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New insights into magnetic derivatives for structural mapping

University of Leeds, U.K.

J. Derek Fairhead and Chris M. Green

GETECH, Leeds, U.K.

Chris MacKenzie

BAFEX Exploration, Windhoek, Namibia

Corresponding author: jdf@getech.com

The first 20% of the full text of this article appears below.

Magnetic and gravity derivatives can be likened to seismic attributes in that they can help define/estimate the physical properties of the source structure causing the anomaly. This contribution looks at the tilt derivative, first reported in 1994 and more recently used to derive the local wavenumber (1997). We will show that the combination of the tilt derivative and its total horizontal derivative are highly suitable for mapping shallow basement structure and mineral exploration targets and that they have distinct advantages over many conventional derivatives. We provide the simple theory behind the derivatives, use a range of simple 2D models to illustrate their response, and apply them to mapping a mineral target in Namibia.

	Tilt derivative (TDR)

 
The physical properties of a magnetic source structure that can be determined from a grid of magnetic data include its shape (plan), location of its edges, depth to top edges, dip, and rock susceptibility contrast. This contribution will focus on the first two—i.e. shape and edge detection. The problems to be overcome in data enhancement are to identify and map (a) subtle anomalies attenuated in the dynamic range due to the presence of high amplitude magnetic anomalies, (b) the continuity of individual bodies where there are lateral changes in susceptibility and/or depth of burial, and (c) the edges of structures by adequately accounting for the nature of the rock magnetization. Rock magnetization is a vector quantity that can consist of both remanent and geomagnetically induced components. The presence of the remanent component can adversely affect the shape of the magnetic field response and result in spurious derivatives, if one has assumed that only the induced component is present. Fortunately, basement rocks are usually dominated by the induced component, whereas mineralized zones often host long-lived remanent components. The following theory and 2D model examples show how these . . . [Full Text of this Article]