Core Orientation & Structural Logging

Core Orientation & Structural Logging: Turning Core into Structural Intelligence

Oriented diamond drill core is one of the most powerful datasets in mineral exploration. When collected and logged correctly, structural measurements become a 3D map of an orebody’s architecture, helping geologists interpret vein systems, faults, folds, lithological contacts, and stress regimes.

Why orient core?

Non-oriented core gives lithology and grade but loses kinematic and 3D orientation data. Oriented core ties planar/linear features to true attitude, enabling accurate structural interpretation.

1. What Is Core Orientation and Why Does It Matter?

Core orientation is the process of determining the true in-situ orientation of geological structures recorded in drill core. Without orientation data, measurements in core remain “apparent,” meaning they cannot be reliably transformed to true dip/dip direction.

Core orientation enables exploration teams to:

• Define vein sets, faults, shear zones, foliation
• Build accurate structural models used in Leapfrog, Micromine, or Seequent Central
• Understand controls on mineralization
• Improve drilling efficiency by targeting high-angle structures
• Support geotechnical domains and slope design

Authoritative reference:

• Clark, I. & Harper, W.V. (2019). Practical Geostatistics & Core Orientation Techniques. Geological Society Publications.

https://geolsoc.org.uk/

Core Orientation Tools and Methods

Most Common Systems

• REFLEX ACT / ACT3 – Widely used; mechanical spear marks bottom-of-hole.
• Devico DeviCore – Gyro-based orientation; high accuracy.
• EasyMark / Ezi-Ori – Manual systems using alignment spear.
• Electronic multi-shot tools – Continuous orientation logging.

Tools mark the Bottom of Hole (BoH) on the core, which becomes the reference for all structural measurements.

Manufacturer references:

• REFLEX ACT: https://imdexlimited.com

• Devico DeviCore: https://www.devico.com

Understanding the Reference Marks

Before structural logging, the geologist must confirm three critical elements:

3.1 Orientation Line

A continuous line drawn along the BoH reference point across every core piece.

3.2 Depth Marker Blocks

Depths must be cross-checked with driller’s records.

3.3 Confidence class

Orientation quality rating (per ISRM / company protocol):

• A = High confidence (continuous core, no rotation)
• B = Moderate (minor breaks)
• C = Poor (core loss/rotation)
• D = Unusable

Logging poor-quality orientation leads to false structural models.

Structural Features to Log in Oriented Core

4.1 Bedding & Foliation

Useful for:

• Determining stratigraphy
• Reconstructing folding
• Understanding kinematics

Indicators include alignment of micas, compositional banding, or sedimentary lamination.

4.2 Veins and Vein Sets

In mineralized systems, veins often control ore.

Log:

• Vein angle to core axis (α angle)
• Orientation line angle (β angle)
• Vein type (QTZ, carbonate, sulfide)
• Fill and selvage alteration
• Vein frequency (vein/veinlet density)

4.3 Faults & Shear Zones

Critical for mineralization pathways.

Parameters:

• Brecciation
• Gouge thickness
• Kinematic indicators (slickensides, Riedel shears)
• Orientation (true dip/dip direction)

4.4 Fractures & Joint Sets

Structural permeability influences:

• Hydrothermal flow
• Groundwater
• Rock mass stability

Record:

• Intensity (fractures per meter)
• Roughness
• Infill (calcite, chlorite, hematite)

4.5 Lithological Contacts

Contacts often become conduits for:

• Shearing
• Mineralized veins
• Alteration fronts

Measure:

• Apparent dip
• Nature (sharp/gradational)
• Thickness of transitional zones

How to Measure Structures in Oriented Core

5.1 α and β Angles

All structural measurements start with these two fundamental angles.

α Angle (Alpha)

Angle between the plane/vein and the core axis.

β Angle (Beta)

Angle measured from the orientation line to the structure.

These are used to compute true dip and dip direction using stereonet software or structural geology tools.

5.2 Measuring Workflow (Step-by-Step)

1. Identify the structure (vein, fault, foliation).

2. Place the core on the orientation line side.

3. Use a core orientation protractor (Kenometer or equivalent).

4. Measure α angle (cross-core).

5. Measure β angle (around the core from BoH orientation line).

6. Record confidence level, quality, and notes.

7. Transform α–β to dip/dip direction using stereonet or software.

Recording Structural Data in Logging Software

Common software platforms:

• LogChief / MX Deposit
• Leapfrog Geo (structural disc + stereonet plug-ins)
• Micromine Geology
• Deswik.GEO

Recommended logging fields:

• Structure type
• α
• β
• Confidence class
• True dip
• True dip direction
• Thickness
• Kinematics
• Infill minerals
• Comments

A consistent structure code library improves model quality and reduces ambiguity.

Applications of Oriented Core in Exploration

7.1 Orebody Modeling

True dips of veins allow construction of:

• Sheeted vein arrays
• Porphyry stockwork zones
• Lode gold shear systems

7.2 Targeting High-Grade Shoots

Many deposits have high-grade plunges controlled by intersection of structures.

Oriented core helps identify these plunges.

7.3 Understanding Stress Regimes

Drill core break angles, fractures, and vein orientations help reconstruct:

• Paleostress
• Current stress fields
• Underground mine design stress models

Reference:

• Hoek, E. (2007). Practical Rock Engineering.

https://rocscience.com

Geotechnical Benefits of Oriented Core

Structural logging supports geotechnical design:

• RMR and Q-system classification
• Kinematic slope stability analysis
• Underground excavation stability
• Fault zone prediction

Oriented core gives true structural sets, improving rock mass models.

Common Mistakes & How to Avoid Them

Mistake 1: Using poor-quality orientation

→ Always classify orientation quality.

Mistake 2: Measuring on the wrong side of the core

→ Always align with the BoH orientation line.

Mistake 3: Confusing α and β angles

→ Use standard protractor tools with clear instructions.

Mistake 4: Logging structures without context

→ Integrate lithology, alteration, and assays.

Mistake 5: Not correcting for core rotation

→ Watch for broken or spiraled core segments.

Conclusion

Core orientation is one of the highest-value datasets in mineral exploration. When executed correctly, it produces a 3D structural framework that guides drilling, improves domain modeling, and reduces exploration risk.

By integrating oriented core data with lithology, alteration, and assay results, geologists can build accurate, predictive geological models that significantly improve the chances of discovery.

References (Reputable Sources)

1. Hoek, E. (2007). Practical Rock Engineering. Rocscience. https://rocscience.com

2. International Society for Rock Mechanics (ISRM). Suggested Methods for Rock Characterization. https://isrm.net

3. Bons, P.D., Elburg, M.A., & Gomez-Rivas, E. (2012). Vein Formation and Structure. Springer. https://link.springer.com

4. IMDEX REFLEX ACT Orientation System. https://imdexlimited.com

5. Devico DeviCore Orientation Tool. https://devico.com

Further reading

• SEG — Courses and guidance on structural geology for exploration