Borehole Electromagnetics –Diffusive and Propagation Transients - Wave Propagation in Drilling, Well Logging and Reservoir Applications - page 349

Figure 8.8e. “Split” magnetic field, Case 5.
Figure 8.8f. “Split,” underside, Case 5.
8.4 Apparent Anisotropic Resistivities
for Electromagnetic Logging Tools in Horizontal Wells
In this example, we discuss the meaning of “resistivity” in typical
resistivity logs. As should be anticipated, petroleum formations encountered in
exploratory drilling are layered, anisotropic and heterogeneous. Yet, when most
logs are presented to the operating company client, a single resistivity is
typically displayed at a given depth. This so-called “apparent resistivity” results
from “force-fitting” receiver phase or attenuation data to a simple isotropic,
homogeneous, infinite medium model that, we emphasize, is arbitrarily
assumed. Now, averages or “ball park” estimates have their roles, e.g., the
average human height is six feet give or take several inches. However, in
modern exploration, typically one finds anisotropies with R v /R h >> 1, say 5 or
10, so that the notion of a single “apparent resistivity' is less meaningful.