Geoscience Reference
In-Depth Information
Infinite line
Loop interior
Loop exterior
Grounded line
(electric bipole)
Long lines
Large loop
Vertical axis loop
Horizontal axis loop
E
y
E
x
Magnetic dipoles
Electric dipoles
(a) Line and Loop Basic Definitions
Electric field
Transmitter and Receiver
Magnetic field
Transmitter and Receiver
Vertical axis, co-planer
Transverse electric dipoles
Transverse axis
Parallel electric dipoles
Horizontal axis, in-line
Line transmitter, dipole receiver
Horizontal axis, tandem, co-planer
(b) Electric and Magnetic Field Measurement Configurations
fIGURe 6.7
Electromagnetic electric field line and magnetic field loop (a) definitions and (b) configurations.
to the distance from the transceiver, as shown in Figure 6.7b, then the electric field transceiver is
called an electric dipole. The designation of a transceiver as a dipole, or conversely a long line, is
determined by the proximity of the observer to the transceiver. A magnetic loop is called a dipole if
the receiver is placed a long distance from the loop relative to the diameter of the loop. A good rule
of thumb is that if the receiver is placed a distance that is greater than ten times the diameter of the
transmitter loop, then the transmitter can be considered to be a dipole. Theoretically, geophysicists
often refer to this as the far-field. In practice, the EM fields are more uniformly distributed in the
far-field, and this makes interpretation of measurements in the far-field much easier than those in
the near-field. The same far-field rule holds for an electric field dipole. Conversely, if the diameter
of a loop is very large, and the observer is very close to the loop, then the “loop” approximates a
