Frogtech Potential Field Enhancments

This page explains the benefits of each filtering technique produced by Frogtech Geoscience.  Some of the filters are proprietary to Frogtech Geoscience and are therefore not discussed in any other text.

The enhancements produced have been developed by Geologists and Geophysicists with extensive experience, who combine the proprietary and standard filters to optimise geological interpretation by providing a greater insight to structural mapping and basement composition.

Enhancements are conducted on the core datasets, which are Digital Elevation Model, Bouguer corrected Gravity, Reduction to the Pole of Total Magnetic Intensity or in regions of low magnetic latitude either the Compound Anomaly of Total Magnetic Intensity or the Modulus of Total Components of Total Magnetic Intensity.

All images below are a small subset from the Australian Top End Potential Field Product.


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Reduction to the Pole (RTP)

The result of the Reduction to the Pole (RTP) is a core dataset and is displayed here to be viewed as the original dataset before an enhancement is applied.

An RTP operation is a magnetic field which would be observed if the given field had been observed with vertical polarisation, that is, as though observed at one of the Earth's magnetic poles. TMI data are routinely reduced to the pole to shift anomalies directly over their source. Therefore the location of sources, particularly source edges, can more readily be determined when the magnetic data has been reduced to the pole.


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First Vertical Derivative (1VD)

The first vertical derivative filter (1VD) reduces the effects of regional anomalies and enhances near surface contrasts by amplifying the high frequency component of the spectrum. The 1VD filter enhances anomalies caused by shallow sources and by the top parts of deep bodies or bodies with a large depth-extent. Anomaly peaks can be used to locate the centres of sources or the steeper sides of bodies. The zero contour lines can be used to locate boundaries of sources. The zero contour lines can be used to locate boundaries of sources.


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Band-pass Filter (BP)

A Band-pass (BP) filter passes frequencies within a certain range and substantially attenuates (reduces) frequencies outside that range. The 20 - 100 km BP filter (illustrated left) emphasises anomalies with wavelengths between 20 and 100 km. However, caution is warranted when interpreting the data, because signal may also arise from the aliasing of sources at other depths.


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High-pass Filter (HP)

A High-pass (HP) filter passes high frequencies, but substantially attenuates (or reduces) frequencies lower than the cut-off frequency. The 20 km HP filter (illustrated left) emphasises anomalies with wavelengths less than 20 km. 


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Low-pass Filter (LP)

A Low-pass (LP) filter passes low frequencies but substantially attenuates (or reduces) frequencies higher than the cut-off frequency. The 20 km LP filter (illustrated left) emphasises anomalies with wavelengths greater than 20 km. However, caution is warranted when interpreting the data, because the signals may also arise from aliasing of shallower sources. 


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Horizontal Gradient North-South (1HDNS)

The Horizontal Derivative (HD) filter reduces regional variation and enhances signals from shallow sources. The anomaly amplitudes preserve the polarity of the original anomaly amplitude. The HD filter calculates the rate of change in a specific horizontal direction. The east-west HD enhances north-south trending structural anomalies and suppresses the structures along the north-south direction.

This enhancement also facilitates interpretation of fault and contact features. It is complementary to the first vertical derivative enhancement. It usually produces a more exact location for faults than the first vertical derivative.


Horizontal Gradient East-West (1HDEW)

The Horizontal Derivative (HD) filter reduces regional variation and enhances signals from shallow sources. The anomaly amplitudes preserve the polarity of the original anomaly amplitude. The HD filter calculates the rate of change in a specific horizontal direction. The east-west HD enhances north-south trending structural anomalies and suppresses the structures along the north-south direction. This enhancement also facilitates interpretation of fault and contact features. It is complementary to the first vertical derivative enhancement. It usually produces a more exact location for faults than the first vertical derivative.


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Automatic Gain Control (AGC)

The Automatic Gain Control (AGC) filter aims to: 1) boost the signal in areas of low field variability and 2) reduce signal amplitude in areas of high variability, by removing the background components and then equalising the signal across the survey area. The AGC filter highlights continuity of structures across areas with different intensity. Since the AGC increases amplitude in areas of low variability it also increases noise in those areas.


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Analytic Signal (AS)

The Analytic Signal (AS) is used to evaluate the major potential field variations and structures. The AS filter calculates the total amplitude of the three directional derivatives (in E-W, N-S and vertical directions). Local peaks in the AS profile are interpreted as edges of source bodies and the shape of the peak contains information about depth to the edge. In the absence of high-frequency noise and aliasing in the data, horizontal locations from Analytic Signal are highly accurate.


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Modulus Filter (MS)

The Modulus (MS), or Horizontal Gradient Magnitude (HGM), filter is a tool for outlining the boundaries of magnetic sources. The filter is calculated from a pair of orthogonal horizontal derivatives. The resultant field consists of positive values. The peaks of horizontal modulus anomalies indicate the edges of a source body. The amplitude of the filtered anomaly retains information about the properties of the sources.


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Tilt Filter (Tilt)

The Tilt filter enhances both strong and weak anomalies at their centres and also to emphasise the edges of broad anomalies. The Tilt filter produces similar shapes to the 1VD, however the amplitudes are greatly condensed to a small range andanomalies appear sharper. For isolated sources, the tilt angle is positive over the source, crosses through zero at or near the edge of a vertical sided source, and is negative outside the source region. The Tilt filter significantly enhances subtle anomalies in areas of relatively flat response, enabling the continuity of major structures to be interpreted. 


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ZS-Edge Filter (Edge)

The ZS-Edge filter is a FROGTECH proprietary filter and has been developed to map source edges. The filter compresses the amplitude range and responds equally well to shallow and deep sources. Also, a greater contribution of the anomaly amplitude over the source is retained, thereby improving anomaly characterisation at the expense of edge sharpness. The numerical range of values for this filter does not correspond to a natural unit of measurement for this data type. Dynamic range can be gauged by colour range, with high (red) to low (blue). 


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ZS-Edgezone Filter (Edgezone)

The ZS-Edgezone filter is a FROGTECH proprietary filter. This filter increases the sharpness of anomalies that indicate the edge of source bodies. The filter results in an anomaly around the edge of a body rather than over the peak. The Edgezone filter compresses the amplitude range and responds equally well to shallow or deep sources, and equally well to strong or weak anomalies. This filter differs from the ZS-Edge filter in that less anomaly amplitude over the source is retained; therefore edge sharpness is increased while anomaly characterisation is compromised. The numerical range of values no longer corresponds to a natural unit of measurement.


ZS-Area Filter (Area)

The ZS-Area filter is a FROGTECH proprietary filter. The ZS-Area filter maps the zonal extent of potential field sources or source regions. The numerical range of values for this filter does not correspond to a natural unit of measurement for this data type. Dynamic range can be gauged by colour range, with high (red) to low (blue).


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ZS-Block Filter (Block)

The ZS-Block filter is a FROGTECH proprietary filter. The filter has the effect of transforming the data into zones that may relate to lithological variations. The filter produces sharp edges and the amplitudes of original anomalies have been normalised and compressed into a reduced range. The numerical range of values for this filter does not correspond to a natural unit of measurement for this data type. Dynamic range can be gauged by colour range, with high (red) to low (blue). 


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ZS-Plateau Filter (Plateau)

The ZS-Plateau filter is a FROGTECH proprietary filter. The Plateau filter depicts the data as a 2D plan of apparent source distribution. The anomalies covered by the source region appear to form a plateau against a background of relatively low amplitude regional anomalies. The numerical range of values for this filter does not correspond to a natural unit of measurement for this data type. Dynamic range can be gauged by colour range, with high (red) to low (blue). 


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Ternary Image – Tilt, MS and ZS-Edge

The CMY ternary image combines the results of the Tilt (cyan), Modulus (magenta) and ZS-Edge (yellow) enhancement filters to produce a highly detailed image. The ternary image highlights the edges of broad anomalies, enhances subtle anomalies in areas of relatively flat response, and shows continuity of major structures and retains the anomaly amplitude over the source thereby improving anomaly characterisation and the mapping of source edges. This image is also of use in accurately locating anomalies as no sun angle is applied to the image.


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Ternary Image – Tilt, MS and ZS-Edgezone

The CMY ternary image combines the results of the Tilt (cyan), Modulus (magenta) and ZS-Edgezone (yellow) enhancement filters to produce a highly detailed image. The ternary image highlights the edges of broad anomalies, enhances subtle anomalies in areas of relatively flat response, and shows continuity of major structures across shallow or deep sources with strong or weak anomalies. This image is also of use in accurately locating anomalies as no sun angle is applied to the image.


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Ternary Image – Tilt MS and AGC

The CMY ternary image combines the results of the Tilt (cyan), Modulus (magenta) and AGC (yellow) enhancement filters to produce a highly detailed image. The ternary image highlights the edges of broad anomalies, enhances subtle anomalies in areas of relatively flat response, and shows continuity of major structures across areas with varying intensity. This image is also of use in accurately locating anomalies as no sun angle is applied to the image.