geophysics as a mapping tool - condor consult geophysical data for... · geophysics as a mapping...

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© Condor Consulting 2014

SOCIETY OF ECONOMIC GEOLOGYEXPLORATION IN 2025

TOOLS AND TECHNIQUES TO EXPLORE UNDER COVER

Using Geophysical Data for Geological and Structural Mapping

Jon Woodhead, Ph.D.Principal GeologistCondor Consulting Inc.Email: [email protected]

October 1‐2 2014Colorado School of Mines, Golden, Colorado


Geophysics as a Mapping Tool• Quick, Cheap and Effective…Why wouldn’t you ?

10 km

Magnetics (1VD) with radiometric ternary Interpretation Framework

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• 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

exploration process (most cases?)

Geophysics as a Mapping Tool


Geophysical Data• What geophysical data can be used?

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Magnetic Expression of Geology


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

and skarn deposits; • Some IOCG’s

Ernest HenryIOCG Cu-Au122Mt 1%Cu 0.5g/t Au

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Indirect Targeting• Yilgarn Craton Au and Ni (Western Australia)

Kambalda Ni deposits

St. Ives Au deposit

Junction Au deposit


Indirect Targeting• Unconformity U (Athabasca Basin, Canada)

Sub-Athabasca basement structure showing structural controls on U deposits

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Direct Targeting• Unconformity U (Athabasca Basin, Canada)

Radiometric anomalies over the Patterson Lake South U deposit

Porgera Au mineDirect Targeting• Epithermal Au / Oil & Gas:

PNG Highlands

Radiometric ternary image (KThU-RGB) draped over topography

Oil & Gas

10 km

Porgera

K%

RTP

Hides & Kariusanticlines

eU

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WABAG 1:250,000 map sheet (1 of 6 sheets)

Regional Geological Studies• New interpretation map series: PNG Highlands (CGG, 2010)


Solid Geology Interpretation• What can we get out of the data?

• Distribution of structures (folds / faults etc.), stratigraphy, intrusions and alteration

• Form surface mapping – which can be used to extend our understanding from the 2D plan view to 3D kinematics

• Relative fault character, timing and displacements• Thin, straight (brittle, shallow, low T);• Broad, discrete breaks (brittle‐ductile: mod P & T); or• Broad, no breaks (ductile ‐ deep, high T).

• Development of tectonic models (local & regional);• Predictive targeting:

• Direct anomaly detection;• Direct structural analogy; and• Indirect structural targeting (new models?);

• Extend our models outside immediate area, to other terranes

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• Key observations

• 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.

Structural Analysis


• 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

Interpretation workflow

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1 km

Demagnetization(Cu-enrichment)

100 m line-spacing, 50 m elevation

400 m line spacing, 150 m elevation

Data resolution and quality• Critical to interpretation outcomes!

• Dictates the scale at which an interpretation can be undertaken and the achievable mapping resolution

conventional gridding gradient enhanced

300 m line-spacing

Magnetic data processing• A variety of derivative products are used to

enhance magnetic boundaries (edge filters) or textural domains (block filters)

• Key products include:• Reduction‐to‐Pole (or Equator)• Vertical Derivatives (1VD, 2VD)• Total Gradient (ASM)• High/Low Pass filters, AGC etc.• Tilt Angle• Pseudo‐depth slicing

‐ frequency filters

• 3D Inversion ‐ unconstrained• Depth or elevation slicing• Isosurfaces (3D DXF’s)

Unconstrained 3‐D inversion

Edge and block filters

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• The magnetic anomaly is complex and needs to be simplified for interpretation

Magnetic data fundamentals

Reduced‐to‐pole (LL‐20)

Reduced‐to‐pole (LL‐50)

N‐S artifacts areattenuated by using a

‘limiting‐angle’ in the RTP(e.g. 20, 30, 40 and 50)

…but at the risk of attenuating

real N‐S signatures

• Example: Reduction‐to‐Pole (RTP)

• Used to ‘simplify’ the data so that the anomaly center is positioned directly over its causative body

• Problems occur at low‐magnetic latitudes (+‐20°) where the field is near horizontal (e.g. Chile, W. Africa)

• This often requires a trial‐and‐error approach to get the best result.

• Usually done by applying an ‘amplitude correction angle’ to minimize the directional amplification or bias (but can amplify noise and artifacts )

40° provides best outcome

Note striping

Magnetic data processing

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• or Reduction‐to‐Equator (RTE)

• At very low latitudes, a simple numerical inversion of the RTE (or the TMI) grids can be used to approximate the RTP

Magnetic data processing

• Many possible models can interpret the same observed anomaly ! (ambiguity)

Ambiguity

“Gravity and magnetic data contains no inherent depth information”

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• The problem of non‐uniqueness

Forward and Inverse Modeling

Forward modeling:• Structure and physical properties are known• Compute the geophysical response• The process is unique

Inverse modeling:• We have the geophysical response but want to know the the structure

and/or physical property of the model e.g. a + b = 7• The process is non-unique

Radiometric data processing

Classified K‐Th‐U

K / Th ratio

U2 / Th ratio

• Radiometric data closely reflects bedrock in residual terranes (vs. transported regolith)

• Ratio and ‘classified’ products may show areas of alteration (e.g. anomalous K)

Ternary K‐Th‐U (RGB)

K‐Th‐U classed as high, med or low (i.e. 33 = 27 classes)

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Add data from other sources (radiometrics and gravity) –

annotate on a separate layer

Finalize map presentation, prepare cross-sections and block model, check geology for internal consistency

Extract line data from 1VD image

Determine movement direction and dips on faults

and dip direction of stratigraphy.

Add geological data from surface geology maps, drilling results – identify formations

and marker units

Improve on unit boundaries, annotate and connect marker

units from magnetics

Interpret faults – breaks, truncations etc. Annotate

accordingly.

Add exploration model parameters

Add geological model parameters

Extract Lithology data from RTP image. Divide area into major lithological packages.

Interpretation Workflow

SRK Consulting

*Methodology by SRK (Australia)


Interpretation Workflow1VD data

Stratigraphic Form Line mapping

Fault/deformation zone mapping (including timing)

Fold mapping (including timing)

Total Field RTP data

Lithology mapping

Integrated interpretation

SRK Consulting

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Integrated interpretation

Regional geophysics

Drillhole data

Published literature

Regional mapping

Local geophysics

Data Integration


Final Products and Outcomes

SRK Consulting

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Semi‐automated interpretation

Salem et al. (2010)

Actual edge of magnetic body

• 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

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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

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• 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

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Final Products

Interpretation Synthesis• Integration of all geological and exploration datasets to

derive structural histories and prioritized exploration targets.

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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)

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high‐K intrusion ?

• Orogenic gold (West Africa)


• 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

Interpretation Guidelines

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• 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.




Additional

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• 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’



Interpretation Guidelines• Don’t ignore the ‘spurious’ data

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Interpretation Guidelines• When relating lithology or stratigraphy to magnetics, think:

• Which rocks contain the magnetic minerals (field evidence, susceptibility measurements, petrology)?

• Do these rocks always contain magnetic minerals in this area?• How and when did the magnetic minerals form?

• Be wary of making the following generalizations:

• “The XYZ Formation is highly magnetic” ‐ Is it?? Always?? Everywhere?? Uniformly??

• “The igneous and metamorphic rocks will be more magnetic than the sedimentary rocks” ‐ They are frequently not!!

• “The mafic rocks will be more magnetic than the felsic rocks” ‐ They are frequently not!!

Think in terms of the magnetic rock body in the ground….not the magnetic field it causes.

(after SRK Consulting)


Interpretation Guidelines• Key Structural Questions:

• What structures occur?• What is their extent?• Relative geometry?• What strain was produced?• What P/T conditions did they form

at?• What is the 3D geometry?• What was the tectonic driving force

& history?• What is the relationship of all this

to mineralization? • Ontario – Superior/Grenville Compilation, 400m line spacing, RTP1VD. Wabigoon subprovince.


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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.


Interpretation

Source: CGG (prev. Fugro)

• Defining magnetic domains

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What is an Airborne Geophysical Survey?

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10 km to 500 km

Typically Fly 400 to 1500 km per day

GPS Satellite Navigation

Specially Modified Survey Aircraft

Tie Lines 5 – 10 x Flight Line

Spacing

(Red Lines)

Flight Lines

100m - 1 km Apart

(Black Lines)

Aircraft Height Determined by Radar or Laser

Altimeter

Fixed-Wing 80-120m

Heli < 80m

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