The use of fractal dimension for texture-based enhancement of aeromagnetic data.
Date
2008
Authors
Dhu, Trevor
Editors
Advisors
Dentith, Michael
Hillis, Richard
Hillis, Richard
Journal Title
Journal ISSN
Volume Title
Type:
Thesis
Citation
Statement of Responsibility
Conference Name
Abstract
This thesis investigates the potential of fractal dimension (FD) as a tool for enhancing
airborne magnetic data. More specifically, this thesis investigates the potential of FD-based
texture transform images as tools for aiding in the interpretation of airborne magnetic data. A
series of different methods of estimating FD are investigated, specifically:
• geometric methods (1D and 2D variation methods and 1D line divider method);
• stochastic methods (1D and 2D Hurst methods and 1D and 2D semi-variogram methods),
and;
• spectral methods (1D and 2D wavelet methods and 1D and 2D Gabor methods).
All of these methods are able to differentiate between varying theoretical FD in synthetic
profiles. Moreover, these methods are able to differentiate between theoretical FDs when
applied to entire profiles or in a moving window along the profile. Generally, the accuracy of
the estimated FD improves when window size is increased. Similarly, the standard deviation
of estimated FD decreases as window size increases. This result implied that the use of
moving window FD estimates will require a trade off between the quality of the FD estimates
and the need to use small windows to allow better spatial resolution.
Application of the FD estimation methods to synthetic datasets containing simple ramps,
ridges and point anomalies demonstrates that all of the 2D methods and most of the 1D
methods are able to detect and enhance these features in the presence of up to 20% Gaussian
noise. In contrast, the 1D Hurst and line divider methods can not clearly detect these features
in as little as 10% Gaussian noise. Consequently, it is concluded that the 1D Hurst and line
divider methods are inappropriate for enhancing airborne magnetic data.
The application of these methods to simple synthetic airborne magnetic datasets highlights the
methods’ sensitivity to very small variations in the data. All of the methods responded
strongly to field lines some distance from the causative magnetic bodies. This effect was
eliminated through the use of a variety of tolerances that essentially required a minimum level
of difference between data points in order for FD to be calculated. Whilst this use of
tolerances was required for synthetic datasets, its use was not required for noise corrupted
versions of the synthetic magnetic data.
The results from applying the FD estimation techniques to the synthetic airborne magnetic
data suggested that these methods are more effective when applied to data from the pole.
Whilst all of the methods were able to enhance the magnetic anomalies both at the pole and in
the Southern hemisphere, the responses of the FD estimation techniques were notably simpler
for the polar data. With the exception of the 1D Hurst and line divider methods, all of the
methods were also able to enhance the synthetic magnetic data in the presence of 10%
Gaussian noise.
Application of the FD estimation methods to an airborne magnetic dataset from the
Merlinleigh Sub-basin in Western Australia demonstrated their ability to enhance subtle
structural features in relatively smooth airborne magnetic data. Moreover, the FD-based
enhancements were able to enhance some features of this dataset better than any of the
conventional enhancements considered (i.e. an analytic signal, vertical and total horizontal
derivatives, and automatic gain control). Most of the FD estimation techniques enhanced
similar features to each other. However, the 2D methods generally produced clearer results
than their associated 1D methods. In contrast to this result, application of the FD-based
enhancements to more variable airborne magnetic data from the Tanami region in the
Northern Territory demonstrated that these methods are not as well suited to this style of data.
The main conclusion from this work is that FD-based enhancement of relatively smooth
airborne magnetic data can provide valuable input into an interpretation process. This
suggests that these methods are particularly useful for aiding in the interpretation of airborne
magnetic data from regions such as sedimentary basins where the distribution of magnetic
sources is relatively smooth and simple.
School/Discipline
Australian School of Petroleum
Dissertation Note
Thesis (Ph.D.) - University of Adelaide, Australian School of Petroleum, 2008
Provenance
Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text.