GPR GPR Applications & Site Assessment 2 — Questions and Answers
Question 1: In a GPR survey for archaeological site assessment, which antenna frequency provides the best balance between resolution and depth penetration for locating buried features at 1–3 meter depths?
- 2.6 GHz
- 400–500 MHz (Correct answer)
- 50 MHz
- 10 MHz
Correct answer: 400–500 MHz
400–500 MHz antennas offer sufficient resolution to detect archaeological features (walls, pits, artifacts) while achieving the 1–3 m penetration depth typical of archaeological surveys.
Question 2: A GPR survey over a suspected underground storage tank (UST) would most likely show:
- A flat reflector at the tank depth with a polarity reversal if the tank is air-filled (Correct answer)
- A series of vertical reflection events
- No detectable signal due to metallic shielding
- Only horizontal banding from antenna ringing
Correct answer: A flat reflector at the tank depth with a polarity reversal if the tank is air-filled
An empty UST produces a strong flat reflector with polarity reversal at the metal-air interface, while a fluid-filled tank shows a strong reflector without polarity reversal.
Question 3: When assessing pavement layer thickness with GPR, the operator uses the EM velocity of each layer to convert TWTT to thickness. Which material has the highest EM velocity (lowest dielectric constant) of those listed?
- Saturated sand
- Dry concrete (Correct answer)
- Water
- Wet clay
Correct answer: Dry concrete
Dry concrete has a relatively low dielectric constant (~4–6), giving it a higher EM velocity compared to water-saturated or clay materials with much higher dielectric constants.
Question 4: A GPR site assessment for a proposed bored pile installation should include scanning to identify:
- Existing underground utilities, voids, and obstructions in the bore path (Correct answer)
- Surface gradient and drainage patterns only
- Above-ground structural elements
- Traffic patterns on adjacent roads
Correct answer: Existing underground utilities, voids, and obstructions in the bore path
Pre-bore GPR scanning locates utilities, voids, and obstructions that could be damaged or cause equipment problems during drilling operations.
Question 5: In a GPR survey for rebar mapping in a bridge deck, the primary deliverable to the client typically includes:
- A raw radargram without interpretation
- A scaled plan view showing rebar layout, depth, and any anomalies indicating corrosion or delamination (Correct answer)
- A list of antenna frequencies used during the survey
- A calculation of concrete compressive strength
Correct answer: A scaled plan view showing rebar layout, depth, and any anomalies indicating corrosion or delamination
Bridge deck GPR reports provide a mapped plan view with rebar depth, spacing, and anomaly locations (low-amplitude zones indicating delamination or corrosion) for maintenance planning.
Question 6: Which GPR application requires the operator to follow OSHA 29 CFR 1926.651 (excavation safety) guidelines before commencing fieldwork?
- Scanning a concrete wall for embedded conduit
- Any GPR survey preceding subsurface excavation (Correct answer)
- Pavement layer thickness surveys on closed roads
- Bridge deck delamination mapping
Correct answer: Any GPR survey preceding subsurface excavation
OSHA 29 CFR 1926.651 requires utility locating and safe digging practices before any excavation; GPR surveys conducted to support excavation must comply with these requirements.
GPR GPR Applications & Site Assessment 2
In a GPR survey for archaeological site assessment, which antenna frequency provides the best balance between resolution and depth penetration for locating buried features at 1–3 meter depths?