Remote Sensing for Geologic
Applications
• Soil Properties
• Mineral and Rock Identification
• Geomorphology (landforms)
• Volcanology
• Coastal Processes
• Fluvial Processes
• Desertification
Remote Sensing of Soils
• A mature, fertile soil is the product of centuries of
physical and chemical weathering of rock, combined with
the addition of decaying plants and organic matter.
• Soil is a mixture of inorganic mineral particles, organic
matter, and pores containing air or water.
• Horizontal layers are called horizons: O,A,E,B,C. In
remote sensing, we see only the upper layer that is
present (not always an “O”).
• EMR in the visible and near-IR only penetrates about ½
of the wavelength of the radiation.
• Longer wavelength microwave energy can penetrate
dry, very sandy soils do depths of up to several meters.
Soil Grain Size
a. Soil Science Society of America and U.S. Department of Agriculture Soil Particle Size Scale
Sand
Clay Silt Gravel
v. fine fine medium coarse v. coarse
0.002 0.05 0.1 0.25 0.5 1 2 mm 76.2
Particle size relative
to a grain of sand
Clay Silt 0.15 mm in diameter
0.15 mm
Sand
b. MIT and British Standards Institute
Silt Sand
Clay Gravel Stones
fine medium coarse fine medium coarse
0.002 0.006 0.02 0.06 0.2 0.6 2 mm
c. International Society of Soil Science
Sand
Clay Silt Gravel
fine coarse
0.002 0.02 0.2 2 mm
Spectral Reflectance of Soils
• Spectral Reflectance of Soils controlled
by:
– Soil moisture: Percentage of sand, silt, and
clay
– Moisture content
– Organic matter
– Mineral contents, including Iron oxide and
carbonates
– Surface roughness
Soil Grain Size
• Different size particles play different roles
in soil:
– Sand (0.05 to 2.0 mm): large air spaces, rapid
drainage of water
– Silt (0.002 to 0.05 mm): enhance movement
and retention of soil capillary water
– Clay (< 0.002 mm): enhance movement and
retention of soil capillary water; carry electrical
charges which hold ions of dissolved minerals
(e.g. potassium and calcium)
Spectral Reflectance from Soils
Soil Texture and Moisture Content
• The void space between soil particles, called
interstitial air space, can hold water from
precipitation. A layer of water can also form
around the particles, called capillary water
• As soil moisture increases, overall reflectance
decreases and water absorption features
develop at 1.4, 1.9, 2.7 microns
• Microwave reflectance increases with increasing
soil moisture
• Sandy soils drain faster and dry by evaporation
faster than clay soils.
The effect of increasing soil moisture
content on spectral reflectance
Soils with a large amount of clay exhibit hydroxyl
Absorption bands at 1.4 and 2.2 microns. 2.2
microns is more useful since it doesn‟t overlap
the water absorption feature.
Clay mineral hydroxyl absorption
band at 2.2 microns
Soil Moisture from Thermal
Sensors
• Water has a higher thermal capacity than
soil and rock.
• Moist soils will change in temperature
more slowly than dry soils.
Soil Moisture and Texture
-Clays hold more •
Percent Reflectance
60 Sand
50
Sand
Sand 0 – 4% moisture content
water more „tightly‟
40
30 5 – 12%
than sand.
20
10
22 – 32%
-Thus, clay spectra •
0
0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5
display more
a.
prominent water
Percent Reflectance
60
Clay
Clay
50
Clay
2 – 6%
absorption bands
40
30
than sand spectra.
20
35 – 40%
-AVIRIS can be useful •
10
0 for quantifying these
0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5
b. Wavelength ( m) absorption features.
Effects of Organic Matter in Soils
Organic matter is a strong absorber of EMR, so more organic matter leads to darker
soils (lower reflectance curves).
Soil Organic Matter
Organic matter content in the Santa Monica mountains mapped using AVIRIS
Effect of Fe-oxide: Increase in
the 600 – 700 nm reflectance,
decrease in 400 – 600 nm
reflectance, decrease in near-
IR (absorption feature at ~875
nm)
Iron Oxide
Iron content in the Santa Monica mountains mapped using AVIRIS
Surface Roughness
• If a surface is smooth (particles smaller than
wavelength), specular reflection is important.
• No return – surface dark – unless sensor correctly
positioned and pointed in specular direction.
• Smooth soil surfaces tend to be clayey or silty, often are
moist and may contain strong absorbers such as organic
content and iron oxide.
• Conversely, a rough surface scatters EMR and thus
appears bright.
• But paradoxically, microwave data of well drained sands
are often very bright, regardless of angle.
Surface Roughness and SAR