Introduction
DUALEM instruments operate through electromagnetic
induction and, thus, require no contact with the ground. Non-contacting operation makes DUALEM
instruments suitable for surveys using a motor vehicle, which enables efficient
coverage of large areas.
Three factors that affect the effectiveness of
motorized surveying are the situation of the instrument relative to both the
vehicle and the ground, the platform for the instrument, and the positioning of
data.
Situation of the
Instrument
The situation of the instrument refers to the
distance between the instrument and the vehicle, the orientation of the
instrument relative to both the vehicle and the ground and the height of the
instrument above the ground.
There are two potential sources of noise from the
vehicle, namely the electrical system of the vehicle and EMI in the metal of
the vehicle. Both types of noise
decrease greatly as the distance increases between the vehicle and the
instrument. DUALEM instruments are
insensitive to noise away from their 9-kHz operating frequency, so the
broadband noise of the electrical system is of less concern than EMI in the
metal of the vehicle.
EMI is generally insignificant if the distance
between the instrument and any substantial area of metal on the vehicle is at
least as large as the transmitter-receiver separation (i.e. 2 m for the
DUALEM-2 and 4 m for the DUALEM-4). The
distance should be increased where signal levels are very low, such as on
deeply frozen ground, or where the application requires high-quality in-phase
measurements, such as UXO detection.
Figure 1 shows a DUALEM-4 being towed on a cart
behind an all-terrain-vehicle. The
distance between the vehicle and the instrument is a little more than the
length of the instrument, which is perfectly adequate for the application of
measuring conductivity (i.e. quadrature) over moderate ground.
The orientation of the instrument relative to the
vehicle is of little importance, but accidental damage to the instrument is
usually less likely if the instrument is aligned with the direction of travel
(as in figure 1). Most operators prefer
orient the instrument with the transmitter closest to the vehicle, and this is
clearly advantageous if a data cable, which attached to the connector near the
transmitter, extends from the instrument to a logging system on the vehicle.
The instrument should be secured so that the data
connector remains vertical on the top of the instrument throughout the
surveying.
Figure 1: DUALEM-4 on a Cart.
(Photo: Alpha Geoscience Pty. Limited)
The distance from the ground to the mid-height of
the instrument should be measured and noted.
To maximize signal levels, this ground clearance should be as low as
practicable. However, some surveyors of
metal or UXO in magnetic ground prefer a ground clearance of as much as 0.5 m,
as the effect of magnetic susceptibility on in-phase measurements decreases
rapidly with increasing clearance.
Instrument
Platform
Carts and sleds are the prevalent types of platform
for DUALEM instruments. The Appendix
notes websites that may contain information about sled materials. A comprehensive description of the sled
shown in figure 2, as originally designed and built by John Holman, is
available from Dualem by request.
Figure 2: Sled for DUALEM-2.
(Photo:
John Holman, USDA-ARS)
Wherever possible, platform materials should be
non-conductive and without magnetic permeability, especially for portions of
the platform nearest the transmitter and receivers. Significant perimeters or areas of conductive material are most
problematic; for example, inflatable tires that incorporate loops of wire
should not be used.
The illustrated cart and sled both use blocks of
plastic foam to reduce vibration of the instrument. Protective padding such as the foam not only will help to
maintain the service life of the instrument, but will also tend to improve the
quality of data.
Some users place a shell around the instrument to
provide additional protection from rocks and vegetation. Figure 3 shows a combined shell and sled
constructed by the University of Florida for surveying irregular terrain in citrus
groves. Foam-rubber sleeves inside the
shell and mounts between the shell and the sled insulate the DUALEM-2 from some
vibration and shock.
Figure 3: Sled with Shell for
DUALEM-2.
(Photo: A. Schumann, University of Florida)
A shell can provide additional benefits in sunny
conditions. A shell that shades the
instrument uniformly will tend to reduce any drift, particularly in
perpendicular (PRP) in-phase, that may otherwise occur. In hot weather, a shell that reduces solar
heating will extend the operating range of the instrument.
Positioning of
Data
GPS positions are used routinely for data from
motorized surveys. Positions can be
assigned to data on the basis of matching time-stamps, or by suitable logging
systems.
A typical logging system consists of a hand-held
computer with two RS-232 ports, along with software that combines serial DUALEM
and GPS input into an integrated file with measurements and positions. DUALEM user manuals contain specifications
for logging systems, and the Appendix contains information about components of
systems that may be commercially available.
Where a measurement is made while the instrument is
moving through an interval between a starting point and an ending point, the
most accurate position for the measurement will lie somewhere between the
points. The exact position depends on
the time that is required to sense the measured quantity and for the instrument
(and logging system) to process the result.
A test of the displacement between the most accurate
position and the recorded position for a measurement uses a conductive object
on the ground. The object must provide
a distinct response to the instrument; an example is a length of wire
connecting two metal stakes that have been driven into moist ground. The instrument is passed at survey speed
over the object twice, in opposite directions, and the apparent positions of
the responses are compared.
Note that the PRP response over a narrow object is
asymmetrical, so to compare PRP positions, the orientation of the instrument
should be kept consistent relative to the positioning coordinate system. For example, if the passes over the object
are from north to south, and then south to north, the transmitter end of the
instrument on both passes should be consistently either north or south of the
receiver end.
Summary
DUALEM instruments are used in motorized surveys to
measure conductivity and detect metal over large areas.
For most surveying, the instrument should be at a
distance from the motor vehicle equivalent to one transmitter-receiver
separation, and at low clearance above the ground. The instrument should be held securely to prevent rotation about
its axis during surveying.
The platform for the instrument should be
non-responsive to the DUALEM transmitter.
The platform should lessen vibration for the instrument, and protect the
instrument from mechanical shock and abrasion.
In sunny conditions, a covering that provides shade may be beneficial.
Generally, GPS is used to position data. Some displacement between the recorded position
and the actual position of measurements may occur, especially at greater
speed. Such displacement can be
measured by surveying in opposite directions over a small target.
