GSSI StructureScan Mini XT, Руководство пользователя

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Geophysical Survey Systems, Inc. StructureScan™ Mini XT
Manual
MN 70-813 Rev E 45
Appendix A: The Dielectric Constant
What is it?
The dielectric constant is number used in GPR to describe how fast the GPR energy moves
through a non-metallic material. GPR waves move so fast (almost the speed of light) that it is not
practical in most cases to use familiar units like miles or kilometers per hour, so we use the dielectric as
an easier method. The range of values goes from 1 (air, the fastest) to 81 (water, the slowest). All of the
concretes, asphalts, rocks, soils, and sediments fall between those extremes.
Why is it important?
GPR doesn’t record depth. It records time of flight. That is the elapsed time
between the antenna sending out the RADAR pulse and then receiving the reflection. Since we know for
how long the pulse traveled, if we can calculate the speed at which it traveled, we can calculate how far
(deep) it went.
How do I know the Dielectric Constant?
You can start off by estimating the number based on the age
of the slab. That is what the chart below is for. The Mini XT also has a function called Auto Depth in the
ScanMax app which will automatically calculate the value for you. Please see Section 5 for more
information on calculating the dielectric.
Concrete Type Dielectric Constant Concrete Age Comments
Very Dry 4.59 5 years + The driest
slabs
Mod. Dry 6.25 At least 1 year Most slab on
grade will be
Mod. Dry
Damp
7.44 3-12 months
Moist 9 1-3 months Slab under
water all of the
time
(swimming
pool, water
tank)
Wet
14.06 Less than 1 month
Please note that these values are approximations.