SUMMARY
While traditional geoelectric array configurations, such as the Wenner–Schlumberger or the dipole–dipole, can provide very good images of 1-D or robust 2-D structures, they are not ...sufficiently sensitive to those inhomogeneities that have a small effect on the surface electrical potential distribution. The detection and description of such inhomogeneities become possible by applying quasi-null arrays, which provide very small (close to zero) signals above a homogeneous half-space. The imaging properties of the members of an array series containing such arrays, the so-called γ11n arrays (n = 1–7), are demonstrated and compared to those of the most popular traditional arrays. Although the field applicability of the quasi-null arrays has been heavily questioned, it was demonstrated by our quasi-field analogue modelling experiments. The quasi-field tests also validated all of the numerical modelling results as follows: (1) many or all of the γ11n arrays were able to detect prisms and vertical sheets located at depths larger than those detectable by traditional geoelectric arrays, including the optimized Stummer configuration; (2) the horizontal resolution of the γ11n arrays proved to be better than the horizontal resolution of traditional arrays; (3) with n increasing, the γ11n arrays proved to be less sensitive to 1-D, but more sensitive to 2-D bodies. In case of high n values, the γ11n arrays may even be entirely insensitive to any 1-D structure. On the basis of the quasi-field experiments, γ11n arrays are expected to be very efficient to indicate bodies, or variations in time that only have a small impact on the surface electrical potential distribution (e.g. caves, mines, tunnels, tubes, cables, fractures, dykes), or small changes in the subsurface conditions (monitoring of dams or waste deposits). Data acquisition by both a traditional and a γ11n array, individual inversion of their data, and a joint interpretation of the results are recommended to obtain both a robust image and fine details of the subsurface.
We have studied a slowly moving loess landslide along the River Danube in South Hungary. In contrast with other efforts, we aimed to determine its fracture system. Due to the homogeneous composition ...of the loess, it seems to be the only possibility to get information about the landslide and its further evolution. Beside of the well-known Electrical Resistivity Tomography (ERT) the so-called Pressure Probe (PreP) method was applied to characterise the supposedly dense fracture system. This method was developed to detect and characterise mechanically weak zones, which may not visible from the surface, and may occur e.g. due to landslides. Fracture zones had been especially well localised by the ERT, enabling the prediction of the positions of future rupture surfaces and thus also the delineation of the endangered zones. PreP was able to give a very detailed image about the surface projection of the fractures. Both methods proved to be good to characterise the fracture system of such a landslide area. Geophysical predictions have been verified also in reality: the mass movements occurred about 1½ years after the measurements. Therefore, to provide early risk warnings and to avoid damage to constructions or endangering human life, the application of the ERT and PreP methods is highly recommended.
ABSTRACT
A slowly moving loess landslide along the River Danube in South Hungary was studied using electrical resistivity tomography. Our aim was to determine the fracture system of the study site. ...Due to the homogeneous composition of the loess, it seems to be the only possibility to get information about the landslide and its further evolution. The applicability of the electrical resistivity tomography technique for such a supposedly dense fracture system was studied by numerical modelling, and the results have been verified in the field. The dip of the fractures could not always been observed, and they could not be explored deeply. However, it was possible to map their surface projection to get the desired information about the structure of the landslide. Fracture zones could have been especially well localized, enabling the prediction of the positions of future rupture surfaces and thus the delineation of the endangered zone. Although the area outside of the already subsided one is not endangered yet, the area which has already started to move is going to break into two. Parts of the about 5 m wide blocks at the front of the landslide may fall or slide down anytime. A large area was assumed to move as one unit. Most of our predictions have been verified by the mass movements that occurred about one and half years after the measurements. The electrical resistivity tomography method proved to be a good tool to characterize the fracture system of such a landslide area, enabling the prediction of future rupture surfaces and also delineation of the endangered area. Its use is therefore highly recommended to monitor landslides to provide early risk warnings to avoid damage to constructions or endangering human life.
ABSTRACT
A new array type, i.e., the γ11n arrays, is introduced in this paper, in which the sequence of the current (C) and potential (P) electrodes is CPCP, and the distance between the last two ...electrodes is n times the distance between the first two ones and that of the second one and the third one. These arrays are called quasinull arrays because they are—according to their array and behaviour—between the traditional and null arrays. It is shown by numerical modelling that, in detecting small‐effect inhomogeneity, these configurations may be more effective than the traditional ones, including the optimized Stummer configuration. Certain γ11n configurations—especially the γ112, γ113, and γ114—produced better results both in horizontal and vertical resolution investigations. Based on the numerical studies, the γ11n configurations seem to be very promising in problems where the anomalies are similar to the numerically investigated ones, i.e., they can detect and characterize, e.g., tunnels, caves, cables, tubes, abandoned riverbeds, or discontinuity, in a clay layer with greater efficacy than those of the traditional configurations. γ11n measurements need less data than traditional configurations; therefore, the time demand of electrical resistivity tomography measurements can be shortened by their use.
In this paper a systematic, semi‐empirical comparison is presented between two‐dimensional geo‐electric models and their inversion images, obtained by using five different electrical resistivity ...arrays and an optimized Stummer configuration. Eight different models (more or less in order of growing complexity) are studied and both noise‐free and noisy data cases are considered. The results show that (1) the quality of the inversion images obtained with traditional arrays depends significantly on the model and on the noise level, (2) among the traditional arrays it is definitely the dipole‐dipole array that provides inversion images mostly similar to the geoelectric models, (3) the inversion images obtained by using the optimized Stummer configuration are even more similar to the original geoelectric model than those obtained by the dipole‐dipole array. It means that the optimized Stummer array is even better than the best traditional array, the dipole‐dipole array, especially in the deepest part of the inversion images. We conclude that in a general field situation the Stummer configuration is good enough for not being forced to search specific configurations. As presented, optimization procedures, involving null arrays could even further improve the quality of the inversion images obtained by using the Stummer configuration.
A new array type, i.e., the gamma sub(11n) arrays, is introduced in this paper, in which the sequence of the current (C) and potential (P) electrodes is CPCP, and the distance between the last two ...electrodes is n times the distance between the first two ones and that of the second one and the third one. These arrays are called quasinull arrays because they are-according to their array and behaviour-between the traditional and null arrays. It is shown by numerical modelling that, in detecting small-effect inhomogeneity, these configurations may be more effective than the traditional ones, including the optimized Stummer configuration. Certain gamma sub(11n) configurations-especially the gamma sub(112), gamma sub(113,) and gamma sub(114)-produced better results both in horizontal and vertical resolution investigations. Based on the numerical studies, the gamma sub(11n) configurations seem to be very promising in problems where the anomalies are similar to the numerically investigated ones, i.e., they can detect and characterize, e.g., tunnels, caves, cables, tubes, abandoned riverbeds, or discontinuity, in a clay layer with greater efficacy than those of the traditional configurations. gamma sub(11n) measurements need less data than traditional configurations; therefore, the time demand of electrical resistivity tomography measurements can be shortened by their use.
ABSTRACT
A new array type, i.e., the γ
11n
arrays, is introduced in this paper, in which the sequence of the current (C) and potential (P) electrodes is CPCP, and the distance between the last two ...electrodes is
n
times the distance between the first two ones and that of the second one and the third one. These arrays are called quasinull arrays because they are—according to their array and behaviour—between the traditional and null arrays. It is shown by numerical modelling that, in detecting small‐effect inhomogeneity, these configurations may be more effective than the traditional ones, including the optimized Stummer configuration. Certain γ
11n
configurations—especially the γ
112
, γ
113,
and γ
114
—produced better results both in horizontal and vertical resolution investigations. Based on the numerical studies, the γ
11n
configurations seem to be very promising in problems where the anomalies are similar to the numerically investigated ones, i.e., they can detect and characterize, e.g., tunnels, caves, cables, tubes, abandoned riverbeds, or discontinuity, in a clay layer with greater efficacy than those of the traditional configurations. γ
11n
measurements need less data than traditional configurations; therefore, the time demand of electrical resistivity tomography measurements can be shortened by their use.
A new array type, i.e., the gamma11n arrays, is introduced in this paper, in which the sequence of the current (C) and potential (P) electrodes is CPCP, and the distance between the last two ...electrodes is n times the distance between the first two ones and that of the second one and the third one. These arrays are called quasinull arrays because they are--according to their array and behaviour--between the traditional and null arrays. It is shown by numerical modelling that, in detecting small-effect inhomogeneity, these configurations may be more effective than the traditional ones, including the optimized Stummer configuration. Certain gamma11n configurations--especially the gamma112, gamma113, and gamma114--produced better results both in horizontal and vertical resolution investigations. Based on the numerical studies, the gamma11n configurations seem to be very promising in problems where the anomalies are similar to the numerically investigated ones, i.e., they can detect and characterize, e.g., tunnels, caves, cables, tubes, abandoned riverbeds, or discontinuity, in a clay layer with greater efficacy than those of the traditional configurations. gamma11n measurements need less data than traditional configurations; therefore, the time demand of electrical resistivity tomography measurements can be shortened by their use.
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