Alteration Index
Geological index used to identify areas of hydrothermal alteration and mineral deposits. The ratio highlights areas where clay minerals and hydroxyl-bearing minerals are present, indicating potential alteration zones.
Used in water detection, and mineral exploration.
When to use
- Mineral exploration target identification in arid and bare-ground regions
- Hydrothermal alteration zone mapping
- Lithological unit discrimination
- Iron oxide and clay mineral mapping
- Pre-field reconnaissance for geological surveys
- Hydrothermal Alteration Mapping
- Iron Ore Detection
Limitations
- Vegetation cover masks underlying mineral signatures — works best on bare ground
- Atmospheric water vapour absorbs in similar SWIR regions, requiring correction
- Particle size and mineral mixtures produce non-linear spectral mixing
- Should be combined with field validation — single-index identification is unreliable
- Different mineral assemblages can produce similar spectral signatures
- Requires sensors with SWIR bands — not available on all platforms
General Formula
Sensor-Specific Formulas
Most-used sensors — click to show code below
| Sensor | Provider | Formula | Band Mapping |
|---|---|---|---|
| USGS/NASA | B6 / B7 | SWIR3→B6, SWIR5→B7 | |
| MAXAR | SWIR3 / SWIR5 | SWIR3→SWIR3, SWIR5→SWIR5 |
Spectral Band Visualization — Landsat 8/9
Code Examples
Adapted for Landsat 8/9 bands —
Frequently Asked Questions
What is the ALT (Alteration Index) and when should I use it?
Geological index used to identify areas of hydrothermal alteration and mineral deposits. The ratio highlights areas where clay minerals and hydroxyl-bearing minerals are present, indicating potential alteration zones. Geological indices identify mineral compositions and lithological features by targeting diagnostic absorption features in shortwave infrared wavelengths. Different minerals produce unique spectral signatures that these indices isolate. ALT is particularly suited for mineral exploration, hydrothermal alteration mapping, geological surveys. The general formula is SWIR3 / SWIR5, which requires SWIR3 and SWIR5 spectral bands.
Which satellite sensors can I use to calculate ALT?
ALT is supported by 2 satellite sensors in our database, including Landsat 8/9, WorldView 3. Each sensor uses different band designations — for example, Landsat 8/9 uses the formula B6 / B7, while WorldView 3 uses SWIR3 / SWIR5. Select a sensor above to see its specific band mapping.
What spectral bands does ALT require and why?
ALT requires SWIR3 (1640-1680 nm), SWIR5 (2145-2185 nm). These specific wavelength regions correspond to diagnostic mineral absorption features caused by electronic transitions and vibrational overtones in crystal lattices.
How do I calculate ALT in Python or R?
Both Python and R code samples are provided above. In Python, use rasterio to load individual band GeoTIFF files and numpy for the arithmetic. In R, the terra package handles raster operations efficiently. The key is to load bands as floating-point arrays to avoid integer division, and to handle division-by-zero cases where the denominator equals zero. For production use, consider applying a valid data mask to exclude no-data pixels before calculation.
What minerals can ALT help identify?
Geological index used to identify areas of hydrothermal alteration and mineral deposits. The ratio highlights areas where clay minerals and hydroxyl-bearing minerals are present, indicating potential alteration zones. For accurate mineral identification, this index should be used alongside other geological indices and validated with field samples or known geology maps. Spectral unmixing or supervised classification using multiple indices typically yields more reliable results than any single index alone.
ALT vs other geology indices
| Index | Name | How it differs |
|---|---|---|
| AKP | Alunite/Kaolinite/Pyrophylite Index | Alternative geology index — different band combination |
| AMP | Amphibole Index | Alternative geology index — different band combination |
| Clay | Clay Index | Alternative geology index — different band combination |
| DOL | Dolomite Index | Alternative geology index — different band combination |
Related Geology Indices
References
Need help choosing?
Ask our AI assistant for sensor recommendations, code examples, or how ALT compares to other indices for your specific use case.