Geoscience Reference
In-Depth Information
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How large is the survey area?
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What is the nature of the site access?
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How will cultural features affect the measurements?
The fundamental questions that must be asked prior to any survey are as follows: (1) What is the
maximum depth of penetration?, (2) What are the line and trace spacing (horizontal resolution)?,
and (3) What is the vertical resolution needed to achieve the goals of a survey?
7.4.1.1
Station and trace Spacing
The survey objectives dictate the depth of investigation, the lateral resolution that must be achieved,
and the orientation of the antennas. However, the depth of penetration and resolution are also deter-
mined by the electrical properties of the material that contains the objects that are the targets of the
survey. These combined factors help to establish the operating frequencies of the antenna that must
be used, and the optimum spacing between measurements on the surface. A good rule of thumb for
establishing adaquate trace measurement spacing is that the spacing should be less than one quarter
of the size of the smallest object that is to be detected by the survey. This value is twice the Nyquist
sampling frequency and should be adequate for most situations. Contrary to popular statements,
it is impossible to “oversample”: oversampling is a myth propagated by lazy people, which is the
spatial analog to the popular myths of “overstudying” for exams, or being “overeducated.” Also, the
rule that “less is best” when it comes to determining the trace measurement spacing is erroneous.
Economics and other practical considerations (e.g., spatial survey accuracy) are the only limitations
to using a very small measurement spacing between traces and lines.
7.4.1.2
depth of penetration
Surveys should be designed so they record a two-way travel time that is twice the amount of time
required to see the deepest object anticipated in the survey area. There is nothing more frustrating
than to run a survey and discover that the objects of interest were below (beyond, in time) the data
that were recorded. The depth of penetration of a radar wave depends upon the electrical proper-
ties and the center-band frequency of the antenna. The theoretical depth of penetration for a given
antenna frequency can be computed from the equation for the skin depth, if the values of conductiv-
ity and permittivity are known. However, from a practical point of view, the only way to know the
depth of penetration for a particular survey site is to make an initial guess based on past experience
and to determine the actual depth of penetration by testing at the site. Dry homogenous rocks and
soils (e.g., beach sand, nonvuggy limestone, granite, etc.), permafrost regions, lignite, and peat bogs,
can yield penetration depths up to 20 m, and sometimes even greater depths. In contrast, the normal
penetration depth for most soils is on the order of 1 to 3 m, with a low range of a few centimeters in
a soil that is predominantly montmorillonite clay, up to several tens of meters for a clean sand.
7.4.1.3
Antenna frequency
There are no fixed rules for determining the optimum antenna frequency range that should be used
for a given survey, but the choice of antenna frequency should be based on the survey objectives
and the electrical properties at the site. Because the depth of penetration decreases as the center-
band frequency increases, the choice of antenna frequency is often determined by the depth of
penetration that is needed. Lower frequencies generally improve the depth of penetration for most
soil and rock types. However, a lower frequency has an adverse effect on the resolution of the radar
wave for shallow investigations. A high-frequency antenna (500 MHz) would provide a detailed
image of very shallow features, but it would not penetrate very deeply. A low-frequency antenna
(50 MHz) would significantly improve the depth of penetration of the radar wave, but the resolution
