Radar Waves

Wave Terminology - amplitude, frequency, and wavelength; velocity equal to frequency times wavelength. Frequencey of radar waves is 10 to 1,000 MHz. Wavelengths are in centimeters to meters.

Radar Wave Velocity - is proportional to the speed of light divided by the square root of dielectric permittivity. Velocities range from 0.3 m/ns for air, 0.03 m/ns for water, and 0.03 to 0.07 m/ns for most sediments and rocks. Velocities are in meters per NANOsecond.

Reflection and Refraction, and Snell’s Law - radar wave energy is reflected or bent across an interface between materials of different conductivities.

Diffraction - new waves are generated in all directions when a wave hits a sudden change in an interface (e.g., fault) that is the same size as the wavelength

Spherical Spreading - energy per area decreases as the wave moves outward in all directions

Absorption - some wave energy is converted into heat energy as the wave propogates through the media. Absorption is frequency/wavelength depended. Short wavelength/high frequency waves lose more energy. Conductive materials (clays, saturated soils) absorb more electromagnetic energy than resistive materials.

Energy Partitioning - energy of incident wave is divided among the reflected and refracted waves

Penetration depth - is dependent on radar frequency and conductivity of the subsurface. Low frequency waves in resistive material (sand) can penetrate as much as 30 meters. High frequency waves in a conductive material (clay) may penetrate only one meter.

Determining Velocity - x2-t2 Method - A plot of x^2 versus t^2 plots on a straight line. The slope of the line is equal to the inverse of velocity squared. Rarely used because the distance between source and receiver is fixed.

Velocity is often inferred from known depths to interfaces from cores or lithology. Velocity is the speed of light divided by the square root of dielectric permittivity. Dielectric properties are primarily determined by water content, dissolved minerals, and expansive clay and heavy-mineral content.

Determining Depth to Interface - Depth to the interface can be determined from the velocity times the intercept time divided by two. Multiple interfaces can be determined by summing each time interval and velocity. Ray paths are nearly vertical so travel time is essentially the intercept time.

Multiples - occur when electromagnetic energy bounces off an interface and returns to the surface more than once. When multiples have enough amplitude to be seen they can be distinguished from primary arrivals because they appear at approximately twice the time as a primary arrival. Multiples occur between air-water and water-sediment interfaces.

Diffractions - Diffractions also appear as noise on GPR records. They have a hyperbolic shape and are symmetrical about the point of diffraction. Reflections from trees or power line noise also appear as point sources.

Resolution - both vertical and horizontal resolution are dependent on the wavelength (centimeters) of the radar energy. Very high resolution data

Common Offset - The offset between source and receive is the same for each "shot". Offset is zero if only one antenna.

Noise - electromagnetic radiation is emitted in all directions (not just downward). Spurious "reflections" can occur from overlying trees, power lines or the back of the vehicle towing the antenna. There also may be a direct wave if 2 antennas are used.

Static Corrections - raw data should be corrected for variations in thickness or velocity of the uppermost layer.

No normal Move-Out - common offset gives a true representation of subsurface geometry without additional processing.