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					The use of curvature in potential-field interpretation
Exploration Geophysics, 2007, 38, 111–119
Phillips, Hansen, & Blakely

Abstract. Potential-field anomalies can be transformed into special functions that
form peaks and ridges over isolated sources. All special functions have a common
mathematical form over an isolated source, which leads to a common equation for
estimating the source depth from the peak value and the curvature at the peak.
Model-specific special functions, usually calculated from a transformed version of a
potential field, are used to estimate the locations of very specific source types.
Model-independent special functions calculated from an observed or transformed
potential field can be used to estimate the locations of a variety of source types.
Vertical integration is a particularly useful transformation for reducing the effects of
noise and increasing the coherency of solutions from model-independent special
functions. For gridded data, the eigenvalues and eigenvectors of the curvature
matrix associated with a quadratic surface that is fitted to a special function within
3×3 windows can be used to locate the sources and estimate their depths and
strikes. Discrete source locations estimated in this manner can be connected into
lines that follow contacts, faults, and other mappable features based on distance
and azimuth criteria. These concepts are demonstrated on aeromagnetic data from
the Albuquerque basin of New Mexico, USA.
The use of curvature in potential-field interpretation, Exploration Geophysics, 2007, 38, 111–119. Phillips, Hansen, & Blakely.
The use of curvature in potential-field interpretation, Exploration Geophysics, 2007, 38, 111–119. Phillips, Hansen, & Blakely.
The use of curvature in potential-field interpretation, Exploration Geophysics, 2007, 38, 111–119. Phillips, Hansen, & Blakely.
The use of curvature in potential-field interpretation, Exploration Geophysics, 2007, 38, 111–119. Phillips, Hansen, & Blakely.
The use of curvature in potential-field interpretation, Exploration Geophysics, 2007, 38, 111–119. Phillips, Hansen, & Blakely.
The use of curvature in potential-field interpretation, Exploration Geophysics, 2007, 38, 111–119. Phillips, Hansen, & Blakely.
The use of curvature in potential-field interpretation, Exploration Geophysics, 2007, 38, 111–119. Phillips, Hansen, & Blakely.
From: The use of curvature in
potential-field interpretation
Exploration Geophysics, 2007,
38, 111–119. Phillips, Hansen &
Blakely.




Comparison of results
A composite view of the estimated contact and fault locations (Figure 4d) shows
how the total gradient solutions from the half vertical integral of the TMI (in
green) and the local wavenumber solutions from the first vertical integral of the
TMI (in blue) typically plot close together. The horizontal gradient solutions
from the reduced-to-pole field (in red) tend to be the most coherent and most
easily interpreted. They are typically offset from the blue and green solutions,
most likely due to non-vertical dips on the faults and contacts. Model studies
(Phillips, 2000) indicate that, for magnetisations collinear with the inducing field,
the offset of the HGM solutions should be in the down-dip direction.
From Phillips’: USGS_SFDEPTH GX – as implemented in Oasis Montaj
The following model-specific special functions are supported :

Assumed_Source_Type           SI Transform Model-Specific_Special_Function

Vertical_Magnetic_Contact     0 RTP      HGM of RTP magnetic field
Vertical Magnetic Sheet       1 RTP     ABS of RTP magnetic field
Horizontal Magnetic Sheet      1 RTP+VI HGM of VI of RTP magnetic field
Horizontal Magnetic Line       2 RTP+VI ABS of VI of RTP magnetic field
Vertical Magnetic Line         2 RTP+VI ABS of VI of RTP magnetic field
Magnetic Dipole               3 RTP+VI ABS of VI of RTP magnetic field
Vertical Density Contact      -1 VD     HGM of VD of gravity field
Vertical Density Sheet         0 VD     ABS of VD of gravity field
Horizontal Density Sheet       0 None   HGM of gravity field
Horizontal Density Line        1 None   ABS of gravity field
Vertical Density Line         1 None    ABS of gravity field
Point Mass                    2 None    ABS of gravity field

Model-independent special functions include the Total Gradient (TG) and the
Local Wavenumber (LW). These are calculated directly from the potential
field or from a vertical integral (VI) of the potential field. The total gradient
requires that a structural index (SI) be assumed for the source.

				
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posted:9/16/2011
language:English
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