Geology: Converting Rock Density, Velocity and Hardness Scales
Published April 24, 2026
Geology uses specialized unit systems for measuring rock properties: density (g/cm³ or kg/m³), seismic velocity (km/s), hardness (Mohs scale), and porosity (percentage). Understanding these unit conversions enables geologists to interpret well logs, compare mineral properties, identify rock types, and predict behavior of geological formations in engineering projects.
Table of Contents
Understanding the Basics
Rock properties directly influence geology and engineering applications. Density determines whether rocks float (pumice) or sink, and affects stress calculations in deep reservoirs. Seismic velocity—how fast sound travels through rock—varies with density, mineral composition, and fluid content; oil-bearing formations show characteristically lower velocity than water-bearing rocks. Hardness, measured by Mohs scale or other standards, indicates mineral composition and mechanical durability. Understanding conversions between these properties enables geologists to interpret subsurface data and predict rock behavior in drilling and construction.
Density conversions between g/cm³ and kg/m³ are routine in geology: 2.65 g/cm³ (quartz) converts to 2,650 kg/m³. Seismic velocity conversions (m/s, km/s, ft/s) are critical when integrating data from different sources. Mohs hardness is ordinal, not interval—the difference between 1 and 2 is not equal to the difference between 8 and 9, so quantitative comparisons require care.
Geology Units
Density
- g/cm³: Common in mineral identification. Quartz = 2.65 g/cm³; feldspar = 2.56 g/cm³; water = 1.0 g/cm³.
- kg/m³: SI standard for scientific calculations. 2.65 g/cm³ = 2,650 kg/m³ (multiply by 1,000).
- Specific Gravity: Dimensionless ratio to water density. Quartz SG = 2.65 (same numerical value as g/cm³).
Seismic Velocity
- km/s: Standard in seismic surveys. Quartz = 6.0 km/s; shale = 3.5-4.0 km/s; water = 1.5 km/s.
- m/s: SI standard. 6.0 km/s = 6,000 m/s. Used in well-log interpretation and wave equation calculations.
- ft/s: Imperial unit still common in US well logs. 1 km/s ≈ 3,281 ft/s.
Hardness
- Mohs Scale: 1 (talc) to 10 (diamond). Ordinal scale; steps are not equal intervals. Simple field test using common materials.
- Vickers Hardness: Quantitative microindentation test. Values > 100 for hard minerals; non-linear relationship to Mohs.
- Porosity: Percentage of rock volume containing void space. 0% = solid rock; 30% = highly porous (typical sandstone).
Conversion Formulas
| From | To | Multiply By |
|---|---|---|
| g/cm³ | kg/m³ | 1,000 |
| km/s | m/s | 1,000 |
| km/s | ft/s | 3,281 |
Worked Examples
Example 1: Mineral Density
Feldspar has density 2.56 g/cm³. What is this in kg/m³?
2.56 g/cm³ × 1,000 = 2,560 kg/m³. This matches feldspar's typical density; useful for mineral identification.
Example 2: Seismic Velocity
A sandstone formation has P-wave velocity of 4.5 km/s. What is this in m/s?
4.5 km/s × 1,000 = 4,500 m/s. This velocity suggests saturated sandstone; dry sand would be faster (~5.5 km/s).
Practical Applications
Well logging integrates multiple unit systems: density logs measure g/cm³, velocity logs measure ft/s or km/s, porosity logs report percentage. Converting all to consistent units enables integrated interpretation: high density + high velocity + low porosity suggests tight limestone; low density + low velocity + high porosity suggests porous sandstone.
Engineering projects rely on accurate rock property conversions. A dam foundation design requires knowing rock density (for stress calculations) and seismic velocity (for wave propagation in earthquakes). Converting between g/cm³ and field density measurements (kg/m³ or lb/ft³) determines total loads on foundations.
Petroleum exploration uses seismic velocity variations to identify hydrocarbon-bearing formations. Oil-bearing sandstone typically shows lower velocity than water-bearing sandstone due to different fluid properties. Accurate velocity conversions between log data (ft/s) and seismic data (km/s) enable "tying wells to seismic"—matching downhole data with surface seismic images.
Best Practices
💡 Pro Tip: Document Unit Source
Well logs from different eras and operators use different units. Always document the original units from data source before converting. A 5.5 value could be 5.5 km/s (reasonable for rock) or 5,500 m/s (same thing), but documentation prevents confusion.
- Cross-reference with mineral databases: Verify density and hardness conversions against published mineral properties.
- Account for measurement conditions: Density varies with temperature and pressure; seismic velocity depends on fluid saturation.
- Use consistent units in calculations: Convert all inputs to SI units (kg/m³, m/s, Pa) before stress or wave equation calculations.
- Verify porosity estimates: Porosity affects density and velocity; high-porosity rocks are less dense and slower than solid equivalents.
Common Mistakes
⚠️ Mohs Scale Arithmetic
Mohs scale is ordinal, not interval. A mineral with Mohs hardness 6 is not "twice as hard" as one with hardness 3. Do not perform arithmetic on Mohs values; use Vickers hardness for quantitative comparisons instead. An "average hardness" of Mohs scale minerals requires conversion to Vickers first.
Tools and Resources
- Mineral Databases: USGS Mineral Properties, Mindat.org include density, hardness, and optical properties.
- Well Log Software: Petrel, LogPlot interpret well logs with unit conversion tools built-in.
- Seismic Processing: Professional seismic software (Seismic Unix, ProMax) handles velocity unit conversions automatically.
Key Takeaways
- Density: 1 g/cm³ = 1,000 kg/m³; typical rocks 2.5-3.0 g/cm³ (quartz-feldspar); typical water 1.0 g/cm³
- Seismic velocity: 1 km/s = 1,000 m/s = 3,281 ft/s; rock velocity 1.5-6.5 km/s depending on mineral and saturation
- Mohs scale is ordinal—don't perform arithmetic; use Vickers for quantitative hardness comparisons
- Porosity is percentage; affects both density and velocity significantly
- Always document original units from data source before converting to prevent confusion
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