Transcript ERS186: Environmental Remote Sensing

ERS186:
Environmental Remote Sensing
Lecture 9:
Soils
Overview
• Applications
– Soil Science
• Physical Principles
– Reflectance (specular and diffuse scattering)
– Absorption bands
– Dielectric constants
• Sensors
– RADAR
– Thermal
– Hyperspectral
Definitions
• Soil: the weathered material between the surface of
the Earth and the bedrock.
• Soils are composed of different composition and sizes
of particles of inorganic mineral and organic matter
• Particles are about 50% of the soil volume, pores
occupy the rest of the space. Pores can contain air or
water (or ice!)
• Soils have vertical zonation (soil horizons) created
by biological, chemical and physical processes
Soil Horizons
•
•
•
•
•
•
•
O horizon: > 20% partially decayed
organic matter (“humus”)
A horizon: zone of
eluviation/leaching; water leaches
many minerals; often pale and sandy
E horizon: mineral layer with loss of
some combination of silicate clay,
iron, aluminum
B horizon: zone of illuviation;
materials leached from other zones
end up here; often lots of clay and
iron oxides
C horizon: weathered parent material;
mostly mineral
W horizon: water layer; Wf if
permenantly frozen
R horizon: bedrock
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
0.002
Clay
0.05 0.1 0.25 0.5
v. coarse
2 mm
1
76.2
Particle size relative
to a grain of sand
0.15 mm in diameter
Silt
0.15 mm
Sand
b. MIT and British Standards Institute
Clay
Silt
fine
medium
0.002 0.006
Clay
Sand
coarse
0.02
medium
0.2
Gravel
coarse
0.6
fine
0.02
Gravel
coarse
0.2
Stones
2 mm
c. International Society of Soil Science
Sand
Silt
0.002
0.06
fine
2 mm
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)
Soil Texture
100
Cla
y
(%
)
90
10
20
80
30
70
Clay
60
40
30
20
sandy
clay
silty
clay
90
80
60
70
80
Loam
loamy
Sand sand
50
silty clay
loam
clay loam
sandy clay
loam
10
100
40
50
read
read
)
t (%
Sil
• Proportion of sand, silt and
clay in a soil (or horizon),
usually calculated as %
weight for each type of
particle.
• These %s can be broken up
into different soil-texture
classes.
silt loam
sandy
loam
90
Silt
100
60
70
Sand (%)
50
40
read
30
20
10
Soil Taxonomy
• Similar to biological taxonomy -- dichotomous
keys based on soil profiles, soil color, soiltexture class, moisture content, bulk density,
porosity, and chemistry are used to ID different
types of soils.
The Question
• What are the important properties of a soil in
an RS image?
–
–
–
–
Soil texture
Soil moisture content
Organic matter content
Mineral contents, including iron-oxide and
carbonates
– Surface roughness
Exposed Soil Radiance
• Lt = Lp + Ls + Lv
• Lt = at-sensor radiance of a pixel of exposed soil
• Lp = atmospheric path radiance, usually needs to be removed through
atmospheric correction
• Ls = radiance reflected off the air-soil interface (boundary layer)
– Soil organic matter and soil moisture content significantly impact Ls; typically
characterize the O horizon, the A horizon (if no O), or lower levels if A and O
are nonexistant.
• Lv = volume scattering, EMR which penetrates a few mm to cm.
– penetrates approximate 1/2 the wavelength
– Function of the wavelength (so RADAR may penetrate farther), type and
amount of organic/inorganic constituents, shape and density of minerals, degree
of mineral compaction, and the amount of soil moisture present.
Exposed Soil Radiance
Exposed Soil Radiance
Space
SpaceShuttle
Shuttle
Color
-Infrared
Color-Infrared
Photograph
Photograph
SIR
-C Color
SIR-C
ColorComposite:
Composite:
••Red
-band HV
Red ==CC-band
HV
••Green
-band HV
Green==LL-band
HV
••Blue
=
L
band
HH
Blue = L-band HH
Percent Reflectance
Basic Dry Soil Spectrum
100
90
80
Silt
70
60
50
Sand
40
30
20
10
0
0.5 0.7
0.9
1.1
1.3 1.5 1.7 1.9
Wavelength (m)
2.1
2.3
2.5
Key characteristic of soil spectrum: increasing reflectance with increasing
wavelength through the visible, near and mid infrared portions of the spectrum
Soil Moisture
• Water is a strong absorber, so
soils with more moisture will
be darker over most of the
VNIR and SWIR portions of
the spectrum than drier soils.
• The depths of the water
absorption bands at 1.4, 1.9 and
2.7 m can be used to
determine soil moisture.
incident energy
specular
reflectance
dry
soil
a.
interstitial
air space
incident energy
specular
reflectance
b.
specular reflectance
volume reflectance
soil water
wet
soil
Percent Reflectance
Percent Reflectance
Soil Moisture and Texture
60
50
Sand
Sand
0 – 4% moisture content
Sand
40
30
5 – 12%
20
22 – 32%
10
0
0.5
0.7
0.9
1.1
1.3
1.5
1.7
1.9
2.1
2.3
2.5
a.
60
50
Clay
Clay
Clay
2 – 6%
40
30
20
35 – 40%
10
0
0.5
b.
0.7
0.9
1.1
1.3 1.5 1.7 1.9
Wavelength (m)
2.1
2.3
2.5
• Since clayey soil holds
water more tightly than
sandy soil, the water
absorption features will be
more prominent in clayey
soils given the same amount
of time since the last
precipitation or watering.
• AVIRIS can be useful for
quantifying these absorption
features.
Soil Moisture from RADAR
• Different materials
conduct electricity
better than others
(different complex
dielectric constant).
• Higher dielectric
constants (more
moisture) yields higher
RADAR backscatter.
Melfort, Saskatchewan, Canada,
ERS-1: Rainfall was incident on the
lower half of the image but not on the
upper half.
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 from Thermal Sensors
Daedalus thermal image
(night time). If we had
a daytime image to
compare it to, we could
see the amount of
change in temperature
and make inferences on
the soil moisture content
(less change = more
moisture).
Percent Reflectance
Percent Reflectance
Identifying Clayey Soils
60
50
Sand
Sand
Sand
0 – 4% moisture content
40
30
5 – 12%
20
22 – 32%
10
0
0.5
0.7
0.9
1.1
1.3
1.5
1.7
1.9
2.1
2.3
2.5
a.
60
50
Clay
Clay
Clay
2 – 6%
40
30
20
35 – 40%
10
0
0.5
b.
0.7
0.9
1.1
1.3 1.5 1.7 1.9
Wavelength (m)
2.1
2.3
2.5
Soils with a large amount
of clay exhibit hydroxyl
absorption bands at 1.4
and 2.2 m. 2.2 m is
more useful since it
doesn’t overlap the
water absorption
feature.
Soil Organic Matter
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 (Palacios-Orueta et al. 1999).
Iron Oxide
Recall that iron oxide causes a charge transfer absorption in the UV, blue and green
wavelengths, and a crystal field absorption in the NIR (850 to 900 nm). Also, scattering in
the red is higher than soils without iron oxide, leading to a red color.
Iron Oxide
Iron content in the Santa Monica mountains mapped using
AVIRIS (Palacios-Orueta et al. 1999).
Surface Roughness
• If a surface is smooth (particle
size is small relative to
wavelength), we expect a lot of
specular reflection.
– Only sensors positioned at the
correct angle will see the bright
reflectance. All other angles will
see a dark surface (including all
RADAR imagery).
– Smooth surfaces are clayey or
silty and often contain strong
absorbers such as moisture,
organic content, and iron oxide.
• A rough surface generates a lot of
diffuse reflection.
– Conversely, well drained sands
are often very bright, regardless
of angle.
Surface Roughness
• C/X-SAR (C-band) image of
Oxford County, Ontario, Canada:
Conservation tillage (the retention
of crop residue on the soil
surface) can diminish soil erosion.
Conventional tillage produces a
much rougher surface, and
therefore brighter backscatter.
The goal of this study was to
determine if tillage practices
could be identified using SAR
imagery.