Gravity Processing/Enhancement Example

Examples of various gravity enhancements from the Clarence-Morton Basin, NSW, Australia. To read the full report from which these images have been taken go to NSW DPI.

Bouguer Gravity Grid with 500m Upward ContinuationWhen applying high frequency enhancement filters to the Bouguer Gravity grid and its derivatives, a 500m upward continuation filter is also applied to reduce noise levels. 

1VD of 500m Upward ContinuationThe first vertical derivative (1VD) filter enhances near surface contrasts in density by amplifying the high frequency component of the spectrum (linear increasing filter).

AS of 500m Upward ContinuationThe Analytic Signal calculates the total amplitude of the three directional derivatives (in EW, NS and vertical directions). This filter is used to evaluate the major density variations and structures.

Tilt Filter of 500m Upward ContinuationThe tilt filter is defined as the arctangent of the ratio of the vertical derivative to the horizontal gradient magnitude of the field. 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. It responds equally well both to shallow and deep sources.

100km High Pass Filter of Bouguer Gravity

100km Low Pass Filter of Bouguer GravityWavelength cutoff techniques such as highpass, lowpass and bandpass filters are used for anomaly separation.  The parameters for cutoff wavelength filters are designed on the basis of the distributions of radial energy spectra derived from the Bouguer gravity grid (energy spectral analysis). Gravity anomalies comprise sources ranging in depth, vertical and horizontal dimensions, and density. In general, deep-seated bodies, representing regional geological structures, have great width and depth extents, whereas near surface bodies are commonly thin and have small sizes and extents. The spectra of the anomalies caused by these bodies are distinctive.  The spectra from deep bodies is concentrated at the low frequency end of the spectrum compared to shallow and small size bodies with higher frequencies. Identified break-point values in the  spectra in both low and high frequency sections, are converted into wavelengths to build lowpass, highpass or bandpass filters.  Although a complete separation of gravity anomalies is not possible, the technique is a useful tool for discerning ‘shallow’ from ‘deep’ sources, and highlighting pertinent features and structure dominant within the depth interval of interest.  

A 100km high-pass and 100km low-pass filter was applied to the data in this case.