ground penetrating radar (nio)

Presented byNika BhingardeSabna Thilakan(Second Year,ME Found.Engg,GEC,Farmagudi, Ponda)

On

Introduction Ground penetrating radar (commonly called GPR) is a

high resolution electromagnetic technique that is designed primarily to investigate the shallow subsurface of the earth, building materials, and roads and bridges.

Civil engineering applications Probing into soil to detect pipelines and tanks Cavities Thickness determination(Pavement thickness) Locating reinforcement Identifying deterioration(Bridge deck conditions, Rail bed

condition)

Principle An Electro Magnetic

pulse is sent through an antenna, penetrating into the surveyed material

A portion of the energy is reflected back to the antenna when an interface between materials of dissimilar dielectric constants is encountered

Antenna and Frequency Different antennas provide different frequencies,

which usually vary between 25 and 1,500 MHz

Lower frequencies provide greater depth penetration but lower resolution

Frequency

A GPR antenna emits a succession of pulses of the same form and duration at definite intervals.

The frequency around which most of the pulses’ energy is concentrated is called the center frequency, fc.

GPR systems are generally planned to create a frequency bandwidth, df (the pulse width at the frequency domain), which is similar to the center frequency, i.e., df ~ fc.

In the time interval between two successive pulses, the receiving antenna measures the electric field in the incoming reflected puas an analogue signal, which is later amplified and converted to digital form.

The interval at which the incoming signals are sampled is called sampling interval (its inverse is termed sampling frequency,fs)

Relative Dielectric ConstantThe Relative dielectric constant is the ratio of the permittivity of a substance to the permittivity of free space.Permittivity, also called electric permittivity, is a constant and is equal to approximately 8.85 x 10-12 farad per meter (F/m) in free space (a vacuum).

= ε (1.13 x 1011)

The velocity of radio wave in a material is given by

r

cVm

0

r

The reflected signal has information on: how quickly the signal traveled

how much was attenuated

The thickness of a layer is given by:

where

di is the thickness of layer i,

ti the total travel time through that layer,

C is the speed of light and

εr,I the dielectric constant of the layer

2,

i

ir

i

tCd

•The amount of reflected energy at an interface is governed by:

where ρ1,2 is the reflection coefficient and εr1 and εr2 are the dielectric constants

21

21

2,1

rr

rr

Radiowave velocities as a function of relative Dielectric constants

Example

How deep below the surface is an object that is detected at 46 ns in dry sand (Er = 4)?

Answer:

D = (0.3x 46) / sqrt (4)

D = 3.45 m

Therefore the object is about 3.45 m underground

Equipment Consists of:

Antenna Monostatic

Bistatic

Air-couple

Ground-coupled

Control Unit

Display device

Storage device

Data Acquisition GPR surveys are conducted by pulling the antenna

across the ground surface at a normal walking pace.

The recorder stores the data as well as presenting a picture of the recorded data on a screen.

Simplified diagram of (A)the constituents of radar diagram (B)the interpreted section (C) the radardiagram display .Adapted from Butler et al (1991) and Daniel et al(1988)

Advantages and LimitationsAdvantages: The GPR method provides a rapid technique for

locating utilities. Since the data is viewed on a screen on the instrument, the data can be viewed in the field and the locations of anomalies marked on the ground.

Limitations: Site specific

Limitations if dielectric properties are similar

Difficult in thin layers

Manufacturers GSSI –The largest manufacturers of GPR systems

MALA Geoscience

RAMAC