Geoscience Reference
In-Depth Information
subsurface from the source to the sensors, called “geophones.” The energy source is positioned at
the surface or at a shallow depth. Geophones are typically inserted at the ground surface. Seismic
waves move through the subsurface from source to geophone along a variety of direct, refracted,
and reflected travel paths. The velocity of a seismic wave as it travels through a material is deter-
mined by the density and elastic properties for that particular material. Differences in the density
and elastic properties across a subsurface discontinuity feature control the amount of reflected or
refracted seismic energy, and hence the seismic wave amplitudes returning to the surface. Infor-
mation on the timed arrivals and amplitudes of the direct, refracted, and reflected seismic waves
measured by the geophones are then used to gain insight on below-ground conditions or to locate
and characterize subsurface features.
1.3
ASpeCtS of AGRICUltURAl GeophySICS dAtA ColleCtIon
And AnAlySIS
1.3.1 s
e l e c t i n g
t h e
P
R o P e R
g
e o P h y is i c a l
M
e t h o d
A clear goal must be defined in the initial planning stage of a geophysical survey regarding the soil
condition or subsurface feature information that needs to be acquired. In order to choose the proper
geophysical method for monitoring changing soil conditions, consideration must first be given to
the different physical properties responded to by the various geophysical methods and then whether
any of these physical properties are influenced by the soil condition of interest. Delineating a sub-
surface feature with geophysics requires there to be a contrast between the feature and its surround-
ings with respect to some physical property responded to by a geophysical method. To summarize,
the geophysical method selected must respond to a physical property that is in turn affected by
temporal changes in soil conditions or the spatial patterns of subsurface features; otherwise, use-
ful information cannot be obtained on these soil conditions or subsurface features of interest. For
example, soil cation exchange capacity (CEC) will often have a substantial impact on soil electrical
conductivity (or resistivity); therefore, resistivity or electromagnetic induction methods that measure
soil electrical conductivity may be useful for delineating spatial patterns in CEC. On the other hand,
magnetometry methods respond to anomalies in remanent magnetism or magnetic susceptibility,
properties that are not likely to be affected by CEC, and consequently, magnetometry methods
would not be a good choice for delineating spatial patterns in CEC.
1.3.2 i
n v e s t i g a t i of n
d
e P t h
a n d
f
e a t u R e
R
e s o l u t i o n
i
s s u e s
Once a geophysical method is chosen, there are usually options with respect to the equipment and its
setup. The investigation depth required and the size of the feature to be detected are two important
issues that should be taken into account when deciding on the equipment to use and its setup. There
is normally a trade-off between the investigation depth and the minimum size a feature must have
to be detected. Finding a large, deeply buried object or a small, shallow object with geophysical
methods is much easier than locating a small, deeply buried object. One potential example is the
use of GPR to locate buried plastic or clay tile agricultural drainage pipe. The radar signal pen-
etration depth and minimum size at which an object can be detected are both inversely related to
GPR antenna frequency. Low-frequency GPR antennas are better for locating larger deeply buried
objects, and high-frequency GPR antennas are more applicable for small, shallow objects. There-
fore, a GPR unit with 100 MHz transmitting and receiving antennas might work well at finding a
30 cm diameter drainage pipe 2 m beneath the surface in a clay soil, and a GPR unit with 250 MHz
transmitting and receiving antennas is likely capable of finding a 10 cm diameter drainage pipe
0.5 m beneath the surface in a clay soil. But, finding a 10 cm diameter drainage pipe 2 m beneath the
surface in a clay soil is probably an extremely difficult undertaking regardless of the GPR antenna
frequency employed.
