The Difference Between True Widths and Apparent Widths in Drill Results

What Are True Widths and Apparent Widths in Drill Results?

In mining exploration, understanding the difference between true widths and apparent widths is crucial for accurately assessing a mineral deposit's potential. When a drill core intersects a mineralized zone, the reported intersection length can be misleading if not properly qualified.

True width represents the actual thickness of a mineral deposit as it exists in the ground. This measurement reflects the genuine dimensions of the ore body and provides the most accurate picture of the resource's size and potential value. Industry statistics indicate that approximately 35% of junior mining investments fail to clearly distinguish between true and apparent widths in their initial press releases.

Apparent width (sometimes called "downhole width" or "intersection length") is simply the length of mineralization as measured along the drill hole. This measurement is directly influenced by the angle at which the drill hole intersects the mineralization and can significantly overstate the actual thickness of the deposit.

Dr. Alan Richardson, Economic Geologist at the Mineral Exploration Research Centre, notes: "The disparity between true and apparent widths can create a misleading impression of a deposit's size, particularly when drill holes intersect mineralization at shallow angles. A 30-meter apparent width might represent just 15 meters of true thickness."

Why Understanding the Difference Between True and Apparent Widths Matters for Investors

Investors who fail to recognize the distinction between true and apparent widths risk making investment decisions based on inflated resource potential. Historical data from the ASX shows that companies clarifying true vs. apparent widths in initial announcements typically experience 12% less share price volatility following subsequent technical reports.

When companies report impressive drill intersections without specifying whether they represent true or apparent widths, they may inadvertently (or sometimes deliberately) create an overly optimistic impression of their discovery. A 2021 study by Mining Associates found that projects with consistently overstated widths experienced an average value downgrade of 28% during feasibility studies.

According to James Wilson, Mining Analyst at RBC Capital Markets: "We've seen multiple cases where stocks jumped 30-40% on seemingly spectacular intersections, only to decline sharply once technical reports clarified the true dimensions. Understanding this distinction is particularly important in narrow vein deposits where the difference can significantly impact economic viability."

This knowledge gap becomes especially critical during early-stage exploration when investors have limited technical data available for decision-making.

How Does Drilling Angle Affect Reported Mineral Intersections?

The angle at which a drill hole intersects mineralization fundamentally alters the reported intersection length. When a drill hole crosses mineralization at 90 degrees (perpendicular), the apparent width equals the true width. However, this ideal scenario rarely occurs in practice.

Most drill holes intersect mineralization at oblique angles due to:

• Practical drilling constraints (access limitations, topography)
• The need to intersect multiple targets with a single hole
• The irregular geometry of mineral deposits
• The dip and strike of the geological formation

As the intersection angle decreases from 90 degrees, the apparent width increases exponentially. For example:

• A 10-meter thick vein intersected at 60° yields an apparent width of 11.5 meters
• The same vein intersected at 30° yields an apparent width of 20 meters
• At an extreme angle of 10°, the apparent width balloons to 57.6 meters

This geometric relationship explains why some companies might preferentially report spectacular-looking intercepts that actually represent shallow-angle intersections with more modest mineralization.

What is the Geometric Relationship Between True Width and Apparent Width?

The mathematical relationship between true width and apparent width follows trigonometric principles. In simplest terms:

True Width = Apparent Width × sin(θ)

Where θ is the angle between the drill hole and the plane of mineralization.

This formula reveals an important insight: as the intersection angle approaches parallel (0°), the apparent width approaches infinity, creating the potential for dramatically misleading results. Conversely, as the angle approaches perpendicular (90°), the apparent width converges with the true width.

A comprehensive analysis of 150 gold deposits by the Mineral Economics Group (2022) found that the average drill intersection angle across initial discovery holes was just 37°, resulting in apparent widths averaging 1.7 times larger than true widths.

Dr. Helena Zhang, Professor of Mining Engineering at Colorado School of Mines, explains: "The sine relationship creates a non-linear distortion that becomes particularly problematic at shallow intersection angles. Below 30 degrees, even small variations in the intersection angle can dramatically alter the apparent width."

How to Calculate True Width from Apparent Width Measurements

Converting apparent width to true width requires knowledge of the geometric relationship between the drill hole orientation and the orientation of the mineralized structure. Here's a practical approach:

1. Determine the dip and azimuth (direction) of the drill hole
2. Determine the dip and strike of the mineralized structure
3. Calculate the angle between the drill hole and the normal to the mineralization plane
4. Apply the formula: True Width = Apparent Width × sin(angle)

For example:

• Reported intersection: 25 meters @ 5.18 g/t gold
• Drill hole dip: -60° (downward)
• Drill hole azimuth: 270° (west)
• Mineralization dip: 80° (steeply dipping)
• Mineralization strike: 0° (north-south)
• Calculated angle: 62°
• True width: 25m × sin(62°) = 22.1 meters

Industry-standard software like Leapfrog Geo, MICROMINE, and Surpac automate these calculations, but understanding the underlying principles remains essential for investors reviewing mining drilling results.

What Do ASX/JORC Guidelines Require for Reporting Drill Results?

The Australasian JORC Code (Joint Ore Reserves Committee) establishes minimum standards for public reporting of exploration results. While the code doesn't explicitly mandate reporting of true widths, it requires sufficient transparency for investors to make informed assessments.

JORC Clause 19 states: "Public Reports of Exploration Results must contain sufficient information to allow a considered and balanced judgment of their significance." This implicitly requires clarity regarding whether reported widths are true or apparent.

ASX Listing Rule 5.7.2 further requires that companies reporting exploration results must disclose "drill hole collar information" including dip and azimuth—essential information for calculating true widths.

According to Andrew Scogings, JORC Committee Member (2018-2021): "While not explicitly stated, best practice under JORC clearly involves either reporting true widths or providing sufficient geometric information for readers to calculate them. Companies consistently reporting only apparent widths without appropriate context risk regulatory scrutiny."

Statistics from the ASX show that approximately 65% of exploration announcements now include explicit true width information, up from just 38% in 2010.

Real-World Examples: How Companies Report Drill Intersections

Examining how different mining companies report their drill results reveals considerable variation in transparency:

The highest-quality reports provide both the apparent width and either the true width or sufficient information to calculate it. This level of transparency builds investor confidence and reduces the risk of negative surprises in later technical studies.

How to Identify Whether Companies Are Reporting True or Apparent Widths

Determining whether a company is reporting true or apparent widths requires careful attention to disclosure details. Here are key indicators to watch for:

1. Explicit statements (e.g., "True widths represent approximately 85% of reported intervals")
2. Asterisks or footnotes qualifying reported widths
3. Cross-sectional diagrams showing drill hole angles and mineralization
4. Disclosure of drill hole orientation (dip and azimuth) and mineralization dip/strike
5. References to "downhole length" or "intersection length" (indicating apparent width)

An analysis of 200 ASX exploration announcements conducted by Mining Associates (2023) found that 47% clearly specified true widths, 31% provided sufficient information to calculate true widths, and 22% provided inadequate information to determine true widths.

Industry veteran Geordie Mark, Head of Mining Research at Haywood Securities, advises: "When companies consistently report spectacular widths without qualifying whether they're true or apparent, it's a potential red flag. Professional investors immediately look for this distinction, and retail investors should adopt the same scrutiny."

What Impact Do Reporting Differences Have on Resource Estimates?

The consequences of conflating apparent and true widths extend beyond immediate investor impressions—they directly impact resource estimation and project economics. Consistently overestimated widths can lead to:

1. Inflated resource tonnage estimates (by up to 40% in extreme cases)
2. Overstated metal content calculations
3. Underestimated mining costs per ounce/pound of metal
4. Overestimated project net present value (NPV)
5. Unrealistic internal rate of return (IRR) projections

A landmark 2020 study by SRK Consulting analyzed 35 projects that transitioned from resource definition to production. Projects that consistently reported apparent widths without clear qualification experienced an average negative reconciliation of 23% between expected and actual ore tonnage during the first year of operation.

This reconciliation problem directly impacts financial returns. Economic modeling shows that a systematic 20% overstatement in widths can inflate projected NPV by 15-25% and add 3-5 percentage points to projected IRR, creating significant risk for investors.

Common Terminology in Drill Result Reporting: What Investors Should Know

Navigating drill result announcements requires familiarity with industry terminology that signals whether widths are reported accurately:

• Downhole Width/Length: Explicitly indicates apparent width
• Estimated True Width: Calculated approximation of actual thickness
• Intersection Width: Ambiguous term that could refer to either measurement
• Core Length: Typically refers to apparent width
• Horizontal Width: Specialized measurement used for flat-lying deposits
• Mineralized Interval: Ambiguous term requiring clarification

Investors should also understand that terms like "significant intersection" and "bonanza grade" have no standardized definitions and should be evaluated critically within the context of the specific deposit type.

John Kaiser, founder of Kaiser Research Online, notes: "The terminology around drill results has evolved to become more transparent, but still requires investor vigilance. Companies with strong technical teams and high ethical standards typically provide clear differentiation between true and apparent widths."

Case Study: Gold Vein Intersections and Width Reporting

The Fosterville gold mine in Victoria, Australia provides an instructive case study in width reporting. This high-grade narrow vein system exemplifies why the difference between true widths and apparent widths in drill results is critical.

In 2017, Kirkland Lake Gold (now Agnico Eagle) reported several spectacular intercepts including:

• Reported result: 20.6m @ 15.7 g/t gold
• Drill hole dip: -48°
• Mineralization dip: 75°
• Calculated true width: 10.3m @ 15.7 g/t gold

The company clearly indicated the true widths represented approximately 50% of the reported apparent widths. This transparency didn't diminish investor enthusiasm—the stock rose 185% that year—but it provided a realistic foundation for resource modeling.

Had the company reported only the apparent widths without qualification, subsequent resource estimates would have significantly overestimated the deposit's size and potentially led to inappropriate mine design decisions.

How to Interpret Drill Results with Asterisks and Footnotes

Asterisks and footnotes in drill result announcements often contain crucial information about width measurements. Here's how to interpret these qualifiers:

• **"True widths approximately 70-80% of reported intervals"* – A common disclosure indicating reported figures are apparent widths
• "†Results pending further geometric analysis" – Suggests uncertainty about the true geometry
• "‡Interval represents core length; true width unknown" – Clear indication of apparent width reporting
• "§Mineralization appears perpendicular to drilling" – Suggests reported widths are close to true widths

A concerning trend identified by the Mineral Policy Institute is the placement of such qualifiers in locations distant from headline results—often buried in technical appendices or method sections. This practice, while technically compliant with disclosure rules, can mislead casual readers who focus only on headline numbers.

Expert Insight
"Understanding the distinction between Apparent Widths and True Widths in drill results is paramount. It allows you to make informed judgments about a company's findings and their potential value." – Discovery Alert Team

What Questions Should Investors Ask When Reviewing Drill Results?

Savvy investors should systematically assess drill result announcements with these targeted questions:

1. Does the announcement explicitly state whether reported widths are true or apparent?
2. If true widths aren't specified, does the company provide the drill hole angle and mineralization orientation?
3. Are there footnotes or qualifiers that modify the headline results?
4. Does the company consistently report width types across multiple announcements?
5. Are cross-sectional diagrams provided showing the relationship between drill holes and mineralization?
6. Has the company previously experienced significant downgrades between preliminary and definitive resource estimates?
7. Do geological consultants or qualified persons endorse the reporting methodology?
8. How do the reported widths compare to regional geology and nearby deposits?

Investors who consistently apply this analytical framework develop a significant advantage in evaluating exploration results, particularly during early-stage exploration when valuation is most speculative.

How Does True Width Calculation Affect Project Valuation?

The financial implications of width calculations cascade throughout project valuation models. Mining economics research by CIBC Global Mining (2022) indicates that:

1. Each 10% overstatement in width typically translates to an 8-12% overstatement in contained metal
2. Every 1-meter overstatement in average true width reduces actual IRR by approximately 2.5 percentage points
3. Narrow vein operations (<3m true width) are particularly sensitive to width miscalculations
4. Projects with high fixed costs (underground operations) show amplified sensitivity to width accuracy

This sensitivity explains why sophisticated investors apply different valuation multiples to companies with varying levels of transparency. Companies with consistent, transparent reporting of true widths typically command premium valuations reflecting lower technical risk.

According to mining economist Catherine Farrow: "Transparency about true widths signals confidence in the underlying geology. When companies consistently report only apparent widths, it raises questions about whether they're trying to make marginal mineralization look more substantial."

FAQ: Essential Questions About Drill Width Measurements

Q: Can a company determine true width from a single drill hole?
A: No. Accurate true width determination requires multiple holes or prior knowledge of the mineralization orientation. Single-hole reported true widths involve significant assumptions.

Q: Do reporting requirements differ between jurisdictions?
A: Yes. While most major mining jurisdictions (Canada, Australia, South Africa) have similar transparency expectations, the specific requirements and enforcement vary. The Canadian NI 43-101 standard is generally considered the most rigorous regarding width reporting.

Q: How much can apparent width differ from true width?
A: The difference can be dramatic. At extreme angles (e.g., 10-15°), apparent width can be 5-6 times larger than true width. Even at moderate angles of 30-45°, apparent width typically exceeds true width by 40-100%.

Q: Do different deposit types require different reporting approaches?
A: Yes. Massive, disseminated deposits have less geometric complexity than narrow vein systems. For porphyry copper deposits, the distinction is less critical than for high-grade gold veins where economic viability depends heavily on true width.

Q: How do professional geologists verify reported widths?
A: Technical verification involves reviewing drill core photography, structural measurements, core orientation data, and cross-sectional interpretation. Independent technical reports (e.g., NI 43-101) include verification of width calculations.

How to Use Cross Sections to Estimate True Widths

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