4.7 Ground penetrating radar
4.7
Ground penetrating radar
Ground penetrating radar, GPR, is a highresolution geophysical method, which is based on
the propagation of high frequency electromagnetic waves. The GPR method images
structures in the ground that are related to
changes in dielectric properties. In sediments, the
water content primarily causes the changes in
dielectric properties. Therefore GPR can be used
to estimate soil water contents.
Over the last decade GPR has been applied in a
vast number of sedimentary and geohydrological
studies (see table in Neal (2004)), also in glacial
environments (Møller & Jakobsen 2002, Jakobsen
& Overgaard 2002, Bakker 2004, Møller &
Jørgensen 2006).
This section on GPR only contains a brief
description of the methodology; a detailed
description of the GPR method can be found in,
e.g., Davis & Annan (1989), Neal (2004) and
textbooks (e.g., Reynolds 1997).
4.7.1 Physical base
The GPR method operates by transmitting a very
short electromagnetic pulse into the ground
using an antenna. The centre frequency is
typically in the range of 10-2000 MHz. Abrupt
changes in dielectric properties cause parts of the
electromagnetic energy to be reflected back to
the ground surface, where it is recorded and
amplified by the receiving antenna. The recorded
signal is registered as amplitude and polarity
versus two-way travel time (Fig. 4.7.1).
The electromagnetic wave propagates in air with
the speed of light (0.3 m/ns). In the ground the
velocity of electromagnetic waves is reduced
since it is dependent on the relative dielectric
permittivity, εr, the relative magnetic permeability,
μr, and the electrical conductivity, σ. The velocity
of electromagnetic waves in a host material is
given by:
v=
c
1 + 1 + (σ ωε )2
ε r μr
2
(4.7.1)
where c is the electromagnetic wave velocity in
vacuum (0.3 m/ns), ε=εrε0 the dielectric
permittivity and ε0 the dielectric permittivity in
12
free space (8.854·10 F/m), ω=2πf the angular
frequency, where f is frequency, and the
expression σ/ωε is a loss factor. In non-magnetic
(μr=1) low-loss materials, such a clean sand and
gravel, where σ/ωε ≈ 0, the velocity of
electromagnetic waves is reduced to the
expression
v= c
(4.7.2)
εr
The Equations 4.7.1 and 4.7.2 show that the
velocity of electromagnetic waves propagating in
the ground is decreased compared to the velocity
in the air. In low-loss (i.e. resistive) materials the
maximum decrease is a factor of nine, which is
the velocity of electromagnetic waves in fresh
water (0.034 m/ns).
Several processes lead to a reduction of the
electromagnetic signal strength. Among the most
important processes are attenuation, spherical
spreading of the energy, reflection/transmission
losses at interfaces and scattering of energy.
Scattering is due to objects with a dimension
similar to the wavelength and is therefore most
pronounced for higher frequencies. Special
attention should be drawn to the attenuation,
which is a function of dielectric permittivity, ε,
magnetic permeability, μ, and electrical
conductivity, σ, as well as the frequency of the
signal itself, ω=2πf. The attenuation coefficient is
expressed as:
α = ω εμ
1 + (σ ωε )2 - 1
2
(4.7.3)
In low-loss materials, where σ/ωε ≈ 0, the
attenuation coefficient is reduced to
α=
σ
2
μ
ε
(4.7.4)
The attenuation is proportional to the electrical
conductivity, which leads to high attenuation in
materials with high electrical conductivity.
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INGELISE MØLLER
4.7.2 Field techniques
Jol & Bristow (2003) give comprehensive advice
and good practice in GPR field techniques.
Reflection profiling
In reflection profiling mode the antennae are
kept at constant separation, while they are
moved along a profile (Fig. 4.7.1a). The
electromagnetic pulses are transmitted at fixed
time or distance interval. The signal is recorded
and displayed immediately on a computer screen
as GPR profiles, in which the vertical axis is twoway travel time in nanoseconds (ns) and the
horizontal axis is distance along the measured
profile (Fig. 4.7.1b,c).
The GPR data are either collected along a single
profile or in a grid of profiles to obtain 2D or
pseudo 3D information on structures in the
ground. The GPR data can also be acquired along
lines so densely spaced that the line spacing
equals the stepsize along the line. This leads to a
3D data cube, where data also can be displayed
as time or depth slices.
Fig. 4.7.1: Principles of GPR in reflection profiling mode. a) In reflection profiling a set of transmitting antenna
and receiving antenna with constant separation is moved along the profile. The path of some of the reflected
waves is sketched for antenna position 56, 91 and 226 of the GPR profile in (c). b) The received signal of these
antenna positions is displayed in wiggle mode. c) GPR profile acquired with 200 MHz system in a coastal
environment. The horizontal axis displays the distance along the profile. The vertical axis to the left displays the
two-way travel time and the axis to the right displays the converted depth. d) Photo of a GPR system equipped
with 100 MHz antenna. The text on the photo explains the different part of the system.
100
4.7 Ground penetrating radar
To ease the work in the field, the GPR system can
be mounted on a cart or sledge which is towed
by a person (Fig. 4.7.1d) or an all-terrain vehicle
(ATV). The acquisition speed is comparable to
walking speed for the most systems. The
productivity per field day depends on the
individual survey setup and the accessibility in the
field.
If there are topographic changes along the GPR
profiles it is important that the topographic
variation is surveyed precisely, so that the GPR
profiles can be displayed with correct
topography. As a result the reflections will be
displayed with the true dip and geometry.
Common mid point
A common mid point dataset, CMP, is also called
a velocity sounding, since the technique is
commonly used for signal velocity establishment.
In CMP mode the antennae separation is
increased for each recording, while they are kept
over a common mid point (Fig. 4.7.2a).
A CMP plot contains the direct wave transmitted
in the air above the ground, the direct wave
transmitted in the ground and waves reflected
from interfaces in the ground, where the
dielectric properties change (Fig. 4.7.2b,c).
Refracted waves are seldom present in CMP
soundings. This is related to the fact that the
electromagnetic wave velocity decreases with
depth together with increasing water content
with depth.
Fig. 4.7.2:
Principles of GPR in CMP mode. a) In
CMP mode a set of a transmitting antenna (Tx) and
a receiving antenna (Rx) are moved away from each
other. The six first antenna positions are shown with
the path of the reflected wave from the first
reflector. b) Sketch of the path of the most common
waves that is present in a CMP. c) Diagram of the
received signals in a CMP. The horizontal axis
displays the distance between the transmitting and
the receiving antenna. The vertical axis displays the
two-way travel time. d) Photo of a GPR system that
is ready for a CMP sounding.
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INGELISE MØLLER
4.7.3 Data processing
■
Migration will often enhance the display of
the reflections significantly since diffraction
hyperbolas are collapsed and dipping
reflections are moved to the true geometrical
position. Usually simple constant velocity
migration procedures are used. Before the
migration procedure can be applied, the
electromagnetic wave velocities in the
ground have to be determined.
■
Due to attenuation and spherical electromagnetic wave spreading of the signal the
GPR data have to be time gained. Several
procedures can be used. One of the most
common procedures is automatic gain
control, AGC. It equalises the amplitudes all
the way down each trace if it is applied with
at window of one pulse length. If the AGC is
applied with a longer window length it tends
to keep some information on the strength of
the amplitudes of the reflections.
■
One of the last processing steps is the depth
conversion and elevation correction. The
electromagnetic wave velocities in the
ground must be determined before this step.
This is done the best in the field carrying out
a CMP sounding (see next section). Postfieldwork velocity establishment is enabled by
measuring the angle of the limbs of
diffraction hyperbolae.
Jol & Bristow (2003) briefly deal with the
commonly used data processing procedures. Neal
(2004) furthermore discusses the requirements of
proper data processing. The next section points
out a number of these essential processing steps.
Reflection profiling mode
Before the GPR data are ready for interpretation
a few processing steps have to be applied.
■
The first step is simple data editing to correct
mistakes in the field as well as reversing
profile directions, merging files, etc.
■
The first regular processing step is a dewow,
which removes a long waved part of the
signal that is caused by electromagnetic
induction.
■
A correction of the zero time may be the
next step. The zero time may not have been
detected precisely by the instrument in the
field and should therefore be repicked to
ensure correct depths in the profile.
Furthermore, drift of the zero time along the
profile can occur because of temperature
difference between the instrument
electronics and the air temperature or
damaged cables. The drift causes
misalignment of the reflections and the zero
time has to be resampled for all traces along
the profile.
■
If high frequency electromagnetic noise is
present in the GPR profile it can be reduced
by temporal low pass or band pass filtering.
The data can either be displayed in colour mode
or wiggle mode. In wiggle mode the amplitude
variation of each trace is displayed as a curve,
where the positive part of the amplitudes is filled
out (e.g., Fig. 4.7.1c). In colour mode the
amplitudes are colour coded (e.g., Fig. 4.7.3).
CMP mode
■
102
A spatial low pass filter also reduces noise as
well as enhancing flat or only slightly dipping
reflections, plus suppression of rapid
changing features like diffraction hyperbolas
and steeply dipping reflections. A spatial high
pass filter work in the opposite way by
enhancing diffraction hyperbolas and steeply
dipping reflections and suppressing flat lying
reflections. Spatial filters can change the
appearance of the data dramatically and
must be used with great caution.
GPR data in CMP mode are processed in a similar
way as data in reflection mode. Dewow, timezero correction and gain should be applied. Noisy
data can be low pass filtered.
The purpose of a CMP sounding is to estimate
the electromagnetic wave velocity. The signal
velocity in the subsurface just below surface can
be determined from the direct ground wave. In
the deeper part of the ground the velocity
information is obtained from reflected waves.
4.7 Ground penetrating radar
The root-mean-square velocities can be
determined in a semblance analysis or simply by
picking the arrival times of the reflections and fit
2
2
them to a straight line in a time -distance diagram. After estimation of the intercept time at
zero distance the interval velocities can be
determined using Dix’ analyses (Dix 1956).
The water content in a coarse to mediumtextured soil can be estimated when the interval
velocities are determined. Assuming that the soil
is a low-loss material, the relative dielectric
permittivity is determined using Equation 4.7.2.
Thereby the volumetric water content can be
estimated by Topp’s relationship that gives an
empirical relationship between the relative
dielectric permittivity and the water content
(Topp et al. 1980).
4.7.4 Penetration depth
The penetration depth is controlled by the GPR
centre frequency, the electrical conductivity and
the attenuation of the subsurface deposits.
In low-loss (i.e., resistive) deposits a low centre
frequency achieves a large penetration depth
whereas a high centre frequency results in a
lower penetration depth. The literature on GPR
investigations
in
sediments
reports
on
penetration depths of up to about 30-40 m for
40–50 MHz, of 10–25 m for 100 MHz, 5–15 m
for 200 MHz and only a few metres for 500–
1000 MHz. The maximum penetration depths are
obtained in dry clean sand and gravel (e.g., Smith
& Jol 1995, Bakker 2004) or sandstone (e.g., Jol
et al. 2003).
How fast the GPR signal is attenuated depends
primarily on the electrical conductivity of the
ground (cf., Eqs. 4.7.3 and 4.7.4). In highresistive materials the signal is attenuated very
slowly, whereas in conductive materials such as
clay or deposits with saline pore water the
attenuation is very fast and the penetration depth
is decreased significantly. Using a 100 MHz GPR
system on clayey deposits the penetration depth
is limited to a few metres. Application on a
deposit with saline pore water allows a
penetration of a few centimetres only.
4.7.5 Resolution
The vertical resolution depends primarily of the
wavelength, λ, of the propagating electromagnetic wave, which is determined by the GPR
frequency, f, and velocity, v, of the ground
material as λ=v/f. Theoretically, the distance
between two reflectors should at least be ¼ – ½
of the wavelength to be resolved (Sheriff 1995),
though in practice the distance should be ½ – 1
wavelength (Møller & Vosgerau 2006). Using a ½
wavelength, the vertical resolution in dry sand
with a velocity of 0.15 m/ns is about 1.5 m, 0.75
m and 0.19 m for a 50 MHz, 100 MHz and 400
MHz centre frequency, respectively. In saturated
sand with a lower velocity of about 0.06 m/ns,
the vertical resolution is 0.6 m, 0.3 m, and 0.075
m for a 50 MHz, 100 MHz and 400 MHz centre
frequency, respectively.
Figure 4.7.3 displays GPR profiles that are
acquired with both 100 MHz and 200 MHz
centre frequencies. This figure clearly illustrates
that the vertical resolution is increased by
decreased centre frequency.
The lateral resolution depends on more than the
wavelength of the propagating electromagnetic
wave. The depth to the target as well as the
antennae focusing plays a part. Neal (2004)
discusses in detail the different aspects that have
to be taken into account in the evaluation of the
lateral resolution.
4.7.6 Restrictions, uncertainties, error
sources and pitfalls
The strong attenuation in conductive material
such as clay or sediment with saline pore water
restricts the GPR method to be used in
environments with resistive sediments and rocks.
When unshielded antennae are used abovesurface reflections from objects like trees, houses,
power lines and poles above the ground surface
should carefully be identified in the GPR profiles.
At the best the survey should be carried out in
safe distance of obstacles above the ground.
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INGELISE MØLLER
Ground truthing is important to verify the origin
and nature of reflections. Usually exposures,
borehole or cone-penetration test data are used.
The application of GPR in Burval studies is related
to the vulnerability mapping of the near surface
layers (e.g., Møller & Jørgensen 2006).
4.7.8 References
Bakker MAJ (2004): The internal structure of
Pleistocene push moraines. A multidisciplinary
approach with emphasis on groundpenetrating radar. – PhD thesis, Queen Mary,
University of London, 177 pp.
Fig. 4.7.3:
GPR reflection profiles acquired in a
coastal environment along the same line with (a)
100 MHz and (b) 200 MHz system. The 200 MHz
GPR profile in (b) displays a better resolution of the
reflections compared to the 100 MHz profile in (a),
whereas the penetration depth is the largest in the
100 MHz GPR profile. The GPR profiles are migrated
with a constant velocity of 0.06 m/ns and scaled
with AGC with a window of four pulse lengths. The
depth axis is shown with a vertical exaggeration of
2.
4.7.7 Interpretation and application of
GPR
Commonly used interpretation techniques are
radar facies analyses (e.g, Beres & Haeni 1991,
Huggenberger 1993, van Overmeeren 1998) and
radar stratigraphic analyses, where radar
sequence boundaries also are taken into account
(e.g., Gawthorpe et al. 1993, Skelly et al. 2003).
Radar facies are defined as mapable three
dimensional units composed of reflections whose
parameters differ from adjacent units. The
sequence boundaries can be recognised by
identifying the type of the termination of the
reflections. Neal (2004) gives a comprehensive
description of these interpretation techniques.
104
Beres M, Haeni FP (1991): Application of groundpenetrating-radar methods in hydrogeologic
studies. – Ground Water 29: 375–386.
Davis JL, Annan AP (1989): Ground penetrating
radar for high-resolution mapping of soil and
rock stratigraphy. – Geophysical Prospecting
37: 531–551.
Dix CH (1956): – Seismic Prospecting of Oil,
Harper, New York.
Gawthorpe RL, Collier REL, Alexander J, Bridge
JS, Leeder MR (1993): Ground penetrating
radar: application to sandbody geometry and
heterogeneity studies. – In North CP, Prosser
DJ (eds.): Characterization of fluvial and
aeolian reservoirs, Geological Society, London,
Special Publication 73: 421–432.
Huggenberger P (1993): Radar facies: recognition
of facies patterns and heterogeneities within
Pleistocene Rhine gravels, NE Switzerland. –
In: Best J L, Bristow C S (eds.): Braided rivers
Geological
Society,
London,
Special
Publication 75: 163–176.
Jakobsen PR, Overgaard T (2002): Georadar
facies and glaciotectonic structures in ice
marginal deposits, northwest Zealand,
Denmark. – Quaternary Science Reviews 21:
917–927.
4.7 Ground penetrating radar
Jol HM, Bristow CS (2003): GPR in sediments:
advice on data collection, basic processing
and interpretation, a good practice guide. – In
Bristow C S, Jol H M (eds.) Ground
penetrating radar in sediments. Geological
Society, London Special Publications 211: 9–
27.
Reynolds JM (1997): An Introduction to Applied
and Environmental Geophysics. - Wiley,
Chichester.
Jol HM, Bristow CS, Smith DG, Junck MB,
Putnam P (2003): Stratigraphic imaging of the
Navajo Sandstone using ground-penetrating
radar. – The Leading Edge 22: 882–887.
Skelly RL, Bristow CS, Ethridge FG (2003):
Architecture of channel-belt deposits in an
aggrading shallow sandbed braided river: the
lower Niobrara River, northeast Nebraska. –
Sedimentary Geology 158: 249–270.
Møller I, Jakobsen PR (2002): Sandy till
characterized by ground penetrating radar. –
In Koppenjan S K, Lee H (eds.): Ninth
International Conference on Ground Penetrating Radar. Proceedings of SPIE 4758: 308–
312.
Smith DG, Jol HM (1995): Ground penetrating
radar: antenna frequencies and maximum
probable depths of penetration in Quaternary
sediments. – Journal of Applied Geophysics
33: 93–100.
Møller I, Jørgensen F (2006): Combined GPR and
DC-resistivity imaging in hydrogeological
mapping. – In proceedings of 11th
International
Conference
on
Ground
Penetrating Radar, June 19–22, 2006,
Columbus Ohio, USA, 5 pp.
Møller I, Vosgerau H (2006): Testing ground
penetrating radar for resolving facies
architecture changes – a radar stratigraphic
and sedimentological analysis along a 30 km
profile on the Karup Outwash Plain, Denmark.
– Near Surface Geophysics 4: 57–68.
Sheriff, RE, Geldart, LP
(1995): Exploration
Seismology. Second Edition. - Cambridge
University Press, New York.
Topp GC, Davis JL, Annan AP (1980):
Electromagnetic determination of soil water
content: Measurements in coaxial transmission lines. – Water Resources Research 16:
574–582.
van Overmeeren RA (1998): Radar facies of
unconsolidated sediments in The Netherlands:
A radar stratigraphy interpretation method for
hydrogeology. – Journal of Applied
Geophysics 40: 1–18.
Neal A (2004): Ground-penetrating radar and its
use in sedimentology: principles, problems
and progress. – Earth-Science Reviews 66:
261–330.
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