4.7 Ground penetrating radar
4.7 Ground penetrating radar where c is the electromagnetic wave velocity in
vacuum (0.3 m/ns), ε=εrε0 the dielectric
Ground penetrating radar, GPR, is a high- permittivity and ε0 the dielectric permittivity in
free space (8.854·10 F/m), ω=2πf the angular
resolution geophysical method, which is based on
the propagation of high frequency electro- frequency, where f is frequency, and the
magnetic waves. The GPR method images expression σ/ωε is a loss factor. In non-magnetic
structures in the ground that are related to (μr=1) low-loss materials, such a clean sand and
changes in dielectric properties. In sediments, the gravel, where σ/ωε ≈ 0, the velocity of
water content primarily causes the changes in electromagnetic waves is reduced to the
dielectric properties. Therefore GPR can be used expression
to estimate soil water contents.
v= c (4.7.2)
Over the last decade GPR has been applied in a εr
vast number of sedimentary and geohydrological
studies (see table in Neal (2004)), also in glacial
The Equations 4.7.1 and 4.7.2 show that the
environments (Møller & Jakobsen 2002, Jakobsen
velocity of electromagnetic waves propagating in
& Overgaard 2002, Bakker 2004, Møller &
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
This section on GPR only contains a brief
the velocity of electromagnetic waves in fresh
description of the methodology; a detailed
water (0.034 m/ns).
description of the GPR method can be found in,
e.g., Davis & Annan (1989), Neal (2004) and
Several processes lead to a reduction of the
textbooks (e.g., Reynolds 1997).
electromagnetic signal strength. Among the most
important processes are attenuation, spherical
spreading of the energy, reflection/transmission
4.7.1 Physical base
losses at interfaces and scattering of energy.
Scattering is due to objects with a dimension
The GPR method operates by transmitting a very
similar to the wavelength and is therefore most
short electromagnetic pulse into the ground
pronounced for higher frequencies. Special
using an antenna. The centre frequency is
attention should be drawn to the attenuation,
typically in the range of 10-2000 MHz. Abrupt
which is a function of dielectric permittivity, ε,
changes in dielectric properties cause parts of the
magnetic permeability, μ, and electrical
electromagnetic energy to be reflected back to
conductivity, σ, as well as the frequency of the
the ground surface, where it is recorded and
signal itself, ω=2πf. The attenuation coefficient is
amplified by the receiving antenna. The recorded
signal is registered as amplitude and polarity
versus two-way travel time (Fig. 4.7.1).
1 + (σ ωε )2 - 1
The electromagnetic wave propagates in air with α = ω εμ (4.7.3)
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 In low-loss materials, where σ/ωε ≈ 0, the
permittivity, εr, the relative magnetic permeability, attenuation coefficient is reduced to
μr, and the electrical conductivity, σ. The velocity
of electromagnetic waves in a host material is σ μ
given by: α= (4.7.4)
v= (4.7.1) The attenuation is proportional to the electrical
1 + 1 + (σ ωε )2 conductivity, which leads to high attenuation in
ε r μr materials with high electrical conductivity.
4.7.2 Field techniques as GPR profiles, in which the vertical axis is two-
way travel time in nanoseconds (ns) and the
Jol & Bristow (2003) give comprehensive advice horizontal axis is distance along the measured
and good practice in GPR field techniques. profile (Fig. 4.7.1b,c).
The GPR data are either collected along a single
Reflection profiling profile or in a grid of profiles to obtain 2D or
pseudo 3D information on structures in the
In reflection profiling mode the antennae are ground. The GPR data can also be acquired along
kept at constant separation, while they are lines so densely spaced that the line spacing
moved along a profile (Fig. 4.7.1a). The equals the stepsize along the line. This leads to a
electromagnetic pulses are transmitted at fixed 3D data cube, where data also can be displayed
time or distance interval. The signal is recorded as time or depth slices.
and displayed immediately on a computer screen
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.
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
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
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.
4.7.3 Data processing ■ Migration will often enhance the display of
the reflections significantly since diffraction
Jol & Bristow (2003) briefly deal with the hyperbolas are collapsed and dipping
commonly used data processing procedures. Neal reflections are moved to the true geometrical
(2004) furthermore discusses the requirements of position. Usually simple constant velocity
proper data processing. The next section points migration procedures are used. Before the
out a number of these essential processing steps. migration procedure can be applied, the
electromagnetic wave velocities in the
ground have to be determined.
Reflection profiling mode
■ Due to attenuation and spherical electro-
Before the GPR data are ready for interpretation magnetic wave spreading of the signal the
a few processing steps have to be applied. GPR data have to be time gained. Several
procedures can be used. One of the most
■ The first step is simple data editing to correct common procedures is automatic gain
mistakes in the field as well as reversing control, AGC. It equalises the amplitudes all
profile directions, merging files, etc. the way down each trace if it is applied with
at window of one pulse length. If the AGC is
■ The first regular processing step is a dewow, applied with a longer window length it tends
which removes a long waved part of the to keep some information on the strength of
signal that is caused by electromagnetic the amplitudes of the reflections.
■ One of the last processing steps is the depth
■ A correction of the zero time may be the conversion and elevation correction. The
next step. The zero time may not have been electromagnetic wave velocities in the
detected precisely by the instrument in the ground must be determined before this step.
field and should therefore be repicked to This is done the best in the field carrying out
ensure correct depths in the profile. a CMP sounding (see next section). Post-
Furthermore, drift of the zero time along the fieldwork velocity establishment is enabled by
profile can occur because of temperature measuring the angle of the limbs of
difference between the instrument diffraction hyperbolae.
electronics and the air temperature or
damaged cables. The drift causes The data can either be displayed in colour mode
misalignment of the reflections and the zero or wiggle mode. In wiggle mode the amplitude
time has to be resampled for all traces along variation of each trace is displayed as a curve,
the profile. where the positive part of the amplitudes is filled
out (e.g., Fig. 4.7.1c). In colour mode the
■ If high frequency electromagnetic noise is amplitudes are colour coded (e.g., Fig. 4.7.3).
present in the GPR profile it can be reduced
by temporal low pass or band pass filtering.
■ A spatial low pass filter also reduces noise as
well as enhancing flat or only slightly dipping GPR data in CMP mode are processed in a similar
reflections, plus suppression of rapid way as data in reflection mode. Dewow, time-
changing features like diffraction hyperbolas zero correction and gain should be applied. Noisy
and steeply dipping reflections. A spatial high data can be low pass filtered.
pass filter work in the opposite way by
enhancing diffraction hyperbolas and steeply The purpose of a CMP sounding is to estimate
dipping reflections and suppressing flat lying the electromagnetic wave velocity. The signal
reflections. Spatial filters can change the velocity in the subsurface just below surface can
appearance of the data dramatically and be determined from the direct ground wave. In
must be used with great caution. 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 4.7.5 Resolution
determined in a semblance analysis or simply by
picking the arrival times of the reflections and fit The vertical resolution depends primarily of the
them to a straight line in a time -distance - wavelength, λ, of the propagating electro-
diagram. After estimation of the intercept time at magnetic wave, which is determined by the GPR
zero distance the interval velocities can be frequency, f, and velocity, v, of the ground
determined using Dix’ analyses (Dix 1956). material as λ=v/f. Theoretically, the distance
between two reflectors should at least be ¼ – ½
The water content in a coarse to medium- of the wavelength to be resolved (Sheriff 1995),
textured soil can be estimated when the interval though in practice the distance should be ½ – 1
velocities are determined. Assuming that the soil wavelength (Møller & Vosgerau 2006). Using a ½
is a low-loss material, the relative dielectric wavelength, the vertical resolution in dry sand
permittivity is determined using Equation 4.7.2. with a velocity of 0.15 m/ns is about 1.5 m, 0.75
Thereby the volumetric water content can be m and 0.19 m for a 50 MHz, 100 MHz and 400
estimated by Topp’s relationship that gives an MHz centre frequency, respectively. In saturated
empirical relationship between the relative sand with a lower velocity of about 0.06 m/ns,
dielectric permittivity and the water content the vertical resolution is 0.6 m, 0.3 m, and 0.075
(Topp et al. 1980). m for a 50 MHz, 100 MHz and 400 MHz centre
4.7.4 Penetration depth Figure 4.7.3 displays GPR profiles that are
acquired with both 100 MHz and 200 MHz
The penetration depth is controlled by the GPR centre frequencies. This figure clearly illustrates
centre frequency, the electrical conductivity and that the vertical resolution is increased by
the attenuation of the subsurface deposits. decreased centre frequency.
In low-loss (i.e., resistive) deposits a low centre The lateral resolution depends on more than the
frequency achieves a large penetration depth wavelength of the propagating electromagnetic
whereas a high centre frequency results in a wave. The depth to the target as well as the
lower penetration depth. The literature on GPR antennae focusing plays a part. Neal (2004)
investigations in sediments reports on discusses in detail the different aspects that have
penetration depths of up to about 30-40 m for to be taken into account in the evaluation of the
40–50 MHz, of 10–25 m for 100 MHz, 5–15 m lateral resolution.
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 4.7.6 Restrictions, uncertainties, error
& Jol 1995, Bakker 2004) or sandstone (e.g., Jol sources and pitfalls
et al. 2003).
The strong attenuation in conductive material
How fast the GPR signal is attenuated depends such as clay or sediment with saline pore water
primarily on the electrical conductivity of the restricts the GPR method to be used in
ground (cf., Eqs. 4.7.3 and 4.7.4). In high- environments with resistive sediments and rocks.
resistive materials the signal is attenuated very
slowly, whereas in conductive materials such as When unshielded antennae are used above-
clay or deposits with saline pore water the surface reflections from objects like trees, houses,
attenuation is very fast and the penetration depth power lines and poles above the ground surface
is decreased significantly. Using a 100 MHz GPR should carefully be identified in the GPR profiles.
system on clayey deposits the penetration depth At the best the survey should be carried out in
is limited to a few metres. Application on a safe distance of obstacles above the ground.
deposit with saline pore water allows a
penetration of a few centimetres only.
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).
Bakker MAJ (2004): The internal structure of
Pleistocene push moraines. A multidisciplinary
approach with emphasis on ground-
penetrating radar. – PhD thesis, Queen Mary,
University of London, 177 pp.
Beres M, Haeni FP (1991): Application of ground-
penetrating-radar methods in hydrogeologic
studies. – Ground Water 29: 375–386.
Fig. 4.7.3: GPR reflection profiles acquired in a
coastal environment along the same line with (a) Davis JL, Annan AP (1989): Ground penetrating
100 MHz and (b) 200 MHz system. The 200 MHz radar for high-resolution mapping of soil and
GPR profile in (b) displays a better resolution of the rock stratigraphy. – Geophysical Prospecting
reflections compared to the 100 MHz profile in (a), 37: 531–551.
whereas the penetration depth is the largest in the
100 MHz GPR profile. The GPR profiles are migrated Dix CH (1956): – Seismic Prospecting of Oil,
with a constant velocity of 0.06 m/ns and scaled Harper, New York.
with AGC with a window of four pulse lengths. The
depth axis is shown with a vertical exaggeration of Gawthorpe RL, Collier REL, Alexander J, Bridge
2. JS, Leeder MR (1993): Ground penetrating
radar: application to sandbody geometry and
heterogeneity studies. – In North CP, Prosser
4.7.7 Interpretation and application of DJ (eds.): Characterization of fluvial and
aeolian reservoirs, Geological Society, London,
Special Publication 73: 421–432.
Commonly used interpretation techniques are Huggenberger P (1993): Radar facies: recognition
radar facies analyses (e.g, Beres & Haeni 1991, of facies patterns and heterogeneities within
Huggenberger 1993, van Overmeeren 1998) and Pleistocene Rhine gravels, NE Switzerland. –
radar stratigraphic analyses, where radar In: Best J L, Bristow C S (eds.): Braided rivers
sequence boundaries also are taken into account Geological Society, London, Special
(e.g., Gawthorpe et al. 1993, Skelly et al. 2003). Publication 75: 163–176.
Radar facies are defined as mapable three
dimensional units composed of reflections whose Jakobsen PR, Overgaard T (2002): Georadar
parameters differ from adjacent units. The facies and glaciotectonic structures in ice
sequence boundaries can be recognised by marginal deposits, northwest Zealand,
identifying the type of the termination of the Denmark. – Quaternary Science Reviews 21:
reflections. Neal (2004) gives a comprehensive 917–927.
description of these interpretation techniques.
4.7 Ground penetrating radar
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advice on data collection, basic processing and Environmental Geophysics. - Wiley,
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Putnam P (2003): Stratigraphic imaging of the Architecture of channel-belt deposits in an
Navajo Sandstone using ground-penetrating aggrading shallow sandbed braided river: the
radar. – The Leading Edge 22: 882–887. lower Niobrara River, northeast Nebraska. –
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characterized by ground penetrating radar. – Smith DG, Jol HM (1995): Ground penetrating
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