Multi-Elevations Upward Continuation Method to Depth Detection of the Main Crustal Layers in Western Desert of Iraq

Abstract


Introduction
The deep structures and basement depth in western Iraq are challenged due to the lack of deep wells which reached the basement rocks.The information about the basement rock and the earth's crust in the western region depends mainly on the information derived from the gravitational and magnetic methods ( Buday and Jassim, 1987).The potential field methods (gravity and magnetic) are used to explore the subsurface sources, which related to the geological structures (Cordell and Grauch, 1982).Many studies used gravity and magnetic data to determine the depth of the basement rocks in western, and it is found to range from 4-12 km.(C.G.G., 1974;Al-Rawi, 1986;Al-Banna, 1992;Mousa et al., 2017, Al-Banna andAl-Namar, 2019;Al-Hadithi and Al-Banna, 2022a).The crust thickness in the study area range (30 -42.6) according to many studies that used different techniques such as Spectral ratio, converted phases, Travel-Time and gravity modeling, (AlSinawi et al., 1987;Al-Heety, 1990;AlSinawi and Al-Banna 1990;Al-Banna and Al-Heety, 1994;Alsinawi and Al-Heety, 1994;and Rafea et al, 2022).The decrease in crustal thickness and deeper basement rocks in west Iraq (Al-Hadithi and Al-Banna,2022b).
Upward continuation is an important technique in gravity and magnetic processing to determine regional anomalies.It is used to determine the effect of the relatively regional sources which are represent a low pass filter (Henderson and Zietz 1949;Dean, 1957;Syberg, 1972;Kearey et al., 2002).Upward continuation is a mathematical process to separate deep anomalies from shallow anomalies of different geological bodies (Jacobsen, 1987).The present study used the upward technique to identify the main layers in the crust of the study area.

Materials and Methods
The study area covers the southwestern part of Iraq.It is bounded by Latitude 30° 69' to 33° 39' N and Longitude 38° 79' to 43° 56' E (Fig. 1) .The regional geology of the study area characterized by strata dipping toward the northeast which coincides with the surface land slope, while the sedimentary layers from Rutba town dipping toward the west.(Buday, 1980;Konert et al., 2001;Jassim and Goff, 2006;Aqrawi et al., 2010).Al-Ma'aniyah depression area is located above Nukhaib Graben on the stable shelf in the southwestern part of Iraq within Al-Najaf Province.According to the gravity and magnetic interpretation, the basement rocks in the study area, consist of the number of regions grouped as either acidic rocks or ultra-basic rocks (Al-Banna, 1999).The geological information from neighbor countries and from gravity and magnetic data indicate that the basement rock types are Granodiorite, Granite, and Murdama Molasse at the west, Phyllite at the center and Ophiolite and Gabbro at the eastern part of the study area, ( Jassim and Goff, 2006).Technologically, the study area is located on the stable shelf, (Buday and Jassim, 1987), at the same time it is located on the inner platform (Fouad, 2015;Al-Banna and Ali 2018).The eastern part of western desert of Iraq consider as complete isostatic compensation (Al-Banna and Al-Rawi, 1993).The gravity survey in the study area were carried out in 1977 during the program of the General State of Geological and Mineral Investigation to achieved the gravity survey in the western desert ( Abbas, 1985).The original gravity data were reprocessing by Getech Group/ British in 2010.The regional view of the gravity map gives an approximate idea about the sedimentary basin in Iraq (O.E.C, 2005).The gravity values range in the study area from -22 to -64 mgal on a map digitized by a grid (1 X 1 kilometers) (Fig. 2   The TMI data were converted to RTP magnetic map considering the inclination and declination angles values in the study area.This process achieved by using the International Geomagnetic Reference Field (IGRF) (Geosoft, 2015).The RTP values in the study area found to be range 4860 to 5196 nT.(Fig. 4).

The Regional and Residual Anomalies of Gravity and Magnetic Data
The upward continuation filter was applied to obtain the regional and residual anomalies maps of the gravity and RTP magnetic data for twelve elevations (1000,2000,4000,8000,12000,16000,20000,25000,30000,40000,50000 and 55000 meters).The regional gravity maps for six elevations (1000, 8000, 16000, 25000, 40000 and 55000 meters) shows that the gravity values trends to decrease toward the west, (Fig. 5), while the regional RTP magnetic maps for the same six elevations (1000, 8000, 16000, 25000, 40000, and 55000 meters) shows the RTP magnetic values trends towards southwest, (Fig. 6).All the residual gravity maps of the twelve elevations show certain positive and negative anomalies.
The locations of these anomalies are not changed in all residual gravity maps (Figs. 7 and 8), (Fig. 7) and (Fig. 8).Generally there are three negative anomalies, these are west and south Rutba which trending north-south; northeast Al-Nukhib trending northwest-southeast, and north Al-Mainyah trending northeast-southwest.The residual RTP magnetic maps of the twelve elevation shows about six to seven anomalies grouped in three parallel lines trending northwest southeast.The first lines include two anomalies the southern one west Najaf city and the second is southwest of Ramadi city.The second line includes three anomalies, the northwest Al-Maaniyah anomaly at the south, then west Al-Nukhib in the middle part, and the third anomaly to the north.The third line located at the western part of the study area, it seems as one major positive anomaly include many arms trending East-West, North-South, and Northwest -Southeast (Figs. 9 and Fig. 10).

The Residual Gravity and Magnetic Anomalies Relation with Upward Elevation
The total range value (at the color bar) of the residual anomalies of all gravity and magnetic maps obtained by using the twelve upward elevations from the elevation 1000 meters to 55000 meters were summarized in table 1 and table 2 for residual gravity and magnetic data, respectively.The elevation of the upward continuation plotted at the X-axis and the total range of residual anomalies values of gravity and magnetic at the Y-axis, ( fig.Figs.11 and 12) respectively.It is observed that the plotted points have different slopes, therefore a group of straight lines were plotted to pass through the points which have the same slope.Five slopes were plotted, which introduce four intersection points.It is found that the trace of these intersection points on the upward elevation axis (at Figs. 11 and 12), indicted the values 4.4-5.2km, 12.8-13.5km, 27-24 km, and 39-39.6 km.

Discussion
The obtained results concerning the main vertical boundaries in the study area were interpreted with available geological and geophysical information.The obtained depths considered as an average values for the main vertical boundary within the crust in the study area.The first boundary may be coincide with Burj Lst.Formation (Mid-Cambrian).The second average depth value may be coincide with the Basement rock in the study area.The third average depth value may be related to a transition zone within the basement rocks.The fourth depth value may be reflect the depth of Moho in the study area.

Conclusions
The upward continuation technique was applied to the gravity and magnetic data to study the main layers in western part in Iraq.Many upward elevation were used to achieved this aim.The total range of the residual anomalies maps of gravity and RTP magnetic were plotted against the upward elevations.Five segments and four intersection points were obtained for the gravity and magnetic data.The trace of these the intersection points on the upward elevation axis are approximately indicate the following values 4. 4-5 km, 12.8-13.5 km, 27-24 km, 39-39.6km.These values may be indicate the depth of the following layers, the Burj Formation, basement, mid layer in the basement rocks, and the Moho depth.The authors believe it is possible to apply the upward continuation to find the main layers which show abrupt change in the physical properties.The authors also believe that the muli-elevation upward continuation method applied in this study can be considered as a new processing method.

Fig. 4 .
Fig. 4. The RTP Magnetic map of the study area

Fig. 11 .
Fig.11.The Upward Continuation elevation against the total range of the residual gravity anomalies of the study area

Fig. 12 .
Fig.12.The upward continuation elevation against the total range of RTP magnetic anomalies of the study area

Table 1 .
The upward continuation elevation and the Total range of the Residual Gravity Anomaly

Table 2 .
The Upward Continuation and the Total range of the Residual Magnetic RTP anomaly