What is GPR?

•A geophysical non-invasive method, employed to carry out subsurface investigation

•Analogous to X-Rays, which scan our body

Non-invasive methods

•Techniques by which, the test subjects are evaluated without actually causing any sort of damage to them

•Some of widely used non-invasive methods, also called as non-destructive methods in civil engineering include

–Rebound hammer

–Ultrasonic pulse velocity

–Ground penetrating radar

–Tomography

–Impact echo

GPR in Civil Engineering

•Detection of rebars and cover depth

•Detection of cracks within structural elements

•Assessment of moisture

•Evaluation of material homogeneity

•Measurement of depth of hard stratum

•Internal structure

•Depth of elements

•Presence of voids and delaminations within structure

How does it work?

•It is just physics

•Electromagnetic technique which takes advantage of the property that “different materials allow waves pass through them at different speed”

•This contrasting speeds of electromagnetic waves through structural elements aids in detecting the variation of material profiles beneath the test surface

Terminologies

•Dielectric permittivity (ε):

 Defines how strongly a material becomes electrically polarized under the influence of an electric field

  •Relative permittivity or dielectric constant(εr)

Defines the dielectric properties of a material relative to that of free-space

"ε" _r=  "ε" /"ε" _o

Dielectric permittivity of some materials

Frequency range

•GPR works in electromagnetic waves with frequency in range of 10 mHz to 2000 mHz

•The choice of frequency, solely depends upon the depth of penetration required

•Higher the frequency of the emitted wave, deeper the penetration

•But, higher frequency is preferred to fulfil higher resolution requirement

Components of GPR setup

•Transmitter antenna

•Receiver antenna

•Processing unit

•Output unit