Velocity calibration
Definition
The process of estimating the true electromagnetic wave velocity in the survey soil, usually by fitting a hyperbola to a known reflector or by a common mid-point (CMP) acquisition. Velocity determines the depth calculation: depth = (velocity x two-way travel time) / 2.
- Definition
- Measurement of EM wave speed in survey soil, essential for converting radar travel-time to depth.
- Key formula
- Depth = (velocity × two-way travel time) / 2
- Varies by
- Soil type and composition, so calibration must be performed locally at each survey site.
Common questions
What is velocity calibration in GPR surveys?+
It is the process of measuring how fast electromagnetic radar waves travel through the local soil. This is done by fitting curves to known reflectors (targets of established depth) or using common mid-point acquisition methods. Accurate velocity is essential because it directly feeds into depth calculation.
Why is velocity calibration important for depth conversion?+
Depth in a GPR survey is calculated using the formula: depth = (velocity × two-way travel time) / 2. Without knowing the true wave velocity in that specific soil, you cannot convert radar travel time into correct depth, making calibration mandatory before interpretation.
What methods are used to calibrate velocity?+
The two main approaches are fitting a hyperbola curve to a known reflector (a buried object or layer of known depth) and common mid-point (CMP) acquisition, which systematically measures reflections at different antenna spacings to isolate velocity.
Related terms
- Antenna frequency
- The centre frequency of the radar pulse, typically expressed in megahertz. Higher frequencies give finer resolution but penetrate less deeply. Lower frequencies...
- Attenuation
- The reduction in radar signal amplitude with distance traveled. Electrically conductive materials (wet clay, saline soil) convert signal energy to heat, limiting...
- Dielectric permittivity
- A soil's capacity to store and transmit electric field energy. Higher water content raises permittivity and slows the radar wave. The contrast...
- Dielectric permittivity (epsilon)
- A material property that controls how fast electromagnetic waves travel through it and how strongly they reflect at a boundary. Water has...
- Hyperbolic anomaly
- The characteristic arch shape that point-like or cylindrical subsurface targets produce on a radargram. As the antenna passes over a discrete reflector...
- Hyperbolic reflection
- The bowtie-shaped pattern in a GPR time section produced as the antenna passes over a point-like object. The shape is governed by...
- Migration
- A processing step that repositions reflections from their apparent location (a spread-out hyperbola) to their true subsurface location (a point or plane),...
- Radargram
- The cross-sectional image produced by a GPR survey. The horizontal axis represents distance along the survey line; the vertical axis represents two-way...
- Time window
- The total two-way travel time recorded by the GPR per trace, effectively setting the maximum depth sampled. The time window must be...
- Two-way travel time (TWT)
- The time from transmission of a pulse to receipt of its reflection. Depth is calculated as: depth = (velocity × TWT) /...
Explained in these topics
- GPR Signal Physics and Soil Geology EffectsThe process of estimating the true electromagnetic wave velocity in the survey soil, usually by fitting a hyperbola to a known reflector or by a common mid-poi...
- Ground-Penetrating Radar (GPR)The process of measuring the speed of radar waves in the local soil, usually by fitting hyperbola curves to known reflectors or by measuring a target of known...