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Environmental and Exploration Geophysics II
Radar Methods – General Overview
tom.h.wilson
[email protected]
Department of Geology and Geography
West Virginia University
Morgantown, WV
The radar band is loosely taken to extend from
approximately 0.1cm to 100cm. The microwave
region is often used for surface imaging from
airborne or satellite platform.
Radar image of the earth’s surface at 5.4cm
or 20 GHz.
Ground penetrating radar (GPR) systems often operate in
the tens of MHz to GHz region of the spectrum.
25MHz = 12m wavelength (40ns)
50MHz = 6m (20ns)
100MHz = 3m (10ns)
1GHz = 0.3m (1ns)
Times in nanoseconds represent the time it takes
light to travel through 1 wavelength in a vacuum.
Visual
wavelength
image
Shuttle Imaging
Radar - SIR A
~ 25cm
Sabins,
1996
Sabins,
1996
Ground
Surveys
GPR mono-static and bi-static transmitter-receiver configurations.
Note similarity to coincident source-receiver and offset source
receiver configurations discussed in the context of seismic methods
Daniels, J., 1989 &
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Spectral and temporal characteristics of
the GPR wavelet.
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As with seismic data, reflection arrival times are 2-way times
and depth equals ½ the two-way time x average velocity.
Velocity in air is approximately equal to the velocity of light in a
vacuum: c.
c = 3 x 108 m/sec
= 9.84 x 108 f/s
or approximately 1 foot per nanosecond. 1 nanosecond is 10-9th
seconds.
Thinking in terms of two-way times, it takes 2ns to travel 1 ft.
In general the velocity of the radar wave is defined as
V
c
r
where c is the velocity of light in a vacuum (or air), and
r is the electric permitivity of the material through
which the radar wave travels.
Examples of r (see Daniels) are
81 for water
6 for unsaturated sand
20 for saturated sand
The presence of water has a significant effect on velocity.
Typical velocities
V
c
r
c ~ 1ft/ns in air
v ~ 1/2 to 1/3rd ft/ns in unsaturated sand
v ~ 1/3rd to 1/5th ft/ns in saturated sand
is proportional to conductivity - materials
of relatively high conductivity have slower
velocity than less conductive materials.
In our discussions of seismic we recognized
absorption as an important process affecting the
ability of the seismic wave to penetrate beneath the
earth’s surface. High attenuation coefficient
produces rapid decay of seismic wave amplitude with
distance traveled (r).
Ar As er
The same process controls the ability of
electromagnetic waves to penetrate beneath the
earth’s surface. The expression controlling attenuation
is a function of several quantities, the most important
of which are conductivity and permitivity.
Attenuation of electromagnetic waves is controlled by the
propagation factor which has real and imaginary parts.
The real part (the attenuation coefficient) illustrates the
influence of permitivity and conductivity on absorption.
2
( 1 ( ) 1)
2
Note in this equation that increases of
translate into increased attenuation. Also
note that increases of angular frequency
(=2f) will increase attenuation.
The display of radar waves shows
considerable similarity to that of seismic data
Diffraction events are commonly produced by heterogeneity
in the electrical properties of subsurface materials
The diffraction response can be used - as you
would have guessed – to determine velocity.
Remember the ray path geometry
for the diffraction event?
*
For coincident source and receiver acquisition
*
d 2 x z
2
d
t
V
2
x2 z 2
x z
2 2
2
2
V t 4x 4z
2
V t 4x 4z
2 2
2
2
2
2 2
2
V t
x
2 1
2
4z
z
2z
a
V
bz
a 2
Slope of Asymtote =
b V
Average Velocity = 1/2 the reciprocal of the slope
Note that the 0.2 m/ns velocities in
the sand dune complex is pretty
high compared to the above.
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Direct “airwave” arrival
Direct arrival
through surface
medium
Reflection
hyperbola
Smith and Jol, 1995
The characteristics of a common midpoint gather from a GPR
data set look very similar to those from a seismic CMP gather.
Thinning layer response and resolution considerations.
Daniels, J., 1989
Horizontal Resolution: The Fresnel Zone
The Fresnel Zone
Radius Rf
l 2
Rf
2 16
Depth =
Velocity =
dominant frequency =
two-way time =
dominant wavelength =
Fresnel zone radius =
Approximation =
An approximation
Rf
V
4
2000
13500
50
0.296296
270
523.9812
519.6152
4t
fc
Topographic variations must also be compensated for.
Daniels, J., 1989
West Pearl Queen Field Area
Surface along the GPR line shown below was very irregular so
that apparent structure in the section below is often the result
of relief across features in the surface sand dune complex.
GPR data is often collected by pulling the GPR unit
across the surface. Subsurface scans are made at
regular intervals, but since the unit is often pulled at
varying speeds across the surface, the records are
adjusted to portray constant spacing between records.
This process s referred to as rubbersheeting.
Daniels, J., 1989
Smith and Jol, 1995, AG
Smith and Jol, 1995, AG
Increased frequency and
bandwidth reduce the dominant
period and duration of the wavelet
Comparison of the 25MHz and 100 MHz records
Smith and Jol, 1995, AG
Ar As er
We also expect to see
decreased depth of penetration
(i.e. increased attenuation) for
higher frequency wavelets and
components of the GPR signal.
Smith and Jol, 1995, AG
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In the acquisition of GPR data one
must worry about overhead reflections.
Daniels, J., 1989
…. and tree branches!
Daniels, J., 1989
GPR unit
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Time slice map from 3D data volume of radar data. This
is a surface of equal travel time. Disruptions in the
reflection pattern are associated with the waste pit.
Green et al., 1999, LE
Bright red areas define the
location of the landfill; the
orange objects represent
gravel bodies.
The brownish-pink lobes are
high reflectivity objects of
unknown origin.
The view at bottom profiles
the underside of the landfill
and gravel bodies.
Green et al., 1999, LE
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