Geoscience Reference
In-Depth Information
Transmit
primary
Object
secondary
Receiver
vactor sum
Z
Z
Z
Y
Y =
Y +
X
X
X
(b) Received field as a vector sum of the primary and secondary fields
Z
Tilt angle
X
(a) Parameters associated with ellipsoidal representation of the
amplitude and vertical orientation of the field
fIGURe 6.5
The vector nature of the electromagnetic field: (a) the primary magnetic field vector, the sec-
ondary field vector, and their sum; and (b) the ellipsoid that represents the direction, amplitude, and tilt of the
measured field.
If the amplitude and orientation of the field are measured, then the field can be viewed as an
ellipsoid of revolution, as shown in Figure 6.5b. The parameters measured at the receiver are (a) the
maximum amplitude corresponding to the major axis of the ellipsoid; (b) the minimum amplitude,
which corresponds to the minor axis of the ellipsoid; and (c) the tilt angle of the major axis with
respect to the horizontal ground surface. In some cases, the azimuthal (x-y position) orientation of
the major axis may also be measured.
Field instruments are designed to separate (or normalize) the primary and secondary EM fields,
because the secondary field is most important for detecting an object in the subsurface. In addi-
tion to the primary field and secondary field, some systems utilize the in-phase and out-of-phase
components, or the real and imaginary components. The out-of-phase component is also sometimes
referred to as the quadrature component. All of these definitions are related to the complex nature
(oscillating, time-varying) of the EM field when it is referenced to the primary field.
One distinguishing characteristic of various EM methods is their operating domain and the
frequency, or time period, of the signal transmitted and received. The operating domain refers to
time or frequency domain designation, which in theory should provide equivalent results because
the two domains are mathematically related by the Fourier Transform. The principles of the time
and frequency domains are illustrated and contrasted in Figure 6.6. A time domain input signal is
generally a square wave with a positive and negative polarity. The input signal is typically a few
hundred milliseconds long, and may be as long as a second or more; the time period depends upon
the application, with longer times used to investigate deeper into the earth. As shown in Figure 6.6a,
the received signal is no longer a square wave. After the input signal is turned off, there is a decay
and delay of the signal over time. This shape of the decay curve is a function of changes in the signal
as it travels into the earth and encounters objects with different electrical conductivity values. The
received signal (output decay) is recorded on a separate coil and the signal amplitude measured at
different delay times. These amplitude decay signals are interpreted as a function of the subsurface
distribution of electrical conductivity.
