WELLOG AFMAG
WHAT IS AFMAG:
AFMAG is an abbreviation
for Audio Frequency Magnetics”.
The AFMAG method is a
passive electromagnetic method that uses natural electromagnetic sources and
measurement of tilt angle. The method was originally introduced by Ward, S. H.
(1959) as cited in references listed below. The primary electromagnetic field
originates from audio frequency sources like lightning strikes that are
associated with worldwide thunderstorm activity. Other sources of natural and
man-made electromagnetic energy also contribute to the over-all ambient
electromagnetic energy. Extremely Low Frequencies referred to as ELF are used
for detection. The signals are random and are generally related to what is
commonly called static noise and more specifically magnetotelluric noise.
AFMAG signals are
generally composed of a small vertically polarized signal and a much larger
horizontally polarized signal. The exception is in the area surrounding a good
conductor. Because of the change in the direction of the electromagnetic energy
in proximity of a conductive body, the tilt-angle (tipper) technique is used.
A second method employs a receiver having a multi-channel audio spectrum
output.
AFMAG RECEIVER:
A receiver (low noise
audio amplifier) having a gain of 1000 to 2000 is used to amplify the signal
produced by a loop antenna system (search coil). In a typical AFMAG system, two
loop antennas mounted in a fixed 90 degree angle to each other are used.
Surveys are simplified due to the fact that only a receiver
and detection coil system are required to make measurements.
HOW AFMAG WORKS:
The antennas are
positioned vertically, and rotated about the vertical axis to establish “azimuth”
to the conductive body. Azimuth is usually measured as a clockwise angle
relative to true north. The second step in the procedure is to orient the
antenna in the horizontal plane. The antenna is rotated about the horizontal
axis to establish a tilt angle upward or downward. Finally a map is created
using data collected in different areas in order to depict the regional tilt
vectors. Interpretation of the tilt vectors provides the center and estimated
depth to a conductive body. A plot along a survey line will typically show tilt
angle ranging from +20 degrees thru zero to -20 degrees.
Survey lines may also be
plotted on hardcopy showing multi-channel (multi-frequency) AFMAG signal
response over a mineralized zone. Tilt measurements are made at two
frequencies. The ratio of low/high response provides a measurement of the
conductivity of a conductor in the ground.
AFMAG APPLICATIONS:
AFMAG is used in locating
Massive Sulfide mineral Deposits including Volcanogenic Massive Sulfide
deposits. The method is applied in ground surveys and Airborne surveys to
define fault/shear anomalies and areas having contrasting conductivities. One
source discusses the use of two audio frequencies – one low frequency, one high
frequency for example 90 and 340 Hz. A conductivity ratio results from dividing
the LF tilt angle by the HF tilt angle.
References:
Basic Exploration
Geophysics, Edwin S. Robinson, John Wiley and Sons, 1988
Principles of Applied
Geophysics, D. S. Parasnis, Chapman & Hall, Ltd.
1972
Applied Geophysics, W. M.
Telford, L.P. Geldart, R.E. Sheriff, D.A. Keys,
Cambridge University Press, 1982
Mining Geophysics, D.S. Parasnis, Elsevier Scientific Publishing Company, 1973
REVISED 11-24-2018
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