Lecture 4a - Ground Penetrating Radar (GPR)

GPR Basic Principles

• GPR transmits short pulses of electromagnetic radiation (10-1000 MHz) into the ground.
• Antenna transmits pulses that reflect off subsurface features and are detected by a receiving antenna
• Reflections are caused by contrasts in electrical properties, similar to how reflection seismics work with seismic properties.

GPR Data Acquisition and Processing

• GPR data processing and analysis are similar to reflection seismics.
• Radograms (reflection profile plots) display reflected amplitude against two-way travel time.
• Time is measured in nanoseconds (1 ns = $10^{-9}$ seconds) due to the high speed of radar waves.
• Data processing converts travel times to depths using radar wave velocity:
  • $Distance = Speed \, x \, Time$

GPR Profile Interpretation

• GPR profiles show wiggles representing single shot measurements.
• Time increases down the plot, with durations on the order of tens of nanoseconds.
• Hyperbolas are similar to those seen in seismic sections.
• Strong, coherent horizontal signal near zero travel time.
• Depth estimates are made based on radar wave velocity (e.g., 0.12 meters/nanosecond).

GPR Characteristics and Applications

• GPR provides high-resolution data but with limited depth penetration.
• Used for near-surface investigations requiring high detail.
• Applications:
  • Assessing hydrological properties.
  • Identifying geological structures.
  • Determining depths to ice.
  • Evaluating infrastructure (walls, road surfaces).
  • Detecting unexploded ordnance (landmines).

GPR Instrumentation

• Instrumentation includes a transmitter and a receiver.
• The transmitter releases radar pulses into the ground.
• The receiver detects the reflected signals and sends the information to a control and display unit.
• Optical fibers are used to connect the receiver to the control unit to reduce electromagnetic interference.

Antenna Styles

Unshielded Antennae

• Transmit and receive signals from a wide range of directions.
  • More susceptible to noise (e.g., signals from passing cars).
  • Flexible systems because the antennae can be moved apart, allowing different survey styles.
  • Lighter and easier to maneuver.

Shielded Antennae

• Focus the transmitted pulse into the ground.
• Reduce likelihood of interference from external sources.
• Improve signal quality.
• Fixed geometry (transmitter and receiver cannot be separated).
• Heavier due to shielding.

Data Visualization

• Signal amplitude exceeding a threshold is infilled with black on the wiggly trace or scanline.
• Colored regions indicate large amplitude reflections.
• Time increases downwards on the plot.

Waveform Analysis

• First signals represent the transmitted source pulse (artifacts).
• Ground-coupled wave represents the pulse traveling directly from the transmitter to the receiver along the ground surface and is not useful for reflection surveys.
• Useful signals are found at longer travel times corresponding to the first primary reflector.
• GPR pulse is a series of rapidly decreasing amplitude oscillations (triplets).

Profile Plots

• Time-position profile plots visualize GPR data.
• Position is on the horizontal axis, and time is on the vertical axis.
• Plots indicate where large magnitude reflections have been received.
• The direct wave between the receiver and the antenna appears as a consistent horizontal return (artifact).
• Below the direct wave, laterally discontinuous reflectors and a more continuous reflector can be observed.

Radar Wave Velocity and Permittivity

• Radar waves travel at the speed of light in a vacuum but are slowed when traveling through matter.
• Electromagnetic wave speed is calculated as:
  • $v = \frac{c}{\sqrt{\epsilon_r}}$,
    • Where:
      • $v$ = electromagnetic wave speed
      • $c$ = speed of light
      • $\epsilon_r$ = relative permittivity

Permittivity

• Permittivity is a material's ability to electrically polarize and transmit an electric field.
• Relative permittivity is the ratio between a material's permittivity and the permittivity of free space ($\epsilon_0$).
• Relative permittivity is unitless and increases from one.
• When relative permittivity is greater than one, the wave speed in matter is lower than the speed of light in a vacuum.

Electrical Properties of Geomaterials

• Water has a high relative permittivity (approximately 80).
• Sedimentary rocks have relative permittivity ranges going up to the into the few tens.
• Dry materials (sand, granite, salt) have faster wave velocities.
• Cold ice also has a fast wave velocity.

Reflection Coefficient

• Contrast in relative permittivity generates reflection interfaces.
• Reflection coefficient (R) is calculated as:
  • $R = \frac{k<em>2 - k</em>1}{k<em>2 + k</em>1}$,
    • Where:
      • $k_1$ = relative permittivity of the upper layer.
      • $k_2$ = relative permittivity of the lower layer.
• Reflections represent interfaces between regions of different relative permittivity, often due to variations in water content.

Summary

• GPR is a high-frequency electromagnetic imaging method for near-surface investigations (depths of meters to a few tens of meters).
• Data is measured in terms of signal travel times to reflectors generated by contrasts in relative permittivity.
• Converting travel times to depths requires estimating wave velocity, which is a function of relative permittivity.
• Water has a high relative permittivity, so reflections often indicate changes in water content.