Geoscience Reference
In-Depth Information
the ground, in turn producing a secondary electromagnetic field. Part of the secondary field spreads
back to the surface and the air above. The second wire coil acts as a receiver measuring the resul-
tant amplitude and phase components of both the primary and secondary fields. The amplitude and
phase differences between the primary and resultant fields are then used, along with the intercoil
spacing, to calculate an “apparent” value for soil electrical conductivity (or resistivity).
1.2.3 g
R o u n d
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e n e t R a t i n g
R
a d a R
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With the ground-penetrating radar (GPR) method, an electromagnetic radio energy (radar) pulse
is directed into the subsurface, followed by measurement of the elapsed time taken by the radar
signal as it travels downward from the transmitting antenna, partially reflects off a buried feature,
and eventually returns to the surface, where it is picked up by a receiving antenna. Reflections from
different depths produce a signal trace, which is a function of radar wave amplitude versus time.
Radar waves that travel along direct and refracted paths through both air and ground from the
transmitting antenna to the receiving antenna are also included as part of the signal trace. Antenna
frequency, soil moisture conditions, clay content, salinity, and the amount of iron oxide present have
a substantial influence on the distance beneath the surface to which the radar signal penetrates.
The dielectric constant of a material governs the velocity for the radar signal traveling through that
material. Differences in the dielectric constant across a subsurface discontinuity feature control the
amount of reflected radar energy, and hence radar wave amplitude, returning to the surface. As an
end product, radar signal amplitude data are plotted on depth sections or areal maps to gain insight
on below-ground conditions or to provide information on the position and character of a subsurface
feature.
1.2.4 M
a g n e t o M e t R y
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This geophysical method employs a sensor, called a magnetometer, to measure the strength of the
Earth's magnetic field. Anomalies in the Earth's magnetic field indicate the presence of subsurface
features. An anomaly is produced when a subsurface feature has a remanent magnetism or magnetic
susceptibility that is different from its surroundings. A gradiometer is an instrument setup com-
posed of two magnetometer sensors mounted a set distance apart. Gradiometers are typically used
to measure the vertical gradient of the magnetic field, which is not affected by transient magnetic
field changes. In comparison to a single magnetometer sensor, the gradiometer has the additional
advantage of being better adapted for emphasizing magnetic field anomalies from shallow sources.
1.2.5 s
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Self-potential methods collect information on a naturally occurring electric field associated with
nonartificial electric currents moving through the ground. Unlike resistivity methods, no electric
power source is required. Naturally occurring electric potential gradients can arise a number of dif-
ferent ways, including the subsurface flow of water containing dissolved ions, spatial concentration
differences of dissolved ions present in subsurface waters, and electrochemical interactions between
mineral ore bodies and dissolved ions in subsurface waters. Self-potential methods are fairly simple
operationally. All that is required to obtain information on a natural electric field below ground is
the voltage measurement between two nonpolarizing electrodes placed or inserted at the ground
surface.
1.2.6 s
e i s M i c
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Seismic methods employ explosive, impact, vibratory, and acoustic energy sources to introduce
elastic (or seismic) waves into the ground. These seismic waves are essentially elastic vibrations that
propagate through soil and rock materials. The seismic waves are timed as they travel through the
