In this paper a systematic, semi-empirical comparison is presented between two-dimensional geoelectric 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. basis for traditional profiling and sounding techniques they are also important for electrical resistivity tomography (ERT) measurements because the individual arrays serve as a basis for the ERT measurements.ERT measurements, which nowadays play a dominating role in geoelectric probing, should however be handled differently from the individual arrays. In their case the DOI (depth of investigation) introduced by Oldenburg and Li (1999) and the DD (depth of detectability) introduced by values can give information about the depth interval from which one is able to obtain useful information. DOI is the depth, below which any change in the model resistivity has an unobservable effect on the measured signal. The DOI is in theory array-and modeldependent but for the same model, in the case of various arrays, more or less the same DOI value is obtained. At the same time, the DD parameter shows a more significant array-and modeldependence. E.g., the same model by using a given array could be observed from even a four-five times larger depth than by using another array. We supposed a relation between the DD values of the configurations and their imaging properties. The existence of such a relation was verified in this paper.
A new method, the so-called pressure probe (Pre-P) method, has been developed for detecting and characterizing mechanically weak zones which may not be visible from the surface and which may occur, e.g. due to landslides. On a high bank at Dunaszekcső, Hungary, the fracture system of the loess landslide area was investigated by large resolution applying this method and proved that (1) cracks as small as 2-to 3-cm wide are detectable;(2) the fractures follow each other almost periodically; and (3) on the side of the fractures towards the slump, there are less fractured zones whose width correlates with the width of the given fracture. We also demonstrated that on the passive side of the clearly visible fracture, (1) there are also fractures along which future rock displacement is expected; (2) these fractures are at least as wide as the active side fractures; and (3) the blocks there are about twice as wide as those on the active side. A block several meters wide is expected to fall before the main mass movement. The Pre-P method seems to be the most powerful tool to map the fracture system of such landslides because of its speed, simplicity of application, cost and interpretation. The Pre-P profiles and maps of the fracture system of a landslide enable to understand landslide evolution and delineate endangered areas earlier than by other methods.
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.
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.
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