Geophysical Survey Systems, Inc.
SIR® 4000
Manual
MN72-433 Rev F
18
Dielectric
Dielectric constant represents the ratio of permittivity of a substance to
permittivity in a vacuum. We use this term to describe how quickly GPR
moves through materials. Materials that have a lower dielectric value allow
GPR to propagate more quickly. See Appendix C for a chart of dielectric
values of common materials and a deeper discussion of dielectrics. Values
range between 1 (air) and 81 (water).
•
If you know the dielectric value of the material through which you
are surveying, you can enter it here and get an in-field time to depth
calculation.
•
Higher dielectric values mean slower travel time and shallower
penetration. For example, in air, which has a dielectric constant of
1, radar energy will travel at 12 inches per ns. Since the time range
is in two-way travel time, 1 ns on the vertical scale translates to
6 inches if the DIEL is set to 1. The distance traveled per ns is
reduced by the square root of the dielectric constant. The dielectric
constant of water is 81, so that water slows down the radar wave by
a factor o
f 9 (√81=9). The range in water is thus 6/9 inches per ns.
•
The prime determinant of the dielectric constant of soils and
sediments is water content, with wetter areas having a higher value. Generally speaking, water
raises a material’s dielectric constant, and surveys should be performed on dry material whenever
possible.
Soil Type
Several different Soil Type options are included on the SIR 4000. Selecting a Soil Type will
automatically update the Dielectric with a value which is representative of that soil.
Custom:
Automatically appears if the Dielectric Constant is manually changed, or when a hyperbola
fitting or ground truth are performed during data collection. For more information on hyperbola fitting
and ground truth see section 2.4 The Command Bar – Output in Collect Mode.
Depth Range
Depth Range is the vertical scale displayed in whichever units are selected under the Output -> Vertical
Units menu.
•
Depth Range can be set from 1-10000 cm (0.4-3900 in).
•
Modifying Time Range will automatically update the Depth Range.
Time Range
Time Range is the vertical scale in nanoseconds (ns) within which the SIR 4000 will record reflections. It
is proportional to depth viewed because a higher Time Range will allow the energy to penetrate deeper
and return deeper reflections.
•
It is important to remember that the Time Range is two-way travel time, so that a range of 50 ns
means that the deepest possible reflector is at 25 ns.
•
A very long range may require a greater number of samples to create enough data dots for the scan
curve. Refer to the section above to determine the appropriate number of samples/scan.
•
Time Range can be set from 1-20000 ns.
•
Please see Appendix D for a list of common ranges for individual antennas.
•
Modifying Depth Range will automatically update the Time Range.
Material
Dielectric
Constant
Snow/Ice
3.0
Dry Sand
4.0
Pavement
6.0
Rock
8.0
Dry Soil
9.0
Ave. Soil
14.0
Wet Soil
20.0
Wet Sand
25.0
Water
81
Custom
1-81
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