GPR GPR Signal Processing & Analysis 2 — Questions and Answers
Question 1: When converting GPR two-way travel time (TWTT) to depth, which parameter is essential?
- Signal frequency
- Antenna separation
- Electromagnetic wave velocity in the medium (Correct answer)
- Scan rate
Correct answer: Electromagnetic wave velocity in the medium
Depth is calculated as half the TWTT multiplied by the EM velocity, so an accurate velocity for the specific material is required.
Question 2: What is a common indication of a metallic pipe in a GPR radargram?
- A flat, continuous reflector
- A strong hyperbolic reflection with ringing below (Correct answer)
- A series of steeply dipping reflectors
- A gradual decrease in signal amplitude
Correct answer: A strong hyperbolic reflection with ringing below
Metallic pipes produce a strong hyperbolic diffraction pattern due to their point-source geometry and generate multiple internal reverberations (ringing) from the high dielectric contrast.
Question 3: Which software feature allows a GPR operator to overlay processed scan data on a site plan for reporting?
- Signal stacking
- Data merging/georeferencing (Correct answer)
- Hilbert transform
- Velocity inversion
Correct answer: Data merging/georeferencing
Georeferencing and data merging tools allow GPR results to be spatially registered to site drawings or GPS coordinates for accurate reporting.
Question 4: The Hilbert transform in GPR data processing is used to produce:
- A phase-shifted version of the signal to compute instantaneous amplitude (envelope) (Correct answer)
- A depth-converted section from time data
- A frequency spectrum of the wavelet
- A stacked trace from multiple passes
Correct answer: A phase-shifted version of the signal to compute instantaneous amplitude (envelope)
The Hilbert transform computes the analytic signal, whose magnitude gives the instantaneous amplitude (envelope), useful for highlighting strong reflectors.
Question 5: What does a 'flat reflector' with no hyperbolic tails typically represent in a GPR scan?
- A point target such as a rebar
- A planar interface such as a layer boundary or void ceiling (Correct answer)
- A metallic conduit
- An air-filled crack perpendicular to the scan line
Correct answer: A planar interface such as a layer boundary or void ceiling
Flat continuous reflectors indicate planar interfaces (e.g., layer boundaries, voids, or slabs) where the reflection geometry produces a horizontal event rather than a diffraction.
Question 6: In GPR data, 'clutter' most commonly refers to:
- Valid target reflections that are too close together to resolve
- Unwanted reflections from surface objects, reinforcement, or other non-target features (Correct answer)
- Noise introduced by the data acquisition system
- Phase reversals at low-contrast boundaries
Correct answer: Unwanted reflections from surface objects, reinforcement, or other non-target features
Clutter consists of reflections from unintended targets (rebar grids, surface debris, fences) that complicate the identification of the target of interest.
GPR GPR Signal Processing & Analysis 2
When converting GPR two-way travel time (TWTT) to depth, which parameter is essential?