The PROBE-1 Hyperspectral Instrument

Within the realm of exploration, remote sensing does not replace the need for geological knowledge, geochemistry, geophysics, seismic, drilling, etc, which are some of the more conventional tools used for exploration. Remote sensing does, however, identify more exploration targets faster and improves the probability of finding anomalies. Conventional exploration methods would take decades to cover the same ground, and at increased cost, the same task that can be performed by utilizing airborne hyperspectral technology.

Prior to the development of hyperspectral systems in the late nineties, multispectral instruments provided the only remote sensing capability from the air or space. Multispectral means these instruments can only record and analyze a few bands ( 6 to 7) of the spectrum. The ESSI PROBE-1 hyperspectral instrument has the capacity for 128 channels of data from a 6 km. wide swath. The difference is achieved by splitting the light spectrum more times. The resulting improvement in spectral resolution enables a trained geologist to read the chemistry of the object viewed, yielding more substantive information, and enables him to identify what is there instead of merely learning that something is there.

The PROBE-1 can be flown over a range of altitudes to provide pixel sizes ranging from 1 to 10 meters and swath widths from <1 km to 6 km. At 2500 meters, the PROBE-1 has a swath width of 3 kilometers with a ground instantaneous field of view (GIFOV) of 5 meters (known as 5-meter resolution). From 5000 meters, the swath width is 6 kilometers with a GIFOV of 10 meters. Given the PROBE-1's transportability between platforms, the lower pixel sizes are achieved by transferring the instrument to a slower flying platform such as a helicopter, or strategic partner Boeing's heliocourier aircraft.

Many common minerals can be identified with the above performance. Ferrous (Fe2+) and ferric (Fe3+) bearing minerals have spectral features in the visible and near infrared region. Clay minerals can be identified with the spectral features in the shortwave infrared region caused by vibrational overtones of bonds between Al-OH, Mg-OH, and Fe-OH. Carbonate minerals such as calcite and dolomite have features in the shortwave infrared region as well. Other minerals, including some sulfates, can be identified on the basis of signatures caused by water in their molecular structure.


Figure 1
Cuprite, Nevada
Natural Scene

Figure 1 shows a PROBE-1 image from a flight path over Cuprite, Nevada. This is produced in colors chosen to represent a natural looking scene, much like a natural photograph.

Software manipulation and a change in selection of the data and processing over the same area reveal a different look in Figure 2. Now, no longer a natural looking scene, the image reveals where the opalized rock and altered clays, kaolinite and alunite are actually located. The PROBE-1's spectral resolution is sufficiently high that minerals with closely related spectral signatures like those above can be distinguished from one another. This process can go on and on over the same area as one looks for different materials.

Figure 2

Figure 3

Figure 3 shows where further analysis identifies buddingtonite, a much more important pathfinder mineral in geological exploration. It can be identified through the bright red color assigned to its spectral signature (Note: different colors can be assigned to the spectra for the sake of clarity, i.e., the same alunite is blue in one image and magenta in another)

In this same manner, different species of plants and trees can also be mapped.

Technical Specifications
The Probe-1 is a "whiskbroom style" instrument that collects data in a cross-track direction by mechanical scanning and in an along-track direction by movement of the airborne platform. The instrument acts as an imaging spectrometer in the reflected solar region of the electromagnetic spectrum (0.4 to 2.5 nm). In the VNIR and SWIR, the at-sensor radiance is dispersed by four spectrographs onto four detector arrays. Spectral coverage is nearly continuous in these regions with small gaps in the middle of the 1.4 and 1.9 nm atmospheric water bands.
In order to avoid geometric distortions in the recorded imagery, the Probe-1 is mounted on a 3 axis, gyro-stabilized mount. Geolocation of nadir pixels is assisted by the recording of aircraft GPS positional data and tagging each scan line with a time that is referenced to the UTC time interrupts from the GPS receiver.

Spectral Specifications (VNIR through SWIR )