرﻣﻟا)ﻲﺑذﺟﻟاﻲﺳﯾطﺎﻧﻐﻣﻟاو (وھدﺣاوﻊﻘﯾﺎﺑﯾرﻗنﻣروﺧﺻةدﻋﺎﻘﻟا .تﻣﺗﺔﺳارد
اذھﻊﻔﺗرﻣﻟانلﻼﺧإدادﻋنﯾﻌطﻘﻣنﯾﯾﺳﯾطﺎﻧﻐﻣ،أﺎﻣھدﺣيدوﻣﻋرﺧﻷاورﻣﯾةاذﺎﺣﻣﺑهروﺣﻣ .ترﮭظﻛﺷﺑولﺢﺿاو
تارﯾﺛﺄﺗذاوﺷﻟﺔﯾﻠﺣﻣدوﻌﺗﻰﻟإتﺎﺑﺳرﺗﺔﯾﻠﻣرﺔﯾﺻﺎﺻرنوﻠﻟاتاذرﻣﻌﻟا،ﻲﻋﺎﺑرﻟاوﻲﻓةدﻋنﻛﺎﻣأنﻣﺔطﯾرﺧﻟا،تدأ
ﻰﻟإتﺎﮭﯾوﺷﺗﺔﯾﻠﺣﻣةروﺻﻟلﺎﺟﻣﻟاﻲﺳﯾطﺎﻧﻐﻣﻟاﻲﻣﯾﻠﻗﻹادﻗوتﻠﯾزُأةروﺻﺑﺔﻣﺎﺗنﻣطﯾرﺧﻟامادﺧﺗﺳﺎﺑﺢﺷرﻣ
رارﻣﺗﺳﻻادﻋﺎﺻﻟالﺎﺟﻣﻠﻟﻻوعﺎﻔﺗرهرادﻘﻣ١٠٠٠م،ﺎﻣﻛترﺷأتارﯾﻐﺗﻟاﻲﻓطﻣﻧبﯾﺳﺎﻧﻣلﺎﺟﻣﻟاﻰﻠﻋلﻛﺷقﻟﺎﻓوأ
قطﺎﻧﻣفﻌﺿنﻣﺿردﺻﻣﻊﻔﺗرﻣﻟاﻲﺳﯾطﺎﻧﻐﻣﻟا .تﻧﯾﺑﺔﺳاردارﺧﻟاط ﱠﯾﻘﻣﻌﻟاﺔﺛﻼﺛﻟسﻛاوﻋﺔﯾﻟازﻟزمدﻋدوﺟوتارﯾﺛﺄﺗ
ﺔﯾﺑﯾﻛرﺗﺔﺣﺿاوﺔﺟﺗﺎﻧنعﺎﻓدﻧﻻايرﯾﮭﺻﻟاسوردﻣﻟاﺔﯾﺎﻐﻟقﻣﻋ٦٠٠٠م)ﺢطﺳﻟاﯾﭘﻲﻣر(،ﺎﻣﯾﻓتﻧﯾﺑدوﺟوتﺎﻌﻔﺗرﻣ
ﺑﯾﻛارﺗﺔﯾدﻗنوﻛﺗﺞﺗاوﻧةرﺷﺎﺑﻣتارﯾﺛﺄﺗﻟاذھعﺎﻓدﻧﻻاﻰﻠﻋﺢطﺳﻟاروﻛذﻣﻟا . ًﺔﻓﺎﺿإكﻟذﻟ،نﻣﻓدﻘﺗﻌﻣﻟانإنوﻛﯾرﻣﻋاذھ
ﻻاقﺎﺛﺑﻧمدﻗأنﻣرﺻﻌﻟاﻟاﯾﭘﻲﻣرﮫﻘﻣﻋورﺛﻛأنﻣ٦٠٠٠م.
INTRODUCTION
Although, the aeromagnetic maps of CGG (1974) and the unified Bouguer gravity (UBG)
maps by Al-Kadhimi and Fattah (1984) have regional nature, these maps are usually used in
primary stages of delineating the deep geological structures that have special importance in
hydrocarbon explorations. However, many gravity surveys of relatively small scales, which
carried out in the Western Desert (e.g. Al-Bdaiwi, 2004), show mismatching with UBG. The
main reason of this mismatching is attributed to relatively large spacing interval between grid
lines used in UBG; 5 Km in the Mesopotamian Plain and 10 Km in the Western Desert, with
spacing interval of 1 Km between stations. For the purpose of determining deep sedimentary
structures, an important anomaly has been suggested for further detailed land magnetic and
gravity surveys (Al-Bdaiwi, 2010a). The anomaly covers an agricultural area of about
(40 × 28) Km and located south of Al-Kut City. In the aeromagnetic map, the anomaly has
amplitude of 20 nT and 18 Km wave length, trending ENE WSW. The CGG (1974)
interpreted this anomaly as an intrusion of 7.5 Km deep (Fig.1). On the other hand, Bouguer
gravity map expresses a gravity high and relatively broad gravity saddle. It appears to have a
relationship with the magnetic anomaly (Fig.2).This magnetic gravity anomaly may be
related to supra basement magnetic body that could eventuate anomalous structure within the
deeper part of sedimentary column (Al-Bdaiwi, 2010a). The present study focuses on the
gravity and ground magnetic surveys, carried out to follow-up this anomaly, and their results.
The agricultural nature of the studied area, which is covered by very complicated
irrigation and drainage channels, made the accessibility to the measuring points difficult.
Therefore, the field work was executed along polygons rather than regular grid. A Google
Earth image and topographic maps at scale of 1: 100000 were used as a base map to delineate
the polygons along available tracks and paved roads with a measuring step of 500 m, for the
area under investigation. The total number of the measured stations is 868.
Location
The studied area is located between Nasiriya and Wasit Governorates, at a distance of
about 65 Km to the south of Al-Kut city, it occupies Al-Fajir town. Al-Gharraf River passes
through the eastern side of the studied area which is bounded by the following coordinates
(Fig.3).
Longitude
45 º 42'
45 º 55'
45 º 51'
46 º 03'
Iraqi Bulletin of Geology and Mining Vol.9, No.3, 2013 p 85 102
٨٧
Latitude
32 º 02'
32 º 08'
31 º 46'
31 º 50'
Fig.1: Aeromagnetic map of CGG (1974) exhibits the studied anomaly and its relation with
the surroundings. The lower part shows magnetic cross section along the axis of this anomaly
The proposed axis of the magnetic
intrusion (CGG, 1974)
Gravity and Ground Magnetic Follow Uup of Aeromagnetic Anomaly West Al-Garraf
River, South Iraq Hayder A. Al-Bahadily and Ahmed S. Musa
٨٨
Fig.2: Bouguer gravity map of the studied area (after Al-Kadhimi and Fattah, 1984)
٤٦º 00
٣٢º 00
Legend
Base station (Bs1)
Area of no geophysical
measurements
Iraqi Bulletin of Geology and Mining Vol.9, No.3, 2013 p 85 102
٨٩
Fig.3: Location map of the studied area and the distribution of 868 measuring stations
Aim
The present work aims to execute detailed gravity and ground magnetic surveys to
determine deep sedimentary structures that may be resulted from the source of the magnetic
anomaly. It also aims to verify the nature (depth, direction, dip and extension) of the causative
source of the concerned aeromagnetic and gravity anomalies.
GEOLOGY OF THE AREA
The studied area is completely covered by Quaternary sediments of Mesopotamian Plain.
The sediments are different in type; including flood plain sediments (sand, silt and clay), sand
dunes, sand sheet sediments, marsh sediments (mud with organic materials) and shallow
depression sediments (clay and silt clay) (Barwary and Yacoub, 1992 and Deikran and Mahdi,
1993).
Geomorphologically, the studied area has almost flat terrain and the average height of the
ground surface is 11 m (a.s.l.). Al-Gharraf River, which has the main net of irrigation
channels, passes through the eastern side of the area. Active sand dunes of Barchan type cover
the northwestern part of the area (Deikran and Mahdi, 1993); therefore, performing
geophysical measurements in this part was not possible (Fig.3).
Tectonically, the area is related to Mesopotamian Foredeep of the Outer Platform (Fouad,
2012). Jassim and Goff (2006) mentioned that "the studied area is located in the Euphrates
Subzone (the western part of the Mesopotamian Zone, which in turn forms the extreme
eastern unit of the Stable Shelf). The Euphrates Subzone is a monocline dipping to the NE
with short anticlines (< 10 km) and structural noses and it is the shallowest unit in the
Mesopotamian Zone. The basement is generally (7 9) Km deep, but the Subzone has thick
Quaternary sediments compared with the Tigris Subzone. Mesopotamian Zone contains
buried faulted structures below the Quaternary cover, separated by broad synclines. A major
N S trending gravity and magnetic high is located along the Gharraf River, between the
Tigris and Euphrates rivers, which may indicate a buried Hercynian structure". They also
mentioned that the Quaternary sediments alone are up to 300 m thick in the Mesopotamian
Zone. Anticlines and horsts lie beneath undeformed or gently deformed Neogene cover, and
are frequently related to long-lived paleostructures (Aqrawi et al., 2010).
It is worth mentioning that the sedimentary column in Iraq is practically not-magnetic; i.e.
the sedimentary formations of this column did not contain appreciable magnetic minerals to
create discernible anomalies even if these formations were displaced vertically with
considerable throw (Al-Bdaiwi, 2010b). However, some Quaternary sediments, derived from
igneous and/or metamorphic rocks, contain very small quantities of magnetite and hematite
minerals may disturb the magnetic field forming local magnetic anomalies.
PREVIOUS GEOPHYSICAL STUDIES
Except the gravity and aeromagnetic surveys, which covered most of the Iraqi territory and
have regional nature, no particular geophysical study was performed in the studied area. These
surveys are:
Regional aeromagnetic and aerospectrometric survey of Iraq carried out by CGG (1974).
The total magnetic intensity map shows many residual magnetic anomalies that may be related
to intrasedimentary igneous intrusions. A magnetic anomaly appears in aeromagnetic map
have been chosen to be followed up by ground magnetic survey (the present work).
Gravity and Ground Magnetic Follow Uup of Aeromagnetic Anomaly West Al-Garraf
River, South Iraq Hayder A. Al-Bahadily and Ahmed S. Musa
٩٠
Regional gravity surveys; unified in a Bouguer gravity map of Iraq by Al-Kadhimi and
Fattah (1984). The studied area expresses a relatively broad gravity saddle, which has the
same trend as that of the magnetic anomaly.
Interpretation of seismic survey in the area east of Al-Diwaniya (including the studied area)
by OEC (1991) determined four reflectors. Depth maps of these reflectors show increasing
depths from the southwest to the northeast (as shown below), i.e. dipping beds towards the
north east as follows:
- Top of Hartha Formation (1800 – 2150) m
- Top of Shu'aiba Formation (2900 – 3300) m
- Top of Triassic (4500 – 5100) m
- Top of Permian (5700 – 6300) m
Paleostructures however, exist at the top of the Permian only (no structures appear
shallower than 5100 m). Accordingly, it could be concluded that the sedimentary cover
overlaying the Permian has not been strongly affected by the Alpine stresses. Furthermore,
the dip of the strata within the studied area is also estimated to be (1.5 – 2) º.
It is worth mentioning that a similar study to the present work was reported by Amin et al.
(2010), where gravity and geomagnetic surveys were executed in the southeastern part of Iraq
between Amara and Qurna in an area covered by (70 × 120) Km to fill a gap of previously
executed gravity and magnetic surveys. The surveys, which were carried out along polygons
with stations of 1 Km apart, aimed to make regional geophysical mapping. Although, the
magnetic survey displayed the regional picture, but the Bouguer gravity map showed two
impressive anomalies that probably reflect associated hydrocarbon structures. The accuracy of
the gravity measurements was (0.022) mGal.
GEOPHYSICAL AND TOPOGRAPHIC WORKS
The present geophysical work includes gravity and ground magnetic works. Both gravity
and magnetic measurements were acquired at the same station and time. There are however,
few exceptions, especially, when the station has high noise level in gravity; the magnetic
reading has been acquired only, and vice versa. For the sake of accuracy and quality control,
15% of the total measurements were repeated at the same day; as repeated points and 10% of
them were repeated at the different days; as control points. The location, readings and time are
recorded at each station as well as any relevant topographic or geologic information and
details of any visible or suspected source of noise.
The topographic works included stabilizing basic gravity station and stations of
measurements that required positions and elevations. GPS of Garmin Version 12XL type with
accuracy of ± 2 m is used for adjusting the positions, while Digital Total Station devices, type
Topcon, version 721 and type Lica, version 405 was used for calculation of elevations. The
accuracy of elevations is ± 3.7 cm/Km.
It is important to mention two factors that affecting the geophysical surveys. The first one
is the agricultural nature of the area, which has prevented the performance of a regular grid;
therefore, polygons are chosen along the available tracks and paved roads. The second is the
microseismicity related to earthquakes, happened in Oct. 24th (southeast of Turkey), Nov. 29th
and Dec. 24th 2011, have strongly affected on gravimeter and gravity measurements, where the
field work has been completely stopped.
Iraqi Bulletin of Geology and Mining Vol.9, No.3, 2013 p 85 102
٩١
Gravity Survey
Two types of gravimeters have been used in the this work; the first is the modern CG-5
(Sceintrex Autograv System) gravity meter, manufactured by Canadian Sceintrex Limited
Company, with sensitivity of ± 1 µGal. Corrections provided in the system include Tide,
Instrument Tilt, Temperature, advanced Noisy Reading Rejection, Seismic Noise Filter/ FIR
Filter and Near Terrain Corrections. The second gravimeter is LaCost & Romberg model D
gravity meter with sensitivity of ± 1 µGal. The latter instrument was used due to a fault that
affected the first instrument, which had already completed some 20% of the survey. The two
sets of measurements of both gravimeters have been tied and adjusted by adding more control
points among the two sets. Auxiliary base station (Bs.1) (Fig.3) was established inside the
studied area and tied with a reference base station (Bs.0) by the usual looping for interpolation
of the absolute gravity value for each measuring station. The mean error (M) of gravity
measurements is ± 0.033 mGal.
Processing of gravity raw data includes applying the necessary corrections and filtering,
gridding and mapping. Data filtering, gridding and mapping have been accomplished with the
aid of the well-known package of geophysical programs Oasis MontajTM (Geosoft). All the
required corrections including latitude, free air and Bouguer have been applied on the raw
data, and then Bouguer anomaly value for each station has been calculated using the
following formula: gb= gobs + CF– CB– gØ
Where:
gb; Bouguer anomaly
gobs; the observed gravity value
gØ; the theoretical value of gravity
CF; free air correction
CB; Bouguer correction by using density value equal to 2.00 gm/cm3
The corrected data have been gridded using minimum-curvature method, then mapped to
produce Bouguer gravity map for the whole studied area (Fig.4). It is worth to mention that
the corrected data represent local relative gravity survey where the values are not tied to the
national anomaly map (UBG).
Qualitative Interpretation
Bouguer anomaly map of the studied area (Fig.4) displays gravity lows and highs. These
are described hereinafter:
The Gravity Lows: Two gravity lows dominate the studied area; the first is the
equidimensional low; situated at the northern part; pointed out as Ve1, with its southward
extension (marker G3) and the second is situated at the south; pointed out as Ve2.
The – Ve1 has a Bouguer value of − 60.0 mGal and a diameter about 20 Km with maximum
amplitude of about − 2.0 mGal, while the – Ve2 is a part of a regional gravity low as appears
when looking at the surroundings (Fig.2). Actually, the two gravity lows (− Ve1 and Ve2)
are related to the same source i.e. the negative background that dominate the Mesopotamian
Foredeep, as will be discussed later in the results and discussion paragraph.
Gravity and Ground Magnetic Follow Uup of Aeromagnetic Anomaly West Al-Garraf
River, South Iraq Hayder A. Al-Bahadily and Ahmed S. Musa
٩٢
Fig.4: Bouguer gravity map of the studied area
Legend
+Ve1 Gravity high with number
-Ve2 Gravity low with number
_ _ _ _ _ G3 Extension of gravity low
+++++++ G1 Gravity saddle
Iraqi Bulletin of Geology and Mining Vol.9, No.3, 2013 p 85 102
٩٣
The Gravity Highs: Bouguer anomaly map seems to be composed of two gravity highs
(+ Ve1 and + Ve2). The gravity high (+ Ve1), extends eastward; represented by a gravity
saddle; pointed out as marker G1 and trending ENE WSW. The marker has amplitude of
about + 1.75 mGal, while the maximum values of the gravity highs; + Ve1 and + Ve2 are
about + 5.0 mGal and 2.5 mGal, respectively.
Different types of filters have been applied to the Bouguer map to enhance shallow or
deep anomalies (Geosoft, 2010). Low Pass filter (LP) has been applied to the Bouguer map to
show the regional anomalies (Fig.5). The used wavelength cut off is 14 Km and the expected
depth of these anomalies is 8.5 Km. However, vertical derivative filter has been applied to
enhance the residual anomalies (Fig.6). Positive residuals appear superimposed on the main
negative, i.e. markers G7 trending NNW SSE, and the markers G8 and G2, trending N – S,
which may be antiforms structures. Impressive positive residual markers are G5 and G6
trending ENE –WSW. These are superimposed on the main gravity high (the +Ve1, G1
and +Ve2). They may reflect the effect of the causative source within shallower part of the
sedimentary cover. Total Horizontal Derivative (THD) filter; determines the high gradient
areas, which have a direct relation with faults, edges and the boundary of the main effective
structures (Fig.7). The red color delineates the high gradient areas or the boundary in which
the density encounters high changes, whereas the blue color represents the areas of no
changes in density. The high gradient delineates the boundary of the causative source of the
gravity anomaly.
To explain the variation of the gravity field and also the shape and amplitude of the
anomalies along the gravity high and the gravity low, two gravity profiles; AB in the
direction N S and CD in the direction E – W, have been constructed (Figs.8 and 9). Profile
AB shows the variation of the gravity curve across the gravity high. A distance of about
20 Km displays the variation from the gravity low to the gravity high that reflects the boundry
in which the density of the rocks encountered high variations. The imposed rsidual anomalies
on the rigional field are easyily recognized in this profile, too. In Fig. (9), the residual positive
anomalies imposed on the –Ve1 have been foucased. These anomalies are exagurated by
using the first drevative filter, which is in turn smoothed by B-spline filter (the line of blue
color). The positive residual is of interst when looking at antiforms structures.
Gravity and Ground Magnetic Follow Uup of Aeromagnetic Anomaly West Al-Garraf
River, South Iraq Hayder A. Al-Bahadily and Ahmed S. Musa
٩٤
Fig.5: Low pass filter applied to the Bouguer gravity map
Iraqi Bulletin of Geology and Mining Vol.9, No.3, 2013 p 85 102
٩٥
Fig.6: Vertical derivative filter shows residual anomalies of
the studied area
Fig.7: Total Horizontal Derivative filter (THD) of Bouguer map
Gravity and Ground Magnetic Follow Uup of Aeromagnetic Anomaly West Al-Garraf
River, South Iraq Hayder A. Al-Bahadily and Ahmed S. Musa
٩٦
North South
Fig.8: Gravity profile across the gravity high (For location, refer to Fig.4)
Fig.9: Gravity profile across the gravity high (For location, refer to Fig.4)
Magnetic Method
Two magnetometers of Proton type, which measure the total magnetic field known as
Magnetic Measurement System (portable and base station) ENVI PRO with magnetic
sensitivity of ± 0.1 nT, have been used in the survey. Generally, the area has low noise level
and the readings are stable. However, some Quaternary sediments that contain sand (derived
from igneous or metamorphic rocks) been affected the regional field value in some places.
These sediments are able to disturb the local field and form Local Magnetic Anomalies
(LMA) (Al-Bahadily and Yousif, 2012). The accuracy of the magnetic measurements is
± 0.9 nT. It has been calculated depending on 101 control points.
Processing of magnetic row data included applying diurnal correction, normal correction
and filtering, gridding and mapping. Normal correction has been applied on the magnetic
measurements by using the IGRF equations, which is included in Geosoft, updated in 2010.
IGRF provides a reasonable representation of the actual regional field in the surveyed area
(Milsom, 2003). The resulted data have been gridded using minimum curvature method, then
mapped to produce Total Magnetic Intensity Map (TMI).
B
A
East
West
C
D
Iraqi Bulletin of Geology and Mining Vol.9, No.3, 2013 p 85 102
٩٧
Qualitative Interpretation
The TMI map of the studied area (Fig.10) shows a magnetic high situated at the
southwestern part. The high has an extension in ENE – WSW direction for a distance of about
20 Km with amplitude of about 54 nT. The extension is assigned in Fig. (10) as marker M1.
Basically, all the regional magnetic anomalies, which appear in the aeromagnetic map,
including the present magnetic high, are related to the crystalline basement.
The magnetic picture of the studied area is relatively disturbed by superficial or Local
Magnetic Anomalies (LMA) caused by Quaternary gray-sand sediments that contain small
quantities of magnetite mineral (Al-Bahadily et al., 2012 and Al-Bahadily and Yousif, 2012).
The LMA have amplitudes, which did not exceed 6 nT, with short wavelengths and different
shapes. The residual map after subtracting the regional field from the TMI is displayed in
Fig. (10). The effect of the LMA on the total field is eliminated and it is clearly shown in
Fig. (11). However, to remove this effect, the upward continuation filter with 1000 m
elevation has been applied on the TMI map to produce the regional map (Fig.12). Figure (12)
shows a considerable enhancement of the TMI map, where the effects of near surface
magnetic sources is totally removed, the contour lines become smoother and the regional
picture appears much better. Moreover, marker F has been added on this map in the direction
of ENE – WSW, it can be delineated as a fault or boundary between two distinctive materials
of different magnetic susceptibilities. Obviously, two impressive negative anomalies pointed
out as markers M2 and M3 on the TMI map, have been enhanced clearly, when applying the
upward continuation filter. These anomalies are probably related to deep weakness zones
within the magnetic source.
To explain the shape of the magnetic field across and along the magnetic high, two
magnetic profiles in N S direction (Fig.13) and in E W direction (Fig.14) have been
constructed. In Fig. (13), the variations of the total magnetic field from the north and south
with TMI value 45675 nT and 45730 nT, respectively are explained. Clearly, the gradient is
not constant, the highest gradient occurs at a distance around 15000 m and this reflects high
variation in magnetic susceptibility. Moreover, the profile at its southern end refers that the
magnetic high has an extension outside of the studied area. Similarly, in Fig. (14), the
magnetic profile exhibits a continuous increase in the magnetic field towards the west, while
it shows a relative constancy in the field at the middle part of the profile. However, the high
gradient part near to a distance of about 18000 m may refer to the area of high contrast in
magnetic susceptibility.
Gravity and Ground Magnetic Follow Uup of Aeromagnetic Anomaly West Al-Garraf
River, South Iraq Hayder A. Al-Bahadily and Ahmed S. Musa
٩٨
Fig.11: Magnetic map of residual anomalies of the
studied area
Fig.10: Contour of Total Magnetic Intensity (TMI) of the
studied area
Iraqi Bulletin of Geology and Mining Vol.9, No.3, 2013 p 85 102
٩٩
Fig.12: The upward continuation filter applied on TMI map up to elevation 1000 m.
The effect of LMA is diminished
Gravity and Ground Magnetic Follow Uup of Aeromagnetic Anomaly West Al-Garraf
River, South Iraq Hayder A. Al-Bahadily and Ahmed S. Musa
١٠٠
Fig.13: Magnetic profile across the magnetic high in N – S direction
(For location, refer to Fig.12)
Fig.14: Magnetic profile along the magnetic high (For location, refer to Fig.12)
RESULTS AND DISCUSSION
Many recent detailed gravity measurements (i.e. Al-Bdaiwi, 2004 and 2005, and
Al-Bdaiwi et al., 2007) carried out in the Western Desert showed very low resolution of the
Unified Bouguer Gravity (UBG) map (Al-Kadhimi and Fattah, 1984). However, the present
work in addition to a more recently work performed by Amin et al., (2010) in Hor
Al-Hwaizah area, south of Iraq, showed good matching with that of UBG maps (Figs.2
and 4). As a result, the gravity measurements plain of the Mesopotamian of the UBG are more
confident than those of the Western Desert.
On the other hand, it is not easy to interpret the negative anomalies (– Ve 1 and Ve 2),
which appear in Fig. (4), in terms of deficiency in density or even as structural low.
Generally, such lows are related to the negative background that may be related to the effect
of Zagros root as well as the effect of Mesopotamian Basin, i.e. the decreases of gravity field
towards the center of the basin due to thickness increment of the post Lower Miocene
sediments. However, the gravity high superimposed the negative background and separates it
into two gravity lows (– Ve1 and Ve2) and it creates a broad high saddle between them
(Figs.2 and 4).
The gravity high either has the same source of the magnetic high or it has different source.
Hereinafter are two possible interpretations concerning the sources of the gravity and
magnetic highs that could be discussed in this study.
The Magnetic High and the Gravity High are Related to One Source
Generally, there are many factors affecting the resulted shape of the magnetic anomaly;
these are mainly the inclination of the magnetic field, the shape, orientation and inclination of
the magnetic source, and also the interaction with the other magnetic anomalies and the angle
between the profile and the strike of the magnetic body. All these factors are responsible for
the final magnetic picture that appears in Fig. (12). However, because of the similarity in the
position, extension, length and orientation of the magnetic and gravity highs; therefore, they
may be related to the same source, which could be supra basement of basic or ultrabasic
intrusion. This interpretation is based on the general direct proportional relationship between
density and susceptibility of rock bodies, and the coincidence between the gravity and
nT N
S
nT
E
W
Iraqi Bulletin of Geology and Mining Vol.9, No.3, 2013 p 85 102
١٠١
magnetic highs. According to structural maps derived from seismic data; the sedimentary
cover overlying the Permian (~ 6000 m deep) shows no evidence concerning the magnetic or
gravity source, which suggests that the age of the source (the age of the intrusion as
mentioned early by the CGG, 1974) is older than the Permian and it is more than 6000 m
deep. It is worth mentioning that the CGG (1974) mentioned a depth of (7.6 7.8) Km for
this intrusion. However, top of the Permian surface shows impressive structures represented
by a structural nose, which has good matching with the gravity and magnetic highs with
NE SW direction. Moreover, it shows two parallel normal faults and an anticline trending
NW – SE lying in the northern negative of Bouguer gravity map.
The Magnetic High and the Gravity High are Related to Two Sources
Commonly, the Reduction To the Pole (RTP) filter removes the north-side low of dipolar
anomalies and centers the anomaly over the source, and enhances the special correlation
between magnetic anomaly and its geological source (Fairhead and GETECH Group, 2011);
therefore, applying this filter will shift the axis of the magnetic source towards the north
(marker M1 in Fig.15) and it leads to suggest a different source for gravity high. In this case,
the gravity high (marker G1 in Fig.4) should be related to a source situated at relatively
shallower depths within the sedimentary cover, which has more density contrast than that
resulted from the magnetic source; near the basement (the gravitational effect of the marker
M1 in Fig.15). Apparently, the negative area situated at the northern part of the studied area is
not a part of the studied anomaly; it represents a regional gradient as appears in Fig. (1). The
authors believe that the first interpretation (i.e. without applying RTP filter) is more
acceptable because, obviously, Fig. (13) shows a negative gradient towards the north rather
than an expected inflection point, i.e. no inflection point, which normally separates the
positive and negative sides of the magnetic anomaly.
TMI map (Fig.10) shows the magnetic high and its extension in ENE WSW direction
(marker M1). Commonly, the regional anomalies appear in the TMI Map reflect the variations
in basement lithology or topography. Marker M1 is almost related to an intrusion of basic
rocks, because the changes in lithology, which give rise to lateral contrast in susceptibility,
show up in the magnetic contour more conspicuously than topographic features on the
basement surface (Dobrin, 1978).
Gravity and Ground Magnetic Follow Uup of Aeromagnetic Anomaly West Al-Garraf
River, South Iraq Hayder A. Al-Bahadily and Ahmed S. Musa
١٠٢
Fig.15: Reduction to the pole (RTP) map of the studied area.
The axis of magnetic high M1 has been shifted to the north
Iraqi Bulletin of Geology and Mining Vol.9, No.3, 2013 p 85 102
١٠٣
CONCLUSIONS
The following conclusions are obtained from the forgoing qualitative analyses.
Although, the present work is detailed gravity survey, the comparison between the present
Bouguer anomaly map and UBG map (Figs.2 and 4) shows high matching, especially, in the
regional picture or long wavelength anomalies. However, the resolution of the residual
anomalies is much better in the present map.
The intrusion (magnetic high) has no anomalous structures within the sedimentary cover
that overlies the Permian surface. However, it could be a reasonable source of some
paleostructures that appear at the Permian's depth map.
The intrusion is older in age than Permian, it is more than 6 Km deep and it may be
a vertical dipole.
The regional gravity high has approximately the same shape, position and depth of the
magnetic high, which means they appear to have the same source of a basement intrusion.
The residual positive gravity markers (G2, G5, G6, G7 and G8), which are marked on the
Bouguer gravity map, represent the suggested antiforms structures within the studied area.
The residual or the Local Magnetic Anomalies (LMA) in the studied area, which almost
related to Quaternary gray sand sediments, contain small quantity of magnetic minerals and
cover many parts of the Mesopotamian Plain, and they may be the responsible source many
near surface magnetic anomalies that appear in the aeromagnetic map of CGG (1974). The
effect of the LMA can be easily recognized in the northern part of the aeromagnetic map of
the studied area (Fig.1), where they disturb the contour lines evidently.
Marker F in Fig. (12) may be delineated as due to deep seated fault, while the markers M2
and M3 are assigned to weakness zones in the same figure.
ACKNOWLEDGMENTS
The authors wish to express their deep thanks to Mr. Ghalib F. Amin (Senior Chief
Geophysicist, GEOSURV) for his assistance in data processing by using Geosoft software.
They also thank Mr. Varoujan K. Sissakian (Retired Expert, GEOSURV) for his scientific
efforts and revision of the text. Finally, we are grateful to Mr. Hayder H. Taha (Senior
Technical Director, GEOSURV) for his efforts in presentation of the figures and arranging the
text.
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Article
Full-text available
The geological setting of the Mesopotamia Foredeep within the tectonic framework of Iraq, has been reviewed and redefined according to the modern concepts of foreland basins, and new structural boundaries are introduced. The Mesopotamia Foredeep, which is the present day expression of the terrestrial part of the Zagros Foreland Basin, is an integral part of the Zagros Fold – Thrust Belt that lies between the deformational front of the Zagros orogenic belt and the stable interior of the Arabian Platform. The Mesopotamia Foredeep is an elongated epicontinental basin formed above an earlier plat formal and marginal basin. Accordingly, the Phanerozoic stratigraphic sequence of the basin can be broadly categorized into three major tectono-stratigraphic assemblages; Cambrian – Early Permian intraplate assemblage, Late Permian – Middle Cretaceous Neo-Tethys passive margin assemblage, and Late Cretaceous – present foreland basin assemblage. The Mesopotamia Foredeep is a mobile tectonic zone and contains several buried structures including folds, fault and diapiric structures. Recent activity of some of these structures is recorded through their effects on the Quaternary stratigraphy and present geomorphological landforms.
Book
Full-text available
The Petroleum Geology of Iraq by A. A. M. Aqrawi, J. C. Goff, A. D. Horbury and F. N. Sadooni ISBN: 978-0-901360-36-8 424 pages +xvi Format: hardback Publisher: Scientific Press Ltd., PO Box 21, Beaconsfield, Bucks, HP9 1NS, UK This book presents a comprehensive, up-to-date appraisal of the reservoir rocks, source rocks, seals and traps that control Iraq’s petroleum resources. Early chapters review the history of the oil industry in Iraq and outline Iraq’s tectonic setting and evolution. A five-chapter section on stratigraphic elements, arranged by megasequence, is followed by an assessment of Iraq’s petroleum systems. The book provides an invaluable source of information for petroleum geologists and other researchers.
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This is the third edition of this book but it has been extensively revised so that it can almost be described as a new book rather than a revision. The book covers the whole range of applied geographical techniques but has a strong emphasis on seismology. Here, properties of seismic waves, instruments data acquisition on land and sea, data processing and geological interpretation are all covered in separate chapers. Three chapters are used for gravity (principle and instrument) field measurements and reductions interpretation and three for magnetics (principles, surveying techniques, interpretation). All vertical methods are in one chapter. Particular change in this edition includes extensive coverage of data processing of seismic data and the use of elementary calculations to present the basic principles.- P.N.Chroston
Gravity and ground magnetic follow up of aeromagnetic anomaly in the Qala't Sukar
  • H A Al-Bahadily
  • A S Musa
  • A F Jassim
Al-Bahadily, H.A., Musa, A.S. and Jassim, A.F., 2012. Gravity and ground magnetic follow up of aeromagnetic anomaly in the Qala't Sukar, south Al-Kut, Iraq. GEOSURV, int. rep. no. 3368.
Ground magnetic survey of some Pliocence -Pleistocence sandy formations
  • H A Al-Bahadily
  • M A Yousif
Al-Bahadily, H.A. and Yousif, M.A., 2012. Ground magnetic survey of some Pliocence -Pleistocence sandy formations, in Bahar Al-Najaf area, central Iraq. Iraqi Bull. Geol. Min., Vol.9, No.1, p51-64.
Magnetic and gravity measurements and interpretations
  • J M Al-Bdaiwi
Al-Bdaiwi, J.M., 2004. Magnetic and gravity measurements and interpretations, Shaeeb Al-Waledge, Western Desert, Iraq. GEOSURV, int. rep. no. 2879.
Geophysical study and preliminary speculations on the genesis of pyroclastic rocks in Al-Waleed area, Western Desert
  • J M Al-Bdaiwi
Al-Bdaiwi, J.M., 2005. Geophysical study and preliminary speculations on the genesis of pyroclastic rocks in Al-Waleed area, Western Desert, Iraq. GEOSURV, int. rep. no. 2923.
Project report: Gravity and ground magnetic follow-up of Kalat Suker aeromagnetic anomaly
  • J M Al-Bdaiwi
Al-Bdaiwi, J.M., 2010a. Project report: Gravity and ground magnetic follow-up of Kalat Suker aeromagnetic anomaly, south Al-Kut, Iraq. GEOSURV, Achieve of Geophysics Division.
Gravity measurements in north Kharja locality
  • J M Al-Bdaiwi
  • A S Mousa
  • S O Abdulkadir
  • M B Jawad
Al-Bdaiwi, J.M., Mousa, A.S., Abdulkadir, S.O. and Jawad, M.B., 2007. Gravity measurements in north Kharja locality, Al-Waleed area, Western Desert, Iraq. GEOSURV, int. rep. no. 3066.