Attila Novák

Depth of Investigation and Vertical Resolution of Surface Geoelectric Arrays

Depth of investigation and vertical resolution values are determined and tabulated for 30 surface geoelectric arrays that have non-zero response, i.e., a depth of investigation characteristic (DIC) function due to a buried thin horizontal sheet. In accord with experience, results show a general reciprocal relationship between depth of investigation and vertical resolution. The most frequently used arrays in multi-electrode studies (i.e., Wenner-a, Wenner-b, Schlumberger, dipole-axial arrays and the pole-dipole array) offer reasonable compromises between depth of investigation and vertical resolution. Depth of investigation can be increased by using the pole-pole array; vertical resolution can be improved with, for example, the a10 or c10 arrays. Current focussing does not increase the depth of investigation for the horizontal thin-sheet model. The complete set of depth of investigation and vertical resolution values permits exact physical comparison of various geoelectric arrays and provides simple but useful rules for practical geoelectric applications, e.g., how to develop multi-electrode systems with higher vertical resolution, or how to select arrays to satisfy special exploration requirements.

Increasing the effectiveness of electrical resistivity tomography using γ11n configurations

A new array type, the γ11n arrays are 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 and third one. These arrays are called quasi null arrays because they are – according to their array and behaviour – between the traditional and null arrays. It is shown by numerical modelling that in detection of small-effect inhomogeneities these configurations may be more effective than the traditional ones including the optimised Stummer configuration. Certain γ11n configurations – especially the γ112, γ113 and γ114 – produced better results both in horizontal and vertical resolution investigations. On the basis of the numerical studies, the γ11n configurations seem to be very promising in problems where the anomalies are similar to the numerically investigated ones, that is they can detect and characterise, for example,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 also the time demand of electrical resistivity tomography (ERT) measurements can be shortened by their use.

Target detectability depths of DC arrays for various models

We have compared target detectability depths for six different DC geoelectric arrays. Five various 2D inhomogeneity models and two noise levels (5 pc and 10 pc) were assumed. The maximum detectability depths were determined by using the RES2DMOD software. Although the results are model-dependent (they depend both on geometry and resistivity contrast), the best results (namely: the maximum detectability depths) were obtained usually with the pole-dipole (P-DP) and the dipole axial (DP-ax) arrays. The worst results (namely: the smallest detectability depths) were obtained (with one exception) in case of the pole-pole (P-P) and the Wenner-alfa (W-alfa) arrays. The results by using the Wenner-beta (W-beta) and dipole equatorial (DP-eq) array groups are slightly below or above the average. Detectability depth values are comparable exclusively for the same model (partly due to the variable resistivity contrast), but in case of a certain model it can be unambiguously declared, which array is the most effective one.

Dimensional Tensor Invariants in Geoelectric Prospecting

It is studied whether the one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) tensor invariants really behave like invariants in the field that is whether their values are independent from the position of the current electrodes of the tensorial geoelectric configuration and what kind of results they can produce in numerical modelling and in field situations. It was shown that: 1. the invariants are “less and less” invariants with their increasing dimension number depending more and more on the position of the current electrodes. 2. They produced smaller and smaller anomalies both in their size and amplitude making the detection and interpretation of the anomalies more and more difficult. 3a. The 1D image produced for all models stable, reliable results which are ideal for creating a starting model. 3b. In spite of the uncertainty of the 2D data they improved the quality of the fault field image which has been received using the 1D data only. In the sites with building remnants and furnace however the 2D invariant was not able to give extra information to that obtained by the 1D invariant. 3c. Although the interpretation of the 3D results may be rather complicated it proved to be more useful than that of the 2D data both in the building remnants and furnace field studies. In special cases the 3D invariant may refine the 1D image. In summary the 2D invariant which is sensible to the two-dimensional changes of the subsurface (like that of the in the field practice most often used 2D ERT configuration) and which was expected to produce the best results proved to be almost the less useful in these investigations in spite of that the investigated models were more 2D/3D than 1D. Because even for such models the 1D invariant produced the best results its application is recommended. Regarding however that the 2D and 3D invariants may refine the 1D image even if their results are more uncertain joint interpretation of all dimensional invariants could also be worthwhile. Although the refined model is more risky it can be very useful e.g. in studies where the danger factor is high, e.g. because of filtrating of dangerous fluid or fissuring on the wall of a nuclear waste deposit. In such cases it is better to warn redundantly than eventually not recognize real danger. The results of these investigations should be taken into account in every research area, where tensorial measurements could be carried out, e.g. in magnetotelluric research.

Studying the Fracture System of a Landslide by ERT Method

In contrary to former researches which used to stud y the sliding surface or the water content of certain layers of a landslide area we investigat ed the near-surface fracture system of it. It makes possible the delineation of the endangered ar ea and gives information about the inner structure of the landslide. For this aim Electrica l Tomography (ERT) measurements were carried out by different geoelectric configurations . Due to the different conditions of the fractures they can be both conductive and resistive ones and even it is possible that a part of them is conductive while the remaining part is resi stive in the function of the filling material and the time passed since the last rainfall. This f eature and the probably high density of the fractures make the interpretation of the ERT data r ather difficult. The Schlumberger and especially the Dipole-Dipole results proved to be a lmost inutile, while the Pole-Dipole and especially the Stummer array results seem to be muc h more perspective. The results of these arrays were compared to the Pressure-Probe data we got on the same profile. Using the ERT results a lot of fractures were detected and more o r less well localised and on the basis of the fractures the research area could have been divided into several domains which are differently fractured. It emerged among others that even in the not-yet sliding area there are fractures that is a part of this area tends to be m ore and more dangerous, as well.

Depth of Investigation of Dipole-dipole, Noncolinear and Focused Geoelectric Arrays

Investigation depth of various DC geoelectric arrays has always been in the focus of interest of geoelectricians. According to its classical definition (Roy and Apparao 1971), the depth of investigation is the depth of the maximum response due to a horizontal thin-sheet embedded in a half-space, by using a given geoelectric array. On basis of the graph of the thin-sheet response as a function of the depth (from the so-called „depth of investigation characteristics” or DIC function) Edwards (1977) found more realistic to compute the medium depth than the depth of the maximum response. DIC functions have been known so far only for simple colinear arrays, the dipole equatorial array and two focused arrays. Here we provide a summary about the depth of investigation values of various dipole-dipole arrays (for parallel, perpendicular, radial, azimuthal ones), and for the most important noncolinear and focused arrays. Depth of investigation values are computed from both approaches. DIC functions (obtained by a new analytical formula) are also presented, as illustrations. The analytical formula can be used to compute DIC function of any surface geoelectric array. A systematic interpretation of the resulting depth of investigation values provides simple but useful thumb-rules for practical applications.