Using Gradiometric Technique to Prospect Archaeological Features in Tell Al-Deylam, South of Babylon City, Middle of Iraq

An extensive vertical magnetic gradiometry survey was carried out over an area spanning 1,188 square meters in the northwestern section of Dilbat; a recently discovered archaeological city within Tell Al-Deylam located approximately 20 km south of Babylon city, so as to reveal the real image of the archaeological features hidden beneath the earth’s surface. The vertical gradiometric survey was done by the Geometrics-G-858 Cesium magnetometer. The gradiometric results showed three clear longitudinal magnetic anomalies in different places on the magnetic contour map. The main anomaly is located on the edge of the northern part of the study area and extends longitudinally in the northwest and southeast direction. This may indicate the presence of the main outer wall, as it extends lengthwise about 30 m, with a width of 3-4 m, and a depth of about 1 m below the surface of the ground, and may contain the main northern gate of the Dilbat Temple. Besides that, this anomaly displayed approximately a high magnetic value of about + 60 nT, which indicates that the wall was built from baked brick composed of clay minerals rich in iron oxides. Furthermore, the magnetic contour map showed two other magnetic anomalies. The first one is located in the western part of the study area and appears perpendicular to the main magnetic anomaly. Meanwhile, the second anomaly occupied the southern part of the study area and is parallel to the main anomaly. The two anomalies showed approximately lower magnetic strength than the outer wall. So, they may represent the remains of the inner room walls built from mud bricks. The thickness of the two supposed walls is about 2.5 m and they are at a depth one meter from the ground. The findings of this method have been a good guide for new excavation, which led to finding the main wall built from fired bricks.


1-Introduction
Tell Al-Deylam is one of hundreds of archaeological sites spread throughout Iraq whose content has not yet been investigated or studied.The vertical magnetic gradient technique is considered an effective method in exploring modern archaeological sites, as it has several specifications, including that it covers a comprehensive survey of the area, is non-destructive, is fast and cheap, and it can reduce excavation hard work and minimize expending time and money [1].Furthermore, its field data does not require diurnal correction and it can effectively cancel the effect of deep magnetic objects and enhance the signal of near-surface objects such as archaeological structures (fired bricks, walls, kilns, and graves).Vertical magnetic gradiometry has become one of the more commonly used methods in the archaeological geophysics sector in recent years [2].It is a useful method that allows the preservation of the prospection site from disturbance and damage owing to its non-destructive and non-invasive.This technique enables precise identification of the location of the subsurface archaeological remains [3].Magnetic anomalies detected in gradiometric surveys have a major role in determining the location of excavations in unexplored archaeological sites [4].Magnetic surveys involve measuring the magnetic field of the earth just a few centimeters above the surface on an evenly spaced grid of points.These measurements can identify even the slightest differences in concentrations of weakly magnetic iron oxides beneath the surface, which can give rise to anomalies in the mapped data.It's important to note that anthropogenic factors such as filled pits, fired or burned earth, intrusive walls, cellars, or privies can also contribute to these concentration differences [5].However, geophysical surveys can provide important insights and acceptable mapping of the subsurface archaeological situation in previously unexcavated sites.Furthermore, gradiometric surveys considered accurate and economic methods because of their high benefit-to-cost ratio and the high cost of archaeological excavations that leave the archaeological site disturbed and may cause damage to the archaeological bodies.Simultaneously, the resolution and data acquisition efficiency of the gradiometric equipment is continuously evolving, and also the new multi-sensor configurations permits rapid coverage of wide areas.Gradient measurement is perhaps one of the modern geophysical techniques for near-surface prospecting [6].However it is now widely used as an effective geophysical technique in the geophysical investigation of cultural heritage sites.Archaeo-geophysical surveys can also be useful in cultural resource management contexts as archaeological-finding methods for focusing on expensive excavations, minimizing the quantity of excavation required, and saving expenditures.Our goal in this research is to use gradiometry technology to detect the main buried archaeological features and draw their maps to guide new excavations without causing any damage to this new archaeological site [7].

2-Location and topography of the research area
The search area is located on the northwestern corner, as archaeologists expect, of the buried ancient Babylonian palace of Dilbat, which has an area of 4800 m2 upon the archaeological hill named Tell-Al-Deylam.It located at coordinates of longitude lines are (E 44° 28" 04.3' and E 44° 28" 03.0') and latitude lines (N 32° 29 " 46.2' and N 32° 17" 45.5').Tell Al-Deylamis approximately 20 km from the center of Babylon Governorate, and the total area of the hill is approximately 1700000 m2, as shown in Fig. 1.The archaeological hill generally

3-Theoretical Principle
The magnetic method is based theoretically on measuring the vectoral summation of the prevailing earth's magnetic field and the induced magnetic field in the subsurface rocks in the study area to detect magnetic anomalies [9].The magnetic anomaly is resulting from the spatial variation in magnetic susceptibility between the target and the surrounding rocks.In general, any rock containing minerals of positive magnetic susceptibility (+K) that exists in the earth's magnetic field will acquire an induced magnetism field that is directly proportional to its magnetic susceptibility (K), as illustrated in the following equation [10].However, the major part of the measured field in the magnetic exploration method is due to the geomagnetic field and a small part of it comes from the induced field of rocks or any other objects containing minerals with positive magnetic susceptibility [11].The induced field of the underground rock represents an important magnetic anomaly that could be interpreted to reveal the depth and location of the target body.[12].
The vertical gradiometry in the field will measure the difference between the readings of the lower sensor, which is close to the ground surface by 30 cm, and the upper sensor, which is located 130 cm above the ground surface [13].As in the below equation: Vertical gradient = Reading of lower sensor -Reading of upper sensor The vertical gradiometer arrangement makes the lower sensor close to the archaeological object so that records a higher magnetic strength than the upper sensor.The magnetic force is decreased sharply when the distance between the sensor and the archaeological object increases (it decreases by the cube of the distance) [14].Therefore, the gradient technique is effective in highlighting the magnetic effect of archaeological structures close to the Earth's surface [15].At the same time, it is successfully able to cancel the magnetic influence of objects far from the Earth's surface.In addition, the magnetic gradient method will reduce or eliminate the influence of iron objects and buildings near the measurement point.Therefore, it will increase the signal-to-noise ratio, giving high interpretive power to the magnetic data.As a result, this method is effective in identifying the archaeological situation below the surface of the earth, reduces failed excavation operations, and decreases money expended [16].

4-Field work
The Vertical gradiometric surveying was performed utilizing a Geometrics-G-858 cesium magnetometer with a pair of sensors.Fig. 2 illustrates that the lower sensor was 30 cm above the ground, and the top sensor was 100 cm above the lower sensor.
Figure 2. Shows a graphical sketch of the vertical distance setup of the two sensors in the gradiometer Gradiometric surveying was carried out in the April 2023, and using high-precision ground magnetic survey criteria.Throughout the magnetic survey, we are attempting to keep the lower sensor 30 cm off the ground, workers' clothing free of magnetic noise, and the examination area free of surface magnetic noise such as buildings, scrap, electrical wires, and iron fences [17], as shown in Fig. 3.  To ensure precise movement during field recording, the traverse lines have been marked with wooden pegs and tied with ropes.This is particularly important since the distance between adjacent traverses is only 2 meters, as shown in Figure 5.A continuous recording manner has been used to acquire gradiometric data over the survey area so that provides one reading every 0.1 second.Gradometreic surveying was carried out by moving slowly in both north and south directions across the survey region.The magnetic survey took fifty minutes, during which more than 30,000 readings were recorded.
Figure 5.In the field work, the traverses market in earth by wooden pegs and ropes

5-Interpretation of Gradoimetric map of the study area
The resulting gradiometer data did not require further processing procedures.It showed a high signal-to-noise ratio that may be due to the fact that the archaeological objects have a high magnetic susceptibility contrast, in addition to the fact that in the process of magnetic investigation specific and high-precision parameters have been applied [17].A color contour map of the vertical magnetic gradient was drawn by using the MagMap2000 program, which is attached to the instrument, as in Fig. 6.
Figure 6.Gradoimetric color contour map of the study area that is resulted from the magnetic survey.The scale is in meters, as stated in the dimensions of the image that is taken directly from the gradiometer screen.
The gradiometric map showed three clear longitudinal magnetic anomalies at different places.
The main anomaly, marked by letter A inside the rectangular of black solid line on the map, lies on the edge of the north part of the study area and extends longitudinally in the northwestsoutheast direction that may indicate the presence of a main external wall extending longitudinally with a width of 2-3 m and at the depth approximately 1 m under the earth's surface, as seen in Fig. 7.Besides that, this anomaly displayed relatively a high magnetic strength of about + 50 nT, which indicates the wall was built from baked brick and composed of clay minerals rich in iron oxides [18].Furthermore, the magnetic contour map showed two other magnetic anomalies.The first anomaly occupies the western part of the study area, marked by a letter B, and appears perpendicular to the main magnetic anomaly.Meanwhile, the second anomaly lies in the southern part of the study area and is parallel to the main anomaly, marked by a letter C on the map, as shown in Fig. 7.The two anomalies showed lower magnetic strength than the outer wall.So, they may represent the remains of the inner room walls built from mud bricks [19,20].The thickness of the two supposed walls is about 2.5 m and they are at a depth one meter from the ground.Accordingly, to the magnetic results, the graphical plan of the buried archaeological features was drawn, which represents a proposal for the reality of an archaeological structure beneath the surface, as seen in Fig. 8.
Figure 8. Graphical sketch explain the supposed walls according to the gradiometric findings.
The results of the magnetic survey have been a good guide for new excavation, which led to finding the main wall built from fired bricks, as illustrated in Fig. 9. Therefore, the magnetic gradiometric survey method is considered useful and effective in detecting archaeological features buried in the soil near the earth's surface [21].
Figure 9. Real picture of buried archaeological main wall after excavation according to the gradiometric results

6-Conclusion
The vertical gradiometric survey was conducted on 1118 m2 in Tell Al-Deylam, one of the archaeological hills located in the middle of Iraq that has not been investigated yet.The interpretation results confirmed the existence of three longitudinal buried walls at different places in the survey area.The main anomaly is located on the edge of the northern part of the study area and extends longitudinally in the northwest and southeast direction.This may indicate the presence of the main outer wall, as it extends lengthwise about 30 m, with a width of 3-4 m, and a depth of about 1 m below the surface of the ground, and may contain the main northern gate of the Dilbat Temple.Besides that, the wall was built from baked brick and composed of clay minerals rich in iron oxides.The second wall is located in the western part of the study area and appears perpendicular to the main fired brick wall.Meanwhile, the third wall occupies the southern part of the study area and is parallel to the main wall.These two walls may represent the remains of the inner room walls built from mud bricks with a thickness of about 2.5 m and at a depth of 1m under the ground surface.The results of the magnetic survey have been a good guide for new excavation, which led to finding the main wall built from fired bricks.Therefore, the magnetic gradiometric survey method is considered useful and effective in detecting archaeological features buried in the soil near the earth's surface.Therefore, a specialized geophysical survey can help provide data on buried antiquities and better understand the general archaeological situation.It can also be a good guide in identifying areas of special importance for planning future archaeological excavations.

Figure 1 .
Figure 1.Location map and satellite image (Google) of the study area.

Figure 3 .
Figure 3. Gradiometer G-858 cesium Magnetometer in the field work with applying Field measurements were carried out in MagMap bi-directional mode, with continuous recording every 0.1 second.The magnetic survey covered an area of 1,118 m2, was rectangular in shape, 44 m long, and 27 m broad.The area was divided into 22 traverses running north-south.The length of each traverse was 27 m, and the spacing between them was 2 m, as shown in Fig. 4.

Figure 4 .
Figure 4. Shows map of the survey of gradiometric measurement paths and direction of movement.The scale is in meter, as stated in the dimensions of the image that is taken directly from the gradiometer screen.

Figure 7 .
Figure 7. Gradiometric map shows magnetic anomalies that represent buried walls.