Geoscience Reference
In-Depth Information
depths down to 3 or 4 km. A geophysical survey conducted as part of a hydrological investigation
to determine groundwater resources usually has an investigation depth no greater than 300 m.
Geophysical investigation depths for environmental, geotechnical engineering, and archeological
applications typically do not exceed 30 m. Agricultural geophysics tends to be heavily focused on a
2 m zone directly beneath the ground surface, which includes the crop root zone and all, or at least
most, of the soil profile.
With regard to the application of geophysics to agriculture, this extremely shallow 2 m depth of
interest is certainly an advantage, in one sense because most geophysical methods have investiga-
tion depth capabilities that far exceed 2 m. However, there are complexities associated with agri-
culture geophysics not typically encountered with the application of geophysical methods to other
industries or disciplines. One such complexity involves transient soil temperature and moisture con-
ditions that can appreciably alter the values of measured geophysical quantities over a period of days
or even hours. Additionally, physical quantities measured in the soil environment with geophysical
methods often exhibit substantial variability over very short horizontal and vertical distances.
1.2 GeophySICAl MethodS ApplICAble to AGRICUltURe
Geophysical methods can be classified as passive or active. There is no artificial application of
energy with passive geophysical methods. On the other hand, active geophysical methods do require
the artificial application of some form of energy. The three geophysical methods predominantly
used for agricultural purposes are resistivity, electromagnetic induction, and ground-penetrating
radar. All three of these predominantly employed methods are active, and each is summarized
within this topic; resistivity in Chapter 5, electromagnetic induction in Chapter 6, and ground-pen-
etrating radar in Chapter 7. Chapter 8 provides shorter descriptions of three additional geophysical
methods: magnetrometry (passive), self-potential (passive), and seismic (active), all of which have
the potential for substantial future use in agriculture, but at present are being employed sparingly
or not at all for agricultural purposes. To provide an introduction, the six geophysical methods—
resistivity, electromagnetic induction, ground-penetrating radar, magnetometry, self-potential, and
seismic—are all concisely defined as follows.
1.2.1 R
e s i s t i v i t y
M
e t h o d s
Resistivity methods measure the electrical resistivity, or its inverse, electrical conductivity, for a bulk
volume of soil directly beneath the surface. Resistivity methods basically gather data on the sub-
surface electric field produced by the artificial application of electric current into the ground. With
the conventional resistivity method, an electrical current is supplied between two metal electrode
stakes partially inserted at the ground surface, while voltage is concurrently measured between a
separate pair of metal electrode stakes also inserted at the surface. The current, voltage, electrode
spacing, and electrode configuration are then used to calculate a bulk soil electrical resistivity (or
conductivity) value.
1.2.2
e
l e c t R o M a g n e t i c
i
n d u c t i o n
M
e t h o d s
Electromagnetic induction methods also measure the electrical conductivity (or resistivity) for a
bulk volume of soil directly beneath the surface. An instrument called a ground conductivity meter
is commonly employed for relatively shallow electromagnetic induction investigations. In opera-
tion, an alternating electrical current is passed through one of two small electric wire coils spaced a
set distance apart and housed within the ground conductivity meter that 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 subsurface.
This electromagnetic field, called the primary field, induces an alternating electrical current within
