Geoscience Reference
In-Depth Information
and looser soil surface conditions). It is good practice to map fields when they are not very dry. Soil
EC
a
mapping with the Veris unit should not be attempted when the soil is frozen, or in the presence
of any frost layers. Frozen soil has significantly different conductive properties, and the EC
a
data
collected will not be valid. The Veris unit is a rugged and reliable system with no known difficulties
in mapping fields in the spring prior to tillage and planting operations or in the fall after harvest with
heavy standing and flat-lying surface residue conditions (Farahani and Buchleiter, 2004). For ease
of maneuvering, fields are normally traversed in the direction of crop rows, but the resulting map is
not affected by the direction of travel. On average, travel speeds through the field range between 7
and 16 km h
−1
with measurements taken every second, corresponding to 2 to 4 m spacing between
measurements in the direction of travel, respectively. A parallel swather (such as AgGPS Parallel
Swathing Option, Trimble Navigation Ltd., Sunnyvale, CA) mounted inside the vehicle pulling the
Veris unit may be used to guide parallel passes through the field at desired (i.e., 12 to 18 m) swath
widths.
The direct contact method used by EC
a
equipment like Veris has a distinct advantage over the
EMI method in that there is no possibility of ambient electrical (for instance from power lines),
metallic (operator's belt buckle), or engine noise interferences. Other important advantages of ER-
type methods over EMI are that there is no calibration or nulling procedures required prior to map-
ping, and there is no known report of any observed drift in the measured soil EC
a
by ER. Regular
“drift runs” that involve traversing the same location in a field are needed for EMI in order to deter-
mine the drift resulting from air temperature effects on the instrument throughout the day. Because
the electrodes in the ER method directly inject the signal into the soil, changes in air temperature
have virtually no effect on the readings. It is noted that EC
a
data collected with either method
are affected by soil temperature. The most obvious disadvantage of direct ER methods is their
intrusiveness as compared to the nonintrusive EMI methods. The invasive ER method requires solid
contact between the coulters and soil; consequently, dry conditions or irregular microtopography
can prevent contact. Although the distinction between the two differing EC
a
measuring methods of
ER and EMI is important, side-by-side measurements of soil EC
a
by contact electrodes and EMI
methods has given highly correlated values (Sudduth et al., 2003) and has provided similar maps
(Doolittle et al., 2002).
In the case of EMI, EC is measured remotely using a frequency signal in the range of 0.4 to
40 kHz and primarily measures signal loss to determine EC
a
. The EMI measurement is made with
the instrument at or above the soil surface. An EMI transmitter coil located at one end of the instru-
ment induces circular eddy-current loops in the soil (Figure 4.2). The magnitude of these loops is
directly proportional to the EC
a
of the soil in the vicinity of that loop. Each current loop generates
a secondary electromagnetic field that is proportional to the current flowing within the loop. A por-
tion of the secondary-induced electromagnetic field from each loop is intercepted by the receiver
coil, and the sum of these signals is related to a depth-weighted EC
a
.
For TDR, an applied electromagnetic pulse is guided along a transmission line embedded in
the soil. The time delay between the reflections of the pulse from the beginning and the end of the
Receiver
coil
Transmitter
coil
+
+
+
+
Current loops
+
+
Induced current flow in soil
fIGURe 4.2
The principle of operation electromagnetic induction. (From Corwin, D.L., and Lesch, S.M.,
Agron. J.
, 95, 455-471, 2003. With permission.)
