Electrical Tomography Profiles Research Articles

Detection of chlorinated contaminants coming from the manufacture of lindane in a surface detritic aquifer by electrical resistivity tomography

This work presents a case study using electrical resistivity tomography at a site mainly affected by the presence of residual lindane coming from a close landfill constructed by an old pesticide manufacturer. In addition, the landfill also collected chlorinated contaminants derived from the same industry. Lindane itself is a highly toxic contaminant that in small concentrations constitutes a high risk to health, which together with the other compounds, makes it a site with very hazardous characteristics. The geological distribution, formed by a complex detrital deposited over a marls substrate, is known from the existing network of monitoring wells. A geophysical prospecting survey consisting of seven electrical tomography profiles was carried out in the site. The lateral and vertical geometry of the contaminant plume was established, and the preferential flow directions were inferred by means of the resistivity sections obtained. Combining this information with analytical data from water samples, an empirical relationship between the conductivity value and the dissolved chloride content was developed, and the distribution of the concentration of chlorinated contaminants in the surface aquifer by isochloride sections was obtained. The results obtained are a very useful tool as a preliminary stage to remediation trials.

Geotechnical settings of volcanic residual soils and derived engineering problems in El Hierro Island (Spain)

Even today, studies about the properties and geotechnical behavior of volcanic residual soils are still scarce. Increasing the knowledge of this type of soil is important, since it has important economic and security repercussions in the infrastructures to be built in volcanic areas. This paper exposes the geotechnical data of volcanic residual soils acquired during the research investigation campaign carried out for the construction of the upper reservoir of El Hierro Hydro-Wind Plant (Gorona del Viento, S.A.) placed in the inactive crater La Caldera (El Hierro, Canary Islands). In this respect, and as per requirement of a geological study (geological mapping, geochemical analysis, and petrographic study), an exhaustive geotechnical investigation (rotary boreholes, dynamic penetrations of DPSH type, and trial pits) and geophysical surveys (seismic refraction and electrical tomography profiles) were carried out at this site, as well as numerous on-site and laboratory tests. The deficient geotechnical characteristics of these soils implied one of the main engineering problems addressed during the design and construction of the upper reservoir: the high risk of ground settlement. The exhaustive investigation through the combination of different techniques allowed to identify this geotechnical risk in advance and carry out a soil improvement using the most appropriate engineering technique.

Electrical resistivity imaging of the shallow structures of an intraplate basin: The Guadiana Basin (SW Spain)

The Iberian Peninsula has suffered intense Alpine intraplate deformation giving rise to a set of sedimentary basins bounded by mountain ranges. The larger basins, such as the Duero and Tajo in the west and central Iberia, are well documented and their infill and structure are well known. However, the southern, much smaller Guadiana Basin has not been much investigated. The Guadiana Basin is an E-W to NE-SW depression developed on igneous and metamorphic rocks of the Variscan basement that overlays, through thrusting, Cenozoic rocks in its northern margin. It is infilled by no more than 200m of continental deposits. By acquiring nine electrical tomography profiles, eight of them with an array extension ranging between 800 and 900m and one exceeding 2000m, we imaged basement-Cenozoic infill unconformity depth. The resistivity data obtained were inverted to 2D resistivity models from the surface down to a depth of ~100m. This procedure proved useful to assess fault geometries and basin infill thickness because of the generally high resistivity contrast between the Cenozoic rocks and Variscan basement. High-angle faults control basin margins and interior structure. The larger fault displacements were detected at the northern border, where they may exceed 500m of vertical throw. Cenozoic deposit thickness is highly variable and thicker sequences appear along the northern border related to the E-W to ENE-WSW thrusts. The basin occupies the footwall of these thrusts, which are in turn subdivided into smaller blocks limited by E-W to NE-SW faults.

Geotechnical and Geophysical Study of Ain Harrouda New City (Morocco)

A reconnaissance study of soil properties is realized in the implantation area of the future town of Ain Harouda (Morocco). This study used two approaches: geotechnic and geophysic. The geotechnical approach begins with 11 mechanical core drilling and 5 wells using mechanical shovel with sampling for laboratory testing and particularly identification tests. This study showed that the settlement area of the new city of Ain Harrouda consists of a primary base consisting of schists and quartzites. These are surmounted by conglomerates and calcarenites plio-quaternary age. It also determined the intrinsic characteristics of the each geological formation that make up the study area, include water content, density, particle size analysis by sieving, sedimentation particle size analysis, the value of methylene blue (VBS), the equivalent of sand, limits Atterberg. The geophysical approach consists of electrical resistivity technique with 5 vertical electric sounding and 8 electrical tomography profiles. This approach was used to determine the resistivity corresponding to each type of land and also the consistency of the soil. It allowed to distinguish the substratum characterized by high resistivity (more than 3000 ohm.m), and the sedimentary cover represented by three classes of average resistivity: 200 to 1600 Ohm.m for calcareous sandstones, 50 to 100 Ohm.m for silty sands, and very low for clays (less than 50 ohms. m). The results obtained shows that the study area appears healthy and suitable for the construction of the new town of Ain Harrouda.

Multi-method characterisation of an active landslide: Case study in the Pays d'Auge plateau (Normandy, France)

Shallow landslides are among the most frequent natural hazards in the Pays d'Auge plateau (Normandy, France) but no study has yet focused on the functioning of these phenomena at a detailed scale. This study aims to identify the structure and mechanical properties of a representative case study in the region. The main objective is to understand landslide dynamics and behaviour in order to assess triggering conditions and quantify triggering thresholds. The results will help complement the regional landslide hazard mapping based on landslide statistical susceptibility mapping and quantification of triggering thresholds. The landslide morphology and internal structure were identified using a multi-method approach. A morphodynamic map was produced in the field using cartographic GPS to depict the surface morphology and map the estimated landslide activity. These field measurements were completed by an analysis of all available aerial-photo images from the French National Geographic Institute (IGN) to identify the occurrence dates and possible landslide reactivations. The landslide structure was defined using multiple electrical tomography profiles, boreholes, augerings and penetration tests. Despite the overall low electrical resistivity of the landslide materials (i.e. ±<80Ω·m), the electrical profiles showed good agreement with the interpreted structure based on direct observations (augerings and penetration tests). The landslide slip surface, internal morphology and palaeotopography were identified. A finite slope model was used to calculate the landslide safety factor based on the internal structure and geotechnical data. The evolution of this safety factor according to the rainfall and the groundwater levels shows that the landslides are more likely to occur after long episodes of high cumulative precipitations with an important role being played by the preliminary conditions and the rise of the surficial groundwater table level.

Geophysical study of palaeo-incisions in the Šventoji–Būtingė coastal area, north-west Lithuania

Palaeo-incisions (palaeo-tunnels, tunnel valleys, buried valleys, etc.) comprise the most prominent features in the sub-Quaternary surface covered by the Scandinavian continental ice sheets. An exception is the Scandinavian crystalline bedrock region. Palaeo-incisions are objects of Quaternary geology both from scientific and practical point of view and have been previously mapped along the Lithuanian coastal area using gravimetric and transient electromagnetic methods, complemented by traditional drilling. This paper presents an approach for high-resolution identification of the palaeo-incisions in the Šventoji–Būtingė coastal area of north-western Lithuania. Several geophysical methods have been tested, including shallow seismic reflection profiling, electrical tomography and ground-penetrating radar. A combination of shallow seismic reflection and electrical tomography profiling provides an optimal survey strategy for high-resolution investigations of palaeo-incisions.

El método de Polarización Inducida aplicado en la contaminación de suelos por hidrocarburos

The evidence obtained by some authors devoted to electrical methods has shown that when a conventional resistivity survey is performed, it is not possible to differentiate areas with similar electrochemical behavior of both the organic and inorganic origin. Therefore, in recent years the method of Induced Polarization (IP) has been applied to soil contamination problems, showing encouraging results. This technique was tested, working in the time domain through an electrical tomography profile of resistivity and chargeability data, which was conducted at a site contaminated by hydrocarbons located in the north of Mexico City, where stratigraphy and distribution of the contaminant were complex. The hydrocarbons were found in a free and dissolved phase, which provided the conditions for taking soil and water samples. These samples were sent to a laboratory for chemical analysis, which depicted benzene concentration in four wells. The obtained results show that it is possible to define areas of large concentration of pollutants, by correlating electrical information, capacitive and qualitatively, with the information from the Cole-Cole parameters, demonstrating the potential of induced polarization method in pollution problems.

Mapping the susceptibility to sinkholes in coastal areas, based on stratigraphy, geomorphology and geophysics

Sinkholes and land subsidence are among the main coastal geologic hazards. Their occurrence poses a serious threat to the man-made environment, due to the increasing density of population, pipelines and other infrastructures along the coasts, and to the catastrophic nature of the phenomena, which generally occur without any premonitory signs. To assess the potential danger from sinkholes along the coast, it is important to identify and monitor the main factors contributing to the process. This article reports a methodology based on sequential stratigraphic, hydrogeological and geophysical investigations to draw up a susceptibility map of sinkholes in coastal areas. The town of Casalabate situated in the Apulia region (southern Italy), affected by a long history of sinkhole phenomena, is here presented as an example. The approach proposed is based on sequential stratigraphical, geomorphological and geophysical surveys to identify the mechanisms of sinkhole formation and to provide a zonation of the areas in which further sinkhole phenomena may likely occur. Interpretation of the ground penetration radar and electrical tomography profiles has enabled us to identify the potentially most unstable sectors, significantly improving the assessment of the sinkhole susceptibility in the area. The proposed methodology is suitable to be exported in other coastal areas where limestone bedrock is not directly exposed at the surface, but covered by a variable thickness of recent deposits.

Nucleation, linkage and active propagation of a segmented Quaternary normal-dextral fault: the Loma del Viento fault (Campo de Dalías, Eastern Betic Cordillera, SE Spain)

Active faults from the Campo de Dalías (SE Betic Cordillera) allow us to constrain the deformation styles involved in the development of segmented oblique-slip faults. This sector constitutes the widest outcrop of Plio-Quaternary sediments in the northern boundary of the Alboran Sea. It has emerged since the Late Pliocene, and therefore provides recent deformation markers that are not disturbed by erosive processes. The faults started to grow during the Pleistocene, reactivating previous hybrid joints, with a normal-dextral slip. We present a detailed map of the largest fault in the area, the Loma del Viento fault, comprising six onshore segments. Based on field work and aerial photography, the distributions of the contiguous joints have been mapped, and the joints reactivated as faults are identified. Some of these fault segments are hard-linked, and fault slip enhances toward the linkage sectors between them with associated sedimentary depocenters. An electrical tomography profile reveals the wedge geometry of a unit of Pleistocene conglomerates and red silts that were coevally deposited during the fault movement. Long-term slip rate in the central part of the fault is estimated at 0.07±0.03mm/y. In addition, a seismic crisis nucleated close to the Loma del Viento fault during November 2010 was recorded. Moment tensor analysis of the two mainshocks (Mw 3.5 and 4.2) provides a focal solution indicating a N120°E striking right-lateral strike-slip fault. The corrugated morphology of the Loma del Viento fault may have influenced its seismic behavior. Some of the fault segments are oblique to the general motion of the fault. These oblique segments would provide higher resistance against the general fault motion and could lock the fault, leading to accumulate elastic energy.

Surface exposure dating and geophysical prospecting of the Holocene Lauvitel rock slide (French Alps)

Large rock falls and rockslides represent a risk for human communities in mountainous areas as they can cause fatalities and destruction of settlements and infrastructures. Assessing the associated hazard requires constraining the time frequency of such events. Since large rockslides are not common, estimating their frequency requires recording them over a long period of time. The Holocene period then appears as pertinent, which implies that rockslide features have to be dated with absolute chronology methods. This paper presents a characterisation and dating of the Lauvitel rockslide, one of the largest Holocene rockslides in the French Alps. Combining field observation with electrical tomography profiles performed on the rockslide deposit that constitutes the Lauvitel Lake dam allows estimating its volume at a minimum of 12 × 106 m3. In addition, cosmic ray exposure dating using in situ-produced 10Be concentration measurements has been applied to date seven samples collected both on the main sliding surface and on blocks lying on the dam and further downstream. Ages obtained are consistent with a single large rockslide event, which occurred at 4.7 ± 0.4 10Be-ka and formed two distinct deposits. However, from a mechanical point of view, these clearly separated deposits could hardly result from a single movement. A comparison of their reach angles with those reviewed in the literature highlights that the lower deposit must result from rock avalanches larger than 107 m3, while the upper one (the Lauvitel dam) must result from several events smaller than 106 m3. In the context of hazard assessment for land use planning, these events can, however, be considered as a unique event.

Geophysical study of a Pre‐Hispanic lakeshore settlement, Chiconahuapan Lake, Mexico

AbstractResults of a combined geophysical survey carried out along the northern shore of Lake Chiconahuapan in central Mexico are presented. The site contains a series of mounds found between the lakeshore and the ceremonial centre named La Campana‐Tepozoco Hill. Those features are the remains of habitation complexes built by the lake residents about 1100 years ago. Archaeological excavations performed towards the northeastern margins of the lake uncovered rectangular clusters of basaltic rocks forming the foundations of ancient dwellings, surrounded by a barrier (‘bordo’), built to prevent floods. A mound located close to the ceremonial centre was studied using the vertical magnetic gradient (VMG), ground‐penetrating radar (GPR) and electric tomography (ET) methods. Interesting magnetic anomalies were found towards the central portion of the mound and one of these was modelled with a three‐dimensional magnetic polygon. The GPR studies unveiled a circular feature in the central portion of the mound that could be associated with a bordo. A rectangular structure was also interpreted within the limits of that structure, which is probably the location of ancient dwelling foundations. Electric tomography profiles were collected in the area, around the VMG anomaly. The resistivity model computed shows the presence of a high‐resistivity layer, which displays discontinuities within the area defined by the bordo. Presumably, the ancient settlers built their constructions on top of this horizon. The geophysical results reveal a rather complex habitat within the mound. The larger size of the bordo (ca. 30 m in diameter) compared with others already studied by archaeologists makes us believe that the purpose of the site was of administrative use or hosted a wealthy group of people. Dwelling foundations are found within the limits of the barrier. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Seismic noise-based methods for soft-rock landslide characterization

AbstractIn order to better understand the mechanics and dynamic of landslides, it is of primary interest to image correctly their internal structure and their slip surface. Several active geophysical methods are able to provide the geometry of a given landslide, but were rarely applied in 3 dimensions in the past. The main disadvantages of methods like seismic reflection and electrical tomography are that there are heavy to set up and/or to process, and they consequently are expensive and time consuming. Moreover, in the particular case of soft-rock landslides, their respective sensitivity and resolution are not always adequate to locate the potential slip surfaces. Passive methods may represent an interesting alternative particularly for landslides difficult to access, as they require lighter instrumentation and easier processing tools. Among them, the seismic noise based methods have shown increasing applications and developments, in particular for seismic hazard mapping in urban environment. In this paper, we present seismic noise investigations carried out on two different sites, the “Super Sauze” mudslide and the “Saint Guillaume” translational clayey landslide (France), where independent measurements (geotechnical and geophysical tests) were performed earlier. Our investigations were composed of electrical tomography profiles, seismic profiles for surface-wave inversions, H/V measurements, which are fast and easy to perform in the field, in order to image shear wave contrasts (slip surfaces), and seismic noise array method, which is heavier to apply and interpret, but provides (S)-waves velocity profile versus depth. For both sites, landslide bodies are characterized by lower S wave velocity (Vs &amp;lt; 300 m.s−1) and lower resistivity (ρ &amp;lt; 60 Ohm.m) than in the stable part (Vs &amp;gt; 550 m.s−1; ρ &amp;gt; 150 ohm.m). Their thickness vary from a few m to 50 m. Comparison between geophysical investigations and geotechnical data proved the applicability of such passive methods in 3D complex structures, with however some limitations.

Application of a two-dimensional electrical tomography technique for investigating landslides along the Amman-Dead Sea highway, Jordan

Electrical tomography geophysical surveys were conducted with the SYSCAL-R2 resistivity instrument at the location of instantaneous rock failure of a few 1,000s m3 along the Amman–Dead Sea highway, Jordan, providing a ground image for investigating landslide sites which caused material damage and closed the road. This slide occurred along two major fault planes after heavy rainfall in a rock consisting of a succession of shale, marl and marly limestone layers which were folded, tilted and fractured. Three electrical tomography profiles were performed along the axis of the slope to assess the landslide risk in such potential unstable areas. The results obtained, based on two-dimensional inversion of field data, have allowed mapping of the shale zones at depths which are characterized by a lower resistivity value, and have demonstrated that the rupture was initiated at the contact between the shale mass (black colored) and the massive limestone (white, yellowish-brown colored).