Environmental and Exploration Geophysics II

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Environmental and Exploration Geophysics II
Radar Methods General Overview
tom.h.wilson tom.wilson_at_mail.wvu.edu
Department of Geology and Geography West Virginia
University Morgantown, WV
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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.
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Radar image of the earths surface at 5.4cm or 20
GHz.
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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.
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Visual wavelength image
Shuttle Imaging Radar - SIR A ? 25cm
Sabins, 1996
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Sabins, 1996
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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 Sensors and Software
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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.
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In general the velocity of the radar wave is
defined as
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.
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Typical velocities
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.
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In our discussions of seismic we recognized
absorption as an important process affecting the
ability of the seismic wave to penetrate beneath
the earths surface. High attenuation coefficient
? produces rapid decay of seismic wave amplitude
with distance traveled (r).
The same process controls the ability of
electromagnetic waves to penetrate beneath the
earths surface. The expression controlling
attenuation is a function of several quantities,
the most important of which are conductivity and
permitivity.
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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.
Note in this equation that increases of ?
translate into increased attenuation. Also note
that increases of angular frequency (?2?f) will
increase attenuation.
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The display of radar waves shows considerable
similarity to that of seismic data
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Diffraction events are commonly produced by
heterogeneity in the electrical properties of
subsurface materials
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The diffraction response can be used - as you
would have guessed to determine velocity.
How would you do that?
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Remember the ray path geometry for the
diffraction event?
For coincident source and receiver acquisition
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Average Velocity 1/2 the reciprocal of the slope
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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 air-wave 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.
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Thinning layer response and resolution
considerations.
Daniels, J., 1989
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Horizontal Resolution The Fresnel Zone
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The Fresnel Zone Radius Rf
An approximation
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Topographic variations must also be compensated
for.
Daniels, J., 1989
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West Pearl Queen Field Area
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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.
Dune surface topography
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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
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Smith and Jol, 1995, AG
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Smith and Jol, 1995, AG
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Increased frequency and bandwidth reduce the
dominant period and duration of the wavelet
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Comparison of the 25MHz and 100 MHz records
Smith and Jol, 1995, AG
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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
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. and tree branches!
Daniels, J., 1989
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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
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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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