Delineate the Main Structural Trend in Anah and Surrounding Area, Western Iraq Using Pie-Slice Technique on Gravity Data

Abstract


Introduction
The gravity method is based on the observation of changes in the gravitational field caused by horizontal variability of density within the subsurface.It is an important method for several problems that involve subsurface mapping and detecting the main structural trends, as in using Pie-Slice Technique.One of the characteristics of the Fourier transform is the transformation of features with specific directions in the space domain into features with just one direction in the frequency domain (Thorarinsson et al., 1988 and1989).The Pie-Slice techniques were first used on seismic data to eliminate the high velocity noise in the recorded section by Embree et al. (1963).Also used by Mars et al. (1999) to constrain eigenvector filters in the various data domains for processing surface seismic data.Al-Rawi et al. (2009) applied Fan Filter Technique to delineate of main structures and trends of tectonic in southern Iraq and demonstrated two main trends namely NW-SE and N-S with a secondary (NE-SW).Hinze et al. (2013) applied Pie-Slice technique on gravity data to show the ratios of the orthogonal wavenumber that represent the strike orientation, and Huo et al. (2018) used this technique on borehole radar data to cut the low velocity zone associated with reflected guided waves (RGWs).
The current study is aimed to determine the main structural features of gravity data and lineaments (from satellite imagery) within the administrative boundaries of Anbar Governorate, in western Iraq.So, Pie-Slice filters are applied to the gravity data.The result will be presented as a rose diagram, which helps in comparison with the directional analyses for the lineament data (from satellite imagery).This will ensure the delineation of deep and shallow structural features and their relation to the tectonic pattern in the study area.

Tectonic and Geology of the Study Area
The current study area is located in western Iraq, within the administrative boundaries of the Anbar governorate.The area an approximate rectangle with a length of 150 km and width of 130 km and an area of 19500 km 2 , as shown in Fig. 1.Table 1 represents the corner coordinate of the study area in UTM-WGS1984-Zone 38.Tectonically, the study area is located in two parts: Inner Platform (Northern part of Western Desert Subzone) contains the area west and south of the Euphrates River, Alpine compressional deformation can't 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 to its marine counterpart the Arabian Gulf.It is located between the stable continental part (i.e., the Inner Platform) and the Zagros mountain front to the northeast.It is flat, terrain, covered by the Miocene to Holocene restricted marine and continental (molasse) deposits (Fouad, 2015).Fig. 3 shows the main faults and transversal block where Anah-Qalat-Dizeh, Amij-Samara, and Ramadi-Musaiyib faults are located within the study area.According to the surface geological map of the study area, most parts of the region are covered by sedimentary rocks belonging to the Tertiary and Quaternary ages (Fig. 4).The outcropped formations in the study area are Anah, Euphrates and Fatha formations and Quaternary Deposits, which reach to the Euphrates River and consist of clay, shale, sand, and gravel in some sections (Buday and Jassim, 1987).

Available Data
The Bouguer anomaly map (Fig. 5) was gridded minimum curvature method with a 1 km cell size and 2 mGal contour intervals.General description of the gravity map of an area characterized by the gradient of gravity field to east and southeast.In the northwest of the study area, the magnitude of gravity is higher than E (about 30 mGal).The southeastern part of the study area is characterized by short wavelength and low amplitude anomalies, while the high amplitude of anomalies increases toward the northwest.In the northwest part of the gravity map, there is a positive anomaly with a value of about 10 mGal.

Separation of Regional -Residual Gravity Data
The power spectrum technique is used to separate regional from Gravity data.The power spectrum technique, presented by Spector and Grant (1970) and developed by Tselentis and Drakopoulos (1988), is usually used in wavelength analysis and depth.The method is based on taking a 2D Fourier transformation of the potential field data.In the power spectrum technique, the high pass filter is applied to gravity data to get the residual, and the low pass filter is used to get the regional field.

Pie-Slice Technique
The Pie-Slice technique is used to delineate feature location by applying gravity data in the frequency domain and then taking the inverse Fourier transforming to the result.The method is zero-phase filtering that preserves the position of the amplitude peak.It is all pass filter in an overlapped band direction (overlapping wedge-arc filters) (Al-Rawi et al., 2009).Pie-Slice filter applied to pass band direction and reject other direct frequencies.

Results
Applied power spectrum technique on the gravity data map where use low pass filter with a chosen cut-off wavelength of 90 km to get the regional field map (Fig. 6).Generally, the gradients of gravity value are toward east and south east and there is a main positive anomaly in the NW and a main negative one in the SE, also use high pass filter with a cut-off wavelength of 90 km for residual anomalies map (Fig. 7).The Pie-Slice technique was applied to the residual anomalies data with an interval angle of 20 o and an overlap of 10 o and eighteen trend maps were obtained.These show, respectively, how the gravity field's energy magnitude is distributed within a chosen direction.The strength of the aligned data will be reflected in the contour intensity in the filtered data, which in turn depends on how many grid points were used to define the trend on the original map (Mobil, 1980).
Rose diagram is used to reflect the intensity or amplitude values, which describe the length and direction of the features.Then the intensity of the amplitude is transformed into a percent value and subtracted from the threshold value to increase the recognition of the rose.The major trend of counter intensity is in N25E, N15E, E-W, N50W and N30W, while the minor trends N-S as shown in the rose diagram in Fig. 9.The major trends N27E-N15E and E-W are mainly shown in Figs. 8 A6, A7 and A4 of the trending map, respectively.The reason for this direction may be related to the effect of Anha Fault System and the Amij-Samara transverse fault as indicated in Figs. 2 and 3.
The N50W and N30W directions are mainly shown in Figs.8 A11 and A14 trending maps).The reason for this direction may be due to the effect of the structures in the east of the Al-Jaziera subzone, as shown in Fig. 2, which is related to the effects of the Rmadi-Musaiyib Fault, which corresponds to the Najd Fault System, these structures are grabens that developed due to the extension phase during the Late Cretaceous, as show in Fig. 3.
The N-S minor trend as in Fig. 8 A9 trending map may be related to the effect of the Nabitah Fault System.To show the effect of these structures on the surface, Al-Amiri (1978) studied and interpreted structures of Landsat Satellite Imagery for the western desert of Iraq.The interpretation was focused on the lineaments and major structural delineation, a basic map was prepared and it comprises all lineaments trace observed in Landsat satellite Imagery, as shown in Fig. 10 (Al-Amiri, 1978, after Iraq GEOSURV permeation).All lineaments are digitized to get the main trends in the study area and to detect if the subsurface structures effects the surface structures.Then, a rose diagram is used to obtain the main trend of lineaments and comprise that with the results of the Pie-Slice technique.

Conclusions
The Pie-Slice technique is one of the best methods for separating potential field components into their values and directions.This technique is useful to detect the main trends of subsurface structures.The method is applied using gravity data for the study area to pass band direction and reject other direct frequencies.Major trends of subsurface structures are in N25E, N15E, E-W, N50W, and N30W and minor trends are in N-S, which have been defined from the Pie-Slice technique applied on residual map of gravity data.Landsat Satellite Imagery is a useful tool for lineament studies on large scale geological features.Many prominent lineament trends are observed for the same study area, These are N25E-N35E and N55W, and the less prominent direction N45W.In comparison, the results of the Pie-Slice technique that identified directions of subsurface structures and trend lineaments on the surface are shown that nearly coincide with each other.From this result, conclude that the effects of subsurface structures affected on the surface in the study area such as Anah anticline.

Fig. 1 .
Fig. 1.The location map for the study area (Modified from United Nations, 2014)

Fig. 2 .
Fig. 2.The location of the study area on the tectonic map of Iraq(Fouad, 2015)

Fig. 6 .Fig. 7 .
Fig. 6.Gravity map of study area after applied low pass filter with cut-off wavelength of 90 km

Fig. 8 .
Fig. 8. eighteen trending gravity maps are obtained by the Pie-Slice technique with an angle of 20 o and with an overlap of 10 o .