Geophysical Survey Systems, Inc.
SIR® 4000
Manual
MN72-433 Rev F
141
The rule of thumb is to have 10 scans divided by the depth of the shallowest object. So if you want to
image something 10 feet deep, 10 scans / 10 feet = 1 scan/foot. For 5 feet, 10 scans / 5 feet = 2 scans/foot.
For locating structural features in concrete
with the 1.6 GHz antenna, we usually
recommend 60-90/ft. For utility and tank
location, we usually recommend 6-24/ft.
While this is more than the minimum rule of
thumb, GSSI has found these densities to
work well in the widest variety of situations.
The image above shows hyperbolas from
objects of similar size. The hyperbolas vary in
size because, due to the spreading of the radar
signal, a deeper target shows reflections in
more scans than a shallow one.
Lower frequency antennas, like the 200 MHz and 400 MHz, will
sometimes not image targets close to the surface very well. While not
strictly a ‘dead zone’ you should be aware that it may be difficult (but
not impossible!) to see targets in this area. As a general rule of thumb,
this zone is equal to the spacing between the transmitter and receiver
dipoles, but this can vary with soil composition. See the chart at right for
a general idea. If your application requires you to see deep and shallow,
consider surveying the area with two different antennas.
Target Composition
Your ability to see a target depends on the contrast between the dielectric values of the target’s material
and the material that the radar energy was traveling through just before it hit the target. The greater the
contrast between the dielectric values, the more visible the target is. For applications which involve
finding metal targets like rebar, pipes, and drums, this is not a great issue because there will always be a
great contrast. The dielectric of metals is so high that the actual number is meaningless. You will always
have a visible contrast where metals are concerned.
Composition can affect your ability to see things in different ways. For example, the contact between a
dry sand (3-6) and a water table (water being 81) will be easy to image, while the contact between
sandstone (6) and limestone (7-8) will be much more difficult. Also remember that it is the electrical
property of the material that most governs dielectric. Even though concrete and grade are qualitatively
very different, they are made of similar materials and react to radar energy similarly. It is usually
extremely difficult to tell the top of grade from the bottom of the slab. You should practice trying to
image different materials so that you can build up a body of experience. See Appendix D for a chart of the
dielectric constants of different materials.
Frequency
“Hazy” Zone
Inches (cm)
2.6 GHz
0.5 (1)
1.6 GHz
1 (2.5)
900 MHz
4 (10)
400 MHz
6 (15.25)
200 MHz
12 (30.5)
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