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In order to evaluate how much Total Solar Irradiance (TSI) has influenced Northern Hemisphere surface air temperature trends, it is important to have reliable estimates of both quantities. Sixteen different estimates of the changes in TSI since at least the 19th century were compiled from the literature. Half of these estimates are “low variability” and half are “high variability”. Meanwhile, five largely-independent methods for estimating Northern Hemisphere temperature trends were evaluated using: 1) only rural weather stations; 2) all available stations whether urban or rural (the standard approach); 3) only sea surface temperatures; 4) tree-ring widths as temperature proxies; 5) glacier length records as temperature proxies. The standard estimates which use urban as well as rural stations were somewhat anomalous as they implied a much greater warming in recent decades than the other estimates, suggesting that urbanization bias might still be a problem in current global temperature datasets – despite the conclusions of some earlier studies. Nonetheless, all five estimates confirm that it is currently warmer than the late 19th century, i.e., there has been some “global warming” since the 19th century. For each of the five estimates of Northern Hemisphere temperatures, the contribution from direct solar forcing for all sixteen estimates of TSI was evaluated using simple linear least-squares fitting. The role of human activity on recent warming was then calculated by fitting the residuals to the UN IPCC’s recommended “anthropogenic forcings” time series. For all five Northern Hemisphere temperature series, different TSI estimates suggest everything from no role for the Sun in recent decades (implying that recent global warming is mostly human-caused) to most of the recent global warming being due to changes in solar activity (that is, that recent global warming is mostly natural). It appears that previous studies (including the most recent IPCC reports) which had prematurely concluded the former, had done so because they failed to adequately consider all the relevant estimates of TSI and/or to satisfactorily address the uncertainties still associated with Northern Hemisphere temperature trend estimates. Therefore, several recommendations on how the scientific community can more satisfactorily resolve these issues are provided.

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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-[Formula: see text], Wenner-[Formula: see text], 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 [Formula: see text] or [Formula: see text] 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.

The term "null array" is introduced for those electrode configurations where the measured potential difference is zero above a homogeneous half-space when using a measuring dipole M 0 N 0. Different types of null arrays (three-electrode, Schlumberger, and dipole axial/equatorial null arrays) and their corresponding traditional arrays are studied. It was shown in a field study carried out in a karstified limestone area covered by thin sediments that it is possible to obtain geologically meaningful results with null-array techniques. The main features of the null-array data are as follows. (1) Nullarray data appear to be more spatially variable than the classical data. The spatial variability provides information about the presence of karstic fractures in the subsurface; (2) The null-array anomalies caused by nearly vertical karstic fractures in the limestone basement do not decay with depth as quickly as the classical array anomalies. (3) The strike direction of the fractures is much less ambiguous than that found by using classical arrays. Nevertheless, the depth variation of the basement is more reliably observed in geoelectric anomalies obtained using traditional arrays. Therefore a joint use of classical arrays and their corresponding null methods is recommended, because the combined methods provide more information about the subsurface structure.

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SUMMARYWe have found in the geophysical literature more than ninety different surface geoelectric arrays, fulfilling an updated definition (specifying the current feeding, the potential difference measurement and the geometry of the electrodes). Several composite configurations, with widely varying geometry, have also been collected. We have presented the geoelectric arrays in a systematic way and with a unified notation. The classification is based on three divalent parameters: "superposition" of measurements, "focusing" of currents and "colinearity" of the array, creating 8 groups of geoelectric arrays. For the simplest group (the group of nonfocused, nonsuperposed, colinear arrays) we cover all theoretically possible arrays. For the other groups -due to the infinite variety -we collected only the already existing arrays, but it is easy to create further example arrays. The proposed classification may facilitate a systematic comparison of properties of different arrays and inspire testing new arrays, to find optimal configurations for actual field problems. Finally, the classification certainly helps to avoid rediscovering already published arrays.

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.