Which velocity should I use for my GPR investigations and why?
Ground penetrating radar (GPR) equipment measures the time it takes for a transmitted energy pulse to travel from the antenna, through a medium, to a target where the energy wave is scattered, and back to the receiving antenna. The principle is like an echo-sounder or ‘fish-finder’ mounted in a boat. To know the actual distance between the antenna and a target we need to know the velocity of the signal (energy pulse) as it travels through the medium.
The velocity of the investigated ground or construction will vary greatly depending on physical properties of the same. For example, GPR wave velocity varies with the soil type, type of concrete, porosity, and most importantly, the moisture content in the ground or in the construction you are investigating. It is important to understand how the velocity of the signal varies, as this directly will be affecting the depth accuracy of your GPR survey and the accuracy of the interpretation.
GPR equipment has a calibration routine, e.g., hyperbola fitting, to help the operator estimating the velocity of the media in direct contact with the antenna. More about this can be read HERE
Radar wave velocity has a maximum and minimum value, defined by the speed of electromagnetic waves in air and in pure water. In all other earth-bound media, the velocity will be in between these two outer limits. In most situations where GPR is applied, the media will be a mixture of (soil) material, water (moisture) and air. The proportion of these three at a specific measuring site will decide the velocity of the signal. It is important to remember that the velocity at a specific site may vary depending on weather and season.
Air is the fastest media with a velocity of 300 m/µs (or 0.3 m/ns), while water is the slowest media, with a velocity of 33 m/µs (or 0.033 m/ns).
Geological medias, as soils and bedrock or concrete constructions are usually found between 70-140 m/µs (0.07 and 0.14 m/ns). The radar wave velocity is lower if the amount of water is higher. Imagine dry or wet sand. Dry sand holds plenty of air and the velocity is therefore high, usually between 100-130 m/µs. Water saturated sand will for the same reason have a much lower frequency, usually a velocity between 70-90 m/µs.
For peat and other organic material, the velocity is typically low due to the water content. A wet peat could show values like that of a lake, i.e., the velocity will be low, between 40 to 70 m/µs.
In short and as a rule of thumb, dry areas will in most cases show higher signal velocities and wet areas much slower velocities.
In the table below you find some common velocities for different wet and dry material. It is important to remember that tabled values are only to be used as guidance. The actual velocity at site will vary depending on local conditions, weather and season. It is crucial to perform proper velocity estimations at the beginning and during every GPR survey.
Data from Reynolds, 1997, An Introduction to Applied and Environmental Geophysics.
As seen in the table, the feature dialectical constant is also listed.