Two types of portable instruments
have been developed for measuring the apparent electrical conductivity
of the soil: (1) direct contact four-electrode sensors, and (2) remote
electromagnetic (EM) induction sensors. Direct contact four-electrode
sensors can take the form of either insertion probes or surface arrays;
the latter being the most common configuration for mobilized applications.
Examples of such sensors include the modified Martek SCT-10 unit
(used in the George E. Brown Jr., Salinity Laboratory Mobile Wenner four-electrode
system) and the sensor technology used in the Veris 3100 and 2000 XA soil
conductivity systems. Commercial examples of electromagnetic induction
sensors include the Geonics EM-38 and EM-31 meters, both of which can be
easily mobilized. Four-electrode and EM type sensors each present
various advantages and disadvantages with respect to mechanized survey
applications. However, in general, both types of sensors can be used
to accurately map soil electrical conductivity. |
| Currently, all local LCRSAN assessment
programs are using the Geonics Dual-dipole EM-38 system (shown in Figure
1 above). The EM38-DD system includes 2 complete EM-38 units synchronized
to operate simultaneously at 14.6 kHz, allowing for the simultaneous, on-the-go
measurements of both vertical and horizontal dipole conductivity data.
The conductivity data can be output in real-time via a standard serial
RS232 digital interface to an external device (such as a computer or GPS
receiver). |
Like conductivity sensors, there
are basically two types of GPS systems which can be incorporated into most
MSCA platforms; (1) self-contained systems, or (2) receivers specifically
designed for precision agriculture applications. However, the difference
here is not in the GPS receiver technology, but rather the interfacing. |
 Self-contained
GPS systems typically include data loggers and software programs which
allow the user to record, modify, and/or store GPS coordinate data independent
of any other sensors or hardware interfacing. On the other hand,
receivers specifically designed for precision agriculture applications
must typically be connected to (i.e., interfaced with) some type of computer
or electronic controller in order to store and/or process any sort of GPS
coordinate data.
The Trimble Pathfinder Pro-XRS and
Trimble Ag132 GPS systems represent examples of these two types of GPS
systems. The Pro-XRS system (shown in Figure 2) is a self contained
system which can also log data from an external sensor (such as an EM-38),
while the Ag132 is a system requiring external interfacing software.
Both types of systems are currently used within the local LCRSAN assessment
programs.
|
Hardware interfacing considerations
arise due to the need to merge the conductivity sensor and GPS coordinate
data, and/or to control the timing of the data acquisition. In most
cases, the complexity of the hardware interfacing depends on the type and
number of sensors which much be integrated, along with the amount of real-time
data processing which must be performed. Hence, hardware interfacing
often tends to be rather system specific. However, in simple MSCA
systems (systems with only one conductivity meter) it is often possible
to bypass the need for separate hardware interfacing entirely. This
can be achieved if the conductivity meter can directly output real-time
sensor data through an RS-232 serial connection, because most stand alone
GPS systems have the ability to "capture" and merge such digital signal
data directly into the GPS coordinate data file.
Since the DDEM-38 meter outputs all
conductivity readings via a single RS232 serial connection, this meter
can be interfaced directly with a Trimble Pro-XRS GPS system. In
a similar manner, DDEM-38 meter can be interfaced with an on-board computer,
which in turn simultaneously communicates with a GPS system (to acquire
real-time location data along with the real-time conductivity data).
Both types of hardware interfacing techniques are currently used within
the local LCRSAN assessment programs.
|
The final component which must be
specified is the transport platform itself. Again, there are basically
two types of platforms in commercial use; motorized (i.e., self propelled)
systems and platforms which must be towed by an external vehicle.
The Mobile EM sensing systems developed by the George E. Brown Jr., Salinity
Laboratory and the Australian Cotton Research Institute are two examples
of motorized platforms (Rhoades, 1996; Triantafilis and McBratney, 1998).
Likewise, the Mobile Wenner four-electrode system and commercially available
Verris 3100 represent two examples of platforms which must be towed.
In most cases, motorized platforms tend to be more sophisticated, versatile,
and more expensive to develop than towable platforms.
The local LCRSAN assessment programs
currently employ both motorized and towable platforms. Examples of
two different motorized platform designs are shown below.
|
 The
Imperial Irrigation District MSCA system is shown to the left (Figure 3).
This system uses a motorized Spray Coup tractor to carry both a side mounted
hydraulic sampling rig and the DDEM-38 meter (contained within a swivle
mounted PVC tail attached to the back of the rig). An on-board computer
running custom Sandia Research survey data integration software controls
the acquisition of both the EM-38 and GPS survey readings. |
 The
Coachella Valley Resource Conservation District MSCA system is shown to
the right (Figure 4). This system usea a hydraulically driven Lee
Spray Trac to carry both a front mounted hydraulic sampling rig and the
DDEM-38 meter (carried in a rigid mounted PVC tail attached to the back
of the rig). In this MSCA system, a Trimble Pro-XRS GPS unit is used
to control the acquisition of both the EM-38 and GPS survey readings. |
Cited References:
Rhoades, J.D. 1996.
New assessment technology for the diagnosis and control of salinity in
irrigated lands. Proc. Int'l. Symp. on Develop. of Basic Technology for
Sustainable Agric. Under Saline Conditions, December 12, 1996, Tottori,
Japan. pp.1-9.
Triantafilis, J. and
A.B. McBratney. 1998. Development of a Mobile Electromagnetic
Sensing System for soil salinity assessment in irrigated cotton fields.
Proc. 9th Australian Cotton Growers Research Association conference, August
12-14, 1998, Broadbeach, Queensland, Australia. pp 61-64.
