Iraqi Geological

Abstract


Introduction
Geophysical surveys, in particular aeromagnetic survey, considered as one of the first methods which used in delineating subsurface geological structures. Furthermore, aeromagnetic data is used to delineate the depth to the basement, and to fined the depth of the anomalies due to the susceptibility contrast between the basement and the sedimentary cover. Euler, Werner Deconvolution and Analytical signal, the Power spectrum analysis, and (SPI) methods are used to estimate the depth to the basement rocks. CGG (1974), used the Inflection Tangent Intersection method (ITI) to analyze the aeromagnetic map. The average depth of the investigated area was about 8 km. Al-Banna (1992) analyzed the aeromagnetic data of the Al-Jazira Zone and concluded that the average depth of the basement was about 7 km. Al-Yasi et al. (2006) applied the spectrum method to aeromagnetic data in order to estimate the basement depth of the Al-Jazira area in Western Iraq. They concluded that the depth range was about 7-10 km. Mousa (2017) applied SPI, tilt depth, and power spectrum methods on the aeromagnetic data to delineate the depth of the basement and anomalies in the Western Desert of Iraq. In addition, Abdulrahim and Al-Rahim (2019) used the power spectrum for the separation of aeromagnetic data to the regional and residual magnetic fields in order to delineate the boundary of the anomalies. Al-Banna and Daham (2019) applied SPI Technique to the potential data to detect the depth of the basement for Diyala Area. Al-Hadithi (2022) used SPI for aeromagnetic data to delineate the basement depth of Al-Tharthar Lake and the surrounding areas. They concluded that the depth reachs to about 6-11 km. The current research aims to estimate the depth of basement rocks, using SPI and power spectrum methods.

Location and Tectonic of the Study Region
The current area is located in Western Iraq within the administrative boundaries of the Al-Anbar Governorate. The area has an approximate rectangle shape with a length 150km and width of130 km and an area of 19500 km 2 , as shown in Fig. 1. Table 1 represents the corner coordinates of the study area in UTM-WGS1984-Zone 38.

Tectonically
The region is divided into two parts ( Fig. 2): • Inner Platform: (northern part of Western Desert Subzone), which contains the area west and south of the Euphrates River. Alpine compressional deformation cannot be recognized and the effects of Permo-Triassic rifting are not significant (Fouad, 2015).
• The Mesopotamia Foredeep: It is the terrestrial remnant of the Zagros foreland basin that extends southeast towards its marine counterpart "Arabian Gulf". It is located between the Inner Platform and the Low Folded Zone, and considered as a flat terrain a flat terrain (Fouad, 2015).

Fig. 2.
The current study area on the tectonic map (Fouad, 2015).

The Buried Precambrian Basement Rock
The Ad Dawadimi Terrane of the research region is distinguished by low to medium gravity and magnetic data values due to the presence of thick phyllites and is bordered to the east by strongly magnetic ophiolites. The terrane has a general N-S longitudinal trend connected with the Amar (Idsas) collision belt. The high gravity and magnetic associated with Ar Rayan Terrane, as well as magnetic anomalies, might be interpreted as an ophiolitic suture. The terrane is expected to contain Amar Group )volcano-sedimentary rocks) with syn-tectonic gabbro and post-tectonic granites (Jassim and Goff, 2006).  (Jassim and Goff, 2006)

Subsurface Geology
To make a summary of subsurface formations, Anah well Ah-2 was selected. The well Anah was drilled in 1959, and located at Anbar Governorate, to the northwest of Ramadi City near the Anha area. The succession in this well includes formations belong to the Cretaceous and Tertiary. These formations are Euphrates, Anha, Baba, Shiranish, and Hartha. The deepest penetrated Yamama Formation, which found at the depth of 3527 m depth 3527 m.

Available data and Materials
The Total Magnetic Intensity (TMI) map of the area has been mapped by using minimum curvature for a cell size of 2 km (after Iraqi Geological Survey permission) with 10 nT contour intervals (Fig. 5). The (TMI) map of the current area is reduced to the north magnetic pole by applying the Reduction To the Pole transformation by using Oasis montaj, Version 8.4. When converting, the parameters must be entered, which are magnetic inclination I, and declination D, where the declination D is the angle between the magnetic meridian and the geographic meridian; the inclination I is the angle formed between the earth's surface and the planet's magnetic lines. The inclination and declination angle values related to the study region were about I= 50.85 , D 3.15. The RTP map (Fig. 6) shows symmetrical anomalies that lay above their causative bodies and this process will lead to acceptable depth estimation. RTP map is used in general because (TMI) does not represent the right position of the anomaly, for example, the circular anomaly in TMI has two parts, positive and negative and the source body locates between them, wherein Fig. 4. M1, M2, M3, M4, and M5 represent the anomalies which located between positive and negative parts. While RTP map (Fig. 5) shows the anomalies has the positive part only. Generally, the RTP map of the current region (Fig. 6) shows that the high values of the magnetic anomalies are distributed in the north, south, and middle parts of the study area. On the magnetic map, two major low-value anomalies were detected; the first is located on the southeastern part of the map, while the second one is located on the northwestern side. A third small minimum anomaly is located to the south of Anah City.

The Power Spectrum Method
The power spectrum technique is used to separate regional from RTP magnetic data by applying a low pass filter to get regional, then a high pass filter used to obtain the residual field data. This technique was first presented by Spector and Grant (1970) then, later was developed by Tselentis and Drakopoulos(1988). This technique is used in depth estimation by analyzing anomalies wavelength.. The technique is based on transforming the RTP field to the frequency domain and taking a slice profile across the selected anomaly. The slope of this profile shows an amplitude decrease as the wave number increases. The depths to the top basement were determined by using the equations of Spector and Grant (1970), which involves graphing the natural logarithms of amplitude with frequency and determining the gradient of the linear slice.

= ∆y
(2) M1 and M2 are the slopes of the first and second plot linear slice, respectively. The negative sign indicates the depth to the structures, whereas D1, deep depth and D2, shallow depth (Ugwu et al., 2018).

Source Parameters Imaging (SPI)
SPI technique uses to calculate the depth of the structure's top (Thurston et al., 1999). It is a practical, rapid, and reliable tools for estimating the thickness of the sedimentary rock (Ofoha et al., 2016). This method uses gridded magnetic data to automatically calculate source depths, and then it saves the depth solutions (Thurston and Smith, 1997). For magnetic depths, the SPI method makes interpreting magnetic data much easier (Eldosouky et al., 2022). One of the benefits of the Improved SPI approach is that the depths can be displayed on an image that can be easily studied by someone familiar with the local geology. (Shehu et al., 2017).
A represents tilt derivative, where M represents the magnetic field. It is applied in the case of a dike, contact, and horizontal cylinder.

Results
SPI was run by using (Oasis montaj software) to process the RTP data for the study region. The obtained basement depth map (Fig. 7) shows a depth minimum of about 5000 m while the maximum depth of the basement was about 11000 m.

Fig. 7. Depth of basement estimated from SPI of RTP data
The map shows the presence of the main proposed basin in the northwest and southeast parts of the current area, where the basins formed during the Late Silurian and then uplifted in the Late Silurian to Mid Devonian time (Jassim and Goff, 2006). In general, there is decreasing in depth to the east and northeast, while to the south the depth reaches to about 5500 m. In the middle and southwest of the current area, the basement was found about about 7500 m, and at the northeast and south side of the study area there are elongated uplifted basements that may affected on the sedimentary cover which may composed post -tectonic gabbro, diorite, as indicated in Fig. 3.
Comparison of the basement depth map of the study area with previous research such as basement depth that calculated from the aeromagnetic interpretation of CGG (1974) and Jassim and Goff (2006) was shown in Fig. 8A and B, respectively. CGG map is published in Buday and Jassim (1987) and show a depth range of approximately 6.5 to 12 km and depth decreasing at the center and increasing to the east and northwest. The second map of basement depth is the product of complicated iterations between geological estimates and 3D gravity inversion. Outcrops and wells were used to control the modeling (Jassim and Goff, 2006), this map shows a depth range of approximately 5 to 7 Km. To compare the results, the chosen depth for three cities (Rawa, Anah, and Hadithah) are calculated, the average results for this comparison are summarized in Table 2.  (Jassim and Goff, 2006).  Jassim and Goff (2006) derived from 3D inversion of gravity (after Getech andJassim,2002), while CGG. (1974) used the Inflection Tangent Intersection method. Table 3 shows the average depth of these main magnetic anomalies. Fig. 9 shows the selected magnetic anomalies that will calculate depth for them. They are named M1, M2, M3, M4, M5, and M6. 10100 m 7500 m Spectrum Analyses are used to delineate the average depth of magnetic anomalies by using (GET grid software) that transforms data into spectra of two functions (amplitude and frequency). Each part of the spectrum curve has its own slope that is selected by drawing a straight line, the straight lines represent the slopes of the various amplitudes and frequencies. The straight line as shown in Figs. (10 to 15) represents the depth of anomaly, where low frequency with high amplitude reflects the deeper anomalies and high frequency with low amplitude energy reflects the shallower depth (Maus and Dimri, 1996).
Each Spectrum curve that divided into one or two straight lines. The first line generally represents the average depth of the main anomalies; the second line represents the smaller anomalies or shallower depth.

Conclusion
SPI technique was effective method in estimating basement depth in cases of dike, contact, and horizontal cylinder when using aeromagnetic data. The range of basement depth for the study area is (5-11) km. The results show the presence of the main basins which were located at the northwest and southeast side of the study area. Comparing the results of SPI with the previous studies such as the basement depth of CGG (1974) were made by selecting cities in three locations (Rawa, Anah, and Hadithah).The results show an acceptable correlation between these maps. The results of depth estimation using the spectrum analyses method for main anomalies in the study area show a depth range between 7500 and 10100 m, and these results approximately coincide with the depth estimated using the SPI method for the same anomalies.