GPR GPR Applications & Site Assessment 3 — Questions and Answers
Question 1: When a GPR operator uses a 3D grid survey pattern (parallel lines in X and Y directions), the primary advantage over a single-line survey is:
- Greater depth penetration
- A volumetric data set that allows C-scan (plan view) slices at any depth (Correct answer)
- Faster data collection
- The ability to use lower frequency antennas
Correct answer: A volumetric data set that allows C-scan (plan view) slices at any depth
3D grid surveys produce a volumetric data set from which horizontal time slices (C-scans) can be extracted, enabling plan-view visualization of subsurface features at any depth.
Question 2: For a forensic GPR investigation to locate clandestine burials, which feature in the data would suggest disturbed soil?
- A strong metallic hyperbola
- Disrupted or chaotic reflections in an otherwise layered stratigraphy (Correct answer)
- A clear polarity reversal indicating a void
- Regular horizontal reflectors at consistent depth
Correct answer: Disrupted or chaotic reflections in an otherwise layered stratigraphy
Disturbed soil from a burial disrupts the natural stratigraphic layering, creating chaotic, irregular reflections that contrast with the surrounding undisturbed horizontal layers.
Question 3: A GPR survey for a geotechnical investigation in karst terrain is primarily used to detect:
- Bedrock depth and potential sinkholes or dissolution voids (Correct answer)
- Surface drainage patterns
- Vegetation root depth
- Groundwater chemistry
Correct answer: Bedrock depth and potential sinkholes or dissolution voids
In karst terrain, GPR is effective for locating dissolution features, voids, and depth-to-bedrock, which are critical inputs for geotechnical risk assessment.
Question 4: When performing a GPR survey in a known asbestos-containing building for a pre-demolition utility scan, the GPR operator must:
- Decline the project as GPR cannot be used in such environments
- Follow site-specific asbestos management protocols and use appropriate PPE as directed by the site health and safety plan (Correct answer)
- Only scan exterior areas
- Obtain a demolition permit before scanning
Correct answer: Follow site-specific asbestos management protocols and use appropriate PPE as directed by the site health and safety plan
GPR operators working in ACM-containing buildings must comply with the site health and safety plan, including asbestos awareness training and use of appropriate respiratory and personal protective equipment.
Question 5: Which GPR survey deliverable is most useful for a structural engineer evaluating a parking garage deck for maintenance?
- A raw B-scan radargram
- A color-coded amplitude map highlighting low-signal zones indicating potential delamination (Correct answer)
- A list of scan frequencies
- A chart of antenna separation distances
Correct answer: A color-coded amplitude map highlighting low-signal zones indicating potential delamination
Color-coded amplitude maps visually highlight areas of low reflectivity that correlate with delamination, corrosion, or voids, giving the structural engineer actionable spatial data for repair prioritization.
Question 6: A GPR operator discovers an uncharted utility during a pre-excavation scan. The correct immediate action is to:
- Continue excavation carefully to confirm the utility visually
- Mark the utility location on the ground surface, document the finding, and notify the project manager and dig-safe authority before any excavation proceeds (Correct answer)
- Remove the mark and rescan to verify
- Increase antenna frequency to confirm the find
Correct answer: Mark the utility location on the ground surface, document the finding, and notify the project manager and dig-safe authority before any excavation proceeds
Uncharted utilities must be marked, documented, and reported to the project manager and relevant dig-safe authority to prevent accidental strikes before excavation can safely continue.
GPR GPR Applications & Site Assessment 3
When a GPR operator uses a 3D grid survey pattern (parallel lines in X and Y directions), the primary advantage over a single-line survey is: