Hyperspectral remote sensing(Imaging Spectroscopy)

Imaging Spectroscopy Imaging Spectroscopy is the acquisition of images where for

each spatial resolution element in the image a spectrum of the energy arriving at the sensor is measured.

These spectra are used to derive information based on the signature of the interaction of matter and energy expressed in the spectrum.

This spectroscopic approach has been used in the laboratory and in astronomy for more than 100 years.

The imaging spectrometer Hyperspectral images are produced by instruments called

imaging spectrometers.

The development of these complex sensors has involved the convergence of two related but distinct technologies: spectroscopy and the remote imaging of Earth and planetary surfaces.

Hyperspectral vs multispectral spectrometers

Multipspectral spectrometers have a limited number of spectral bands to operate with (7-64).

Using hyperspectral data (about two hundred bands)

• Material on Earth’s and other planetary surface can be identified

• forest inventories can be developed for remote regions to support ecological planning and management.

• Agriculture

Multi- vs. Hyper-

Hyper-: Narrow bands ( 20 nm in resolution) and continuous measurements.

Multi- vs. Hyper- Although most hyperspectral sensors measure hundreds of

wavelengths, it is not the number of measured wavelengths that defines a sensor as hyperspectral.

Rather it is the narrowness and contiguous nature of the measurements. For example, a sensor that measured only 20 bands could be considered hyperspectral if those bands were contiguous and, say, 10 nm wide.

If a sensor measured 20 wavelength bands that were, say, 100 nm wide, or that were separated by non-measured wavelength ranges, the sensor would no longer be considered hyperspectral.

Moffett Field, California, at the southern end of the San Francisco Bay.

AVIRIS acquired the data on August 20, 1992 when it was flown on a NASA ER-2 plane at an altitude of 20,000 meters (65,000 feet) over

One pixel response

The x-axis is channel wavelength in micrometers, also known as microns (one micron = 1000 nm).

The y-axis is radiance, usually expressed in units of microwatts per square centimeter per nanometer per steradian, or uW / (cm^2 * nm * sr).

Why imaging spectroscopy?

Plotting spectra

Spectral reflectance curve Scatter plots

Plotting spectra

Mathematically, each pixel value is the end point of a vector that begins at the origin of the coordinate system.

Spectra with the same shape but differing overall reflectance (albedo) plot as vectors with the same orientation but with endpoints at different distances from the origin.

Shorter spectral vectors represent darker spectra and longer vectors represent brighter spectra.

A reflectance spectrum consists of a set of reflectance values, one for each spectral band.

Each of these bands can be considered as 1-D in an n-dimensional spectral space, where n is the number of spectral bands.

If we plot the measured reflectance value for each spectral channel on its respective coordinate axis, we can use these coordinates to specify the location of a point in spectral space that mathematically represents that particular spectrum.

Plotting spectra

It may be difficult to visualize such a plot foran image involving more than three wavelengthbands, but it is mathematically possible to construct a hyperdimensional spectral space defined by dozens or hundreds of mutually-perpendicular coordinate axes.

Each spectrum being considered occupies a position in this n-dimensional spectral space. Similarity between spectra can be judged by the relative closeness of these positions (spectral distance) or by how small the angle is between the spectral vectors

Hyperspectral Systems

Hyperion- On NASA Earth Observing-1 (EO-1) spacecraft (2000). - Pushbroom system - A 705-km sun-synchronous orbit following 1 minute

behind LANDSAT 7, essentially viewing the same atmospheric conditions.

- 220 10-nm hyperspectral bands between 400 nm (blue) and 2500 nm (SWIR) and records reflectance in 12-bit format.

- 30 m x 30 m resolution; - 7.5 km hyperspectral swath

Another hyperspectral sensor aboard EO-1, called LEISA, with a 185-km swath at 250 m x 250 m GSD, collects 246 bands in the SWIR portion of the spectrum where water vapour absorption is significant. - This data is used to derive atmospheric correction

information for the other sensor datasets.

Hyperspectral SystemsAVIRISThe Airborne Visible InfraRed Imaging Spectrometer (AVIRIS) designed and built by the NASA Jet Propulsion Laboratory (JPL).

A whiskbroom scanner, and it flies on several airborne platforms. The swath and spatial resolution depend on the flying height and over-ground speed of the aircraft.

On the NASA ER-2 jet at 20 km above MSL, AVIRIS covers an 11 km swath at 20 m resolution.

The sensor records radiance levels at 12-bit radiometric resolution for 224 10-nm bands between 400 nm (blue) - 2500 nm (mid-IR).

Hyperspectral Systems

HyMapHyVista Corporation, of Sydney, Australia, commercially operates a 128-band whiskbroom hyperspectral scanner.

Spectral range of 400 - 2500 nm, with a bandwidth of 15-20 nm rather than 10 as for AVIRIS and Hyperion.

It records data in 12 - 16 bit format.

Spatial resolution depends on flying height, but ranges from 3 - 10 meters under typical conditions.

Hyperspectra of pyroxenes

What would happen to the absorption features in the enstatite-ferrosilite series (Mg2Si2O6 -Fe2Si2O6) as we move towards ferrosilite from enstatite?

Enstatite-ferrosilite

(MgFe)2Si2O6

(Ca,Fe)(MgFe)Si2O6

What would happen to the absorption features as we move down from hedenbergite to pigeonite?

Why sub-calcic hedenbergite has M2 absoprtion at 2300 nm while enstatite-ferrosilite have M2 absorption at 1850 nm?

Fe+2 CF @ M1 site

Fe+2 CF @ M2 site

Subcalcic-Hedenbergit

e

Fe+2 CF @ M1 site

Fe+2 CF @ M2 site

Fe+2 CF @ M2 site

Fe+2 => Fe+3 IVCT

Fe+2 CF @ M2 site

Fe+2 CF @ M2 site

Hedenbergite

Ca(MgFe)Si2O6

IVCT – Intervalence charge transfer, when an excited 3d electron is transferred between neighboring cations in adjacent coordination sites, transient changes in valencies occur, and the process is IVCT

Spectra of some pyroxenes (Wo (Wollastonite) – 100% Ca; En (Enstatite) – 100% Mg; Fs (Ferrosilite) - 100% Fe)

Molecular vibration of Hydroxyl (OH)

Symmetric stretch (v1 mode 3652 cm-1 or 2.8 μm)

First overtone of v1 at 1.4 μm - Diagnostic

Combination of Metal-OH bend + v1 between 2.2-2.4 - Diagnostic

OH – 2v1 Mg-OH Fe+2 CF splitting

Silicates - Amphiboles

Fe2+ - Ti4+ / Fe2+ - Fe3+

Olivine – Mg silicate Serpentine – MgFe silicate hydroxide

Silicates – Hydrated

and hydroxide