Two-Way Travel Time (TWT)
Definition
The time from transmission of a pulse to receipt of its reflection. Depth is calculated as: depth = (velocity × TWT) / 2. The factor of two accounts for the outward and return journey.
- Definition
- Time from pulse transmission to receipt of its reflection
- Depth formula
- depth = (velocity x TWT) / 2
- Reason for factor of two
- Accounts for outward and return journey
- Context
- Ground-Penetrating Radar surveys
Common questions
Why does the velocity term in the depth formula matter so much for accuracy?+
Radar wave velocity varies with soil type and moisture content, so using an incorrect velocity estimate for the actual ground conditions can shift the calculated depth of a buried target substantially even when the TWT measurement itself is accurate.
How is TWT actually recorded during a GPR survey?+
The GPR unit continuously transmits pulses and records the elapsed time until each reflection returns, building a profile of TWT values across the survey grid that the operator later converts to depth using an estimated or calibrated 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...
- 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...
- 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...
- 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...
- Velocity Calibration
- The process of estimating the true electromagnetic wave velocity in the survey soil, usually by fitting a hyperbola to a known reflector...