3D Architecture · 3D Architecture Geophysical methods and advanced inversion methods 1 predictive...

Enabling Technologies

3D Architecture

Geophysical methods and advanced inversion methods

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predictive mineral discovery*Cooperative Research Centre

A legacy for mineral exploration science

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A. Gradient inhydraulicpotential

B. Permeability

C. Solubilitysensitivity to P, T, C

D. Spatialgradient of P, T, C

E. Time (duration)

Key Parameter

is reflected in

ExplorationMineral System

scale-dependent translation

5 Questions1. Geodynamics2. Architecture3. Fluid

reservoirs4. Flow drivers &

pathways5. Deposition

Terrain Selection

Area Selection

Drill Targeting

Slide after: A. Barnicoat

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Gravity Profile

2D section

White more dense

Boschetti et al 1998

Other 2D sections

White more dense

Slide after: L. Ailleres

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Let us consider a set of drilling intersections

After Caumon, 2007A very conservative geologist’s interpretation.

After Caumon, 2007A conservative geologist’s interpretation.

After Caumon, 2007An optimistic geologist’s interpretation.

After Caumon, 2007A very optimistic geologist’s interpretation.

After Caumon, 2007An extremely optimistic geologist’s interpretation.

After Caumon, 2007A geophysicist’s interpretation.

After Caumon, 2007The reservoir engineers interpretation.

After Caumon, 2007

Slide after: L. Ailleres

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Gravity/magnetic data

Inversion

Applyconstraint

Create reference

Buildmap

3D reference model3D geological map andphysical properties

3D inversion model

???

Analyse and update Slide after: Richard Lane

Solid geologySeismicStructureDrilling(et cetera)

Potential field data - Gravity

Gravity variations → rock density

density contrast

shape of the spatial variation in the gravitational acceleration to identify the location and shape of the ore body

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gravity anomaly: variations in the background gravity field produced by local geologic structure or a model of local geologic structure

a gravimeter is any instrumentdesigned to measure spatial variations in gravitationalacceleration

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Material Density (gm/cm^3)

Air ~0Water 1Sediments 1.7-2.3Sandstone 2.0-2.6Shale 2.0-2.7Limestone 2.5-2.8

Granite 2.5-2.8

Basalts 2.7-3.1

Metamorphic Rocks 2.6-3.0



Factors that Affect the Gravitational Acceleration:

Temporal Based Variations - time dependent- Instrument drift- Tidal effects

Spatial Based Variations - space dependent- Latitude - Elevation - Slab - Topographic

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Gravity - Magnetics

Similarities:

both measure potential fields

Identical physical and mathematical representations can be used to understand magnetic and gravitational forces

point mass ↔ magnetic monopole

The acquisition, reduction, and interpretation of gravity andmagnetic observations are very similar.

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Gravity - MagneticsDifferences:

rock densities vary little ↔ magnetic susceptibility can vary much (amongst different rock types, within a given rock type) → difficult to determine rock types on the basis of estimated

susceptibilities

gravitational force is always attractive ↔ magnetic force can be either attractive or repulsive

Unlike point masses in gravity, single magnetic point sources (monopoles) always occur in pairs → dipoles

at least two possible sources for magnetic field (induced/remanent)

magnetic field is highly time dependent

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Potential field data - Magnetics

Magnetics measures magnetic field variations→ magnetic susceptibility→ determined by the physical properties of the magnetic

material, unitless constant

Dia- / Para- / Ferromagnetism

Magnetic anomalies are a function of 2 independentparameters:

subsurface distribution of susceptibilityorientation of the Earth's main magnetic field

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I = кH

Potential field data - Magnetics

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Magnetized Sphere North Pole Equator Northern Hemisphere

Potential field data - MTmeasures electric and magnetic fields

main parameter derived from MT is restivity.

main factor affecting resistivity is lithology, but also pore fluid, P, T

mapping of basins, geothermal prospects, structures

data are normally integrated with whatever other informationis available (gravity, magnetics, geology, and borehole).

in lieu of seismic, in lead of seismic, in conjunction with seismic

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Potential field data

(a) Interpretation of MT data

and

(b) seismic section from Turkey

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Worms

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• edges in a "wavelet transform" of the underground terrain• an above ground 3D mirror image of a geological boundary • Irregularities - Density contrasts – points of maximum gradient

at successive heights• Position (x, y), height (z) and amplitude (w) of a gradient• Visualised over range of heights, points merge into sheet-like

surfaces (worm sheets)• Height and amplitude of worm sheet is a function of the

geological source

Worms

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* =

* =

One unit upward continuation

Five units upward continuation

Upward continuation

Courtesy of: F. Horowitz

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After: Holden et al 2001

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Sources(L×M×N)

Field(M×N)

Worms

Archibald et al 1998

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Worms -Visual interpretation


Holden et al 2001

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Synthetic spherical/plug intrusions (A) Infinite vertical column.(B) Finite truncated parabolic

cone. (C)Infinite outward diverging

cone.

Worms - Visual interpretation Holden et al 2001

Synthetic dipping dykes

(A) Vertical dyke. (B) Dyke dipping 60o to the right. (C) Dyke dipping 30o to the right.

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Worms - Visual interpretationHolden et al 2001

Synthetic plunging folds (A)with vertical axial planeand vertical axes. (B)with vertical axial plane andaxes plunging 60o to the right.(C)with vertical axial plane andaxes plunging 30o to the

Synthetic dipping contacts on highdensity body (A) vertical contact. (B) Contact dipping to the right

with footwall high density. (C) Contact dipping to the right

with hanging wall high density.

Example

Yilgarn Gold Faults:

Existing digital coverage

Issues of quality andreliability

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Slide after: Barry Murphy

Example

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Length

Zwt Max (Height)

Worm Data

Wallaby

Sons of Gwalia

Log Zwt/Log Length10km radius of deposit

Bierlein et al 2006

ReferencesHolden, D., Archibald, N., Boschetti, F. , and Jessell, M. (2001). "Inferring

Geological Structures Using Wavelet-Based Multiscale Edge Analysis and Forward Models." Exploration Geophysics 31(4): 000-000.

Boschetti, F., Horowitz, F.G., and Hornby, P. (1998). "Ambiguity Analysis and the Constrained Inversion of Potential Fields." manuscript submitted to Geophysical Journal International: 30 p.

Archibald, N. J., Gow, P., and Boschetti, F. (1998). "Multiscale edge analysis of potential field data." Exploration Geophysics 30: 38-44.

Bierlein, F. (2006). "Distribution of orogenic gold deposits in relation to fault zones and gravity gradients: targeting tools applied to the Eastern Goldfields, YilgarnCraton, Western Australia." Mineralium deposita 41(2): 107-126.

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3D Architecture · 3D Architecture Geophysical methods and advanced inversion methods 1 predictive...

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