User's Manual
GPR4 Pavement Scanner
–
User Manual
D
-
000002
-
000022
Earth Science Systems, LLC
February
2
9
,
2019
Copyright © 2019 ESS, LLC. All rights reserved.
Page 18 of 27
5 Theory
5.1 Theory of Operation
The pavement scanner uses ground penetrating radar (GPR) and infrared (IR) sensing to
characterize the surface beneath the sensing unit [1]. The GPR sends a low-energy impulse of
electromagnetic (EM) energy towards the surface. Some of this energy penetrates into the
subsurface and some is reflected back towards the sensor unit. The amount of reflected energy
depends on the dielectric constant of the pavement, which in turn depends on the level of
compaction. The amplitude of reflected EM wave is given by
a
a
i
r
A
A
R
ε
ε
+
−
==
1
1
where R is the reflection coefficient, A
r
is the reflected wave amplitude, A
i
is the incident wave
amplitude, and ε
a
is the dielectric constant of the asphalt. The Pavement Scanner measures
both the incident and the reflected wave amplitudes, which allows the dielectric constant of the
asphalt to be calculated from the equation above. The asphalt dielectric constant ε
a
is related to
the porosity (or air fraction) of the pavement according to the CRIM equation (complex refractive
index method).
airairbindbindaggagga
εφεφεφε
++=
Here, ε
agg
, ε
bind
, and ε
air
are the dielectric constants of the aggregate, binder, and air
respectively, and ø
agg
, ø
bind
, and ø
air
are their volume fractions (note that ø
air
is also referred to
as porosity).
While these relationships provide an understanding of the processes involved, the
dielectric constant and volume fractions of the binder and the aggregate are not usually known.
In practice, the relationship between the reflected wave amplitude and porosity is determined
empirically using a polynomial curve fit and measurements taken from cores. The procedure
starts by taking measurements with the Pavement Scanner and then coring those locations to
determine their actual level of compaction. These data are used to calibrate the scanner
response for a given type and batch of paving material. The relationship used is
2
210
RaRaaC ++=
where C is compaction, density, or porosity, and a
0
, a
1
, and a
2
are calibration coefficients
determined by fitting this relationship to the calibration data. Because compaction, density, and
porosity are closely related parameters, this relationship can be used for all of them (although
the calibration coefficients will be different for each parameter. In practice, users will select and
use one of these parameters.
The IR sensor measures the amount of IR energy being radiated from the asphalt
surface. The amount of IR energy emitted by the surface