An Attempt to Delineate Subsurface Ain Aldawar Cavity Using 2D Electrical Resistivity Imaging Method in Haditha Area, Western Iraq

Abstract


Introduction
Construction projects rely heavily on the stability and integrity of the underlying geological structure.Onе of the main engineering risks associated with construction is the existence of underground cavities and voids, particularly in regions underlain by carbonate rocks.This phenomenon poses various challenges to infrastructure such as buildings, roads, and pipelines, and can еvеn lеad to land subsidence.Thе subsurface cavity encompasses all subsurface features including voids, cavities, caves, fissures, lenses, karsts, and sinkholes (Owеn, 1983).Thеsе cavities occur due to a variety of factors, including natural processes like the dissolution of soluble rocks such as limestone or gypsum, as wеll as the dissolution of sediment dеposits such as gypsum soil (Al-Zarah, 2007).Human activities such as mining or tunnеling can also contribute to the formation of these conditions.Carbonate rocks, prevalent in most western deserts, are particularly sensitive to acidic solutions.Limestone has a low or constant dissolution ratе in acidic water (0.0001 N H2SO4) (Othman еt al. 2020).Typically, cavities in limestone are uniformly shaped and larger than those in gypsum, with depths ranging from 1 to 15 m (Abеd еt al.

2021(.
A variety of geophysical methods are employed to detect subsurface cavities and voids.These include; ground penetration radar (GPR), electrical resistivity method, seismic survey, and electromagnetic method.These methods yield specific types of data regarding the ground composition, allowing engineers to identify risks beneath the surface (Greenfield, 1979;Bates, 1973;Dutta et al. 1970).Electrical resistivity is particularly noteworthy for its widespread use in geophysical and civil engineering to explore subsurface cavities and areas of structural weakness (Dahline, 2001;Hassan et al. 2018;Al-Hetty et al. 2021;Abbas et al. 2022a).The resistivity imaging involves sequential horizontal and vertical measurement with progressively increasing electrode spacing.The accuracy of the resistivity distribution data for the subsurface layers depends on the specific electrode array used (Loke and Barker, 1996).Recent advancements in this field include the development of modern inversion algorithms that generate resistivity images, which can illustrate both 2D and 3D representations of the subsurface.
In western Iraq, several studies have implemented resistivity techniques to map underground cavities and voids; Abed et al. (2021) used 3D resistivity imaging to detect and visualize the Um El Adam cavity within a complex geological setting, demonstrating the efficacy of the method in detecting and characterizing subterranean caves.Abbas et al. (2022b) employed 2D and 3D resistivity imaging to examine subsurface weak zones in Ramadi City.They recognized a bed of gypsum soil at about 2.5m depth with resistivity values reaching from 400 to 112 ohm.m.Al-Jbouri et al. ( 2022) developed 2D electrical resistivity imaging, exactly dipole-dipole arrangement, to determine the depth and sizes of a subsurface cavity in the carbonate rocks.This section showed a cavity depth of about 11 m up to the covering of the cavity, whereas the real measured depth at the a similar place was about 15 m.

Geological Setting
The current study is done to detect the subsurface cavity, known as Ain Aldawar, within the Euphrates Formation situated on the right bank of the Euphrates River in Allows village, east of Haditha city, western Iraq (Fig. 1).Tectonically, the study area is located in the Salman Zone, which is a segment of the stable shelf (Jassim and Goff, 2006).The Euphrates Formation (Early-Middle Miocene) within this cavity is 14 meters thick and consists of multiple layers from the base to the top: (1) a 5 m thick basal brecciate base layer made of limestone, (2) overlying well-bedded limestone rocks transitioning into dolomitic limestone rocks, and chalky limestone rocks with minor shale in the upper parts, which are 9 m thick.
The study aims to outline the extension and image of the dimensions of underground cavities using the 2D resistivity imaging technique.

Materials and Methods
The study utilized the ElectreII Pro program to establish 2D resistivity survey parameters, including a-spacing, n-factor parameters, length of the traverse, number of electrodes, and depth of investigation before fieldwork (Fig. 2).Field surveys were conducted in the eastern part of the Ain Al-Dawar cavity using the Terrameter SAS 4000 instrument to collect 2D resistivity data.Four 2D resistivity traverses were employed using a Dipole-Dipole array with a spacing of 3m, an n-factor of 6, and the distance spacing between traverses was 15m.With a total length of 57 meters, 20 electrodes, and 138 readings that provided data coverage for a single traverse, these traverses were carried out from the North to the South (N-S), reaching a depth of investigation of 11.75 meters.All 2D traverses were carried out above the cavity, and 2D resistivity data were gathered manually, with cables moving from electrode to electrode along the measurement path traverse.

Results and Discussion
RES2DINV software was used to process and interpret the 2D resistivity image data.The robust constrained inversion method used to characterize sharp boundaries of cavities, voids, and cracks (Busby, 2000;Loke, 2020).
Because the first traverse (T-1) is located at the beginning of the cave, the inverted model (Fig. 3) only depicts a small portion of the cavity.The results show the highest resistivity values at distances of 7-25 m and depths of 5-12 m, which are interpreted as cavities.The second high resistivity values were found at distances of 34-48 m and depths of 4-12 m, indicating cavities filled with sediment or dry, whereas the low resistivity values indicated weathered zones or voids filled with water caused by heavy rain in the area.
The inverted model of the second traverse (T-2) (Fig. 4) showed large parts of the cavity because located traverse over the cave about 15 m west of the T-1 with variable resistivity value from 159 to 2100 Ω.m.The results show first the high resistivity values at distances 9-13 m and a depth of 2.7 -7 m, the second high resistivity values at distances 15 -28 m and a depth of 2.7 -8 m indicator as a major part of the cavity or large voids, while the third high resistivity values at distances 32 -54 m and a depth of 2.7 -12 m indicator as a major part of the cavity.The section shows variation in resistivity value for long traverses with shallow depths that may represent weathered zones filled with water of sediments.
The 2D inverted model shows a large part of the cavity along the third traverse (T-3) because this traverses over the cave.The results show the high resistivity values at a distance of 7-18 m and depth of 0.5-4 m interpreted as a lag void and weak zones.The high resistivity value extends to a distance of 20 to 35 m and a depth of 1-12 m interpreted as a cavity dry or filled with sediments while the beginning of the southern and northern sides of the 2D invesion model shows weak zones filled with water with a distance of 7 to 12 m,41-50 m and depth 1.5 to 4.5 m.The last high resistivity value extends to a distance of 37 to 54 m and a depth of 3-12 m interpreted as a major cavity (Fig. 5).

Conclusions
The 2D resistivity models revealed numerous cavities and weathered zones within the Euphrates Formation.The electrical resistivity imaging traverses indicated the presence of cavities at a depth ranging from 5-12 meters and with a width of approximately 40 meters.These cavities exhibited higher resistivity values compared to surrounding rocks.Additionally, the existence of several small caves and voids near the surface of the ground was observed, indicating a higher risk for construction projects in the study area.The inverted model showed differences in cavity location due to variations in traverse location over the cave.

Fig. 1 .
Fig.1.Location of 2D resistivity traverse along the area of the cave, which noted in the red circle

Fig. 2 .
Fig.2.Parameters of 2D resistivity survey in the present study by ElectrePro program.
Figure (6) shows a 2D resistivity model of the fourth traverse (T-4), which located west the study survey within a small part of the cavity because the position of this traverse is on the end of the cave.The results show two cavity zones, the first at distance between 15-27 m and a depth of 2-8 m, the second at a distance of 32-48 m and a depth of 1.5 -9 m.Many weathered zones present with shallow depth with variable resistivity values from 2.97 to 23 Ω.m.The 2D resistivity model shows the cavities and voids with sharp boundaries and high resistivity values compared to surrounding rocks.

Fig. 3 .
Fig.3.Inverted model of 2D resistivity for the first traverse

Fig. 5 .
Fig.5.Inverted model of 2D resistivity for the third traverse

Fig. 6 .
Fig.6.Inverted model of 2D resistivity for the fourth traverse