Geophysical Survey Systems, Inc.
SIR® 4000
Manual
MN72-433 Rev F
140
to-depth estimations. If you have never worked with soils before, you should consult the US Department
of Agriculture soils website at
http://soils.usda.gov/
. The site has a number of free and low cost
resources including soil maps of most of the United States and guides to help you understand soils.
Site Accessibility
Simply put, can you feasibly work in the proposed area? Is the site in the middle of a dense thicket of
trees, or is it the outside of a tall building, or a tight elevator shaft? Remember that GPR and geophysics
depends on your ability to see contrasts in the data. The area has to be large enough for you to collect
enough data to be able to make an interpretation. For example, if you need to survey an area in advance of
an 18” utility trench, you want to make sure that you have some coverage over areas outside of that trench
so you can see normal conditions. Always give yourself some elbow room.
C.2: Targets
The type of targets you are trying to find will govern your choice of antennas, setup parameters, or even
the feasibility of radar for the application. There are two main criteria to consider:
•
Target Size
•
Target Composition
Target Size
All things being equal, antenna choice determines how deeply you are able to penetrate and the minimum
size of the targets that you are able to see. Lower frequency antennas see deep, but the minimum target
size that they can see is larger. Rather than focus on what each antenna can see, the table below lists the
appropriate antenna by application and depth range.
Frequency
Sample Applications
Typical Max Depth
Feet (meters)
Typical
Range (ns)
2.6 GHz
Structural Concrete, Roadways,
Bridge Decks
1 (0.3)
10
1.6 GHz
Structural Concrete, Roadways,
Bridge Decks
1.5 (0.5)
10-15
900 MHz
Concrete, Shallow Soils,
Archaeology
3 (1)
10-20
400 MHz
Shallow Geology, Utility,
Environmental, Archaeology
9 (3)
20-100
200 MHz
Geology, Environmental
25 (8)
70-300
100 MHz
Geology, Environmental
60 (20)
300-500
Antennas by Application
Radar is also not a continuous measurement along a survey line. The system takes readings (scans)at a set
spacing. If your scan spacing is too wide, you risk not hitting your target with enough scans to draw a
recognizable hyperbola, or worse, missing the target altogether. Generally, you need a minimum of 10
scans to draw a recognizable hyperbola.
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