Basic Ground Penetrating Radar Theory

1. Basic Ground Penetrating Radar Theory

2. GPR LIMITATIONS • Penetration depth and ability to resolve targets at depth is strongly dependent upon the local soil properties. Highly conductive soils can render the GPR method ineffective. • There must be a sufficient electrical contrast between the target and the host materials • Interpretation of GPR data can be subjective. The experience of the interpreter is very important.

3. Penetration depths Antenna (MHz) In soil (m) In rock (m) 25 25 40 50 20 30 100 12 20 200 8 15 500 3.5 5 1000 1.5 3 Average penetration depths of radar signals in high resistivity geological environment absent of low resistive layers.

4. 0 1 2 D E P T H 3 4 5 6 Deep utilities must have larger diameters than shallow utilities in order to be detected with GPR Deep utilities must have larger diameters than shallow utilities in order to be detected with GPR

5. The GPR technique • GPR is an electromagnetic method that detects interfaces between subsurface materials with differing dielectric constants. Your Easy Locator GPR system basically consists of: • An antenna, which houses a transmitter and receiver. • A monitor, which processes the received signal and produces a graphic display of the data. The transmitter radiates repetitive short-duration electromagnetic signals into the earth from the antenna moving across the ground surface. Electromagnetic waves are reflected back to the receiver by interfaces between materials with differing dielectric constants. GPR wave propagation from transmitter (Tx) and reflection to the receiver (Rx).

6. How GPR works • GPR is, in principal, similar to sonar • equipment (fish finders) found in boats • The transmitter emits a “train” of • electromagnetic impulses which • propagate through the media • Reflection (i.e. scattering) occurs where • the electrical properties of subsurface • materials change • The receiver picks up the “back-scattered” • signal and displays it on a monitor

7. ? Depth [m] Time [ns] Length [m] GPR signatures

8. Data Examples & Interpretation

9. GPR APPLICATIONS • CIVIL/STRUCTURAL ENGINEERING • Utilities (pipes, cables), rebar and voids. Pre-studies for Horizontal Directional Drilling (HDD) • Transportation: Roadways and railroad tracks, ice thickness, • bridge deck and bridge fundation studies. • ENVIRONMENTAL • Hazardous waste mapping, underground storage tanks (UST), • Sedimentology studies, Bathymetry. • GEOTECHNICAL • Stratigraphic mapping, cavities and sinkholes, groundwater, mining hazards, fracture detection, Earth dam studies, foundation studies, tunnel investigations. • MILITARY • Ordinance detection, runway integrity, clearing of trenching routes • ARCHAEOLOGY • site mapping, grave detection, artifacts

10. The most important markets for radar… • Utility detection • Pipe and culvert inspections • Concrete and NDT • Road and bridge deck investigations • Geological mapping • Ice, snow and glacier • Borehole radar

11. The GPR performance GPR is primarily affected by the conductivity and dielectric permittivity of the mediums. GPR works best in resistive, sandy or gravely soil types. Difficult, conductive types are typically composed of silts and clays or contains salt water. Depth of investigation is limited by signal attenuation of conductive soil but also dependent on the antenna selected. The difference in radargram between good and bad soil conditions

12. Electrical Conduits Shallow antenna Unknown Comparison between the SHALLOW and MID antenna. Water Main Force Main Electrical Conduits Unknown Mid antenna Water Main Force Main

13. Interpretation: • Metallic water pipes shows sharper • Sewer line is large enough to show both top/bottom reflection • Note radiuses of the signatures • Trench shows

14. Thank you!