Geology Reference
In-Depth Information
(a)
(b)
Transmitter
Receiver
R
R
Fig. 11.10
(a) A simple induction log.
(b) A focused induction log.
about twice the transmitter-receiver separation. This
particular focused system has the disadvantage that spu-
rious apparent resistivities are produced at boundaries,
but this effect may be compensated by employing addi-
tional coils.
See Section 9.6 for the application of time-domain
electromagnetic techniques in borehole surveys.
11.5 Induction logging
The
induction log
is used in dry holes or boreholes that
contain non-conductive drilling fluid which electrically
insulates the sonde.The wallrock is energized by an elec-
tromagnetic field, typically of about 20 kHz, which gen-
erates eddy currents in the wallrock by electromagnetic
induction. The secondary EM field created is registered
at a receiver which is compensated for direct coupling
with the primary field and which allows a direct estimate
of apparent resistivity to be made. The set-up is thus
similar to the surface moving coil-receiver EM system
described in Section 9.5.
The two-coil system shown in Fig. 11.10(a) is unfo-
cused and the induced EM field flows in circular paths
around the borehole, with a depth of investigation of
about 75% of the transmitter-receiver separation. Litho-
logical boundaries show up as gradual changes in appar-
ent resistivity as they are traversed.When combined with
information from other logs, corrections for invasion
can be made from standard charts.
Clearer indications of lithological contacts can be ob-
tained using a focused log such as that shown in Fig.
11.10(b), in which two extra coils are mounted near the
receiver and transmitter and wired in series with them.
Such an arrangement provides a depth of penetration of
11.6 Self-potential logging
In the
self-potential
(
SP
)
log
, measurements of potential
difference are made in boreholes filled with conductive
drilling fluid between an electrode on the sonde and a
grounded electrode at the surface (Fig. 11.11).
The SP effect (Section 8.4.2) originates from the
movement of ions at different speeds between two fluids
of differing concentration. The effect is pronounced
across the boundary between sandstone and shale, as the
invasion of drilling mud filtrate is greater into the sand-
stone. Near the borehole there is a contact between
mud filtrate in the sandstone and pore fluid of different
salinity in the shale. The movement of ions necessary to
nullify this difference is impeded by the membrane
polarization effect (Section 8.3.2) of the clay minerals in
the shale. This causes an imbalance of charge across the
