Borehole Electromagnetics –Diffusive and Propagation Transients - Wave Propagation in Drilling, Well Logging and Reservoir Applications - page 347

The results in Figures 8.8a to 8.8f, characterizing the formation one
diameter away from our coil, display longitudinal magnetic fields. “Red”
indicates the strong fields associated with the coil core. On the other hand,
“yellow, green, and blue” represent decreases in strength as the coil wire itself is
approached, while depressions in the blue “fabric” describe the “turning” of
field lines as they form closed loops. In this work, transmitter and receiver coils
are circular with planes perpendicular to the tool axis, however, they may in
general take any closed or open form; minor source code changes to the J s
vector which excites A will be required. Differences are seen, but the most
pronounced is shown for Case 5, where the transmitter coil resides across two
adjacent layers. Here, the magnetic field in Figure 8.8e is “split” by strong
heterogeneities, the way woodcutters split fire logs. Its underside in Figure 8.8f
highlights this bias. This result bears strong implications in both resistivity and
NMR logging. Instrument readings, and possibly, interpretation software, are
likely to smear individual layer results, suggesting instead, averaged formation
properties. Consequently, saturation estimates can be calculated incorrectly, and
oil and gas producing zones can be bypassed unintentionally.
Figure 8.8a. Axial magnetic field, Case 1.