Introduction:

Conductivity meters are electromagnetic induction tools that, like magnetometers, are
used extensively to detect buried munitions. Conductivity meters have an advantage
over magnetometers in that they are not limited to detecting ferrous items. They are
also useful in detecting non-ferrous metallic items. When a metallic object is
subjected to a varying magnetic field, eddy currents are induced within the object.
Conductivity meters detect buried munitions by measuring the secondary magnetic
field produced by these eddy currents. The performance of conductivity meters is
seriously degraded in areas underlain by shallow, mineralized groundwater. Since
conductivity meters generate an electronic signal they are "active" devices. There are
basically two types of conductivity meters:

Frequency Domain Conductivity Meters
Frequency domain conductivity meters produce electromagnetic waves that pass
through the subsurface, causing eddy currents to form. The intensity and phase of
those eddy currents is a function of ground conductivity. Buried debris and/or
disturbed soil have conductivities different from the surrounding natural soil. It is
those conductivity differences that frequency domain conductivity meters detect.

Frequency domain instruments are useful for detecting large buried caches of
munitions, detecting disturbed earth associated with pits and trenches, and are the
best geophysical tool available for detecting very small, very close objects such as
the metal firing pins in plastic land mines buried just beneath the ground surface.
However, since the resolution ability decreases dramatically with depth, frequency
domain conductivity meters are not optimum for detecting individual, deeply buried
munitions. Most commercial coin detectors are frequency domain conductivity
meters.

Time Domain Conductivity Meters
Time domain conductivity meters produce and measure an electromagnetic wave similar to that of frequency domain systems. A major difference is in the system waveforms used. Typically, a half-duty cycle waveform is used, and measurements made during the time the transmitter is off.
The instrument locates metal by inducing a current in the ground and observing its
decay with time. The detector portion of the instrument is tuned to sense only a
specific portion of the response curve, which greatly reduces noise and improves
signal detection for certain buried objects.