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2D Filtering

Oasis montaj’s 2D Filtering extension offers a set of 2D filters aiming to emphasize subtle features, separate regional

residual fields, separate signals representing different depths layers, remove noise, and produce different field

components as an aid for interpretation.

Namely these filters are:

1. MAGMAP: Fourier domain filters

2. Matched Filter: Depth slice separation filter

3. Analytic signal: Resolve cross section of two dimensional potential field structures

4. Generalized derivative: Curvature response of potential field structures

5. Tilt Derivative: Rough estimate of horizontal and vertical location of magnetic sources

6. Gradient to total magnetic intensity: Magnetic field reconstruction from gradient components

Oasis montaj Fourier Domain 2D Filtering

The montaj™ MAGMAP Filtering tool provides a 2D FFT filter library to allow

the application of common Fourier domain filters to grids. MAGMAP rapidly

processes and enhances gridded datasets by applying a wide range of robust

geophysical and mathematical filters. This tool lets you easily define your own

filters, modify the filter specific parameters and apply any number of filters

together in a single operation. A Spectral filter editor provides an interactive

means of modifying the filter parameters and instantly observe the effect both

on the radially averaged power spectrum (if symmetrical) and the 2D preview

window.

The MAGMAP Filters for gridded data can be divided into 2 categories:

Potential Field Filters

• Reduction of magnetic data to the magnetic pole or equator,

• Differential reduction of magnetic data to the magnetic pole appropriate for

very large grid,

• Upward/downward continuations to a different datum,

• Apparent magnetic susceptibility calculation for magnetic field,

• Apparent density calculation for residual gravity field,

• Optimum Weiner depth filter,

• Conversion between different directional components of the magnetic field.

Use MAGMAP Filtering to:

• Process and enhance gridded datasets by

applying a wide range of two dimensional

Fast Fourier Domain filters (FFT filters),

• Apply any combination of geophysical

filters, and/or mathematical filters,

concurrently,

• Modify selected filter parameters, using

the Interactive Spectral Filter Editor, and

observe live the effect of the filter on the

radially averaged power spectrum and the

preview window,

• Define and apply custom filters,

• Customize settings to your dataset • Interpret grids using spectral analysis

products (2D and radially averaged power

spectra) models or other sources.

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General Purpose Filters

• Integer and fractional derivate in X, Y, or Z directions

• Integration in X, Y & Z

• Low/High Bandpass filters

• De-corrugation and directional filters,

• Four different potential field components calculations,

• Custom radially symmetrical filters

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Interactive Filter Editor

An Interactive Spectral Filter Builder enables users to

design and fine tune filter parameters and instantly

observe the effect of the filter(s) in the preview window,

collectively or individually.

Only the radially symmetric filters, such as low/high pass

filters will be displayed on the spectrum graph. The initial

power spectrum, the final power spectrum, the filter

coefficients of the currently selected radially symmetric

filter and the filter coefficients of the combined filter will

be displayed in real time.

The 2D output preview window is resizable and is used to

view individual or combined effect of the filters.

Matched Filter

The Matched Filter tool leverages MAGMAP to help define interactively a set of complementary filters, each filter emphasizing the signal of a defined depth slice. The matched filters are tapered bandpass filters that collectively cover the entire spectrum.

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Horizontal and vertical derivatives of potential field data are routinely

measured, and can be calculated from the observed total magnetic intensity.

The next set of tools utilize a combination of these derivatives to enhance

specific signatures in the data and are an essential part of the interpretation process.

Analytic Signal The Analytic signal of a magnetic field is a common interpretation tool that

helps resolve the cross section of two dimensional potential field

structures.

.

Generalized Derivative This derivative operator is a linear combination of the horizontal and

vertical field derivatives, normalized by the analytic signal amplitude. It is

indicative of the curvature response of potential field structures.

Tilt Derivative The tilt derivatives can be used to roughly estimate the horizontal and

vertical location of magnetic bodies without taking into consideration the

nature of the magnetic source. The nature of the source can then be further investigated.

Gradient to Total Magnetic Intensity The residual magnetic field can be reconstructed from gradient

components, and used in conjunction with other data during the

interpretation process.

References

Cooper G., Cowan D.R., A generalized derivative operator for potential field data, Geophysical Prospecting, (2011) Vol

59, pp. 188-194

Hardwick, C.D., "Two Methods of Gradient-Enhanced

Gridding – A Discussion", Technical Paper., (2004)

Miller H.G. , Singh V.J., "Potential Field Tilt - a New Concept for Location of Potential Field Sources", Journal of Applied Geophysics, vol. 32 (1994), no. 2-3, pp. 213-217.

Verduzco B. et al., "New Insights into Magnetic Derivatives for Structural Mapping", The Leading Edge, vol. 23 (2004), pp. 116-119.

Salem A., Williams S., Fairhead J.D., Smith R., Ravat D., "Interpretation of magnetic data using tilt-angle derivatives", Geophysics, vol. 73 (2008), no. 1, pp. L1-L10.

Phillips J., Designing matched bandpass and azimuthal

filters for the separation of potential field anomalies by

source region and source type, ASEG extended abstract,

(2001)

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