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6.1 Depletion alteration halo around a dyke

This model shows the results of emplacing a dyke in an area of refolded folds. The refolded fold patterns are similar to those seen in the type D model of section 3.7. The density and susceptibility values are modelled as depletion haloes where the rock properties are varied as a function of distance from the dyke, before returning to normal as the distance away increases.
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	LOAD JPEG IMAGE Block diagram of unaltered geology Block diagram showing the geology of the model with dyke intrusion but no alteration halo. The top layer has been removed to show the internal structure of the model. Colours are used simply to highlight the structures.
LOAD JPEG IMAGE Block diagram showing density variations in altered geology Block diagram showing the geology of the model with dyke intrusion and alteration halo. The top layer has been removed to show the internal structure of the model. Colours are used to demonstrate density variations, using a rainbow look up table.		LOAD JPEG IMAGE Block diagram showing magnetic susceptibility variations in altered geology Block diagram showing the geology of the model with dyke intrusion but no alteration halo. The top layer has been removed to show the internal structure of the model. Colours are used to demonstrate magnetic susceptibility variations, using a rainbow look up table.

Link	Gravity	Magnetics	Magnetics	Magnetics at Pole	Magnetics at Pole 1VD
No alteration	LOAD JPEG IMAGE	LOAD JPEG IMAGE	LOAD JPEG IMAGE	LOAD JPEG IMAGE	LOAD JPEG IMAGE
	Note the constructive interference between the dyke and the background layers that produces a local high where the dyke cuts the layers.				Note the appearance of a variation in intensities along the dyke.
Alteration	LOAD JPEG IMAGE	LOAD JPEG IMAGE	LOAD JPEG IMAGE	LOAD JPEG IMAGE	LOAD JPEG IMAGE
	Note the destructive nature of the alteration halo.	Note the destructive nature of the alteration halo.	Note the destructive nature of the alteration halo.	Note the destructive nature of the alteration halo.	Note the appearance of a variation in intensities along the dyke.

Key	Survey Specifications	Scales
yellow k = 10-2 r = 3.5 blue k = 10-2 r = 2.5 dyke k = 10-2 r = 3.5 image width 10,000 m	inclination 50° or 90° intensities 50,000 or 70,000 gamma flying height 80 m	max min max min

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