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ERS186: Environmental Remote Sensing

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Pores can contain air or water (or ice! ... Water is a strong absorber, so soils with more moisture will be darker over most ... Water has a higher thermal ... – PowerPoint PPT presentation

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Title: ERS186: Environmental Remote Sensing


1
ERS186Environmental Remote Sensing
  • Lecture 9
  • Soils

2
Overview
  • Applications
  • Soil Science
  • Physical Principles
  • Reflectance (specular and diffuse scattering)
  • Absorption bands
  • Dielectric constants
  • Sensors
  • RADAR
  • Thermal
  • Hyperspectral

3
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

4
Soil Horizons
  • O horizon gt 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

5
Soil Grain Size
6
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 (lt 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)

7
Soil Texture
  • 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.

8
Soil Taxonomy
  • Similar to biological taxonomy -- dichotomous
    keys based on soil profiles, soil color,
    soil-texture class, moisture content, bulk
    density, porosity, and chemistry are used to ID
    different types of soils.

9
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

10
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.

11
Exposed Soil Radiance
12
Exposed Soil Radiance
13
Basic Dry Soil Spectrum
Key characteristic of soil spectrum increasing
reflectance with increasing wavelength through
the visible, near and mid infrared portions of
the spectrum
14
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.

15
Soil Moisture and Texture
  • 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.

16
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.
17
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.

18
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).

19
Identifying Clayey Soils
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 doesnt overlap the
water absorption feature.
20
Soil Organic Matter
  • Organic matter is a strong absorber of EMR, so
    more organic matter leads to darker soils (lower
    reflectance curves).

21
Soil Organic Matter
  • Organic matter content in the Santa Monica
    mountains mapped using AVIRIS (Palacios-Orueta et
    al. 1999).

22
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.

23
Iron Oxide
  • Iron content in the Santa Monica mountains mapped
    using AVIRIS (Palacios-Orueta et al. 1999).

24
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.

25
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.
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