Aspects of multicomponent resistivity data and macroscopic resistivity anisotropy

New induction-logging hardware makes it possible to obtain both resistivity and resistivity-anisotropy data. Resistivity anisotropy, the ratio of vert ical to horizontal resistivity, is the macroscopic effect of thinly layered formations in which logging tools have insufficient vertical resolution to properly resolve the individual beds, or laminae. Generally, there are two special types of layering. Laminated shaly sands. The sediment consists of thin-bedded sand/shale sequ ences (anisotropy in these sands originates from the contrast of shale and sand resistivity). Finely layered anisotropic sands. The sand is composed of layers of differe nt grain sizes/sorting (anisotropy in these sands originates from the resis tivity contrast associated with the different water saturation). The interpretation of conventional resistivity data is well understood. How ever, interpretation of vertical resistivity and resistivity anisotropy is not well understood and is often counterintuitive. We have used forward mod eling to illustrate the effects of porosity variability in layered formatio ns. For example, we have investigated the porosity-layering effect, which v aries from an average porosity of 20 p.u. by +/-5, 10, and 15 p.u. Three ty pes of layering were considered: graded bedding and square- and sin(2-)poro sity variation with depth. The modeling shows that sharp bed boundaries cre ate the maximum resistivity anisotropy for any two component resistivity di stributions. The new induction-logging hardware comprises three mutually orthogonal, tra nsmitter-receiver coil configurations that measure all data necessary to de rive both resistivity and resistivity anisotropy of the formation in vertic al. deviated, and horizontal wells. Simple models illustrate the physics. a nd the tools' capabilities are demonstrated with a synthetic example, Based on the petrophysical analysis of porosity contrast and layering type, a re sistivity model is constructed. and the too[ responses of this model are co mputed. With inversion techniques, both resistivity and resistivity anisotr opy can be recovered.