Introduction and History Electromagnetic Wave Propagation
Velocity Wavelength Attenuation Dispersion
Rocks, Soils and Fluids: Electrical Properties Magnetic Properties
Environmental Influences Heterogeneity, Anisotropy and Scale Radar Equation
Antenna Polarization
![[Under Construction]](images/undercon.gif){kind=small}
Electromagnetic waves are polarized. This
means the electric field is a vector pointed in a particular direction. Generally,
the magnetic field is perpendicular to the electric field, and the direction of
propagation is perpendicular to both electric and magnetic field directions. This is
a different context than used previously for magnetic or dielectric polarization
properties in which polarization was describing energy storage properties of materials.
Most commercial ground penetrating radar systems use linearly polarized antennas.
The most common antenna arrangement is with the electric fields of the transmitter
and receiver antennas aligned in parallel with each other, parallel with the earth, and
towed in a traverse direction perpendicular to the electric field direction. This
results in a wave propagating perpendicular to the surface of the earth, into the earth.
If such an arrangement is pulled across a buried metallic pipe (or wire or rebar)
with the electric fields aligned parallel to the length of the pipe, the pipe appears in
the ground penetrating radar data as an excellent reflector, with a hyperbolic shape (the
shape is the result of the antenna pattern and geometry of traverse motion). If the
antennas are rotated 90 degrees, so they cross the pipe with the electric field direction
at right angles to the long axis of the pipe, the pipe disappears (it's still there, but
very tough to see). This latter arrangement is a good way to see past rebar in
concrete. The electric field parallel alignment is also an excellent
polarization to measure subsurface layering. If the antennas are reconfigured with
one antenna electric field perpendicular to the other (called cross-polarized), they
become insensitive to layering and more sensitive to scatterers like pipes, independent of
alignment of the pipe compared to the antennas. In general, three sets of
measurements are required with antennas oriented (relative to the electric fields)
parallel to the surface of the earth and: 1) parallel to each other and perpendicular to
traverse direction, 2) parallel to each other and parallel to traverse direction, and 3)
perpendicular to each other (cross-polarized). The change in polarization that
occurs as a propagating wave encounters a contrast in electromagnetic properties (and is
scattered) is described by the Stokes-Mueller Matrices.
There are also other kinds of antennas and polarization, such as
left and right hand circularly polarized logarithmic spiral antennas... (more)
(references)
(illustrations)
Fresnel Reflection Snell Angle Stokes-Mueller Matrices Poincare Sphere
Antennas Coupling Near / Far Fields Waveguides Multipathing Resonance
Survey Design Contrast Geometry Resolution Depth of Investigation Orientation
Data Acquisition Data Processing Modeling Interpretation Uncertainty
Applications: Noninvasive Surface Borehole Airborne Satellite and Space