Electromagnetic induction (EM)
instruments are utilized in many different types of geological
and environmental applications. These include shallow soils mapping,
soil-salinity mapping, ground water investigations, and the detection
and delineation of waste pits and associated subsurface contaminants
from acids, salts or VOC’s. They have also been used extensively
for the detection of conductive geologic media such as clays and
ferrous mineral deposits, as well as for the detection of resistive
geologic media like gravel deposits. In addition, the systems are
used for near-surface archaeological investigations and the detection
of buried structures such as building foundations, as well as for
the detection of highly conductive metallic objects like steel
drums, tanks, large metallic utilities and other nondescript buried
ferrous metallic objects.
EM Method
EM instruments contain two sets of coils that are located within
opposite sides of the tool. One set of coils is used to transmit
a primary magnetic field, which generates electrical current in
the ground. The created current then generates a secondary magnetic
field, which is sensed by the coils in the receiver end of the
instrument. Data is then collected on a control unit indicating
the conductivity of the earth.
The fundamental principle of electromagnetic induction is the
measurement of the change in mutual impedance “Q” (or
mutual coupling) between a pair of coils above the earth. For the
most part, symmetrical, moving source dipole-dipole frequency domain
instruments are used to measure subsurface conductivity. They operate
by driving a transmitter coil with an AC current at audio frequencies
to generate a sinusoidal time-varying magnetic field. A receiver
coil is positioned on or near the surface of the earth some distance
away from the transmitter coil. The transmitted time-varying magnetic
field generated by the transmitter coil induces secondary currents
to flow in the subsurface, which in turn generate a secondary (induced)
magnetic field. Both the induced secondary field, along with
the primary field, is detected and recorded at the receiver coil.
The magnitude of the secondary field is broken into two orthogonal
components. The two components of the secondary magnetic field are
the In-phase (real component) and the Quadrature or out-of-phase
component (imaginary component). For instruments operating within
the Low Induction Number (LIN) approximation, the magnitude of the
Quadrature component of the secondary field is linearly proportional
to the apparent conductivity, sa. In the absence of a highly
conductive material (e.g. metallic targets) in the subsurface, the
magnitude of the in-phase component is dependant on the magnetic
susceptibility of the subsurface.
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