In Visualization tips for geoscientists: MATLAB, Part III I showed there’s a qualitative correlation between occurrences of antimony mineralization in the southern Tuscany mining district and the distance from lineaments derived from the total horizontal derivative (also called maximum horizontal gradient).
Let’s take a look at the it below (distance from lineaments increases as the color goes from blue to green, yellow, and then red).
However, in a different map in the same post I showed that lineaments derived using the maxima of the hyperbolic tilt angle (Cooper and Cowan, 2006, Enhancing potential field data using filters based on the local phase) are offset systematically from those derived using the total horizontal derivative.
Let’s take a look at the it below: in this case Bouguer gravity values increase as the color goes from blue to green, yellow, and then red; white polygons are basement outcrops.
The lineaments from the total horizontal derivative are in black, those from the maxima of hyperbolic tilt angle are in gray. Which lineaments should be used?
The ideal way to map the location of density contrast edges (as a proxy for geological contacts) would be to gravity forward models, or even 3D gravity inversion, ideally constrained by all available independent data sources (magnetic or induced-polarization profiles, exploratory drilling data, reflection seismic interpretations, and so on).
The next best approach is to map edges using a number of independent gravity data enhancements, and then only use those that collocate.
Cooper and Cowan (same 2006 paper) demonstrate that no single-edge detector method is a perfect geologic-contact mapper. Citing Pilkington and Keating (2004, Contact mapping from gridded magnetic data – A comparison of techniques) they conclude that the best approach is to use “collocated solutions from different methods providing increased confidence in the reliability of a given contact location”.
I show an example of such a workflow in the image below. In the first column from the left is a map of the residual Bouguer gravity from a smaller area of interest in the southern Tuscany mining district (where measurements were made on a denser station grid). In the second column from the left are the lineaments extracted using three different (and independent) derivative-based data enhancements followed by skeletonization. The same lineaments are superimposed on the original data in the third column from the left. Finally, in the last column, the lineaments are combined into a single collocation map to increase confidence in the edge locations (I applied a mask so as to display edges only where at least two methods collocate).
If you want to learn more about this method, please read my note in the Geophysical tutorial column of The Leading Edge, which is available with open access here.
To run the open source Python code, download the iPython/Jupyter Notebook from GitHub.
With this notebook you will be able to:
1) create a full suite of derivative-based enhanced gravity maps;
2) extract and refine lineaments to map edges;
3) create a collocation map.
These technique can be easily adapted to collocate lineaments derived from seismic data, with which the same derivative-based enhancements are showing promising results (Russell and Ribordy, 2014, New edge detection methods for seismic interpretation.)