Optimising surface mapping of elongated geological features from

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Optimising surface mapping of elongated geological features from
Optimising surface mapping of elongated
geological features from FTG – clustering
D FitzGerald, H Holstein, H Gibson
Group of auto-solvers for elongated geological sources (fault planes, dykes,
volcanic sheets, horst-blocks, and/or anticlinal hinge zones)
Builds on previous work by FitzGerald & Holstein (SEG 2014 workshop)
New developments:
Anisotropic recursive clustering technique after Ouillon et. al, 2008
GUI Intrepid v6.0
Attempts to valid our case study, geological field observation
Diagonalising the Gradient Tensor
Gravity gradient tensors may be subjected to an Eigen decomposition :
rotation matrix of Eigenvectors
diagonal matrix of Eigenvalues
α
• 3 principal components
Special case
If gyy goes to zero, we have a
flattening of the principal axes
2D geological character
• ellipse (2D)
 g 'xx



g 'yy




g
'
zz 

Case Study: Dry Hills, Nevada - FTG
2005 Bell Geospace
CMQ Resources
Geology
Rift corridor, extensional
Low density, Palaeozoic sediments
Prominent ENE &
older NNW fault systems
Near-surface,
young basalt /andesites
Dohrenwend et al., 1996
Stage 1: Tensor Convolution: Find 2D sources
Signal is the tensor (single band)
Terrain corrected
Cell size: 40 m
Samples: 172 205
Nulls: 165 179
Moving window (4x4):
Eigensystem is solved in every cell
If 9/16 or more found with
Mid-eigenvalues 0 +/- 1 Eötvös (<noise)
Then solutions saved to
spatial points database
New, reduced database of 2D character ‘special case’
solved eigensystems – Dry Hills
3,150 samples remain from ~172,000
X,Y
Strike
location
within the DTM
Eigenvalues
Tilt
Eigenvectors
Stage 1 results, Dry Hills
Strike (or azimuth)
Elongated geological-source body
&
Tilt of top-of-body
Tilt of top-of-body
Mean: 2.4 degrees
Std dev: 14.2 degrees
Azimuth: solved strike of the 2D body
Colour: amplitude Max/Min eigenvalue
Stage 1: Dry Hills location: Structural geology validation !
Samuel Siebenaler, MSc thesis, 2010
Field observations: Permian sediments
Fold hinge-line strike and plunge orientations:
Plunges/tilts < 30°towards NW through NE
Stage 2: Apply clustering
GUI
Stage-1 output database
DTM
‘Tensor Gradients’ method
Run the 3D viewer
Cluster centres are
found:
(for like-polygonal zones)
Backdrop is the DTM
Further clustering,
reductions of locations,
(strengthening
evidence for likepolygonal zones)
2 methods merge:
Cluster centres are the
optimum places to do
dip calculations
(by profile
constructions)
[perspective view on DTM]
Finish of clustering:
Dry Hills: Best-fit dips estimated for over 60 fault planes:
majority are steep
field geology validation: Steep-dips generally observed
Take home points
Our group of auto-solvers for full tensor gravity gradiometry depend upon:
•
•
•
•
density contrast
high quality DTM
ability to treat the full tensor as a signal (not just components)
reliably grid the full tensor at cells < ¼ line spacing
We are looking for a next case study for this workflow …
Thank you
Des FitzGerald, Horst Holstein, Helen Gibson

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