HR: 1330h
AN: H42B-1080    [PDF]
TI: Characterization of Fractured Zones With Multidirectional Electrical Methods
AU: * Alte da Veiga, N
EM: ndaveiga@ci.uc.pt
AF: Centre for Geophysics and Department of Earth Sciences, Instituto Geofisico, Universidade de Coimbra, Av. Dias da Silva, Coimbra, 3000-134 Portugal
AB: In granitic bedrocks the network of joints and localized fractured zones are the only features with water storage and water conduction capabilities. Fractured zones can range from a few meters to tens of meters wide (in the case of important fault zones) or even hundreds of meters wide when major faults are concerned. These fractured zones imprint an anisotropic character to granitic massifs. Electrical apparent resistivity measurements show rock anisotropy dependence and have been used namely in the determination of fracture orientation. The field technique consists in performing apparent resistivity measurements along a number of azimuths around a common point and plotting the results in a polar diagram. Some recent approaches make use of the Wenner array (Busby, 2000; Watson and Barker, 2002). In late to post-tectonic Hercynian granites of central Portugal, fractures are the only broad anisotropic rock features. Field surveys were conducted in these areas using both the Schlumberger and the square arrays. Measurements performed with these arrays show a strong dependence on the orientation of subvertical structures. In theoretical macroanisotropic situations Schlumberger multidirectional measurements are in accordance with the anisotropy paradox, which means that apparent resistivity is higher along the strike direction of the structure and lower at right angles. Square array multidirectional measurements yield the opposite effect. Field surveys have shown that Schlumberger and square array results are dependent not only on the strike of the structure, but also on the width of the structure and on the position of the array centre. This has been confirmed by analytical and numerical modelling. Fractured zones may have a single character or they may correspond to limited, localized macroanisotropic situations with alternating domains of stronger/weaker fracturation intensity. Schlumberger multidirectional results may be misleading in differentiating the two situations, but square array results are clearly discriminating. When Schlumberger arrays are centred inside a single fractured zone, the longitudinal apparent resistivity is higher than the transverse apparent resistivity (for array spacings larger than the zone width). The results can be much the same in the case of localized macroanisotropic situations. On the other hand, and as long as array spacings do not exceed much the width of the fractured zone, multidirectional square array apparent resistivities for localized macroanisotropic fractured zones are consistent with the theoretical situation. This is not the case with single type structures. Moving the centre of the arrays may lead to completely different results. Thus, along with strike determination, it is possible to evaluate the width and the macroanisotropic versus single character of a fractured zone. Busby, J.P., 2000. The effectiveness of azimuthal apparent-resistivity measurements as a method for determining fracture strike orientations. Geophys. Prosp., 48, 677-695. Watson, K.A. and Barker, R.D.,2002. Use of the azimuthal offset Wenner technique to characterise a single dipping interface. Europ. J. Environm. Engin. Geophys., 7, 103-120
DE: 0925 Magnetic and electrical methods
DE: 1829 Groundwater hydrology
DE: 8010 Fractures and faults
SC: Hydrology [H]
MN: 2003 Fall Meeting