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

 

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