Im vwwwv

Time

Time

Receive coil

Receive coil

Primary signal

Time decay channels

Time

(a) Time Domain Input-output

Primary signal

Time decay channels

Time

Time wvwwv

Time

(a) Frequency Domain Input-output Signals

FIGURE 6.6 The time and frequency domains transmit and receive signals: (a) time domain transmits (input) and receives (output) signals, and (b) frequency domain transmits and receives signals.

Frequency domain systems utilize an input signal that is a sinusoidal wave with a fixed frequency (a square wave time domain signal is composed of many frequencies), as shown in Figure 6.6b. The signal recorded at the receive coil is delayed (called a phase shift) in time with a decreased amplitude. The amplitude and phase of the received signals are used to interpret the propagation path and induction of the signals that have traveled from the transmit coil, through the earth, to the receive coil. Some frequency domain systems transmit and receive several different frequencies in sequence, as illustrated by the low-frequency and high-frequency signals shown in the figure. Lower-frequency signals penetrate more deeply into the earth than higher-frequency signals; hence, the depth of investigation can be somewhat predicted by the chosen frequency. Typical frequencies range from a few hundred Hz for intermediate-depth investigations (approximately 10 to 100 m) to several hundred kilohertz for very shallow soil conductivity determinations.

So far, we only illustrated the use of coils for transmitting and receiving EM fields. However, in many geophysical applications, EM signals are transmitted and received by straight wires (commonly called lines) or by loops (or coils) of wire. Most lines have electrodes in the ground at each end of the line, as shown in Figure 6.7. If the grounded ends are very close together with respect r

Infinite line r

Infinite line

Grounded line (electric bipole)

Long lines

Grounded line (electric bipole)

Long lines

Loop interior

Loop exterior

Large loop

Large loop

Electric dipoles

Electric dipoles

Vertical axis loop

Vertical axis loop

Horizontal axis loop

Magnetic dipoles

Horizontal axis loop

Magnetic dipoles

(a) Line and Loop Basic Definitions

Electric field Transmitter and Receiver

Electric field Transmitter and Receiver

Transverse electric dipoles

Transverse electric dipoles

Parallel electric dipoles

Parallel electric dipoles

Line transmitter, dipole receiver

Magnetic field Transmitter and Receiver

Magnetic field Transmitter and Receiver

Vertical axis, co-planer

Vertical axis, co-planer

Transverse axis

Transverse axis

Horizontal axis, in-line

Line transmitter, dipole receiver

Horizontal axis, tandem, co-planer (b) Electric and Magnetic Field Measurement Configurations

Horizontal axis, in-line

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

(a) Ground conductivity
(d) Shallow-intermediate depth continuous mode person carried

(b) Shallow-intermediate depth

(e) Intermediate depth fixed station

(e) Intermediate depth fixed station

(c) Shallow-intermediate depth (person-pulled, or ATV towed)

(b) Shallow-intermediate depth

(c) Shallow-intermediate depth (person-pulled, or ATV towed)

(f) Towed array with GPS

FIGURE 6.8 Examples of commercially available time- and frequency-domain electromagnetic (EM) systems: (a) ground conductivity system (www.geonics.com/html/products.html), (b) frequency-domain intermediate-depth EM system (www.geophex.com/GEM-2/GEM-220home.htm), (c) shallow-intermediate-depth time-domain system (www.geonics.com/html/metaldetectors.html), (d) shallow-intermediate-depth person-portable system (www.geophex.com/GEM203/GEM-320home.htm), (e) intermediate-depth fixed station (www.geonics.com/html/conductivitymeters.html), and (f) towed array of five frequency-domain sensors (www.geophex.com/GEM-2/GEM-220home.htm).

line from the observer's viewpoint, and the EM fields measured by the observer are approximately the same as those that would be measured from a long line. In most cases, the manufacturer supplies an instrument with the transmitter and receiver located at a fixed size/spacing ratio, and the geophysicist does not have to be concerned with interpretation problems associated with not being in the far-field.

For purposes of agricultural applications, the most practical systems are handheld, or cart-mounted loop-loop systems. Examples of a few of the systems currently available are shown in Figure 6.8. Note that some of the systems operate in the time domain, and other systems utilize multiple frequencies and are called frequency domain systems.

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