Application of Multi-Channel Analysis Surface Waves and Electrical Resistivity Tomography Methods to Identify Weak Zones at University of Mosul, Northern Iraq

Abstract


Introduction
The connection between human activities and geological position is one of the causes of the progressive increase in sinkhole dangers and weaknesses at several sites (Lazzari, et al. 2010).When the collapses happen in urban centers, they pose a major natural danger to human safety and infrastructure.(Cardarelli et al. 2010;Krawczyk et al., 2012).In the city of Mosul, sinkholes often occur in areas with karstic geology which are comprised primarily of limestone, dolostone, and gypsum (soluble rocks), therefore, many places of the new buildings at the university of Mosul are underlain by rock types that are susceptible to dissolution and over time the movement of water may lead to develop of cavities and sinkholes within these rocks.The study area is situated above superficial (infill materials) and river terraces deposits.Large areas within the university and adjacent areas are composed of clastic sedimentary rocks such as (sands, gravel, and conglomerate) are haven't solubility and karst formation (Al-Dewachi, 2005).
The river terrace deposits exhibit a great mechanical and spatial heterogeneity and a very high stiffness variance within the ground (Colmenares et al., 2018).Within the lifetime of an engineered structure, new caves or sinkholes may form due to both geological and human activities.Man-made sinkholes are frequently seen near-continuous water flow, where dewatering efforts can disrupt regional groundwater flow patterns, causing subsidence and increase void formation.thus, dewatering accelerates sinkhole formation by raising hydraulic gradients and flow velocities, which speeds up the erosion of unconsolidated materials.Many variables contribute to engineering structure failures, such as building collapse or subsidence, including poor and inadequate construction materials and incompetent foundation soils or sediments.
According to many authors (e.g., Zhou et al., 2002;Ivanov et al. 2013;Samyn et al. 2014) in urban areas, invasive methods may be high cost, time consumption and may exacerbate sinkhole development.geotechnical and environmental projects that employ geophysical subsurface imaging techniques to mapping residential geohazards, such as sinkholes and subsoil void in built-up areas are often led in and around structures that are above, on, or under the ground including (e.g., foundations and concrete slabs) (Dobecki, 2010).The traditional geophysical measurements may not be effective (Cardarelli et al. 2010;Krawczyk et al. 2012).The near-surface features, such as cavities and/or inhomogeneities in foundation geo-materials, are the main sources of hazards in civil engineering disciplines (Domenico and Sergio, 2009;Araffa, 2010;Al-Heety and Shanshal, 2016).
Geophysical survey can be used to investigate geotechnical engineering issues.It can be carried out to collect information about physical and dynamic properties of soil and rock underlying (and sometimes bordering to) the site to design earthworks and foundations for proposed structures and repair of distress to earthworks and structures caused by subsurface conditions.These aims will be accomplished by utilizing near-surface geophysical methods such as Seismic Refraction Tomography (SRT), Multichannel Analysis of Surface Waves (MASW), Electrical Resistivity Tomography (ERT), and Ground penetration radar (GPR).From building site studies to dam inspections, geophysical methods are employed in a wide range of applications to investigate geological structures, estimate soil/rock physical features, and calculate the physical and geotechnical parameters of rock formations.(Othman, 2005;Soupios et al., 2007;Al-Saigh and Al-Heety, 2014, and Al-Heety et al., 2021, Ghanem et al., 2021).The MASW approach, which is based on the dispersion properties of the Rayleigh wave, may be used to determine the distribution of Shear-wave velocity or shear moduli of the rock or soil layers.A Rayleigh wave is a dispersive surface wave with numerous frequency components, each wave traveling at a specified velocity parallel to the Earth's surface (phase velocity).The wavelengths of the frequency components determine their penetration depths, each component's propagation velocity is proportional to the medium's SV-wave velocity.Many researchers have shown that there was no consistent difference between the results of MASW method results and those produced by traditional borehole surveys.(Xia et al., 2000;Xia et al., 2002;2006;Di Fiore et al., 2016).
The Electrical Resistivity techniques such as ERT, as a non-invasive technique for subsurface characterizations, yield temporal and spatial variations of various physical properties of the subsurface strata and structures, fluid composition or water content (Park et al., 2014;Arjwech and Everett, 2015;Gardi et al., 2018).The application of ERT can provide 2D geoelectrical images of subsurface resistivity distributions from which features of contrasting resistivity can be located and characterized.The main advantage of ERT survey is produce high-resolution images of the subsurface.Because of the sensitivity of resistivity to changes in hydrogeological, and lithology (e.g., water saturation, pore fluid composition, and clay content) and geological features.Furthermore, when investigating indicate the presence of voids and sinkholes that might pose major threats to buildings or civil engineering systems, ERT images are very helpful.(Kaufmann 2014;Samyn et al., 2014;Giampaolo et al., 2016).The engineering issues at the lecture hall (cracks, joints, and crawling) have been reported.The problems were first noticed two years ago after the lecture hall had been built.The floor starts developing a crack between the wall and the floor, separating from the baseboard, and started to find gaps between the floor and the wall outside construction.The cracks appeared around door frames, ceiling, corners, and along the walls.Also, some concrete structures outside the building were observed to fall and started to crack or sink.The main objective of this paper is to recognize the possible reasons of the observed cracks and fractures and survey the near-surface soil and rock to identify the presence of voids or other types of anomalies in the ground of the lecture hall building by utilizing two geophysical techniques.With this aim, we present the results obtained through combined geophysical methods involving non-invasive seismic MASW and ERT to clearly understand and support the quality of the interpretation.To provide information regarding the safety of constructions, they are well suited for surveying foundation soils and identifying possible causes of settlement.

Site Description and Geological Setting
A large lecture hall was established five years ago belongs to the college of Administration and Economics (Called Mosul Hall) one of many new halls and new collage constructed at the northeastern region of the University of Mosul Campus, on the left bank of the Tigris River, North of Mosul city (Fig. 1).It was built to address the issue of insufficient accommodation.This structure has geotechnical problems, which manifested as cracks and fractures of variable proportions (Fig. 2).Tectonically, Mosul city is situated at the Foothill zone according to the tectonic division of Iraq (Jassim and Goff, 2006) (Fig. 3a).Geologically, The Fat'ha Formation (M.Miocene) and Quaternary sediments are mostly exposed rocks on the eastern part of the Mosul city (Al-Naqeeb and Sulaiman, 2008) (Fig. 3b).The Fat'ha Formation consists of cyclic sediments of green marl, limestone, and gypsum, with reddishbrown claystone of it is upper member.The foundations of most construction rest on these highly karstified rocks (gypsum and limestone).
The geology and climate conditions surrounding Mosul areas are favorable to the formation of dolines, caves, and sinkholes (Al-Dewachi, 2005).The terraces are constructed of several rock types, such as extremely coarse sandy conglomerate, silty conglomerate, and clayey conglomerate.The difference in terraces thicknesses reflects different amounts of erosion controlled by morphology.The river terraces around the Mosul University area consist of an alternation coarse-grained carbonate or/and silica cemented conglomerates, gravel, pebbles, sands, clay, and marls.The pebbles consist of limestone, silicate, and igneous and metamorphic rocks, in gypsiferous, carbonate, and sandy (silica) cement in addition to other non-soluble grains occur as well.The maximum thickness of the outcrop is ranging between (2m and 15 m), while some subsurface drilling is up to 30 m thickness (Al-Dewachi, 2005).The conglomerates often crop out and form ridges and steps at the University and adjacent area.The Tigris river has four terraces; the study area is located within the third terrace as shown in Fig. 4. The river terraces lie unconformably on the Fat'ha Formation bedrocks, with a basal erosional contact.(Fig. 5) show typical caves on a conglomeritic rock outcrop, near Mosul university, while Fig. 6 show conglomeritic rock outcrop, nearly to the cracked building inside Mosul University.Jassim and Goff, 2006); (b) part of the 1:1 million scale geological map of Iraq shows the study area (modified after Sissakian and Fouad, 2015).

Materials and Methods
The objectives of this study, as stated in the introduction, are to obtain an appropriate geological characterization of the site and to discover any possible cavities or subsurface voids surrounding the structure.The study area peculiarities have to be taken into consideration during the planning phase of the geophysical measurements.the two geophysical methods were conducted successively.MASW data were collected and assessed, and in the second stage, according to the MASW results, ERT surveys were performed at the same locations.First, the two 2D MASW lines are parallel and the other one orthogonal to them was designed in addition to the same strategy carried out to the 2D-ERTs survey.

Multi-Channel Analysis Surface Waves (MASW) Survey
The MASW measurements were carried out at the bounding of the lecture hall building from three sides to cover the whole possible area as shown in Fig. 7.The surface wave records were collected using a linear array of 12 vertical geophones with a natural frequency of 4.5 Hz with 2.0 m geophone intervals, and 5 m source to receiver offset, Therefore, the length of each array is 22 m.Standard roll-along (SRA) techniques provided shot gathers with a consistent spread geometry every 2 m for a total of 10 shots points along the MASW lines.Thus, the total length for MASW lines is 40 m.The geophones were connected to ABEM Terraloc Mk6 v2 seismograph multichannel data-acquisition, and 10 kg sledgehammer impacted on a metallic plate to generate seismic energy.To increase the signal-to-noise ratio, each shooting point received 5 to 10 hammer hits.The recording period was 1000 ms, with sample intervals of 25 ms.The MASWs and ERTs lines were deployed such that geophones and electrodes array in the adjacent to sidewalks.Spiked geophones were used in fair compacted infill materials areas for MASW-1 and 2 while high compacted areas in MASW-3.Fig. 8 shows roll-along data acquisition system for generating 2D Vs Pseudosections.The depths of investigation extended few meters of about 12m below the ground surface.A summary of the main parameters related to the MASW and ERT field measurements is provided in Table 1.The MASW method consists of four main components (Miller et al., 1999): acquiring multichannel records (or shot gathers) with roll-along data acquisition, dispersion curve imaging, and curves estimations (from each record) (Park et al., 1998;Xia et al., 2007).Dispersion-curve inversion to obtain a 1D (depth) shear-wave velocity (Vs) profile for each record (Xia et al., 1999), and assembling multiple 1D results into 2D or 3-D images (Miller et al., 1999;Miller et al., 2003) using interpolation algorithms such as Kriging.Fig. 8 shows examples of the MASW data acquisition and processing results.The MASW field data were processed to obtain 2D shear-wave velocity section is performed using SeisImager/SW programs from Geometrics company (Pickwin, WaveEq, GeoPlot modules) and Surface Wave Analysis Wizard automatically calls on functions from these three modules to walk through the processing flows for surface wave data analysis.The MASW data was recorded with 2m geophone intervals for each spread array with standard roll-along provided shot gathers every 2 m for a total of (10) shots point along the MASW line.All recorders for individual lines opened at once using Pickwin, the survey geometry parameters (source and receiver location, receiver interval) have been updated.The next step used filters (high-cut and low-cut filters) to remove noise caused by wind, traffic, and other sources.(Fig. 9) shows filtered shots gather recorded for the first shot gather in three MASW's lines and corresponding dispersion curves.Next step, the Common Mid-Point (CMP) cross-correlation gathers of multi-channel and multi-shot surface waves were calculated.The first and last distances were determined automatically using the first and last coordinates of the receiver spread, and the bin size is two times the receiver interval.Next recognizing the surface wave (Rayleigh wave) from each shot gather and set parameters for transformation CMP cross-correlation gathers from space-time domain (tx) to phase velocity-Frequency (c-f) domain to generating the dispersion image and testing some parameters to find the best dispersion image.The minimum and the maximum frequency were (5 and 35) Hz while Min. and Max.phase velocity were (35 and 800) m/sec.
The necessary dispersion features (fundamental mode) were picked to estimate dispersion curves.The dispersion curve is shown as a graph between phase velocity and frequency.The picking of dispersion curves was performed automatically and any bad picks can be manually deleted later or edit picks).Then these pick points are saved as a dispersion-curve file, which is later viewed and inverted by WaveEq module, to obtain 1D(depth) Vs profiles for each record.The module includes a set of dispersion curve editing methods for removing low-quality, noisy, and higher-mode choices that are common on dispersion curves and can skew or create inversion instabilities.From the produced dispersion curves, 1DVs layer model profiles were generated.This 1Dmodel will be inverted.The goal of the inversion is to find a 1DVs layer model whose calculated dispersion curve would reasonably match well the observed dispersion-curve points in terms of Root Mean Square (RMS).The inversion starts with an initial Vs with depth model from which a dispersion curve is calculated and compared to the picked dispersion-curve points.By default, the program calculates the depth of the initial model using a one-third-wavelength approximation.Once the initial model is calculated, the number of curves used in the model is reported.Next Wave Eq. to run the inversion after setting the number of iterations for the inversion.The software will iterate a certain number of times, and this iterative procedure will continue until the generated curve fits the selected dispersion-curve points relatively well.The inversion process was carried out using a non-linear (least-squares) technique.The WaveEq module reports the matching error (RMSE) of the calculated and observed dispersion curves in units of time (ms or %).Once the inversion is complete, the final Vs curve is displayed.The measured and calculated curves of the MASW's lines (1, 2, and 3,) were overlapped using the RMSE method in the inversion operation.It was sufficient to perform 10 iterations at most.The RMSE values varied from 3.4% and 4.6%.The surface location of each 1D Vs profile was assigned to the center of the geophones spread.The final step is assembling the multiple 1D Vs final results (10 profiles for each line) into a 2D Vs pseudo section in a successive demand on the receiver station position using Kriging interpolation algorithms.Herein golden software Surfer 17.1 was used in the final production of 2D Vs pseudo-cross-section and later for ERT profiles.Fig. 9 shows filtered shot records with corresponding dispersion images and picked curves.

Electrical Resistivity Tomography (ERT) Survey
The ERT data were collected during 16-17, March 2021 by using Terameter SAS 4000 resistivity meter manufactured by ABEM supported with multi-electrode survey systems.The length of each ERT profile was 60 m, using 41 electrodes spaced at 1.5 m.After considering the benefits and disadvantages of each array arrangement, (Loke and Barker 1996;Zhou et al. 2002;Dahlin andZhou 2004, Shanshal andAl-Heety, 2020), the Wenner array electrode was employed for ERT measurements.Wenner array is relatively sensitive to vertical changes in the subsurface resistivity below the center of the array (Loke,2004), Wenner arrangement has a strong pictorial ability to determine depth compared to other arrangements.If the survey area is a high-noise area and needs good vertical accuracy, and survey time is limited, it is preferred to use the Wenner array.A total of 191 measurements were taken for a total of 8 levels.Fig. 11 shows the measured apparent resistivity pseudo-section for ERT profiles.
The recorded resistivity data sets for ERTs were processed and inverted by utilizing RES2DINV software by Geotomo Software (Loke and Barker, 1996;Loke,2004;Loke 2020).For the model roughness constraint, it uses a robust data weighting and L2-norm (in addition to a non-linear smoothness-constrained least-squares inversion method (Loke and Barker, 1996).Griffiths and Barker (1993) described in details the philosophy and technique of the routines applied in the software.The inversion method constructs a model of the subsoil using rectangular prisms and determines the resistivity value for each of them, minimizing the differences between the observed and calculated apparent resistivity.As a result, the RMS errors for the final models utilized for interpretation were 3.1%, 4.5%, and 2.8 % for ERT-1, 2, and 3, respectively.The inverted resistivity models obtained using the RES2DINVx64 software of ERTs profiles are depicted in Fig. 11.

MASW Lines
The calibration and correlation for both geophysical survey (MASW and later ERT) results were made by using geological information from more than locally four outcrops exposed face and excavation, as well as available geological and geophysical studies that carried-out in Mosul and within Mosul University Campus particularly nearby the current study area.Fig. 12 shows litho-strata constructed by taking the geological information from outcrop photographs taken nearby the study area during the fieldwork.It shows three distinctive lithological units: superficial soil on the top, river terraces in the (middle), and green marl bed at the bottom, which is visible from Fig. 12.The litho-strata clearly show the variation of thickness and depth of different layers.These strata will be used to correlate the geophysical results with the geological information from the outcrops.Three MASW lines were collected along two parallel lines and another one perpendicular to them bounding the lecture hall building from three sides as shown in Figs. 1 and 7. Fig. 13 depicts 2D shear wave velocity pseudosections obtained from MASW-1, 2, and 3 respectively.Three layers were outlined in the study area which includes infill materials (mixing of sands, clays, gravels and pebbles), river terrace (Early Pleistocene), and upper member green marl bed of Fat'ha formation (Middle Miocene).The shear wave velocities (Vs) ranging from 210m/s to 420 m/sec was found to be in good agreement with the infill materials.The associated low velocity down to river terraces top contact may be attributed to weathering of the medium because of karstification.The Vs ranging from 430 m/s to 840 m/s were found to be in good overall agreement with river terrace deposits.It is characterized by a gradual increase of mechanical properties with depth.Lastly, Vs larger than 840 m/s was found to be in good overall agreement with a competent marl bedrock.These Vs range in compatible with previous geophysical result such as (MASW and shear refraction) inside Mosul University Campus (Al-Saigh and Al-Heety, 2014).

MASW-1 Line (NNE-SSW)
MASW-1 line is located along the right side of the structure.It crossed over an area supposed to have subsurface conditions conducive to sinkhole/subsidence and possibly related to the structural damage such as cracks and fractures observed at the structure.The shear wave velocity pseudo sections for MASW-1 Fig. 13 exhibits that Vs values distribution is inhomogeneous (lateral and vertical variations).The first layer is characterized by low shear wave velocity ranges values (210m/s to 420 m/s).This is because poor compacted fill materials were noted through inserting geophone spikes and electrodes.Considering the local geology, it is reasonable to suggest that the infill material layer between the ground surface and about 2.5 -3.0 m of depth are vertically consistent.The next layer is attributed to a river terrace (fairly competent rock) which has Vs ranging from 430 to 840 m/s t's distinguished by a gradual increase in mechanical features as depth increases.between about 2.5-6.0 m the shear wave velocity drops in the top contact of this layer.
The related low velocity may be attributed to weathering of the medium because of karstification/sinkhole. several small-scale isolated low-velocity closures exist at this zone which seems localized, having a shape that could be a result of downward movement of sediments replacing more compacted/competent material(terrace) with less compacted materials from a shallower layer.There is no doubt that the main reason for this anomaly is behind the initiation, development, and formation of the sinkhole in the terrace conglomerate rocks.Furthermore, from fieldwork observation beside the building (right side) we assumed that the presence of water-pipe leaks, rainwater pool, and poorly compacted infill materials are the main reasons that are responsible for the development of pathways for the transport of unconsolidated clastic sediments and the development of depressions and cavities/sinkholes.As a result, this condition may cause hazardous stability problems and have a significant impact on the mechanical behavior of the building's structure.Underneath these levels, the third layer which is the marl bed (competent rock) belonged to the upper member of the Fat'ha Formation causes obvious increase in seismic velocity and makes a difference from the strata above and reached the maximum penetration depths of MASW survey.

MASW-2 (WNW-ESE)
MASW-2 line is perpendicular to other parallel lines.The subsurface imaging along this line extends from the end of MASW-1 west of the northwest (WNW) corner to near the east-southeast (ESE) corner of the structure.The interpretation of this line is also the same for MASW-1 (Fig. 13).The variations in velocity seem to track the lithological properties of subsurface material.Noteworthy features with low shear wave velocity are immediately evident on this line.These features are likely related to subsidence activities beneath the structure.Undulating (high to low) velocity features within river terraces are indicative of localized dissolution features or localized pockets of rock that are more susceptible to dissolution.The river terrace is well delineated by increasing shear velocity and the boundary between terrace and bedrock (marl bed) can be delineated.This feature can be interpreted as a stiffer, more resistive layer that has been replaced by relatively low velocity and likely much more mobile materials.All these features or velocity anomalies may be an area currently subsiding, as indicated by lower velocity and therefore weaker materials.

MASW-3 (NNE-SSW)
The MASW-3 line is located along the left side of the structure.Fig. 13 shows MASW-3-line 2D Vs pseudo sections.This MASW line shows no significant velocity anomaly, just the first layer (infill materials) has a higher velocity value (>250 m/s) than the other two lines.This is because of high compacted fill materials and drier.Also, this situation was noted through fieldwork when inserting geophone spikes and electrodes (hard ground), These results correspond to the lack of the building from cracks and fractures in this side.Fig. 14 shows proposed general lithological interpretation of shear wave data of MASW-1, allowed to recognize the near-surface geology.The current data suggest subsidence activities are or have been active along the right side of the structure.
This layer classification and interpretation allowed cavity and sinkhole areas at the top contact of the terrace to be detected by a sudden and drop in shear-wave velocity.These velocity droppings are generally limited in dimension to a few meters, revealing the presence of potentially isolated sinkhole These are in strong agreement with historical karst-related surface problems in the conglomeratic rock of the University of Mosul and adjacent area (Al-Dewachi, 2005), and situated within area favorable to the form dolines, high concentration of sinkholes, voids, and cavities that can aid subsidence in the area.In addition, Al-Saigh and Al-Heety (2014) concluded according to engineering and elastic parameters such as (Poisson's ratio, shear modulus, Young's modulus, concentration index, material index, and the stress ratio) that the terrace is slight to fairly competent and not suitable layer for foundation purpose.

ERT Profiles
The ERT data were utilized to give the distribution of electrical characteristics in the subsurface soil, delineate high resistivity zones that may be attributed to hard rock and dry clayey sand, silty sand, sand, gravel, and conglomerates lithology such as river terraces, and delineate the low resistivity zones associated with silty clay and marly clay such as Fat'ha formation clays and water-saturated soil/deposits.ERT-1, 2, and 3 penetrated about 10 m in depth.RMSE ranges from 2% to 4.5 % reach to 10 iterations for all profiles and it indicates a good data quality.Fig. 15 shows ERT-1, 2, and 3 in a logarithmic scale to represent the resistivity values after using surfer software.

ERT-1 (NNE-SSW)
ERT-1 corresponds to right side of the structure and reflects the geotechnical issue.The results showed variation in resistivity values ranging from 5 Ω.m to 600 Ω.m.These results illustrate the most significant findings of the ERT and could describe the cause of the engineering problem in the construction.The ERT-1 section consists of four electro-layers: The top thin electro-layer is characterized by a resistivity range between 50 Ω.m to 70Ω.m with a variable thickness from 0.5m and 1.5 m. which indicates a media with mixing clay, fine sand, and gravel.This zone is interpreted as poor compacted infill materials.The second electro-layer have a very low resistivity value (highly conductive) ranging between 2 Ω.m to 20 Ω m that extends horizontally from NNE to SSW beneath ERT-1 profile at depth of 0.5m to 3.5 m, which might be interpreted as a result of increasing water and clay and silt content and/or the presence of a high concentration of water (high water-saturated zone), where prober water leakage is concentrated at the center of this profiles.Also, the accumulation of rainwater pools in this small local depression, or the reason for this decrease in resistivity) noted in shear wave velocity ( can be attributed to the water pipe leakage underneath the building's right side.This high conductive zone may indicate the presence of geological dangers such as subsidence, a cavity or sinkhole filled with clay or silt, etc.The third electro-layer is considered by a high-resistivity value >100 Ω.m reflects a dry conglomerate rock and gravels belonging to river terraces.The fourth electro-zone is characterized by a low-resistivity value < 20 Ω.m associated with the water-saturated marl bed.These stratigraphic units are visible on the face of the outcrop excavation closest to the survey area.

ERT-2 (WNW-ESE)
ERT-2 corresponds to the west-northwest (WNW) corner (backside) of the structure, and is also, probable closest to the geotechnical issue.A series of resistivity zones similar to those of the ERT-1 profile is obtained, with the exception that the first electro-layer is considered by increasing resistivity values.Also, the lower zone of the marl bed does not appear, it may be found deeper than the penetration depth.A zone of low resistivity with an elliptical shape below the center of the profile is shown >20 Ω.m and can also be interpreted as a sinkhole due to water leakage.which means that it is a continuation of this sinkhole towards the southeast as shown in a logarithmic scale image (Fig. 15).

ERT-3 (NNE-SSW)
ERT-3 corresponds to the SSW corner (left side) of the structure.The results show that the top layer has a high resistivity value ranging between 50 Ω.m to 100 Ω.m.This layer is attributed to the dry and high compacted fill materials.Underneath this zone, very low resistivity values which are corresponding to clayey or marly bed (high water-saturated zone).In comparison to the marl of the Fat'ha Formation, the river terrace deposit is considered very permeable from a hydrogeological standpoint.surface water and groundwater will infiltrate through the river terraces and accumulate at their lower boundary with the marl of Fat'ha Formation.There is a small anomaly below beginning meters of profile with low resistivity with elliptical shape can also be interpreted as a small sinkhole.This interpretation agrees with shear velocity anomalies MASW-3 (Fig. 13).It is worth declaring that small-scale low, intermediate, and high resistivity anomalies (discontinuities) can be seen clearly along with all profiles.The general geologic setting that was extracted from the 2D ERT profiles is in quite an agreement within situ observation of the sediments from a natural subsurface cross-section observed at the adjacent site (Fig. 12).These stratigraphic units are visible on the outcrop and excavation closest to the study area.There are several outcrops, excavations, previous geophysical studies were performed on the investigated at the Mosul University and surrounding areas, allowing calibration of the electrical responses.Resistivities values were found ranges between 70 Ω.m to 800 Ω.m and 80 Ω.m to 320 Ω.m corresponding to river terraces whereas found ranges between 1 Ω.m and 70 Ω.m and 1 Ω.m to 80 Ω.m corresponding to the upper part of Fat'ha formation (Al-Mashhadany, 2020;Al-Heety and Shanshal, 2016) respectively.Mutiab (2000) found that resistivities values ranges (50-100) corresponding to (sand and gravel) and (Sand and silt), and value ranges (2-50) corresponding to Fat'ha clays and Quaternary (fluvial and alluvial clays).Whereas found ranges between 80 Ω.m to 400 Ω.m corresponding to gravels and conglomerates and found ranges between 400 Ω.m to 1000 Ω.m corresponding to clastic deposits.The subsidence/sinkhole may become out because the water seeps into the near-surface layers from water pipe leakage, in addition, and accumulating rainwater in some depressions in the adjacent area.The clay minerals belong to the first and second layers are saturated with water in the winter season, and they remain nearly stable.When the hot weather of summer appears, evaporation drives water out of the underlying strata.This process, in addition to loads of buildings, causes cracks features in the structures.The recent subsidence occurs with related to some artificial reasons such as careless foundation base layer conditions such as the mixing (mixture design) and compacting or aging of underground water pipelines.A cavity begins to develop as leaked water erodes the soil surrounding pipelines.This reduces the bearing capacity of the soil layer above the cavity, and hence, the ground collapses to form a sinkhole.

Combined Interpretation of MASW and ERT Data
A visual qualitative evaluation for a comparison of cross-sections of MASW data with other geophysical methods such as ERT data can often be useful to solve some ambiguities in interpretation.The combination interpretation of two data has been carried out to aim to obtain a more detailed and reliable reconstruction and enhance the identified near-surface anomalies imaged of the geological structures by both techniques.The MASW data can be useful to recognize the extent of near-surface layers because shear wave velocity significantly decreases in weathered rocks comparative to consolidated ones, Moreover, alternation of sinkholes/Cavities and weak zones and consolidated stiff rocks can be highlighted by lateral changes of velocity.Geophysical properties of different soil/rock types, particularly Vs and electrical resistivity, give enough difference to characterize soil/rock types (Inazaki, 2007).Fluctuations in water content (saturation), porosity, and permeability alter the resistance of the path an electrical current follows in the subsurface producing spatial differences of electrical resistivities, such as when a transition from unconsolidated clastic deposits such as sands to clays (Archie, 1942).Changes in soil compaction over time are responsible for the spatial variations of Vs (Bitri et al., 2013, Whiteley andCaffi, 2014) or the contrast in sediment stiffness between various sediment types (Cercato et al., 2010).Changes in Vs and resistivity readings as a consequence of temporal variations in water saturation can be used to identify soil.Temporal variations, such as a decrease in Vs or a change in resistivity, might signal the development of weaknesses.
Most of the resistive anomalies along profiles ERT-1, 2, and 3 as shown in Fig. 15 agree with the boundaries of the seismic shear velocity anomalies in Fig. 13.For example, the resistive anomaly (decrease in resistivity) along profile ERT-1 between depth 0.5m to 3.5 m are corresponding with the velocity anomaly in MASW-1 (drop-in velocity) at the same positions.Which was interpreted as saturated zone and weak zone (probably sinkhole) respectively.ERT-2 profile (Fig. 15) has a low resistive zone, this zone corresponding with the relative moderate shear velocity anomaly as shown in Fig. 13.The high shear velocity MASW-3 (Fig. 13) is by highly resistive anomalies along profile ERT-3 corresponding to dry and high compacted infill materials.The most interesting features can be seen along MASW-1 and ERT-1 (right side) which as adjacent to the position of beginning wall cracks and tiles are the low resistivity anomalies corresponding with relatively low shear velocity zones.These resistive anomalies could be interpreted as the saturated zone (weak zone) beneath the building.Another interesting resistive anomaly trending and shear velocity detected along MASW-2 and ERT-2, (back) and can be interpreted as the same profiles above.The shallow highly resistivity and low shear wave velocity anomalies are reflecting subsurface features possibly having man-made infill materials such as resistive anomalies corresponding with relative low shear wave velocity anomalies of ERT-3 and MASW-3.By contrast, some conductive anomalies (low resistivity zone) correspond with relatively low shear velocity anomalies such as MASW-1, 2 and ERT-1, 2. This can occur when the infill materials have a higher moisture content than the surroundings and might refer to sinkhole/cavity or caused subsidence by the effect of water pipe leakage.As illustrated in the fence diagram (Fig. 16), both MASW and ERT profiles appear to exhibit similar behavior of velocity and resistivity for most of the subsurface materials encountered.
To demonstrate the combination mentioned above, inversion results of arbitrary (center of line) 1D Vs and 1D ERT profiles form (MASW-1 and ERT1) have been used to compare the resistivity and shear-wave velocity response of the subsurface model as shown in Fig. 17.There are infill materials, a relatively thinner layer on the top followed by more compacted sediment (river terrace) layers, and in other areas being more saturated, and finally, the last layer as the bedrock having considerably higher values for both models.The two inverted models are corresponding to the same subsurface geology and, naturally, the response of the two different methods would suggest different models considering the two physical properties that are measured which are affected by different parameters such as groundwater saturation and consolidation.The top contact of the river terrace (corresponding to 430 m/s) as mapped on the 2D MASW Vs pseudo section for MASW-1 is shown to match fairly well with the top of the terrace as mapped on ERT-1 profiles.In addition, the top contact of the marl bed (greater than 840 m/s) was mapped, and it well matches the top of the marl bed mapped on ERT-1 profiles.The variations between the tops of beds on ERT and MASW can be attributable to several factors: Changes in moisture content, clay content, and porosity are the main triggers for the ERT technique.The MASW tool, on the other hand, reacts to variations in shear strength.Lower shear strength increased moisture and clay content, and higher porosity are all characteristics of soils.As a result, using ERT and MASW data, the recognized top of the bed is often at equivalent depths.generally, The Variance's in-depth estimations are attributed to the presence of very stiff moist soils (recognized as a rock by MASW data and soil by ERT data) or to the presence of very moist, porous rock (identified as a rock by MASW data and soil by ERT data).Furthermore, the 1DMASW Vs assigned to each depth interval indicate the average subsurface velocity (at that depth interval) along the 22-meter length of the MASW geophone array.The average resistivity across a 1.5 m period is represented by the ERT resistivity value applied to a certain depth interval (separation between electrodes).As a result, the MASW data are more reliable than the ERT data.

Conclusions
Geophysical surveys in the field of geotechnical engineering are considered to describe subsidence features of unknown natural origin, provide imaging analysis of possible reasons for subsidence.MASW and ERT surveys were conducted at the surrounding lecture building at Mosul University Campus to investigate the nature and distribution of shallow subsurface soil/rock.Three MASW and ERT profiles were collected along two parallel profiles and another one perpendicular to them bounding the lecture hall building from three sides.The MASW data show three layers: the first layer is characterized by low shear wave velocity which is attributed to infill materials, the second layer is attributed to a river terrace (fairly competent rock).In-depth about 2.5m and 6.0 m the Vs drops in the top contact of this layer.The causes of low velocity could be due to weathering of the medium because of sinkhole.ERTs profile shows four electrical zones, the first zone has 50-70 Ω.m with a variable thickness from 0.5 to 1.5 m which indicates as infill materials.The second zone has very low resistivity value, with depth of 0.5m to 3.5 m, which might be interpreted to be increased clay, silt and water content (high water-saturated zone).The third zone is characterized by a high-resistivity value (>100 Ω.m) that could be related to a dry conglomerate rock and gravels belong to river terraces.The fourth zone have a low-resistivity value (>20 Ω.m) associated with water-saturated marl bed.We could be definitively correlated the resistivity and velocity anomalies to sinkhole activity in were identified and characterized using combined geophysical methods.The variations in Vs (low velocity) and resistivity (conductive zone) within the river terrace were detected and proposed to be indicative of dissolution and the subsidence responsible for structural damage implied by the change in the velocity and resistivity.The roughness of the top terrace surface strongly influences the nature of the velocity and resistivity values.This roughness is suggestive of dissolution or erosional activity.
The reasons for the noticed cracked and fractured structure was predicted after fieldwork observations: (1) the infill materials layer below and nearby the structure may have a problem due to the pooling of rainfall water, which is leakage, penetrated and collected within in terrace bed, or at the contact between terrace and marl bed and might be wash out the matrix and cement supporting the strength of bed that loads of the structure; (2) a probable lack of uniformity in the infill materials layer under shallow foundations impacted by a high amount of water such as water pipe leakage and this reason might have been the primary cause of the structure subsidence.
The geophysical surveys revealed that settlements were caused by at least two factors; the downward migration of the upper infill materials bed into the weathering of the lower bed (river terraces), and the possible densification of the upper fill under its weight.Analysis and interpretation of the results from MASW and ERT data have confirmed that infill materials are existing at a depth between the ground surface and 3.5 m under and alongside the cracked structure.This bed of unconsolidated materials is influenced by the water from the vicinity area.

Fig. 1 .Fig. 2 .
Fig. 1.Satellite image shows the study area and MASW lines and ERT profiles locations (same red lines)

Fig. 3 .
Fig. 3. (a) Tectonic map of the North Iraq (modified after Jassim and Goff, 2006); (b) part of the 1:1 million scale geological map of Iraq shows the study area (modified afterSissakian and Fouad, 2015).

Fig. 7 .
Fig. 7. (left) Building site map shows the layout of MASW and ERT profiles; (right) geometry of MASW field acquisition Fig. 10 a, b presents 2D shear wave cross-sections obtained from the MASW-1.

Fig. 8 .
Fig. 8. Examples of the MASW data acquisition and processing results

Fig. 9 .
Fig. 9. Filtered records for the first shot for MASW lines (top) and dispersion images and picked curves correspond to each shot gather (bottom).The dark blue zones indicate the fundamental mode

Fig. 10 .
Fig. 10.(a) Dispersion curves and (b) 2D Vs section of MASW-1 clearly shows the development of anomalies of reduced lower Vs materials that are extending downward into more competent, higher Vs (lime green color) materials (terraces), The orange-red color is the approximated depth to bedrock (marl bed) (Vs >840m/s)

Fig. 12 .
Fig. 12. Outcrop exposed face photograph was taken neighboring the study area

Table 1 .
Active seismic MASW and ERT data obtained parameters