Integrated Geophysical Methods to Discriminate the Locations of the Subsurface Weakness Zone Areas in the State Company for Glass and Refractories at Al-Ramadi City, Iraq

Abstract


Introduction
The weakness zone simply can be defined, weakly lithological materials properties which as a layer, zone or vein where the rock mass properties are meaningfully poorer than in the surrounding materials (Carrière et al., 2013).The weakness zones in the subsurface can be included cavities, voids, fractures and remains that may be fully or partially filling with sediments that have a high gypsum content and secondary gypsum or interbedded with other materials such as soil, sand, silt and clay (Salman et al., 2020a).These features can lead to significant damage to infrastructure and buildings constructed above them.Ground Penetrating Radar (GPR) and Electrical Resistivity Imaging (ERI) techniques are among the most common and significant tools within the geophysical methods.They are applied in shallow investigations to detect, identify, and map subsurface materials and fillings, especially in the construction and infrastructure features (Al-Hetty et al., 2021).ERI measures resistance by employing two potential electrodes M, and N fixed to the ground while inserting current from the resistivity meter into the ground using two stainless steel electrodes A, and B with specific parameters that control the depth of investigation and resolution (Telford et al., 1990;Ernstson and Kirsch, 2006;Loke, 2018;Salman et al., 2020b).
Maxwell's equations are significant for understanding the performance of GPR because they describe the relationship between electromagnetic material properties and electromagnetic wave propagation, allowing for quantitative characterization (Baker et al., 2007).The transmitting antenna sends out an electromagnetic pulse (EM pulse) that travels into the subsurface and bounces off point sources or interfaces due to differences in the relative permittivity.The energy is then reflected or scattered back to the surface and recorded by the receiving antenna (Aziz and Othman, 2017).
Several authors have Integrated these techniques to determine the subsurface conditions such as; (Benito et al., 1995) that have applied them to determine the natural and human induced sinkholes in gypsum terrain, the results state that the sinkhole morphologies give some idea of possible subsurface processes as well as an indication of the final mechanisms involve in sinkhole development and the (GPR) anomalies provides the voids at 4-5 m deep.Accordingly, (Boubaki et al., 2011) were used both techniques for buried cavity detection at the Abbaye de l'Ouye, France, the results provided idyllic recognition of the subsurface to detecting of the cavities and provides useful subsurface image.(Saharudin et al., 2017), on the other hand, that used techniques including seismic and ERI to detect the filled and air cavities at University Saints, Malaysia, the results give a good identification to determine the filled cavities and air cavities based on the velocity distribution and resistivity values.(Mohamed et al., 2019), however, have been Integrated both GPR and ERI for detecting near-surface caverns at the Duqm area, Oman.The results indicate two separated caverns within the karstic unit filled with materials based on the physical properties of the caverns than the host rock.
The objective of this paper is to detect, assess, and produce 2D maps of the weakness zones using the electrical resistivity technique with a Dipole-dipole array.Besides, applied ground-penetrating radar along the same traverse as the ERI profiles to combine both techniques and enhance the results of the current study.

Location and Geological Setting of the Study Area
The investigated area is situated in the Al-Anbar Governorate's, Al-Ramadi City, five kilometers west of the city and it aligns with the Al-Warar River, which is a branch of the Euphrates River, (Fig. 1).

Fig. 1. A satellite image shows the investigation's area
According to Jasim and Goff (2006), the investigation's region is situated in the Mesopotamian Zone of the Nubian-Arabic Platform's stable shelf from the west.The region is structurally surrounded by the Amij Samarra -Halabcha Transvers Fault, which passes across the most area, which is a primary subsurface structural lineament.The Quaternary deposits, Injana, and Fatha formations make up the geologic sequence.These deposits comprise of sabkha deposits, gypcrete, gypsiferous soil, marl, siltstone, claystone and fine sandstone with secondary gypsum (Sissakian and Mohammed, 2007;Abbas et al., 2022).
The investigated area has been influenced by the rising of the groundwater levels due to its locating along with Al-Warar River besides the quaternary deposits that formed with deposit of the gypsiferous, gypcretes and secondary gypsum soils.This is due to the interaction of the gypsiferous soil with the groundwater and leads to the dissolution of the gypsum in water or soil, resulting in the form of weakness zones, leakage areas, corrosion of ferroconcrete and making up a condition like chemical weathering processes, (Figs. 2 and 3).

Data Acquisition and Processing
The electrcial resistivity profiles were achieved using the Syscal Pro resistivity meter device, which is one of the most commonly used devices for acquiring resistivity profiles.using a Dipole -dipole array in three electrical profiles of two-dimension were carried out (Fig. 4, Table.1).There is a fixed distance between each of these four electrodes in the Dipole-dipole array.This array can provide valuable information about lateral variations because of its high sensitivity, although it has limited coverage of vertical heterogeneity (Al-Ane, 1998;Loke, 2022;Salman and Al-Rahim, 2023).A LMX 200 GPR unit equipped with a 250 MHz antenna was arrayed at three selected stations in conjunction with the 2D ERI profiles (Fig. 4) to identify weakness zones, voids, and cavities, supporting the study's results.The field parameters were summarized in Table 2.The processing of ERI data consists of two stages: initially, Pro-Sys ll software is employed for editing, adjusting and removing any inaccurate values from the readings.Subsequently, the data is uploaded into the RES2DINV program, which generates an inverse model of the electrical resistivity data readings (Table 3) lists the processing parameters that were used on the data.GPR data processing is performed using the mat-GPR software, which is a software package designed to run within the MATLAB environment and features a user-friendly graphical user interface (GUI), (Tzanis, 2010).Similar to other software tools used for GPR data processing, mat-GPR offers a wide range of tools, filters, and the capability to model 2D profile sections and 3D (GPR) slices.The processing sequence involves multiple stages, as illustrated in Fig. 5.

Results and Discussion
The inverse model of the Dipole-dipole array at station-1 (Fig. 6) revealed the resistivity distribution of the subsurface.It detected a weakness zone along the profile at a depth ranging approximately from 3 to 7.5 meters.Furthermore, it identified two expected cavities filled with sediments.The entire profile indicated the presence of a single weakness layer.In contrast, the GPR profile (Fig. 7) displayed an anomaly with a weak signal due to several factors affecting the survey.However, the top section of the GPR profile indicated the presence of a weakness zone along with the middle section.
The inverse model of the Dipole-dipole array at station-2 (Fig. 8) clearly depicted the resistivity distribution along the entire profile.An expected cavity with high resistivity was evident at a depth of approximately 12 meters.Additionally, a single layer with moderate resistivity values was identified as a weakness zone.This can be interpreted as an indication that the groundwater has affected certain regions within the study area as a tunnel, while other regions remain unaffected.Similarly, the GPR profile at station-2 (Fig. 9) detected the anomaly identified at the upper layer of the weakness zone.A hyperbolic response appeared in the section at about 15-20 meters.This is caused by passing the survey line through an iron structure placed on the top of the survey line.This result can also lead to the fact that the GPR energy can leak out to the air and reflect back to be visually present in the section due to the surrounding structures such as buildings, trees, poles and fences.
The inverse model of the Dipole-dipole array at station-3 (Fig. 10) displayed the resistivity distribution of the weakness zone at a depth ranging approximately from 3 to 5 meters.It also detected two expected cavities in the center of the inverse model, located at depths of about 5 meters and 10 meters, respectively.The (GPR) profile at station-3 (Fig. 11) strongly supported the presence of the two expected cavities and successfully detected the weakness zone in the area.

Conclusions
The electrical resistivity and ground penetration radar techniques were employed to investigate the subsurface of the study site and the results of the two geophysical techniques were integrated.The conclusions of the investigations confirmed that the primary lithology led to the identification of two distinct weakness zones.
The first weakness zone was located in the quaternary deposits at a depth of approximately 2.5 to 5 meters, while the second weakness zone was detected in the Injana Formation deposits at a depth ranging approximately from 7 to12 meters.
The electrical resistivity method utilizing the Dipole-dipole array provides an ideal and powerful subsurface image for detecting weakness zones with precise structural information.Therefore, the electrical resistivity method proves to be a promising technology for detecting and identifying weakness zones.On the other hand, ground penetration radar offers a useful technique for locating weakness zones, although it may not provide detailed and accurate information about the subsurface due to the optimal antenna requirements, the signal-to-noise ratio, and other aspects.
The lithological properties of Al-Ramadi area consist mainly of inter-bedded layers rich in gypsiferous deposits.Gypsiferous soils are known to be among the soils that can lead to various issues affecting the stability of substructures and engineering soil mechanisms.
These issues include soil swelling, the formation of cavities, fractures and voids.In Al-Ramadi area, groundwater levels vary from shallow to deep during the recharge season, which can result in a range of problems due to the interaction between water and the gypsiferous layers.
This study suggests the need to delineations all potential areas within the glass and refractories factory situated along the factory's riverfront.It is essential to identify the subsurface leakage responsible for the damage to the infrastructure in the thermal insulation area.Furthermore, conducting a geotechnical engineering study is recommended to identify weak layers and conduct appropriate engineering treatments such as cement grouting and injection before commencing the development of infrastructure and facilities.

Fig. 2 .
Fig. 2. The seepages zone within crypt of a glass processing unit.

Fig. 4 .
Fig. 4. Sites of the selected stations of the 2D electrical and GPR profiles

Fig. 5 .
Fig. 5.The applied processing sequence of the GPR data.

Table 3 .
The parameters of processing applied to the ERI data.