Specifications

University of Pretoria etd – Combrinck, M (2006)
power law decays; straight lines with equal slopes, independent of the actual conductivity
values. Figure 2.7 shows the h
v
1
θ
r
(t) component as a function of time, once again
comparing free space with conducting space responses. The delayed and reduced peaks
with straight-line behaviour in the late time are still present, but the most interesting feature
is the sign change that occurs in this response when a conducting space is introduced. The
last component of interest is the h
u
1
θθ
(t) in figure 2.8. This component doesn’t exist for
the free-space case and presents the magnetic field components associated with galvanic
currents generated in the sphere. These responses are comparable in magnitude to the
fields generated by the vortex currents. The sign changes observed in figure 2.7 can be
explained by thinking of the sphere acting as a secondary source of smoke-rings. Smoke-
rings (or more probably shells) move out from this source in all directions and the moment
they pass the receiver position a sign change is recorded in the measured radial component
(figure 2.9).
Figure 2-9 “Reflected” smoke-rings. Electric field intensity is presented as sections for 20
consecutive time channels. The cyan and black contour lines present the electric field intensity
for a homogeneous 50 Ohm.m half space and indicate the well-known diffusive smoke-ring
behaviour. The colour contours indicate electric field intensity of currents for a 5 Ohm.m prism
(black rectangle) in a 50 Ohm.m half space as percentage of the half space response. The
“reflected” currents are clearly visible in channels 13 to 20.
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