Two-dimensional Electrical Resistivity Tomography Research Articles

Electrical resistivity tomography array comparisons to detect cleared-wall foundations in brownfield sites

Electrical resistivity surveys are commonly used to detect and characterize near-surface buried objects in commercial developments of brownfield sites. Two-dimensional electrical resistivity tomography (ERT) profile arrays predominate in such surveys owing to their relatively rapid deployment, good penetration depths and fast data collection rates. However, there is a need to test the optimum array types in such surveys. A scaled model was used to simulate a large cleared-building wall foundation in gravel-fill at a test facility, before multiple 2D ERT profiles were acquired using different array configurations. Results were used to generate 2D resistivity models using both least-squares smoothness-constraint and robust inversion. Two-dimensional profile array comparisons showed that the Wenner and dipole–dipole arrays were the best in detecting the cleared-wall foundation, although dipole–dipole arrays better delineated the top of the wall foundation. This study suggests that both Wenner and dipole–dipole array configurations should be utilized to detect buried wall foundations for 2D resistivity surveys.

Two-dimensional electrical impedance tomography (EIT) for characterization of body tissue using a gauss-newton algorithm

The gauss-newton algorithm is applied to reconstruct a two-dimensional image of Electrical Impedance Tomography (EIT) using python program. The study aimed to determine the characteristics of body tissues either conductive or resistive properties through the tissue structure which is displayed in the form of images so we can distinguish between one tissue and another. In this study, it was a fabricated hardware system, phantom, and software system. The EIT technique is performed by injecting a low constant electrical current of 1 mA at 50 kHz frequency on a pair of electrodes on 16 copper electrodes (Cu) surrounding a phantom filled with ground meat as conductor of electricity, flat bone, and cord bone of chicken as anomalies. The principle of the neighboring method achieved voltage measurements from the other electrode pair. The measurement data is processed with the python program using gauss-newton algorithm to obtain a reconstructed image of the tissue structure. The characteristics of body tissue can be identified from the color distribution, position, shape, and size of the object in the reconstructed image so it can distinguish between one tissue and another. The original analysis of body tissue either conductive or resistive properties could be reconstructed by two-dimensional electrical impedance tomography conjunction with the gauss-newton algorithm.

Shallow aquifer potential mapping in the foothills of Churia in Eastern Gangetic Plain of Saptari District, Nepal

The population living in the foothills of Churia, the study area, faces severe water scarcity due to limited access to surface water. The only alternative is groundwater, and there is extensive use of groundwater for domestic and agricultural purposes. Groundwater in some areas is limited and very limited information pertaining to the aquifer. In this context, this research tries to answer the pertinent questions concerning the aquifer properties, depth and thickness of the groundwater aquifer layer and groundwater prospects for irrigation. The objective of this research is to identify the aquifer properties and groundwater prospects using two-dimensional electrical resistivity tomography methods (2D-ERT). Five 2D-ERT profiles were used to collect information to a depth of around 40 m, with minimum electrode spacing of 5 m over a profile length of 240 m. The survey was carried out using Wenner electrode configurations. The resistivity tomogram sections were interpreted using RES2DINV software, which provides output in the form of resistivity contours that are used to draw up the lithological and geological information. The result of the survey indicates that the areas with good potential for groundwater access exist mostly along the southeast to central east parts of the study area, where there is gravel and sand with resistivity values between 100 and 500 Ω m. Areas along the northwest with clay and silty clay formations have low resistivity values (< 70 Ω m) and low groundwater potential. While the northeast has high resistivity values (> 2000 Ω m) due to sand formation underlain by boulder and cobbles, it also has limited groundwater potential due to the limited depth (< 6 m) of sand and low storage capacity. The study reveals a variation in lithological formation, including a patchwork of clay, silty clay, sand, gravel and boulder materials, with varying groundwater potentials. When developing shallow tube wells in the study area, spatial variation in the productive aquifer, hydraulic conductivity of groundwater flow within and near the tapped aquifer and geological and lithological material need to be considered.

Mapping soil layers using electrical resistivity tomography and validation: Sandbox experiments

Knowledge of the geologic structure at a field site is a useful piece of information for hydrologic modeling since it can serve as more site-specific prior information about the hydraulic parameter patterns at the site than generic spatial statistics. Widely accepted electrical resistivity tomography (ERT) survey for mapping subsurface anomalies could be a viable tool for acquiring this information. However, their ability to delineate geologic structures has not been thoroughly investigated. In this study, two-dimensional ERT numerical experiments were first conducted to study the effects of boundary conditions on the dipole-dipole and pole-pole array configurations. An ERT setup subsequently was implemented in a sandbox consisting of complex layers of different sands. A continuous copper wire was installed along the sides of the sandbox to impose potential boundaries. Using data collected with the pole-pole array in the sandbox under different degrees of drainage and the Successive Linear Estimator (SLE) algorithm, we show that ERT yields electrical conductivity estimates of complex layer structures with small uncertainties. In addition, using SLE with physically meaningful correlation scales as prior information can lead to an electrical conductivity field that is consistent with visually observed layer structures. The correlation scale concept also was demonstrated to provide guidance to the design of the electrode spacing in the surveys. Moreover, the estimated field was validated by predicting electrical potential fields from two independent ERT surveys using electrodes at different locations. Results of this study suggest that the combination of ERT and SLE is a viable geophysical survey tool for mapping geologic layer structures. Research SignificanceThis study develops a method to implement prescribed potential boundaries for enhancing the pole-pole ERT survey, illustrates the importance of correlation scales and develops an approach for validating ERT results.

Baseline evaluation of freshwater\u2013saltwater interface in coastal aquifers of Badagry, south-western Nigeria

This study focused on the evaluation of the baseline condition of saline water–groundwater interface in the coastal aquifers along Badagry, south-western Nigeria. Geologically, Badagry lies within the coastal sands and recent alluvium of the Dahomey Basin. Two-dimensional electrical resistivity tomography (2D-ERT) data along 15 survey lines were acquired in the study area, adopting the Wenner electrode arrays system with minimum spacing of 10 m. Ninety-one water samples were also collected, and their physical parameters were measured using hand-held devices. The (2D-ERT) data were processed and interpreted with the aid of DiProWin software. Sandy topmost layer, freshwater sand, brackish water sand and saline water sand were delineated from the study. Brackish water sand and freshwater sand layers were dominant in areas with high proximity to the Atlantic Ocean located in the south, which were not observed in the northern part of the mapped area. The study established the freshwater–saltwater interface at a depth of 10 m and above, in areas around the coast, while the interface occurs at depth greater than 50 m in areas 3–5 km from the coastal area. Observations from the pH and the TDS show that 87.3% of the samples are slightly acid, while less than 12.7% of them are slightly alkaline, while the TDS vary from 8 to 520 mg/l. The EC of the samples varies from 13 to 1140 µS/cm. All water samples are fresh groundwater, which confirms the occurrence of freshwater aquifer even in areas closer to the Atlantic Ocean.

Electrical resistance analysis of material using a colour bar scale in two-dimensional electrical impedance tomography (EIT)

Image reconstruction by adding colour bar scale has been implemented in the two-dimensional Electrical Impedance Tomography (EIT) to distinguish the electrical resistance of the material. The EIT technique is performed by injecting a low constant electric current of 1 mA at 20 kHz frequency on a pair of electrodes of 16 copper electrodes (Cu) surrounding a medium filled with saline solution, aluminium, PVC, and PE material. Voltage measurements from the other electrode pair were achieved by the principle of the neighbouring method. The measurement data is processed with the python program using gauss-newton algorithm to obtain a reconstructed image of materials. The results from the reconstructed image show that the higher the colour bar number, the lower the electrical resistance, while the lower the colour bar number, the higher the electrical resistance. The difference in electrical resistance of material can be obtained from analysis of colour bar scale in two-dimensional electrical impedance tomography.

Landfill leachate assessment by hydrological and geophysical data: case study NamSon, Hanoi, Vietnam

The main objective of this study was to assess the impact of the NamSon landfill on subsurface geological structure and hydrological environment by geophysical techniques and hydrochemical analysis of surface and groundwater. The electrical resistivity tomography (ERT), self-potential (SP) and very low frequency (VLF) methods were used for the investigation of geological structure near the landfill. Three profiles (900 m long in total) of the two-dimensional ERT, VLF density sections and 180 SP data points scattered throughout the study area near the disposal site constituted the basis of the data used in analysis. Additionally, surface water and groundwater samples were collected from six sites in the area for the chemical analysis. Interpretations of geophysical data show a low resistivity zone (< 15 Ωm), which appears to be a fully saturated zone with leachate from landfill. The results of the geophysical investigations are not always fully confirmed by the results of hydrochemical analysis. The quality of water in the vicinity of the landfill dramatically decreased over the year (2015–2016) and actions should be taken to inverse this negative trend.

Correcting the distortion of apparent resistivity pseudosection produced by 3D effect of the embankment geometry

Two-dimensional electrical resistivity tomography data acquired on the crest of the embankment are distorted by the 3D effect of the embankment geometry, reservoir water level and abutment. The distortion affects seriously the final solution of the 2D inverse problem. By comparing the apparent resistivity pseudosections from a 3D and 2D electrical resistivity model of the embankment, the distortion degrees of the apparent resistivity pseudosections along the axis on the crest were estimated for the cases of reservoir and which does not contain water. The obtained results indicate that the distortion degree acquired in the case of a reservoir that contains water was much less than that in the case of the reservoir that does not contain water. In the case of reservoir that contains water, the apparent resistivity pseudosections of the P–P and ED–ED arrays had the largest distortion degree and of D–D, W–S and P–D arrays had the smallest distortion degree. In the case of the reservoir that does not contain water, the apparent resistivity pseudosection of P–P array had the smallest distortion among all arrays. Through modeling investigation, a correction process to reduce the distortion of the apparent resistivity pseudosection was proposed. The correction process was tested in the embankment model, and two field works were carried out in the To Lich River in Hanoi and Khuan Cat embankment in Lang Son province, Vietnam. It is possible to bring the distortion of the apparent resistivity pseudosection down to 2.8–13.9%, depending on the type of electrode arrays and the type of reservoirs, containing or does not contain water. The distortion correction of the apparent resistivity pseudosection is strongly recommended before doing the 2D inverse interpretation.

Why 1D electrical resistivity techniques can result in inaccurate siting of boreholes in hard rock aquifers and why electrical resistivity tomography must be preferred: the example of Benin, West Africa

Hard rock aquifers are of particular importance for supplying people with drinking water in Africa and in the world. Although the common use of one-dimensional (1D) electrical resistivity techniques to locate drilling site, the failure rate of boreholes is usually high. For instance, about 40% of boreholes drilled in hard rock aquifers in Benin are unsuccessful. This study investigates why the current use of 1D techniques (e.g. electrical profiling and electrical sounding) can result in inaccurate siting of boreholes, and checks the interest and the limitations of the use of two-dimensional (2D) Electrical Resistivity Tomography (ERT). Geophysical numerical modeling and comprehensive 1D and 2D resistivity surveys were carried out in hard rock aquifers in Benin. The experiments carried out at 7 sites located in different hard rock groups confirmed the results of the numerical modeling: the current use of 1D techniques can frequently leads to inaccurate siting, and ERT better reveals hydrogeological targets such as thick weathered zone (e.g. stratiform fractured layer and preferential weathering associated with subvertical fractured zone). Moreover, a cost analysis demonstrates that the use of ERT can save money at the scale of a drilling programme if ERT improves the success rate by only 5% as compared to the success rate obtained with 1D techniques. Finally, this study demonstrates, using the example of Benin, that the use of electrical resistivity profiling and sounding for siting boreholes in weathered hard rocks of western Africa should be discarded and replaced by the use of ERT technique, more efficient.

A novel post-processing scheme for two-dimensional electrical impedance tomography based on artificial neural networks.

ObjectiveElectrical Impedance Tomography (EIT) is a powerful non-invasive technique for imaging applications. The goal is to estimate the electrical properties of living tissues by measuring the potential at the boundary of the domain. Being safe with respect to patient health, non-invasive, and having no known hazards, EIT is an attractive and promising technology. However, it suffers from a particular technical difficulty, which consists of solving a nonlinear inverse problem in real time. Several nonlinear approaches have been proposed as a replacement for the linear solver, but in practice very few are capable of stable, high-quality, and real-time EIT imaging because of their very low robustness to errors and inaccurate modeling, or because they require considerable computational effort.MethodsIn this paper, a post-processing technique based on an artificial neural network (ANN) is proposed to obtain a nonlinear solution to the inverse problem, starting from a linear solution. While common reconstruction methods based on ANNs estimate the solution directly from the measured data, the method proposed here enhances the solution obtained from a linear solver.ConclusionApplying a linear reconstruction algorithm before applying an ANN reduces the effects of noise and modeling errors. Hence, this approach significantly reduces the error associated with solving 2D inverse problems using machine-learning-based algorithms.SignificanceThis work presents radical enhancements in the stability of nonlinear methods for biomedical EIT applications.

Characterization and Delineation of Gypsum Karst Geohazards Using 2D Electrical Resistivity Tomography in Culberson County, Texas, USA

The Delaware Basin of W Texas and SE New Mexico is the western subdivision of the Permian Basin and a northern extension of the Chihuahuan Desert. The major evaporite unit within the Delaware Basin is the Castile Formation, which consists of gypsum/anhydrite and is highly susceptible to dissolution and karstification. Manifestations of karst within the Castile outcrop are abundant and include sinkholes, subsidence features and caves, both epigene and hypogene in origin. Land reconnaissance surveys conducted during 2015 and 2016 documented abundant karst landforms near major thoroughfares in Culberson County, Texas. Two dimensional (2D) electrical resistivity surveys were conducted at four sites to characterize and delineate karst related hazards, both laterally and vertically, associated with the road. The electrical resistivity data were collected with a multi-electrode earth resistivity meter using a dipole-dipole array configuration. The resistivity data were then processed using EarthImager2D to produce inverted profile sections of each site. Two-dimensional electrical resistivity tomography was shown to be an effective non-invasive method in detecting solution conduits, soil filled voids, and fractured bedrock in the shallow subsurface in addition to those directly observed on the surface.

Dynamic Monitoring of Water in a Working Face Floor Using 2D Electrical Resistivity Tomography (ERT)

The roof and floor of a water-rich area can be explored using traditional two-dimensional (2D) electrical resistivity tomography (ERT), but this method cannot be applied to a water-rich internal working face. In this paper, the resistivity of an internal working face was determined using 2D ERT by laying electrodes and cables in two adjacent roadways and using an equatorial dipole device. The water inrush process of the goaf floor in the study area was monitored with pre-implanted electrodes and cables at the front of the mining working face and was analyzed using the processed results of four acquired data sets. The location of the water inrush from the floor was determined based on the change in the resistivity of the floor and the hydrological variations at the observational hole, which provided the basis for how to stop the water inrush.

Spatial Controls of Turf-Banked Solifluction Lobes and Their Role for Paraglacial Adjustment in Glacier Forelands

In glacier forelands, solifluction is a major and fast-responding paraglacial process in the adjustment of low- or moderately-inclined moraine slopes. However, the spatial pattern of the factors that control solifluction and their interactions are poorly understood. In this study, we combine geomorphic and vegetation mapping, analysis of borehole material and time domain reflectometry with two-dimensional (2D) and three-dimensional (3D) electrical resistivity tomography (ERT) to quantify the controlling factors and evaluate their influence on sediment transport. The investigated turf-banked solifluction lobe is located on the distal slope of a 1920s lateral moraine in the foreland of the Turtmann glacier in the southern Swiss Alps. Our data demonstrate the interaction between material, soil moisture and vegetation, which results in geomorphometric features such as (i) frontal and lateral risers, (ii) a lobe tread and (iii) a ridge. Lobe size indicates intense solifluction movement in the last 100 years. The interaction of controlling factors results in a negative feedback which stabilises the lobe and, thus, limits paraglacial sediment transport to frost heave processes on the ridge. Copyright © 2016 John Wiley & Sons, Ltd.

An electric and electromagnetic geophysical approach for subsurface investigation of anthropogenic mounds in an urban environment

Scientific interest in mounds as geomorphological features that currently represent topographic anomalies in flat urban landscapes mainly lies on the understanding of their origin, either purely natural or anthropogenic. In this second circumstance, another question is whether traces of lost buildings are preserved within the mound subsurface and can be mapped as remnants testifying past settlement. When these landforms have been modified in centuries for civilian use, structural stability is a further element of concern. To address these issues we applied a geophysical approach based on a very low frequency electromagnetic (VLF-EM) technique and two-dimensional electrical resistivity tomography (2D-ERT) and integrated it with well-established surface survey methods within a diagnostic workflow of structural assessment. We demonstrate the practical benefits of this method in the English Cemetery of Florence, Italy, whose mixed nature and history of morphological changes are suggested by archival records. The combination of the two selected geophysical techniques allowed us to overcome the physical obstacles caused by tomb density and to prevent interference from the urban vehicular traffic on the geophysical signals. Eighty-two VLF-EM profiles and five 2D-ERTs were collected to maximise the spatial coverage of the subsurface prospection, while surface indicators of instability (e.g., tomb tilt, location, and direction of ground fractures and wall cracks) were mapped by standard metric survey. High resistive anomalies (>300 and 400Ωm) observed in VLF-EM tomographies are attributed to remnants of the ancient perimeter wall that are still buried along the southern side of the mound. While no apparent correlation is found between the causes of tomb and ground movements, the crack pattern map supplements the overall structural assessment. The main outcome is that the northern portion of the retaining wall is classed with the highest hazard rate. The impact of this cost-effective approach is to inform the design of maintenance and restoration measures based on improved geognostic knowledge. The geophysical and surface evidence informs decisions on where interventions are to be prioritised and whether costly invasive investigations are needed.

On the potential for a bottom active layer below coastal permafrost: the impact of seawater on permafrost degradation imaged by electrical resistivity tomography (Hornsund, SW Spitsbergen)

This paper presents the results of two-dimensional electrical resistivity tomography (ERT) of permafrost developed in coastal zone of Hornsund, SW Spitsbergen. The measurements were made using the Wenner-Schlumberger electrode array with an electrode spacing 5m for overview and 1.5 or 1m spacing for detailed imaging. Using the ERT inversion results, we studied the ‘sea influence’ on deeper parts of the frozen ground. During the investigations we tested hypotheses that the operation of seawater on shoreface may cause changes in the shape of the coastal permafrost base, and that the impact of seawater on more inland permafrost depends on the shape of the shoreline (differently in the embayment, and differently in a headland exposed to the open sea). Our study was inspired by previous ground temperature measurements conducted in several boreholes located in study area which captured the propagation of ground heat waves from the base of permafrost. Our resistivity models indicate a major differentiation in terms of resistivity of permafrost in the coastal zone. The resistivity measures obtained in deeper layers of ground were so low (<100Ω·m) that in the ‘warm permafrost’ conditions they exclude a possibility of freezing the coastal sediments and bedrock from the side of the sea. Low values continue further inland, going down under the surface layer of permafrost with higher resistivity. We interpret this situation as an influence of seawater's temperature and salinity on deeper parts of permafrost. Based on the measurements conducted within two years, we stated a change in the distribution of resistivity, both in the active layer, and in coastal front of permafrost in deeper parts of the ground. As observed in the inverse models, the geometric arrangement between the fields of extreme resistivity indicates the existence of a bottom active layer by the permafrost base, depending on thermal and chemical characteristics of seawater. The measurements conducted in the embayment, as well as on the headland exposed to the operation of storm waves, proved strong differences in the scale of the impact of seawater on permafrost.

Two-dimensional electrical resistivity tomography (ERT) and time-domain-induced polarization (TDIP) study in hard rock for groundwater investigation: a case study at Choutuppal Telangana, India

Groundwater investigation in a crystalline rock is a crucial task. A study was carried out at Choutuppal Telangana, India, under the pivotal research project of societal relevance. High-resolution electrical resistivity tomography (ERT) and time-domain-induced polarization (TDIP) dataset were collected in a granitic terrain to solve the groundwater problem as people are facing acute shortage of drinking water in the study area. The interpreted results derived from two-dimensional (2D) inverted resistivity models revealed substantial resistivity contrast between the weathered and massive granite and delineated three groundwater prospects zones, where the degree of weathering of fractured granite decreases with depth. On the other hand, the induced polarization (IP) results reflect marginal chargeability contrast, which indicates groundwater prospect zone. The basement of the hard rock aquifer system is clearly delineated showing very high resistivity with a range from 5000 to ~4 × 105 Ohm.m, which is confirmed by drilling at two places. Both the wells are drilled during the month of April and June, 2013, which are productive with a yield varying from 82.14 to 105 l/min. This study may help in future planning for groundwater exploration strategy and development for groundwater resources.

Electrical resistivity imaging and hydrochemical analysis for groundwater investigation in Kuala Langat, Malaysia

ABSTRACTIntegrated two-dimensional electrical resistivity imaging (ERI) and hydrochemical surveys were used to investigate the groundwater alluvial aquifer in Kuala Langat, Malaysia. The study in the Langat basin considered the thickness of the aquifer, the depth of the bedrock, the regions influenced by seawater intrusion, and the monitoring of water levels. The resistivity imaging results show that the upper layer consists of clay, while the second layer is an aquifer whose thickness varies mostly in the range of 10–30 m, and in some cases extends to 40 m. The bedrock depth varies from 30 to 65 m. The chemical analyses were carried out on groundwater samples from nine boreholes collected between 2008 and 2012. The analyses indicate that the total dissolved solids (TDS) exceed 1000 mg L-1 near the coastal area and are often less than 500 mg L-1 further inland. The ERI and hydrochemical analyses reveal that groundwater in the study area, especially towards the coast, is a mixture of brackish and fresh waters.EDITOR D. Koutsoyiannis; ASSOCIATE EDITOR M.D. Fidelibus

Integrated approach for pavement deterioration assessments in a low latitude crystalline basement of south-western Nigeria

An integrated approach involving electrical resistivity (ER) technique and geotechnical analysis was employed to assess the deterioration of pavement along Iwo–Osogbo road in south-western Nigeria underlain by quartzo-syenite, migmatite gneiss, and pegmatite crystalline rocks. The ER involved the use of two-dimensional (2D) electrical resistivity tomography and one-dimensional (1D) vertical electrical sounding (VES) to provide an overview of the ground conditions and probe the subsurface profile, respectively. Geotechnical analysis involving compaction, Atterberg limits, grain-size distribution (GSD), and California bearing ratio (CBR) were also carried out on the recovered samples from four test pits, two pits each dug at the smooth and deteriorated pavement portion. Whilst moisture-related distresses originating beneath the pavement were investigated through suction distribution determined from estimated soil–water characteristic curves (SWCC) employing physico-empirical model approach. At the deteriorated pavement segment; the 2D pseudosection indicated the material at and near-surface (depth less than 2.49 m) was of low resistivity values ranging between 69 and 153 Ωm, with upper bound values near surface. The resistivity further decreases with depth indicating increase in proportion of clay and or water content. The other half of the pseudosection is characterized by relatively high resistive material whose resistivity values ranged between 103 and 227 Ωm at and near surface (depth less than 1.27 m) as well as alternating bands of low (20 to 70 Ωm) and relatively high resistivity (103 to 337 Ωm) materials of varied thickness at greater depths. Conversely, the pseudosection obtained from the smooth pavement segment was characterised by high resistivity (>212 Ωm) at and near surface (depth ≤3.5 m) over the entire length. Iterative interpretation of 1D VES data indicated three to four layers earth model without association or relationship between a particular apparent resistivity curve type and deteriorated or stable portion of the road. The moisture content and Atterberg limit parameters namely plastic limit and liquid limit plots against depth suggest the soil environment beneath the pavement as unsaturated. The GSD curves indicate the soils are gap-graded comprising sand-sized and coarse particle and corroborated by GSD curve fitting. The CBR under soaked condition ranged from 16.5 to 41.1 % indicating that the subbase soil could not withstand much load. The CBR varied between 20.8 and 72.0 % indicating that the subgrade soils have a relatively medium strength whilst the subbase soil has a very high strength. The SWCCs generally indicated that the moisture content decreases with increasing matric suction resulting in suction gradient capable of generating flow. Thus, the pavement deterioration could be attributed to improper handling of the flow initiated by the suction gradient.

Mapping underground layers in the super arid Gidron Wadi using electrical resistivity tomography (ERT)

Along the arid Arava, southern Israel, acacia trees are considered keystone species. Based on survival of acacias through drought years that have not seen a single rainfall or flood, it is assumed that acacias endure the local harsh conditions by drawing water from deep underground water reservoirs. Studying water use of acacias has so far been done by exposing root systems or by isotopic water measurements, both problematic methods.We present a new application of a two-dimensional direct current electrical resistivity tomography (ERT) to measure the electrical resistivity of the subsurface. We applied the ERT device to two fixed 100 m transects cutting across (north-south) and along (west-east) the super arid Gidron Wadi during March, April and August 2013. An inversion model was used to create a map of electrical resistivity of the layers below ground, an indirect indicator of the underground water content. We identified a conductive layer located at ∼7–10 m below ground in all surveys. Although we could not identify changes to this apparent perched aquifer, such a layer may be a secondary source of water for acacias that can explain their survival in drought years. Further ERT-aided studies are needed to correlate acacia distribution and ecophysiological state with perched aquifers, even if quantitative analysis of their replenishment in such desert environments is not trivial.

Preliminary Assessment of Submerged Beachrock and Tsunamigenic Deposit, Hasır Island, Marmara Archipelago, Turkey

Ertek, A.; Kılıç, E.; Erginal, A.E.; Ekinci, Y.L., and Demirci, A., 2015. Preliminary assessment of submerged beachrock and tsunamigenic deposit, Hasır Island, Marmara Archipelago, Turkey.A preassessment of coexisted submerged beachrock and a fossil-laden near-shore deposit on the coast of Hasır Island, SW Marmara Sea, is presented based on depositional characteristics, two-dimensional electrical resistivity tomography (ERT) survey and accelerator mass spectrometry (AMS) radiocarbon ages. ERT-derived geophysical images clearly showed the subsurface position and contact relationship of submerged beachrock under the studied beach. Textural features of beachrock are typical solely of marine-phreatic cementation, including consecutively developed cement fabrics, i.e. micrite coatings, radial aggregates consisting of scalenohedral high-Mg calcites, and reticulated needles of aragonite. The intertidal cementation of beachrock took place between 2940 and 2470 YBP when the level of the Marmara Sea was about 1.5 m lower than that of the present. Its purely submerged nature is likely concerned with rise in sea level in pursuit of the cementation period. Though dated between 2340 and 1590 YBP, the fossil-rich near-shore deposit behind the studied beach could be of a tsunamigenic origin based on its sequence characteristics typical of such a high-energy event.