GPR GPR Applications & Site Assessment 1 — Questions and Answers
Question 1: When scanning a concrete slab to locate post-tension cables, the GPR operator should primarily be concerned with:
- The dielectric constant of the formwork
- Accurately mapping cable positions to avoid saw-cutting or coring through them (Correct answer)
- Measuring the exact diameter of the cables
- Determining the concrete mix design
Correct answer: Accurately mapping cable positions to avoid saw-cutting or coring through them
Post-tension cables are under high tension; cutting one is extremely dangerous, so GPR scanning maps their locations precisely before any cutting or coring work.
Question 2: What GPR antenna frequency range is most appropriate for scanning reinforced concrete slabs up to 18 inches thick?
- 10–50 MHz
- 100–250 MHz
- 900 MHz–2.6 GHz (Correct answer)
- 5–10 GHz
Correct answer: 900 MHz–2.6 GHz
High-frequency antennas (900 MHz to 2.6 GHz) provide the resolution needed to distinguish rebar, conduit, and cables in concrete at typical slab depths.
Question 3: During a utility detection survey, a GPR operator identifies a hyperbolic reflection that is accompanied by strong ringing and a polarity consistent with a high dielectric contrast. This most likely indicates:
- A PVC pipe filled with air
- A metallic pipe or conduit (Correct answer)
- A clay-filled void
- A tree root
Correct answer: A metallic pipe or conduit
Metallic utilities produce high-amplitude hyperbolae with strong ringing and clear polarity signatures because of their near-total reflection of GPR energy.
Question 4: The American Concrete Institute (ACI) and ASNT guidelines recommend that GPR concrete scanning be performed by:
- Any licensed engineer without special GPR training
- Personnel certified to an appropriate GPR qualification level (Correct answer)
- The structural engineer of record only
- A building inspector
Correct answer: Personnel certified to an appropriate GPR qualification level
ACI and ASNT guidelines require GPR concrete scanning to be performed by personnel who hold appropriate GPR certification or qualification to ensure reliable results.
Question 5: Which condition would most significantly reduce GPR effectiveness when scanning for utilities in urban soil?
- Dry sandy soil
- High clay content with elevated moisture (Correct answer)
- Limestone bedrock
- Asphalt surface layer
Correct answer: High clay content with elevated moisture
Wet clay has a very high dielectric constant and high electrical conductivity, which rapidly attenuates GPR signals, severely limiting depth penetration.
Question 6: When performing a GPR scan over an area suspected of containing a void beneath a concrete pavement, the void would most likely appear as:
- A strong, flat reflector with a polarity reversal at the void-air interface (Correct answer)
- A hyperbolic diffraction with ringing
- A continuous low-amplitude band
- A gradual decrease in signal brightness
Correct answer: A strong, flat reflector with a polarity reversal at the void-air interface
A void beneath concrete creates a strong flat reflection at the concrete-air interface with polarity reversal (concrete to air = lower dielectric), indicating an air-filled cavity.
GPR GPR Applications & Site Assessment 1
When scanning a concrete slab to locate post-tension cables, the GPR operator should primarily be concerned with: