Geoscience Reference
In-Depth Information
electrodes (current dipole) and one pair of potential electrodes (potential dipole). Similar to a gal-
vanic contact system, a capacitively coupled system determines ρ
a
(or σ
a
, EC
a
) using measured
electric current, measured voltage, and array characteristics, which in this case, are the coaxial
cable dipole lengths and the distance between the two dipoles. The spacing between the two dipoles
governs the soil investigation depth, given that the dipole lengths remain constant.
As previously mentioned, the transmitter in a capacitively coupled resistivity measurement sys-
tem generates alternating electric current. The higher the AC frequency, the better the capacitive
coupling, and the more electric current is coupled to the ground. However, higher frequencies also
generate some unwanted electromagnetic effects that adversely influence resistivity measurement.
A transmission frequency of between 12 kHz and 20 kHz is a good compromise between the need
for a low enough frequency to avoid unfavorable electromagnetic effects on resistivity measurement
and the need for a high enough frequency to get good current coupling to the ground.
There are some advantages and disadvantages for capacitively coupled resistivity measurement
systems when compared to galvanic contact systems. Capacitively coupled systems can be used in
areas covered with pavement, whereas galvanic contact systems, with electrodes that need to be
inserted into the ground, usually cannot be employed in these settings. In soil environments with
high resistivity, it is often difficult when using galvanic contact continuous measurement systems to
adequately transfer electric current from a current electrode into the ground, a problem previously
referred to as contact resistance. This problem does not occur with capacitively coupled systems;
therefore, these systems are a good choice for use in high-resistivity soil environments. Conversely,
capacitively coupled systems do not work well in low-resistivity soil environments because the
potential dipole voltage becomes too small to be measured reliably. Increasing the amount of elec-
tric current transferred into the ground would solve this problem, but this tactic is not easily accom-
plished with capacitively coupled systems. Furthermore, due to limitations on the amount of electric
current that can be transferred into the ground, the maximum investigation depth achievable with
a capacitively coupled system is around 20 m, which is much less than what can be obtained with a
galvanic contact system.
The OhmMapper TR1 (Geometrics, Inc.) displayed in Figure 5.8 is an example of a capaci-
tively coupled continuous resistivity measurement system. The current and potential dipoles can
be 5 or 10 m in length and are composed of two coaxial cables integrated with either transmitter or
receiver electronics. The transmitter generates a 16 kHz alternating current. The OhmMapper TR1
also has a battery power supply, a data logger console, and rope that separates the two dipoles from
one another (Figure 5.8). The depth of investigation can be adjusted by lengthening or shortening
the rope separating the current and potential dipoles. Continuous ρ
a
measurements are collected at
time intervals as short as 1 sec. The OhmMapper TR1 can be integrated with a GPS receiver for
accurate determination of the resistivity measurement locations.
Transmitter
Receiver
Rope
Tow Link
Optical Wand
Weight
Data
Logger
Console
Tow Cable
fIGURe 5.8
OhmMapper TR1 capacitively coupled continuous resistivity measurement system. (Courtesy
of Geometrics, Inc., San Jose, California.)
