lecture08

ERS186:Environmental Remote Sensing: 

ERS186: Environmental Remote Sensing Lecture 9: Soils

Overview: 

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

Definitions: 

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: 

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: 

Soil Grain Size

Soil Grain Size: 

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: 

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.

Soil Taxonomy: 

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.

The Question: 

The Question What are the important properties of a soil in an RS image? Soil texture (proportion of sand/silt/clay) Soil moisture content Organic matter content Mineral contents, including iron-oxide and carbonates Surface roughness

Radiance of Exposed Soil: 

Radiance of Exposed Soil 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

Exposed Soil: 

Exposed Soil (Wavelength of C-band is approximately 5 cm; L-band, 30 cm.)

Interpretation of last slide…: 

Interpretation of last slide… There is water in the rivers (reason: glitter in color IR) Moderate wind possibly from upper left is ruffling water producing small ‘capillary’ waves (reason: that’s the direction of blown sand from dunes in image, glitter is spread out indicating ruffled surface, rivers ‘yellow’ in radar composite indicating high return in C band and L band HV but low return in L band HH) Pattern of water channels apparent beneath sand dunes. Dark red soil surface on left side of composite radar image might have low density of scattered, possibly loose, surface pebbles (reason: dark red indicates moderate C band return, little L band return; so the surface is smooth on a scale of 30 cm but a little rough on a scale of 5 cm; And the slight roughness in C band is depolarizing the signal, converting H polarization to V much like what a small pebble might do.) The blue areas of soil are probably hard packed fairly smooth clay. (reason: The lack of red suggests a smooth specularly reflecting surface at a scale of 5 cm, one sufficiently smooth that it tends not to depolarize the incident signal; the L band HH signal is returned by the very minor roughness – very minor undulations not rocks - on the soil surface; if the surface included rocks, presumably depolarization would occur in both C and L bands and we would observe more red and green mixed together with the blue.) I do not have an explanation for the predominately green areas in the center of the image.

Basic Dry Soil Spectrum: 

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

Soil Moisture: 

Soil Moisture Water ‘coats’ particles, filling air spaces and reducing the amount of multiply scattered light, so soils with more moisture will be darker in the VNIR and SWIR than drier soils. Moist soils will also be darker in the SWIR region where water absorption increases significantly with increasing wavelength. The depths of the water absorption bands at 1.4, 1.9 and 2.7 m can be used to determine soil moisture.

Soil Moisture and Texture: 

Soil Moisture and Texture Clays hold more water more ‘tightly’ than sand. Thus, clay spectra display more prominent water absorption bands than sand spectra. AVIRIS can be useful for quantifying these absorption features.

Soil Moisture from RADAR: 

Soil Moisture from RADAR Dielectric constant of water is about 80. (sq. root of dielectric constant = index of refraction = 9 for water at longer radar wavelengths) Dielectric constants of anything dry – dry soil, for example – are much, much smaller, generally less than 5. Thus, adding water (=80) to anything dry (<5) dramatically increases the dielectric constant of the mixture of the two when measured at radar wavelengths. The bigger the difference in the speeds of light in air and soil, the bigger the reflection at their interface; Thus, Higher dielectric constants (more moisture) yield dramatically higher RADAR backscatter. (Also remember that radar images are usually display using a log rather than linear scale. So the differences displayed in this image are huge.) 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: 

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: 

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).

Identifying Clayey Soils: 

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 doesn’t overlap the water absorption feature.

Soil Organic Matter: 

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: 

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

Iron Oxide: 

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

Surface Roughness: 

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. Why?

Surface Roughness: 

Surface Roughness C/X-SAR (l=5/1 cm) image of Oxford County, Ontario, Canada: Crop residue on the soil surface can diminish soil erosion. Conventional tillage produces a much rougher surface than ‘no-till,’ and therefore brighter backscatter. The goal was to determine if tillage practices could be identified using SAR imagery. A Philosophical Comment Do you think the various tillage practices could be identified? At regional to global scales? With certainty? Except in geology, remote sensing generally involves a mapping of one observation to many possible causes. So reduced backscatter might indicate ‘no-till’ in Oxford County on one date but not necessarily ‘no-till’ in ag fields anywhere else - around Davis, for example. However, by collecting observations at multiple wavelengths, times, dates, view angles, sun angles, polarizations, etc. and taking account of scene contextual information, we usually are able to narrow the range of possibilities – and ultimately extract good quality information about the scene.