ERS186: Environmental Remote Sensing

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ERS186Environmental Remote Sensing

• Lecture 9
• Soils

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Overview

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

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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

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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

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Soil Grain Size
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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)

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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.

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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.

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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

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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.

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Exposed Soil Radiance
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Exposed Soil Radiance
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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
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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.

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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.

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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.
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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.

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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).

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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.
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Soil Organic Matter

• Organic matter is a strong absorber of EMR, so
  more organic matter leads to darker soils (lower
  reflectance curves).

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Soil Organic Matter

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

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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.

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Iron Oxide

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

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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.

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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.