Environmental Science & Engineering - www.esemag.com - May 2003
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Hyperspectral imaging technologies key for oil seep/oil-impacted soil detection and environmental baselines

by Jim Ellis,
Ellis GeoSpatial

Landsat™ image as a total infrared colour image (healthy vegetation reddish orange) of the area mapped with airborne hyperspectral imagery in Southern California. The detected oil seeps are east of the lake.

Natural oil seeps can help exploration professionals discover new oil reserves hidden beneath the Earth’s surface, creating wealth for companies and governments. At the same time, society’s global use of hydrocarbonbased products has resulted in countless spills and leaks over the past century, polluting soils, water, vegetation and air. The locations of many of these oil seeps, oil spills, and oilimpacted soils are not known.

Exploration opportunities across the globe are being missed and environments continue to be degraded as these oil-impacted sites go unmapped. By mapping and monitoring these areas using new imaging technologies and advanced software packages, exploration professionals can find new seeps indicative of oil accumulations at depth and environmentalists can assess a broad range of surface conditions.

Energy and oil companies monitor their assets and develop environmental baselines to better understand the impact of development on the environment.

The power of hyperspectral imagery for asset mapping Energy and oil companies are responsible for significant assets, including oil fields, refineries and tank farms, pipelines, and exploration acreage. With all of these assets comes the responsibility of regularly and uniformly assessing the environmental conditions across the acreage – including the location and extent of oilimpacted sites. This type of monitoring can be a time-consuming process without the appropriate imaging, mapping and analysis tools.

Typically, detection of oil-impacted sites is conducted as part of an overall environmental baseline that includes mapping of vegetation types, vegetation stress, soil conditions, land use/land cover, infrastructure and water conditions. Satellite and airborne spectral imaging sensors are often used to document vegetation condition and geology across oil fields.

Hyperspectral imaging sensors have sufficient spectral resolution to identify different surface materials based solely on spectral characteristics. They record narrow wavelength bands of visible light as well as longer, near-infrared and short wave-infrared light, providing vast amounts of spectral information at the sub-pixel level. Hyperspectral sensors continuously measure the intensity of solar energy reflected from materials as it varies across different wavelengths. More than 100 measurements are typically recorded for each pixel, each representing the material’s response to a different wavelength of light.

We are using NASA’s experimental Hyperion hyperspectral satellite and several commercial airborne sensors to map environmental conditions across high-value assets for the energy industry. The unique spectral signatures that characterize pixels with different vegetation types and their levels of stress, disturbed soils, water conditions, manmade materials, and oil-impacted surfaces, are organized into spectral libraries that can be applied globally. Spectral libraries are essential for environmental baselines. The pixels that are used to characterize a feature of interest need to be accurately located on a map and the image and groundspectra measurements obtained.

In 1999, a cooperative research project was proposed by me, organized by Geosat and sponsored by Chevron, Exxon and Shell to determine the viability of hyperspectral technology for detecting oil seeps and oil-impacted soils. We knew that if we were successful, we would be developing a new mapping tool that would have global application for the energy industry.

The Geosat project proved that sophisticated airborne and ground hyperspectral sensors were capable of detecting oil seeps and oil-impacted soils. We used the sophisticated algorithms of ENVI Software to extract the subtle hydrocarbon signature from the airborne hyperspectral datacubes. The research project demonstrated that facility managers, engineers, environmental scientists and geologists could use these technologies to obtain traditional maps and to detect oil-impacted sites, subtle variations in vegetation vigour, different plant types and differences among disturbed and engineered soils.

Geologic mapping For exploration, we use remote sensing imagery to effectively map variations in rock type, structure, vegetation and fracture density; map more subtle variations at the surface; and consider direct detection of oil seeps as part of the exploration tool kit. Oil accumulations trapped beneath the surface of the earth can leak, resulting in oil seeps at the surface. Detecting and accurately mapping the location, size, and pattern of oil seeps can improve subsurface modelling and prioritize expensive seismic and drilling programs.

ENVI Software was selected to enable this sophisticated mapping project, as the software’s features are especially key for extracting information from hyperspectral data at the subpixel level. Using subpixel and unmixing algorithms, we are able to detect small amounts of hydrocarbon-based material within a pixel. This is a critical capability that enables mapping of a subtle, yet very important feature, from hyperspectral data.

For mapping targets that have unique and strong spectral signatures, such as the clay minerals associated with hydrothermal alteration, our analysts use ENVI to easily, rapidly and systematically derive some information from hyperspectral images. During processing, a series of image maps are developed which reveal pixels containing a spectrally unique feature of interest – such as an oil-impacted surface, vegetation type, iron or carbonate mineral. Ultimately, the image maps are converted into geographic information system (GIS) layers with attributes for helping the client identify specific features within an area of study.

Helping exploration and the environment Finding new natural oil seeps can be very important for advancing exploration opportunities. When these seeps are near coastal communities, they also cause pollution and are of concern to the environmentalist. In Southern California, as decades of production depletes the pressure within oil reservoirs buried beneath the ocean floor, natural oil seeps along the coast are leaking less and the amount of pollution is decreasing. Here the environment is improving at the same time energy companies and governments are reaping the monetary benefits of production.

Sophisticated satellite and airborne hyperspectral sensors, coupled with powerful image processing software and GIS, enable both exploration and environmental professionals to improve their understanding and mapping of high-value assets. These technologies are being used for detecting natural oil seeps in support of exploration and oil-impacted surfaces for environmental applications. Once located, characterized and loaded into a GIS, these oily sites are integrated with other maps and databases to improve the management of the asset.

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