Rotational Invariants And Lineament Analysis
Introduction
The 5 independent Tensor components measured using FTG technology can be represented as a nine component tensor matrix and from there analysed using invariant techniques first described by Pederson & Rasmussen in 1990 *. Although there are many possible combinations of the Tensor components, Bell Geospace currently employ 3 such combinations to extract detailed geology.
The first of these isolates signature patterns associated with 'whole' anomalies such as salt bodies, igneous intrusives, kimberlite pipes, ore bodies and fault blocks. The 2nd works on the horizontal components only and isolates signature patterns associated with contact geology, be it structural and / or stratigraphic. The 3rd estimates geological strike directions from such contact information.
The computation of the Rotational Invariant Tensor enhances signature patterns associated with target geology from intrusive igneous bodies and salt domes to fault blocks, making it suitable for targeting potential ore bodies. This is performed on wavelength filtered tensor components that best capture the signature pattern arising from the geology of interest. The technique has been most instrumental in focusing the explorer's eye to anomalies of interest in the drill planning stage.
Lineament analysis is a semi-automated technique that identifies maxima and minima anomalies as lineament trends from the horizontal components, be they associated with stratigraphic or faulted contacts in the sub-surface. The technique computes a series of Invariant Tensors that combines the horizontal tensor components that have been previously wavelength filtered to focus on key depth intervals. The result is a mechanism of imaging fault / lineament patterns as they vary with (pseudo) depth.
The 3rd computation estimates the geological strike direction lending it for use in more main stream structural geological analysis techniques to determine stress-strain relationships.
A paper presented by Bell Geospace at the ASEG 2007 show in Perth describes examples of using the Invariant Technique in a FTG Gravity Interpretation workflow. Murphy & Brewster demonstrate the method's ability to image the complex geometric shape of a geological body.
* Pedersen, L.B., and Rasmussen, T.M., 1990, The gradient tensor of potential field anomalies: some implications on data collection and data processing of maps: Geophysics, 55, 1558-1566.