5.1Concepts—This guide summarizes the
equipment, field procedures, and data processing methods used to
interpret geologic conditions, and to identify and provide
locations of geologic anomalies and man-made objects with the GPR
method. The GPR uses high-frequency EM waves (from 10 to 3000 MHz)
to acquire subsurface information. Energy is propagated downward
into the ground from a transmitting antenna and is reflected back
to a receiving antenna from subsurface boundaries between media
possessing different EM properties. The reflected signals are
recorded to produce a scan or trace of radar data. Typically, scans
obtained as the antenna(s) are moved over the ground surface are
placed side by side to produce a radar profile.
5.1.1 The vertical scale of the radar
profile is in units of two-way travel time, the time it takes for
an EM wave to travel down to a reflector and back to the surface.
The travel time may be converted to depth by relating it to on-site
measurements or assumptions about the velocity of the radar waves
in the subsurface materials.
5.1.2 Vertical variations in propagation
velocity due to changing EM properties of the subsurface can make
it difficult to apply a linear time scale to the radar profile
(Ulriksen 5.2Parameter Being Measured and Representative
Values:
5.2.1Two-Way Travel Time and Velocity—A GPR
trace is the record of the amplitude of EM energy that has been
reflected from interfaces between materials possessing different EM
properties and recorded as a function of two-way travel time. To
convert two-way times to depths, it is necessary to estimate or
determine the propagation velocity of the EM pulses or waves. The
relative permittivity of the material (εr)
through which the EM pulse or wave propagates mostly determines the
propagation velocity of the EM wave. The propagation velocity
through the material is approximated using the following
relationship (see full formula in Balanis (32)):
It is assumed that the magnetic
permeability is that of free space and the loss tangent is much
less than 1.
5.2.1.1Table 1 lists the relative permittivities
(5.2.1.2 If the relative permittivity is
unknown, as is normally the case, it may be necessary to estimate
velocity or use a reflector of known depth to calculate the
velocity. The propagation velocity, Vm, is calculated from the
relationship as follows:
5.2.1.3 Methods for measuring velocity in
the field are found in 6.7.3. Note that measured velocities may
only be valid at the location where they are measured under
specific soil conditions. If there is lateral variability in soil
and rock composition and moisture content, velocity may need to be
determined at several locations.
5.2.2Attenuation—The depth of penetration is
determined primarily by the attenuation of the radar signal due to
the conversion of EM energy to thermal energy through electrical
conduction, dielectric relaxation, or magnetic relaxation losses.
Conductivity is primarily governed by the water content of the
material and the concentration of free ions in solution (salinity).
Attenuation also occurs due to scattering of the EM energy in
unwanted directions by inhomogeneities in the subsurface. If the
scale of inhomogeneity is comparable to the wavelength of EM
energy, scattering may be significant (Olhoeft (33)). Other factors that affect
attenuation include soil type, temperature (Morey (34)), and clay mineralogy
(Doolittle 5.3Equipment—The GPR equipment utilized for
the measurement of subsurface conditions normally consists of a
transmitter and receiver antenna, a radar control unit, and
suitable data storage and display devices.
5.3.1Radar Control Unit—The radar control unit
synchronizes signals to the transmitting and receiving electronics
in the antennas. The synchronizing signals control the transmitter
and sampling receiver electronics located in the antenna(s) in
order to generate a sampled waveform of the reflected radar waves.
These waveforms may be filtered and amplified and are transmitted
along with timing signals to the display and recording devices.
5.3.2 Real-time signal processing for
improvement of signal-to-noise ratio is available in most GPR
systems. When working in areas with cultural noise and in materials
causing signal attenuation, time-varying gain is necessary to
adjust signal amplitudes for display on monitors or plotting
devices. Filters may be used in real time to improve signal
quality. The summing of radar signals (stacking) is used to
increase effective depth of exploration by improving the
signal-to-noise ratio.
5.3.3Data
Display—The GPR data are displayed as a continuous profile
of individual radar traces (Fig.
2). The horizontal-axis represents horizontal traverse
distance and the vertical-axis is two-way travel time (or depth).
Data are commonly presented in wiggle trace display, where the
intensity of the received wave at an instant in time is
proportional to the amplitude of the trace (see Fig. 2), or as a gray scale or color
scale display, where the intensity of the received wave at an
instant in time is proportional to either the intensity of gray
scale (that is, black is high intensity, and white is low
intensity; see Fig. 3) or to
some color assignment defined according to a specified color-signal
amplitude relationship.
5.4.2.4Polarization—The type and alignment of
polarization of the vector electromagnetic fields may be important
in receiving responses from various scatterers. Two linear,
parallel polarized, electric field antennas can maximize the
response from linear scatters like pipes when the electric field
(typically long axis of the dipole antenna) is aligned parallel
with the pipe and towed perpendicular across the pipe. Similarly,
alignment with the rebar in concrete will maximize the ability to
map the rebar, but alignment perpendicular to the rebar will
minimize scattering reflections from the rebar to see through or
past the rebar to get the thickness of concrete. Similar
arrangement may be made for overhead wires and nearby fences.
Cross-polarized antennas (perpendicular to each other) minimize the
response from horizontal layers.
5.4.3Interferences Caused by Ambient, Geologic, and
Cultural Conditions:
5.4.3.1 Measurements obtained by the GPR
method may contain unwanted signals (noise) caused by geologic and
cultural factors.
5.4.3.2Ambient and Geologic Sources of
Noise—Boulders, animal burrows, tree roots, or other
inhomogeneities can cause unwanted reflections or scattering of the
radar waves. Lateral and vertical variations in EM properties can
also be a source of noise.
5.4.3.3Cultural Sources of Noise—Above-ground
cultural sources of noise include reflections from nearby vehicles,
buildings, fences, power lines, lampposts, and trees. In cases
where this kind of interference is present in the data, a shielded
antenna may be used to reduce the noise.
(1) Scrap metal at or near
the surface can cause interference or ringing in the radar data.
The presence of buried structures such as foundations,
reinforcement bars (rebar), cables, pipes, tanks, drums, and
tunnels under or near the survey line may also cause unwanted
reflections (clutter).
(2) In some cases, EM
transmissions from nearby cellular telephones, two-way radios,
television, and radio and microwave transmitters may induce noise
on the radar record.
(3)Other Sources of
Noise—Other sources of noise can be caused by the EM
coupling of the antenna with the earth and decoupling of the
antenna to the ground due to rough terrain, heavy vegetation, water
on the ground surface, or other changes in surface conditions.
5.4.3.4Summary—All possible sources of noise
present during a survey should be noted so that their effects can
be considered when processing and interpreting the data.
5.4.4Alternate Methods—The limitations
previously discussed may prohibit the effective use of the GPR
method, and other methods or non-geophysical methods may be
required to resolve the problem (see Guide D6429).
Note 1:The quality of the result produced by applying this
standard is dependent on the competence of the personnel performing
the work, and the suitability of the equipment and facilities used.
Agencies that meet the criteria of Practice D3740 are generally considered capable of
competent and objective testing/sampling/inspection/etc. Users of
this standard are cautioned that compliance with Practice
D3740 does not in itself
assure reliable results. Reliable results depend on many factors;
Practice D3740 provides a
means of evaluating some of those factors.
1. Scope
1.1Purpose and Application:
1.1.1 This guide covers the equipment,
field procedures, and interpretation methods for the assessment of
subsurface materials using the Ground Penetrating Radar (GPR)
Method. GPR is most often employed as a technique that uses
high-frequency electromagnetic (EM) waves (from 10 to 7000 MHz) to
acquire subsurface information. GPR detects changes in EM
properties (dielectric permittivity, conductivity, and magnetic
permeability), that in a geologic setting, are a function of soil
and rock material, water content, and bulk density. Data are
normally acquired using antennas placed on the ground surface or in
boreholes. The transmitting antenna radiates EM waves that
propagate in the subsurface and reflect from boundaries at which
there are EM property contrasts. The receiving GPR antenna records
the reflected waves over a selectable time range. The depths to the
reflecting interfaces are calculated from the arrival times in the
GPR data if the EM propagation velocity in the subsurface can be
estimated or measured.
1.1.2 GPR measurements as described in
this guide are used in geologic, engineering, hydrologic, and
environmental applications. The GPR method is used to map geologic
conditions that include depth to bedrock, depth to the water table
(Wright et al 1.2Limitations:
1.2.1 This guide provides an overview of
the GPR method. It does not address details of the theory, field
procedures, or interpretation of the data. References are included
for that purpose and are considered an essential part of this
guide. It is recommended that the user of the GPR method be
familiar with the relevant material within this guide and the
references cited in the text and with Guides D420, D5730, D5753, D6429, and D6235.
1.2.2 This guide is limited to the
commonly used approach to GPR measurements from the ground surface.
The method can be adapted for a number of special uses on ice
(Haeni et al 1.2.3 The approaches suggested in this
guide for using GPR are the most commonly used, widely accepted,
and proven; however, other approaches or modifications to using GPR
that are technically sound may be substituted if technically
justified and documented.
1.3Units—The values stated in SI units are
to be regarded as standard. The values given in parentheses are
provided for information only and are not considered standard.
Reporting of test results in units other than SI shall not be
regarded as nonconformance with this standard.
1.4This guide offers an organized collection of
information or a series of options and does not recommend a
specific course of action. This document cannot replace education
or experience and should be used in conjunction with professional
judgment. Not all aspects of this guide may be applicable in all
circumstances. This ASTM standard is not intended to represent or
replace the standard of care by which the adequacy of a given
professional service must be judged, nor should this document be
applied without consideration of a project’s many unique aspects.
The word “Standard” in the title of this document means only that
the document has been approved through the ASTM consensus
process.
1.5This standard does not purport to address all of
the safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish
appropriate safety, health, and environmental practices and
determine the applicability of regulatory limitations prior to
use.
1.5.1It
is the responsibility of the user of this standard to follow any
precautions in the equipment manufacturer's recommendations and to
establish appropriate health and safety practices.
1.5.2If
this standard is used at sites with hazardous materials,
operations, or equipment, it is the responsibility of the user of
this standard to establish appropriate safety and health practices
and to determine the applicability of any regulations prior to
use.
1.6This international standard was
developed in accordance with internationally recognized principles
on standardization established in the Decision on Principles for
the Development of International Standards, Guides and
Recommendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
Standard Practice for Minimum
Requirements for Agencies Engaged in Testing and/or Inspection of
Soil and Rock as Used in Engineering Design and Construction
Standard Guide for Site Characterization
for Engineering Design and Construction Purposes
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