Geoscience Reference
In-Depth Information
GPS
P
3
P
1
F
P
4
P
2
S
C
R×4
R×3
R×2
R×1
Tx
Array offset: R×1 = 5.0, R×2 = 7.5, R×3 = 10.0 m, R×4 = 12.5 m
Rx = receiver
Tx = transmitter
P = potential dipole = 5 m
C = current dipole = 5 m
S = separation (rope) = 2.5 m
F = distance between GPS & array = 4.2 m
fIGURe 5.9
Schematic of OhmMapper TR4 with one current dipole and four potential dipoles. (Courtesy
of Geometrics, Inc., San Jose, California.)
The OhmMapper TR1 mimics a single galvanic contact dipole-dipole electrode array and there-
fore has only one receiver (potential) dipole. Different OhmMapper systems can have up to five poten-
tial dipoles. Figure 5.9 is a schematic that illustrates the setup for a system, the OhmMapper TR4,
having four potential dipoles. These systems will accordingly mimic up to five dipole-dipole electrode
arrays at a time, with each array having a different length and therefore different resistivity measure-
ment investigation depth. The value of these multiple potential dipole OhmMapper systems is in their
ability to continuously measure both lateral and vertical soil resistivity variations simultaneously.
5.9 fIeld dAtA ColleCtIon ModeS
As previously implied in Section 5.5 where the most common electrode arrays were discussed,
there are two main modes for resistivity data collection that are used in the field: one is “constant
separation traversing,” and the other is “vertical electric sounding” (Reynolds, 1997). With con-
stant separation traversing, the spacing between electrodes remains constant as the whole electrode
array is moved along a transect (Figure 5.10a). Again, for the Schlumberger, Wenner, and dipole-
dipole arrays, apparent resistivity, ρ
a
, measurement locations are referenced to the midpoint posi-
tion between the two outer electrodes. Constant separation traversing resistivity measurements are
usually collected along a series of transects forming a grid that covers a study area, thereby making
this data collection mode ideal for highlighting horizontal variations in ρ
a
. The development of
continuous resistivity measurement equipment has made it possible to efficiently conduct constant
separation traversing surveys on large agricultural fields.
Information regarding resistivity changes with depth can be obtained through vertical electric
sounding. The procedure for vertical electric sounding involves keeping the midpoint of the resistiv-
ity array stationary, while the overall array length is successively increased (Figure 5.10b). As the
electrode array is lengthened, electric current penetrates further into the subsurface, providing a
greater depth of investigation. Consequently, changes in ρ
a
that occur with successive increases in
the electrode array length indicate variations of resistivity with depth. Vertical electric sounding is
done with either conventional equipment or computer-controlled multielectrode systems.
Horizontal and vertical resistivity data can be collected together. One procedure based on con-
ventional equipment or a computer-controlled multielectrode system is to conduct a complete vertical
electric sounding at each regularly spaced location along lines within a grid. Collection of horizontal
and vertical resistivity data together can also be accomplished by conducting several resistivity sur-
vey passes over a study area. For each successive pass, the electrode array is lengthened, and inves-
tigation depth is increased. Continuous measurement resistivity systems are especially applicable to
this type of approach. Additionally, with the continuous measurement resistivity systems that have
