Geoscience Reference
In-Depth Information
(a) Ground conductivity
(b) Shallow-intermediate
depth
(c) Shallow-intermediate depth
(person-pulled, or AT V towed)
(d) Shallow-intermediate
depth continuous mode
person carried
(e) Intermediate depth
fixed station
(f ) To wed array with GPS
fIGURe 6.8
Examples of commercially available time- and frequency-domain electromagnetic (EM)
systems: (a) ground conductivity system (www.geonics.com/html/products.html), (b) frequency-domain
intermediate-depth EM system (www.geophex.com/GEM-2/GEM-220home.htm), (c) shallow-intermediate-
depth time-domain system (www.geonics.com/html/metaldetectors.html), (d) shallow-intermediate-depth
person-portable system (www.geophex.com/GEM203/GEM-320home.htm), (e) intermediate-depth fixed
station (www.geonics.com/html/conductivitymeters.html), and (f) towed array of five frequency-domain
sensors (www.geophex.com/GEM-2/GEM-220home.htm).
line from the observer's viewpoint, and the EM fields measured by the observer are approximately
the same as those that would be measured from a long line. In most cases, the manufacturer sup-
plies an instrument with the transmitter and receiver located at a fixed size/spacing ratio, and the
geophysicist does not have to be concerned with interpretation problems associated with not being
in the far-field.
For purposes of agricultural applications, the most practical systems are handheld, or cart-
mounted loop-loop systems. Examples of a few of the systems currently available are shown in
Figure 6.8. Note that some of the systems operate in the time domain, and other systems utilize
multiple frequencies and are called frequency domain systems.
6.4 fIeld opeRAtIon ConSIdeRAtIonS
Field procedures for EM measurements follow the basic design that must be employed for most
geophysical methods. The process of making a series of geophysical measurements in the field is
commonly called a survey. The field procedure for a specific objective is determined by answering
a number of questions, including the following:
•
What are the objectives of the survey?
•
What is the nature of the subsurface environment?
•
What are the electrical properties of the materials at the site?
•
How large is the survey area?
•
What is the nature of the site access?
•
How will cultural features affect the measurements?
