Lineaments Tracing of Magnetic Sources Depending on the Aeromagnetic Data: Case Study in Salman Basin and Surrounding Areas, Southern Desert of Iraq

Abstract


Introduction
The magnetic method is one of the most useful techniques for figuring out surface and subsurface geologic structures.The aeromagnetic analysis is used to help explain issues such as regional geological structure, illustration of rock boundaries and edges, location of possible areas of rock differentiation, and mineralization ( Eldosouky et al., 2017).The magnetic susceptibility of sedimentary rocks is the lowest, while that of metamorphic and acidic molten rock's middle and essential volcanic rock, is the highest (Kearey et al., 2002).When magnetic anomalies are observed over sedimentary territory, they are frequently created by fundamental basement (volcanic and/or metamorphic) rocks or volcanic structures such as meddling plugs, dykes, sills, magma flows, and volcanic centers (Gunn, 1997).Lineament detection can help the mapping of geological structures (Yeomans et al., 2019).Lineaments are prolonged mappable straight or curvilinear highlights of a surface whose segments adjust in straight or about straight connections and maybe the expression of subsurface folds, breaks, or faults.
Lineaments express themselves geomorphologically in the form of straight stream valleys and aligned valley segments (O'leary et al., 1976) .Optical remote sensing data (this data does not provide subsurface geological information) and Geophysical methods especially aeromagnetic data can be used to Detect deep-seated, and shallow linear structural features (Eldosouky et al., 2017).
The study area is located at in the southern part of Iraq, between Southwest (Al-Samawa city, Euphrates River), northeast Dhiqar, west of Najaf Governorate, and the Iraq-Saudi border in the south.Fig. 1 shows the boundary location for the study area, and Table1 determine the coordinate in longitude-latitude for the points that locate the study area.This area is separated from the regional groundwater basin known as Al-Shbecha-Salman Basin that covers an area of 29,518 Km 2 inside Iraq (Hassan and Al-Kubaisi, 2002).Structurally, the study area lies within the Platform of the Arabian Plate and in particular within the Salman Zone (Buday and Jassim, 1987), where the Cenozoic succession is exposed and slopes gently towards the Mesopotamian basin; beyond the Euphrates River to the north of the area.Several previous papers have been presented the linear structural features detection.Numan (1997) studied the Lineaments appearing on Landsat images of the western and southern deserts representing the stable shelf of Iraq.Lineaments can be processed and extracted manually or automatically from remotely sensed images, magnetic, gravity and seismic maps (Madi and Zhao, 2013;Baiyegunhi et al.2017).Mousa and Al-Rahim (2016) determined several Lineaments of the Western Part of the Iraqi Western Desert using Aeromagnetic and Gravity Data.Owolabi et al., (2021) studied the lineament and lithologic characterization impacting groundwater prospecting.
The present study was focused on the detection of deep-seated and relatively shallow lineaments using aeromagnetic data and Centre Exploration Targeting (CET) GX extension available in Geosoft, together with the filter zero contour of tilt derivative (TDR) over the Salman basin.TDR is similar to the local phase, except in the denominator it uses the absolute value of the horizontal derivative.TDR is defined as follows: TDR=tan -1 (VDR/THDR), It was defined by (Miller and Singh, 1994).The tilt angle is positive when over the source, near the edge as the vertical derivative is zero and the horizontal derivative is maximum and is negative outside the source region.From the zero value of the TDR, the outline of the edges can be identified.The tilt amplitudes have values ranging between −π/2 to +π/2.Its zero-contour line is located over or close to the edges of the faults (Miller and Singh, 1994).

Geological Setting of the Study Area
The stratigraphic column within the study area comprises formations from oldest to youngest, Umm Er Radhuma Formation, Rus Formation, Dammam Formation, Euphrates Formation, Ghar Formation, Dibdibba Formation, Zahra Formation, and Quaternary deposits.All the exposed rock units belong to the Cenozoic (Sissakian, 2000).The oldest formation is Umm Er Radhuma (Middle-Late Paleocene), This formation is exposed on the Iraqi-Saudi Arabian border in the study area, and is subdivided into two members depending on the lithological, and faunal variations.It ranges from 310 to 450 m thick anhydrite, and dolomitic limestone, mostly white or buff microcrystalline and permeable (Al-Mubarak and Amin, 1983).The formation slopes gently northwards and northeastwards.It is overlain by the Rus Formation (Early Eocene).It's around 90m thick of Rus formation, it's mostly consisting of anhydrite with some limestones and marl alternate in its middle parts.The lower part of the Rus formation consists of shale, limestone-dolomitize, and soft chalky limestone, the upper part of this formation.This lithology is found mainly in the deep wells of the Mesopotamian basin (Bellen et al., 1959).According to Sissakian, (2000), the main source of groundwater in the region is Dammam Formation (Fig. 2).It is overlain by the (Upper-Middle Eocene) Dammam Formation, which is ranges 150 -200m thick, it consists mainly of mutable carbonate limestone (dolomitic, or chalky) rocks.The bottom of this formation consists of marl beds -chalky limestone and gray-green waxy shale layers (Jassim and Goff, 2006).Euphrates Formation in (Early Miocene) is underlain by unconformable within Dammam Formation in the Southern Desert.It consists mainly of basal breccias and is clear by strata of pebbles, and carbonates cobbles.Ghar Formation: (Early Miocene), consists of 2-3 m basal breccia, rare anhydrite, gypsum, clay and sandy calcareous interbreed, rarely fossiliferous, and sand and gravel.The Ghar Formation is exposed, as a discontinuous ridge until NE of Salman town, as overlap within the Euphrates Formation, and unconformably overlies several formations.The Dibdibba Formation is exposed between, Busaiya, Iraqi-Kuwait, and Saudi Arabia's borders.It contains major sandstone, conglomerate sandstone partly cemented by limestone, and gravel (grey and brownish-grey), in the SW of the Busaiya town, which are popular in the area.The Zahra Formation (Pliocene-Pleistocene) forms of white and pink limestone, occasionally sandy marl, and calcareous sands.The formation within the Southern Desert outcrops in a low topographic area concerning the surrounding older rocks (Jassim and Al-Jiburi, 2009).It forms parts of the floor of the Salman topographic depression (Tamar Agha, 1984).The magnetized basement rocks of Iraq are not exposed at the surface, also, there are no boreholes dug to penetrate the whole thickness of the sedimentary cover.Therefore, available information concerning the depth and properties of the basement rocks are predicted indirectly from some geophysical studies.The model of basement rocks depth was conducted with the support of geological information which depended on the thickness of the mega sequence (Getech and Jassim, 2002 in Jassim andGoff, 2006).The depth to the basement rocks in Iraq is the consequence of complicated iteration between geological estimates and 3D inversion of gravity data.The modeling was dependent on outcrops and drilled wells from Iraq and adjacent countries.There are few studies on the lineaments in the study area; the main reliance is on the reports and studies of the Iraqi Geological Survey and the Geology of Iraq (Jassim and Gof, 2006).The study area is located between several major lineaments (Jassim and Gof, 2006).The Euphrates fault Zone (Najd Fault System), borders the study area from the north and goes alongside the Euphrates River.The fault zone, which is made up of a sequence of step faults and grabens, marks the transition between the Quaternary Mesopotamian Plain, and the rocky desert.Tar Al Jil Fault Zone (Najd Fault System) runs parallel to and to the south of the Iraqi-Saudi border.It's part of a Palaeocene escarpment that faces a depression filled with Mio-Pliocene clastics and freshwater limestones.Takhadid-Qurna Fault Zone (Transversal System), divided the study area into two parts.

Aeromagnetic Data Processing and Interpretation
Compagnie Générale de Géophysique (CGG), conducted an aeromagnetic survey of Iraq in 1973.GETECH Group conducted re-processing to convert all of the aeromagnetic tapes into a standard Geosoft database format and it is focused on TDR after transforming total magnetic intensity (TMI) data to reduction to the pole (RTP) grid (Fig. 4A).Also, the same technique is applied on TMI data extracted from satellites using facilities attached with Geosoft software (Fig. 4B).Analytical signal filter to the TMI is also available by satellite in Geosoft with a cell size of 5000 m.The data was corrected and plotted as a total aeromagnetic intensity map, which was reduced to the northern magnetic pole (RTP) to locate the maximum of the anomalies directly over the center of the causative body.

Regional and Residual Maps of Magnetic Data
Before interpreting the magnetic anomalies to characterize their geological sources, it is usually important to separate a portion of the field delivered by the structure of concern.The processing and analysis available data from satellite images and aeromagnetic data from CGG include information in this research, data grid spacing 500m, flight line 140m, line spacing 2km, other regions 5km, and line direction N 30˚ E, Tie line spacing 10 km in N 60 ˚ W direction.
Separating residual field from total magnetic field of the magnetic data into regional and residual is only appropriate if there is a clear distinction between both anomalous categories and the research area includes all of the residual structure's impacts.If not, a portion of the residual field can be removed with the regional, resulting in an inaccurate interpretation (Fairhead, 2011).The residual maps for magnetic data were calculated by applying upward continuation for 2 km (equal to cell size that was utilized for gridding of the considered area) and after that subtracted the upward grid from the magnetic fundamental grids.For the regional inspection which is very imperative in tectonic studies, satellite reduction to a pole of Earth Magnetic Anomaly Grid (EMAG2) from TMI is concerned (Fig. 3, and 4B).

Lineaments Extraction from Aeromagnetic Data
CET Grid GX extension was developed by the University of Western Australia (Holden et al., 2010).The aim of utilizing CET is to find any fractures or faults that may occur within the study area.Two procedures are included: one for evaluating locations with muted magnetic responses, in which texture analysis might improve local data contrast first to discover discontinuities and another for recognizing data edge structures.Texture analysis characterizes the local neighborhood of each image location.The standard deviation provides an estimate of the local variations in the data.At each location in the grid, it calculates the standard deviation of the data values within the local neighborhood.Features of significance often exhibit high variability concerning the background signal.When interpreting the output, values that approach zero indicate very little variation, whereas large values indicate high variation (Holden et al., 2010).

Analytical Signal applied total Magnetic Intensity Map (TMI)
When applied to Total Magnetic Intensity data, Analytical Signal (AS) aims to simplify the fact that tilted magnetic buried bodies typically have positive and negative peaks, making it easy to identify the exact location of the causative body (Fig. 3).Computed x-, y-, and z-derivatives with amplitude independent of magnetization direction were used to obtain the analytic signal that illustrates the structure linear extension features within the basin (Roest et al., 1992).The result of the amplitude domain analytic signal is shown in (Fig. 3).There is a color gradient from purple to blue signifying high magnetic susceptibility to relatively low respectively.The region with the lowest amplitudes within the study area are related to low susceptibility or relatively deep basement.When compared the map in (Fig. 3) with the depth map of the basement calculated by (Jassim and Goff, 2006), it seems that the purple color may be related to relatively low basement depth, while the blue color which can be observed out of Iraq in Saudi Arabia may be related with relatively high depth.

Reduction-to-Pole
RTP is a common part of magnetic data processing, especially for large-scale mapping.The RTP procedure can transform a magnetic anomaly caused by any source into the anomaly that the same source would produce if it were located at the magnetic pole and magnetized simply by induction.RTP also helps in the interpretation of magnetic data by removing the impact of magnetic latitude on anomalies.The area is usually characterized by both high and low magnetic closures, which could be due to a variety of causes such as variations in depth, magnetic susceptibility, and lithology.The RTP maps of the aeromagnetic data for the study area inside Iraq (Fig. 4A) and outside Iraq in Saudi Arabia (Fig. 4B) are shown.The magnetic anomalies that show low amplitude are possibly had a deeper depth, which related with blue color RTP map with a cell size of 3000 m-for the study area inside and outside Iraqi are shown in (Fig. 4B).The part of the study area inside Saudi Arabia has a maximum with NW trend followed by complex circular minima.

Regional and Residual Magnetic Anomaly Maps of RTP
The residual map got by removing the computed regional data (long-wavelength anomalies) allows embedded residual anomalies in the original aeromagnetic map to be seen (Fig. 5A).The research areas, residual magnetic anomaly map showed that the study's major magnetic anomaly trends are primarily in the NW-SE and NS directions, with values ranging from -4.60 to 4.25 nT.The residual outline uncovered several positive and negative closures, which reflect the Basement structure's impact on the susceptibility of rocks.The most positive anomalies are found within the center of the study area, while the main negative anomalies with a minimum value are displayed with blue color (Fig. 5A).The Saudi lands adjacent to the study area have low magnetic anomalies, that may reflect the relatively low depth of Basement and/or differences in magnetic susceptibility (Fig. 5B).The magnetic map from the satellite (EMAG2) is shown in (Fig. 5B).The EMAG2 map has the character of a regional aeromagnetic map.The magnetic anomalies have two parts, the positive ones are in the middle and west inside Iraq, and outside of Iraq towards Saudi Arabia has NE trend, while the negative one has NW direction inside Iraq and a circular negative one in Arabia (Fig. 5B).

Application TDR of RTP Inside and Outside Iraq
To locate magnetic source edges for both shallow and deep structure contacts and faults.In this research, the Tilted Derivative (TDR) was applied to RTP magnetic data.It's worth noting that in TDR maps, the edges fault boundaries of the magnetic source, are defined by a zero-contour value.Utilizing amplifying weak signals and suppresses strong signals, which can be distinguished and recognized (Fig. 6A).To identify the edges of magnetic sources for both shallow, and deep structures contacts, faults.TDR filter was applied to Earth magnetic anomaly map of 3000 m hight for the RTP map of both inside and outside of Iraq (Fig. 6B).

Aeromagnetic Lineaments Map
CET technique is used to draw the lineament system, which is effective in showing local intensity changes and enhancing locations of discontinuity in aeromagnetic data (Fig. 7.A, and B).The resultant lineaments were superimposed on zero contour line of TDR.Most of the extracted lineaments correspond to the specified faults, indicating that faults caused complexity in the structures in this region.The TDR zero values map (Fig. 7.A and B) respectively show the spatial distribution of maxima, minima, (NW-NE and NS-EW) lineaments trends, the new lineaments were discovered using the CET grid analysis are visible as fault or contact.Fig. 7.C, D, E, and F maps show (NW, NE, NS, and EW) lineaments trends in Earth magnetic anomaly Map (EMAG2) up3000m-RTP of the study area (blue polygon) and outside Iraqi area, the new lineaments were extracted using the CET grid analysis are visible as lineaments.

Conclusions
The low magnetic values areas with blue color represent very low amplitudes and/or deeper magnetic sources, whereas the high magnetic value areas with purple color show relatively high susceptibility and/or arising from basement intrusion at relatively low depths; these areas are mostly found near Salman, Shawiya, Takhadeed, Ansab, and Amgar.Depending on the geology of the study area, the types of rocks are anhydrite, dolomitic limestone, shale, basalt breccias, and sandstone.There is activity and influence of groundwater within the study area, some karst processes have occurred, which indicates the association of anomalies with a probable saturated karst cavity or an empty karst cavity.The residual map of RTP data shows more positive magnetic anomalies which differ in size, direction and position.The original aeromagnetic data has been reduced to the northern pole (RTP).A rapid change in underlying geologic conditions and/or lithological characteristics is defined by visual inspection of the RTP magnetic map.This map, on the other hand, shows a variety of anomalies with diverse frequencies and amplitudes that reflect a variety of causal sources as well as varying compositions.Whereas, the regional map of RTP shows directions varying between N-S and E-W.These directions are well confirmed by the geological data with a dominance of the NE-SW direction.As a result, the deep main linear features follow the general trend of the main faults or contact that formed the Salman Basin.The new lineaments were discovered using the CET grid analysis, zero contour of TDR.There are two systems of lineaments in the present study area.The first set is taken the NW-NE direction perpendicular to the extensional structures, while the second set has the NS-EW direction.

Fig. 3 .
Fig.3.Analytical signal of TMI after gridding the raw data of the Salman Basin, Iraq

Fig. 5A .
Fig.5A.Residual map of RTP with cell size equal to 500 m for Salman Basin

Fig. 5B .
Fig.5B.Residual map of RTP -up 3000m of Salman Basin and surrounding area from the earth magnetic anomaly map

Fig. 7A .
Fig.7A.Tilt Derivative of RTP map shows NW-NE lineaments trends of the Salman Basin

Fig. 7C .
Fig. 7C.Earth magnetic anomaly Map (EMAG2)-up 3000m of RTP map shows NW lineaments trends of the Salman Basin and surroundings area

Fig. 7E .
Fig. 7E.Earth magnetic anomaly Map (EMAG2)-up 3000m of RTP map shows NS lineaments trends of the Salman Basin and surroundings area

Table 1 .
Coordinates of the boundary locations of study area