This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
• Mapping geology…..why/how is it possible?• Airborne surveys allow rapid and inexpensive coverage• Coverage is continuous• Measurements are a direct representation of the
physical property of underlying rock units
• What attributes can be resolved?• Stratigraphy, Structure, etc.• Burial depth and 3D orientation of structures• Metamorphism, Alteration, Mineralization etc.
• When/where is it required• Wherever the geology is regarded as a key driver of the
Direct verses Indirect Targeting• Indirect targeting (most common)
• Geological interpretation and structural analysis of magnetic data assists us by: ‐ Identifying favorable hosts (and source) rocks for mineral deposits‐ Highlighting faults/folds that provide fluid conduits and traps for mineralization
• Direct targeting• Kimberlites;• BIF’s;• Some porphyry, epithermal
• Associated features:• Folds (parallel / oblique);• 2nd order sediment depo‐centers / basins;• Intrusives / extrusives;• Veining / alteration; and• Recognition of fault hierarchy (1st, 2nd order etc.)
• Strain variations inferred from block geometries:• Structures at block margins; and• Strain within blocks (possible modified stress fields)
• Regional context:• Relationship to regional structures;• Setting (e.g. basin, mobile belt, arc setting etc.);• Orogenic events at inferred time of faulting?• Reactivation of older structures, etc.
• Interpretation workflows typically consists of 3 stages:
• Observation / Planning• Decide on scope (time needed/available), scale, and expected outcome• Assess the availability and quality/resolution of the geophysical data• Obtain complimentary geological data (maps, sections, reports/papers, etc.)
and geophysical/satellite data (SRTM, Aster, regional magnetics etc.)
• Data Processing and Compilation• Processing and imaging of all digital data (incl. 3D inversions)• GIS compilation of data (using common projections and formats)
*May include printing of base maps for overlay interpretation
• Interpretation• structural framework (i.e. trends and discontinuities)• domain delineation (i.e. areas with common character)• integration of secondary data (e.g. published geology)• map compilation
• A semi‐automated process can be employed to rapidly derive the position and accurate extent of magnetic sources by utilizing the ‘Tilt Angle’ grid
• The Tilt Angle is the ratio (restricted to ± 90 degrees) of the vertical and horizontal derivatives and is used to define the edges of magnetic sources
• The ‘zero’ angle provides a good approximation of source edges, while the horizontal distance between the +45 and ‐45 contours gives an approximation of depth.
Forward model Tilt depth output
4 km deep
8 km deep
Magnetic anomaly map• Individual magnetic sources are extracted as discrete
tilt angle values and colored by relative magnetic susceptibility (here dikes are colored black)
• Provides improved spatial resolution
15
Interpretation workflow
Sources
Fabric
• Solid geology interpretation is built on geophysically‐constrained contacts(magnetic domains), thus limiting interpretation bias
• Each domain can be directly characterized by its geophysical attributes
*Classification of geophysical domains based on ‘average’ magnetic intensity values
Domains
Classification
1 km
Magnetic Domain Classification• Magnetic ‘sub‐domains’ can be assigned value statistics
according to the original (RTP) magnetic data
• The output is a geophysical domain classification and structural interpretation map (not necessarily lithologically assigned)
1 km
16
• Geophysical domain classification provides a proxy for geology in covered areas or in the absence of outcrop (or no detailed mapping)
• Provides a detailed representation of the magnetic data in a geological context that can be used to as a base for field mapping
The patterned overlay shows the magnetic ‘texture’ based on the total gradient
Color domains (9 classes) represent the main variations in magnetic amplitude, i.e. reflect major rock units
Interpretation outcomes
1 km
marble, carbonate schist
quartziteironstone
schist alluvium
Interpretation vs field mapping Geophysical Domain Classification Published geology (1:100k)
Magnetics Radiometrics
1 km
17
Final Products
Interpretation Synthesis• Integration of all geological and exploration datasets to
derive structural histories and prioritized exploration targets.
18
Exploration Targeting
‐ Large‐scale fold closure on regional shear‐zone, with proximal intrusions‐ Local concentration of dikes suggest deep‐seated cross‐structure‐ Association with anomalous magnetic units and elevated K (? felsic intrusions)
• Guiding Principals• Airborne geophysical data shows the continuous distribution of magnetic
radioactive, conductive/resistive or dense minerals, from which the geometry of the rock units can be resolved
• Interpretation is designed to translate the geophysical data into a form suitable to a broad exploration user group. The aim is to improve our knowledge in such a way as to significantly improve exploration efficiency and success
• Structural geology is a natural partner since it also relies on geometrical patterns
• Models (structural/stratigraphic) are needed to guide the process, but…
• Interpretation needs to be objective (i.e. the data needs to tell its own story and not be used to confirm/reject pre‐existing geological mapping).
• Not an end in itself, but is a first step towards a coherent 3D geological model
• Guiding Principals (cont.)• The objective is to produce a map which depicts all relevant and appropriate
geophysical features in the context of the ‘known’ geology.
• This product is designed to be used in conjunction with the outcrop geological map to develop a 3D ‘solid geology’ interpretation.
• This integrated product is a record of the combined geological and geophysical observations, from which a range of interpretations and analysis can be derived
• The map should provide the user with a relatively non‐interpretive set of observations from which the user can also develop their own ideas
• The knowledgeable user will use the this map in conjunction with the original raw data to test hypotheses and progressively build and refine the geology of a project area.
• Mechanics of Interpretation• Flexibility is required to take into account a wide range of geophysical responses
that can be observed across different geological environments
• Data resolution and spatial limitations should guide outcomes – may require a trade‐off between aesthetics and geological data since there is generally more information‘ in the data than can be presented in a single map product.
• Multiple scales are needed to integrating the regional with the detail
• Workflows are required that involve three or more distinct stages(a ‘crack map is not an end in itself’)
• Recycling – the best work usually comes from the second (or third!) pass.Thinking time and revision is essential!
• “Geophysics does not lie”. There are very few non‐geological contributions to airborne geophysical data ‐ don’t dismiss unexpected features as ‘spurious’
Geological Models for Mapping• Geological models are a primary component of the interpretation
process for producing maps;• In order to present an interpretation of the geology of the region,
there needs to be a coherent framework of stratigraphic and structural principles that form the basis of the interpretation;
• A coherent regional geological pattern can be followed through the series of maps;
• Individual structures can be interpreted in both a time and space context. We can interpret when and how faults moved;
• The spatial distribution of mineral deposits in relation to structures becomes evident, and can be related to the geological evolution of the region, not just the geometry;
• Areas and structures with potential for reactivation at later times become apparent; and
• Cross‐sections can be developed which provide a realistic 3D form consistent with the geological models.