Geoscience Reference
In-Depth Information
ε
−
+
ε
r
2
r
1
T
=−
1
ε
ε
r
2
r
1
where ε
r
1
is the dielectric constant of the first layer through which the radar wave is traveling
before it encounters the subsurface interface, and ε
r
2
is the dielectric constant of the second
layer on the opposite side of the interface.
Electrical Conductivity
The capability of a material to transmit electric current.
Electrical Resistivity
The capability of a material to oppose the transmission of electric current.
Electric Current
Flow of electric charge. Charge carriers can be electrons or dissolved electro-
lytes (ions). Electric current is generally either unidirectional (direct current) or alternates in
direction with a given frequency (alternating current).
Electric Field
(Also referred to as the
electric potential gradient.
) A vector quantity having a mag-
nitude at any point defined by the change in electric potential per unit distance.
Electric Potential
Potential energy for a unit electric charge due to its position at some point within
an electric field.
Electric Potential Difference
The difference in electric potential between two points in an electric
field.
Electrolyte
A chemical compound capable of dissociating into ions when dissolved in solution.
These ions can then act as electric charge carriers for the transmission of electric current
through the solution. The common dissolved ions found in soil solution are SO
4
2−
, Cl
−
,
HCO
3
−
, NO
3
−
, PO
4
3−
, Ca
2+
, Mg
2+
, K
+
, Na
+
, and NH
4
+
.
Electromagnetic Induction (EMI) Methods
Geophysical investigation methods used to measure
subsurface electrical conductivity (or its inverse, electrical resistivity). An instrument called
a ground conductivity meter is commonly employed for the relatively shallow investiga-
tions needed for agricultural purposes. In regard to operation, an alternating electrical cur-
rent is passed through one of two small electric wire coils spaced a set distance apart and
housed within the ground conductivity meter, which is positioned at, or a short distance
above, the ground surface. The applied current produces an electromagnetic field around the
“transmitting” coil, with a portion of the electromagnetic field extending into the subsur-
face. This electromagnetic field, called the primary field, induces an alternating electrical
current within 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 resultant 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).
Electromagnetic Wave Skin Depth
The depth beneath the ground surface at which a plane elec-
tromagnetic wave's amplitude has been reduced to 1/
e
(approximately 37 percent) of its
surface value.
Field Capacity
An initially
saturated or near-saturated soil that is freely capable of being drained
reaches a volumetric water content called “field capacity” when all the possible gravity
drainable water has been removed from the soil.
Field-Scale Experiments
(With respect to agriculture.) Experiments often conducted on farms,
where the land area being investigated is of a field size generally associated with tradi-
tional agricultural practices. Field-scale experiments will, when needed, utilize typical farm
equipment.
Fragipan
A high bulk density layer within the soil profile that can impede root growth and the
vertical movement of water.
Frequency
Wave cycle repetition rate.
