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Geophysics foundations:Quick overview:

GPR outline

GPR is the only common geophysical survey that responds to subsurface changes in dielectricproperties as well as changes in conductivity. At industrial sites, both physical properties might

be expected to vary across the water table, in regions of oil contamination, and where fillmaterials are variable. GPR responds by reflecting or scattering off boundaries where thesephysical properties change. The velocity of signals in the ground is determined mainly bydielectric permittivity, and velocity is necessary for converting echo travel times into distanceto the reflector. The attenuation of GPR signals is affected primarily by conductivity, sovariations in penetration depth should provide some information about the various regions of materials.

GPR can be referred to as radio echo sounding. The seismic analogy is often helpful:

urveying involves in!ecting a short pulse of energy into the ground and recording echoes.

The GPR source energy is electromagnetic" therefore, relevant physical properties, namely conductivity an

dielectric permittivity, are electrical. Results are usually presented similarly to seismic data, as position versus #$way travel time.

%iggle trace presentation is shown below right. Amplitude of the trace represents voltage on the antenna,

and echoes appear as &wiggles&, indicating that radio energy that was emitted from the source antenna hasechoed from some interface. 'ore commonly the traces have one half the trace filled in (the variable areastyle of plot).

GPR concept involving &common offset& instrument

configuration. *choes arrive from targets that are within rangeof signals. The signals echo off surfaces that are perpendicular

to the signals+ direction of travel.

Radar section resulting from the survey shown tothe left.

ata are commonly presented as echograms along a survey line with line position along the hori-ontal axis andtwo$way signal travel time along the vertical axis. Gray$ or colour$scale images are also common, in which thecolour is scaled to signal amplitude. n order to convert the signal+s travel time to depth, a velocity for the signalsin the grond is needed. /elocity is dependent mainly upon dielectric permittivity, although this approximation failin very conductive soils. f the velocity (i.e. the ground+s permittivity) cannot be estimated , it is advisable to tryand measure the velocity using a particular field configuration called a common mid$point (0'P) or wide angle

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reflection 1 refraction (%ARR). A table of velocities and attenuation rates from the instrument manual is givenbelow.

MaterialVelocity(m/ns)

AttenuationdB/m

Air 2.3 2

ce 2.45$2.46 2.24

ry oil 2.47

ry and 2.47 2.24

Granite 2.43 2.24 $ 4.2

ry alt 2.43 2.24 $ 4.2

ry Roc8 2.4#

9imestone 2.4# 2. $ 4.2

%et Roc8 2.4

0oncrete 2.2; $ 2.4#

Pavement 2.4hales 2.2< 4 $ 422

ilts 2.26 4 $ 422

%et oil 2.25

%et and 2.25 2.23 $ 2.3

0lays 2.25 4 $ 322

=resh %ater 2.233 2.4

ea %ater 2.233 4222

Ground penetrating radar survey data are most easily interpreted when measurement stations are very closetogether. n fact, if target interfaces dip by more than a few degrees, they may be invisible if station spacing is toocoarse. >ne other important survey design consideration is the choice of antenna, which determines the centrefre?uency of signals, and hence the penetration depth and vertical resolution. f several antennas are available, it ia good idea to survey a short line with as many different fre?uencies as possible to establish the optimum antennafor the !ob.

The figure above shows the instrument+s operating console and laptop computer in a cart with the transmitter andreceiver antennas arranged in a common offset configuration behind. The three components are lin8ed with fiberoptic cables.

@ B0 *arth and >cean ciences, =. Cones. 2