KIRTLAND AIR FORCE BASE ALBUQUERQUE, NEW MEXICO ...

APPENDIX B
Frequency Domain Measurements:
― The frequency domain EMI technique is a geophysical method in which a time-varying
electrical current is induced into the ground with a transmitter coil, and the resultant
magnetic fields are measured with a receiver coil. The flow of electrical current in the
ground can be interpreted in terms of the apparent ground conductivity.
― The transmitter coil induces small amplitude currents (eddy currents) in the earth. Most
EMI techniques employ frequencies from a few-hundred to several-thousand hertz (hz).
These eddy currents produce a magnetic field that is measured by a receiver coil located a
fixed distance from the transmitter coil. Under certain constraints, the measured response
is proportional to the ground conductivity. The quantity actually measured at the receiver
coil is the voltage of the signal, and is expressed in mvs, or equivalently in parts per
thousand of the transmitted signal.
― The electromagnetic instrument response at the receiver coil can be divided into two
distinct components differing in phase by 90 degrees. The in-phase response tends to
emphasize the presence of metallic materials. The quadrature (i.e., ―out-of-phase‖)
response most readily characterizes the apparent ground conductivity. The signal
amplitude for the quadrature and in-phase responses are dependent upon the transmitter
frequency, transmitter/receiver separation distance, and the conductivity of the ground.
Under certain constraints, the quadrature response is linearly related to the ground
conductivity.
― Conductivity contrasts in the earth can be caused by natural phenomena such as lithologic
changes, or by man-made phenomena such as disturbed ground, buried materials, or
contaminants in the soil or groundwater. Generally, man-made materials that are metallic
can produce large contrasts in conductivity with the surrounding geologic material.
― The EMI quadrature phase data are most useful for determining lateral changes in
conductivity that may be related to manmade waste materials and/or natural geologic
conditions. The factors that primarily affect the ground conductivity are the subsurface
moisture content, conductivity of the moisture, and type of material in the vicinity of the
instrument. EMI in-phase data are most useful for providing information on surface and
subsurface metallic material.
― The maximum depth of investigation of the EMI instrumentation is primarily a function of
the frequency employed and the transmitter/receiver coil separation. The EM-31
manufactured by Geonics Limited, utilizes a fixed frequency of 9.8 kilohertz (Khz) and a
coil separation of 12 ft. The maximum depth sensitivity of the EM-31 is approximately 18
ft. The EM-38, another system manufactured by Geonics Limited, uses a frequency of
14.2 Khz and a coil separation of 3 ft to achieve a maximum penetration (i.e. depth of
sensitivity) of approximately 5 ft. These 2 instruments are primarily used to costeffectively
delineate lateral conductivity changes at a site.
― If it is necessary to determine the change of conductivity with depth, other EMI
instrumentation exists. These techniques are often referred to as ―electromagnetic
sounding‖ techniques in the geophysical community. The data can be useful for
determining the approximate vertical contact between landfill materials and native
geology if a sufficient contrast in conductive properties exists. The method is not very
Kirtland AFB
SOPs for Field Investigations B-68 April 2004