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					                                                                            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
                                                                              12
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
Jørgensen 2006).
                                                        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
                                                        expressed as:
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)
                                                                            2
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)
                                                             2   ε

                c
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.
                    2




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      INGELISE MØLLER




      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.




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
      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.
          induction.
                                                             ■   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.
                                                             CMP mode
      ■   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.




102
                                                                         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
                                         2        2
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
                                                       frequency, respectively.

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.




                                                                                                              103
      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 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,
            GPR
                                                                 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.




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                                                                       4.7 Ground penetrating radar




Jol HM, Bristow CS (2003): GPR in sediments:          Reynolds JM (1997): An Introduction to Applied
    advice on data collection, basic processing          and Environmental Geophysics. - Wiley,
    and interpretation, a good practice guide. – In      Chichester.
    Bristow C S, Jol H M (eds.) Ground
    penetrating radar in sediments. Geological        Sheriff, RE, Geldart, LP     (1995): Exploration
    Society, London Special Publications 211: 9–         Seismology. Second Edition. - Cambridge
    27.                                                  University Press, New York.

Jol HM, Bristow CS, Smith DG, Junck MB,               Skelly RL, Bristow CS, Ethridge FG (2003):
   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. –
                                                         Sedimentary Geology 158: 249–270.
Møller I, Jakobsen PR (2002): Sandy till
  characterized by ground penetrating radar. –        Smith DG, Jol HM (1995): Ground penetrating
  In Koppenjan S K, Lee H (eds.): Ninth                  radar: antenna frequencies and maximum
  International Conference on Ground Pene-               probable depths of penetration in Quaternary
  trating Radar. Proceedings of SPIE 4758: 308–          sediments. – Journal of Applied Geophysics
  312.                                                   33: 93–100.

Møller I, Jørgensen F (2006): Combined GPR and        Topp GC, Davis JL, Annan AP (1980):
  DC-resistivity imaging in hydrogeological              Electromagnetic determination of soil water
  mapping. – In proceedings of 11th                      content: Measurements in coaxial trans-
  International     Conference    on    Ground           mission lines. – Water Resources Research 16:
  Penetrating Radar, June 19–22, 2006,                   574–582.
  Columbus Ohio, USA, 5 pp.
                                                      van Overmeeren RA (1998): Radar facies of
Møller I, Vosgerau H (2006): Testing ground              unconsolidated sediments in The Netherlands:
  penetrating radar for resolving facies                 A radar stratigraphy interpretation method for
  architecture changes – a radar stratigraphic           hydrogeology. – Journal of Applied
  and sedimentological analysis along a 30 km            Geophysics 40: 1–18.
  profile on the Karup Outwash Plain, Denmark.
  – Near Surface Geophysics 4: 57–68.

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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      INGELISE MØLLER




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