This
page is designed to provide a brief overview of Ground-penetrating
Radar. There is also a short FAQ that will
hopefully help you to decide if GPR is right for you and your project.
If you have further questions, plea
se contact
us.
Introduction to GPR mapping
Ground-penetrating
radar data are acquired by transmitting pulses of radar energy into the ground
from a surface antenna, reflecting the energy off buried objects, features,
or bedding contacts and then detecting the reflected waves back at the ground
surfac e with a receiving antenna. When collecting radar reflection data,
surface radar antennas are moved along the ground in transects within a surveyed
grid and a large number of subsurface reflections are collected along each line.
As radar energ y moves through various materials, the velocity of the waves
will change depending on the physical and chemical properties of the material
through which they are traveling. The greater the contrast in electrical
(and to some extent magnetic) propert ies between two materials at an interface,
the stronger the reflected signal, and therefore the greater the amplitude of
reflected waves. When travel times of energy pulses are measured, and
their velocity through the ground is known, distance (or d epth in the ground)
can be accurately measured. Each time a radar pulse traverses a material
with a different composition or water saturation, the velocity will change and
a portion of the radar energy will reflect back to the surface and be recorde
d. The remaining energy will continue to pass into the ground to be further
reflected, until it finally dissipates with depth.
The GPR system used in most of our work is a Geophysical
Survey System Inc. (GSSI) Subsurface Interface Radar-2000 (SIR-2000) that
employs antennas housed in a fiberglass sled. Radar energy is transmitted
to and from the radar control system and computer by a cable. All data
is collected digitally a nd transferred to CD-ROM as an archive.
The success of GPR surveys in archaeology is largely dependent
on soil and sediment mineralogy, clay cont
ent, ground moisture, depth of
burial, and surface topography and vegetation. Electrically conductive
or highly magnetic materials will quickly dissipate radar energy and prevent
its transmission to depth. The best conditions for energy propag
ation
are therefore dry sediments and soil, especially those without an abundance
of clay.
The
depth to which radar energy can penetrate, and the amount of resolution
that can be expected
in the subsurface, is partially controlled by the
frequency (and therefore the wavelength) of the radar energy transmitted.
Standard GPR antennas propagate radar energy that varies in frequency from
about 10 megahertz (MHz) to 1000 MHz. Low f
requency antennas (10-120
MHz) generate long wavelength radar energy that can penetrate up to 50
m in certain conditions, but are capable of resolving only very large buried
features. In contrast, the maximum depth of penetration of a 900
MHz antenn
a is about one meter or less in typical materials, but its generated
reflections can resolve features with a maximum dimension of a few centimeters.
A trade off therefore exists between depth of penetration and subsurface
resolution. In most s
urveys we use a 500 MHz antenna, which produces
data of good resolution at depths ranging from 30-40 cm to over 1 meter.
Ground-penetrating
radar data are acquired by transmitting pulses of radar energy into the ground
from a surface antenna, reflecting the energy off buried objects, features,
or bedding contacts and then detecting the reflected waves back at the ground
surfac e with a receiving antenna. When collecting radar reflection data,
surface radar antennas are moved along the ground in transects within a surveyed
grid and a large number of subsurface reflections are collected along each line.
As radar energ y moves through various materials, the velocity of the waves
will change depending on the physical and chemical properties of the material
through which they are traveling. The greater the contrast in electrical
(and to some extent magnetic) propert ies between two materials at an interface,
the stronger the reflected signal, and therefore the greater the amplitude of
reflected waves. When travel times of energy pulses are measured, and
their velocity through the ground is known, distance (or d epth in the ground)
can be accurately measured. Each time a radar pulse traverses a material
with a different composition or water saturation, the velocity will change and
a portion of the radar energy will reflect back to the surface and be recorde
d. The remaining energy will continue to pass into the ground to be further
reflected, until it finally dissipates with depth.
The
depth to which radar energy can penetrate, and the amount of resolution
that can be expected
in the subsurface, is partially controlled by the
frequency (and therefore the wavelength) of the radar energy transmitted.
Standard GPR antennas propagate radar energy that varies in frequency from
about 10 megahertz (MHz) to 1000 MHz. Low f
requency antennas (10-120
MHz) generate long wavelength radar energy that can penetrate up to 50
m in certain conditions, but are capable of resolving only very large buried
features. In contrast, the maximum depth of penetration of a 900
MHz antenn
a is about one meter or less in typical materials, but its generated
reflections can resolve features with a maximum dimension of a few centimeters.
A trade off therefore exists between depth of penetration and subsurface
resolution. In most s
urveys we use a 500 MHz antenna, which produces
data of good resolution at depths ranging from 30-40 cm to over 1 meter.