SEMINER PRESENTATION ON ELECTRICAL RESISTIVITY TOMOGRAPHY

electrical resistivity tomography ppt

Abstract
Electrical Resistivity Tomography (ERT) has become one of the most applied and user-favourable geophysical technique in geomorphological research. Multiple character of the technology using various electrode arrays significantly reduces measurement time and is suitable for applications even in hardly accessible mountain areas. ERT can be used for various problems concerning slope deformations and accompanying landform assemblages. Our study shows that the best results are obtained in sites with high resistivity contrasts in subsurface environment associated with e.g. changes of lithology or ground water conditions. Good results, which significantly extrapolate geomorphic and speleological investigations, were obtained in elevated gravitationally spread ridges with the presence of crevice-type caves. Resistivity soundings in such settings bring a new insight into the internal structure of deeply disrupted mountain ridges. Very promising seems to be also delimitation of bodies of active, water-saturated landslides or lacustrine deposits behind landslide dams. On the other hand, some problems with the interpretation of ERT record are associated with the study of internal structure and depth of old inactive landslides situated in relatively homogenous flysch layers. Although ERT sounding results must be interpreted critically, they always shed some light on the internal characteristics of slope deformations and in combination with other methods they create a reliable base for the analysis of so far unrecognized features of slope deformations.

INTRODUCTION
Geoelectrical techniques belong to the most applied geophysical methods in a broad spectrum of environmental studies (e.g. D u r a s et al. 2005; M a i l l e t et al. 2005; S o u p i o s et al. 2006, etc.). Electrical Resistivity Tomography (ERT or DC-tomography) has recently become the most frequently used geoelectrical application for geomorphological purposes due to its relative simplicity and time effectivity. This technique was used for the investigation of morphotectonics (S u z u k i et al. 2000), weathering studies (B e a u v a i s et al. 2006), fluvial geomorphology (M a i l l e t et al. 2005), permafrost detection (H a u c k and K n e i s e l 2006) or exploration of underground karst structures (Z h o u et al. 1999). ERT seems to be especially suitable for the identification of depth and internal structure of various types of slope deformations (e.g. B a t a y n e h and A l - D i a b a t 2002; L a p e n n a et al. 2003; B i c h l e r et al. 2004; L e b o u r g et al. 2005; D r a h o r et al. 2006; P e r r o n e et al. 2006; Sass 2006; Sass et al. 2008; Van Den Eeckhaut et al. 2007, etc.). In comparison with ground penetrating radar and seismic reflection Schrott and Sass (2007) consider ERT to be the most successful geophysical method for the study of internal characteristics of active and dormant landslides. Slope deformations are extremely abundant features in the region of the Flysch Carpathians (S t a r k e l 1997). However, their internal structure and depth remain usually enigmatic features (see e.g. M a r g i e l e w s k i 2006) and most cross-sections describing deep-seated landslides in this region are based only on tentative, surface-related investigations (e.g. geomorphic mapping). The aim of the presented study is to show ERT advantages and limitations when studying the structure of various types of slope deformations and their accompanying landform assemblages. The study is based on numerous ERT measurements which were performed on slope deformations in mountainous regions of the Czech and Slovak parts of the Flysch Carpathians.

METHODOLOGICAL BACKGROUND
ERT method, which belongs to geoelectric, geophysical methods, is based on the application of electric current into analyzed bedrock and measurement of the intensity of electric resistivity to its conduit. Basically, it gives us information on electric resistivity properties of analyzed material towards passing electrical current (L a z z a r i et al. 2006; S a s s et al. 2008). This data collection is realized by means of 4 electrodes localized in a line (Fig. 1). Two end electrodes emit electric current whose run in the bedrock has character of a part of arc of a circle. The other two electrodes, localized between the emitted electrodes, measure the bedrock electric resistivity in a certain point under the surface (S a s s 2006; S c h r o t t and S a s s 2008). In this way measurement of all possible combinations of electrodes in the profile is carried out automatically while the function of individual electrodes changes as emitting and measuring functions alternate. This measuring algorithm, called Schlumberger, is the most commonly used in geomorphologic applications at the present time (S c h r o t t and S a s s 2008). Its application is especially recommended in the research of horizontal structures (e.g. alluvial plain), however, its use brings quality results also with regard to vertical structures (e.g. tension cracks) (D r a h o r et al. 2006). Absolute maximal depth in which we are able to measure electric resistivity is theoretically given by maximum spacing of emitting electrodes.Electrical Resistivity Tomography is an earth resistance technique conducted along a series of single lines or profiles, rather than area surveys. A small electrical current is applied to a series of probes laid out across a site, producing a cross-section of data indicating a broad stratigraphy of archaeology and deeper features. The electric resistivity tomography can then be combined with topographical data to provide an accurate representation of the underlying soils.

Electrical Resistivity Tomography accurately records depth information and assesses deeply buried archaeological remains such as souterrains or elements of multiphase sites including surfaces, water courses and buried walls. This technique also allows geophysical investigation in landscapes which are not generally amenable for other types of survey such as woodland, marshland and areas of dense vegetation.

Electrical Resistivity Tomography allows for the assessment of features such as

Souterrains
Ditches
Walls
Structures
Water features
Mounds
Cavities / Voids / Caves

Electrical resistivity tomography (ERT) or electrical resistivity imaging (ERI) is a geophysical technique for imaging sub-surface structures from electrical resistivity measurements made at the surface, or by electrodes in one or more boreholes. If the electrodes are suspended in the boreholes, deeper sections can be investigated. It is closely related to the medical imaging technique electrical impedance tomography (EIT), and mathematically is the same inverse problem. In contrast to medical EIT however ERT is essentially a direct current method.

A related geophysical method, induced polarization, measures the transient response. The technique evolved from techniques of electrical prospecting that predate digital computers, where layers or anomalies were sought rather than images. Early work on the mathematical problem in the 1930s assumed a layered medium (see for example Langer, Slichter). Tikhonov who is best known for his work on regularization of inverse problems also worked on this problem. He explains in detail how to solve the ERT problem in a simple case of 2-layered medium. During the 1940s he collaborated with geophyicists and without the aid of computers they discovered large deposits of copper. As a result, they were awarded a State Prize of Soviet Union.

When adequate computers became widely available the inverse problem of ERT could be solved numerically, and the work of Loke and Barker at Birmingham University was among the first such solution, and their approach is still widely used.

With the advancement in the field of Electrical Resistivity Tomography (ERT) from 1D to 2D and now-a- days 3D, ERT has explored many fields. The applications of ERT include fault investigation, ground water table investigation, soil moisture content determination and many others. In industrial process imaging ERT can be used in a similar fashion to medical EIT, to image the distribution of conductivity in mixing vessels and pipes. In this context it is usually called Electrical Resistance Tomography, emphasising the quantity that is measured rather than imaged.

LECTRICAL RESISTIVITY TOMOGRAPHY (ERT)

Electrical Resistivity Tomography (ERT) is an advanced geophysics method used to determine the subsurface’s resistivity distribution by making measurements on the ground surface. ERT data are rapidly collected with an automated multi-electrode resistivity meter. ERT profiles consist of a modeled cross-sectional (2-D) plot of resistivity (Ω·m) versus depth. ERT interpretations, supported by borehole data or alternate geophysical data, accurately represent the geometry and lithology and/or hydrology and/or petrology of subsurface geologic formations.

ERT measures resistivity. Resistivity, measured in Ω·m, is the mathematical inverse of conductivity. It is a bulk physical property of materials that describes how difficult it is to pass an electrical current through the material. Resistivity measurements can be made with either an alternating current (AC) or a direct current (DC). As resistivity measurements are frequency dependant, care must be taken when comparing resistivity values collected using different techniques.

Clay materials, metallic oxides, and sulphide minerals are the only common sedimentary materials that can carry significant electrical current through the material itself. As such, the resistivity of most near surface sedimentary materials is primarily controlled by the quantity and chemistry of the pore fluids within the material. Any particular material can have a broad range of resistivity responses that is dependant on the level of saturation, the concentration of ions, the presence of organic fluids (such as non aqueous phase liquids, NAPLs), faulting, jointing, weathering, etc.

The general principals that ERT is based on have been in use by geophysicists for almost a century. Recent advances to field equipment and data processing procedures have made rapid 2D surveys routine and 3D surveys possible. Old-style 1D resistivity surveys are still common and are useful on many occasions, but encounter interpretation problems in areas of complex 2D or 3D geology.