The Evolution of Remote Sensing

• Apr 01, 2002

Environmental site assessments, commonly referred to as Phase I through Phase III audits, are often required by insurance companies, financial institutions, real estate companies and other stakeholders as a prerequisite for property transactions. Phase I audits are used to determine if liabilities exist as a result of potential site contamination. Phase II and III audits involve corrective action processes. The methodology for conducting site assessments can be time consuming, tenuous and occasionally erroneous when decisions are based on random judgments of the terrain coupled with some type of "scientific" analysis for verification. Additionally, contaminants and the source of contaminants associated with adjoining properties that may affect the assessment are difficult to ascertain and evaluate with a significant degree of confidence due to perceived intrusion on private property, as well the often litigious mind-set.

Digital Airborne Imaging Spectrometers (DAIS) are remote sensors that can provide high-resolution hyperspectral imaging data of the terrain below. The data gathered is legally defensible in liability lawsuits, cost and logistically efficient in Phase I audits for large or multiple sites, and effective at providing drill location information for Phase II and III audits.

Hyperspectral imaging using remote sensors can provide data to detect and identify contaminated areas on a site, as well as provide the same data to adjoining sites. The latter can be important in assessing the source of a contaminate that may be found on site. In addition to establishing an anomaly (contamination) map to assist environmental engineers in well location and distribution, remote sensing can be used for periodic monitoring to determine if wells need to be moved or eliminated, if corrective action is effective and if migratory contamination is involved.

Remote sensing technology has been used for years to study the earth's surface as well as the atmosphere. In forestry, hyperspectral imaging scanners are used in aerial mapping and analysis of deforestation, reforestation, forest fire monitoring, invasive species identification, and fulfillment of other management requirements.

As an environmental tool, the technology is used to monitor wildlife habitats, flora distribution, detection and mapping of buried toxic waste and other related tasks. It is used in hydrology to evaluate potential water resources, flood areas and non-point contamination. Other uses include precision agriculture, coastal zone management, wetland monitoring, and exploration for minerals and petrochemical resource prospecting.

At present, remote sensing has not been used for environmental site assessments by engineering companies. The main reason is the majority of remote sensing services, which mostly involve satellite imagery, are provided to academic and governmental institutions under research and development grants, research and development contracts, and other sources of funding. The availability of airborne hyperspectral imaging services to the private sector is a recent development.

Digital Airborne Imaging Spectrometers operate by gathering light that is reflected or energy emitted from the earth's surface. The sun provides energy in the form of a continuum of discrete wavelengths over the entire electromagnetic (light) spectrum. Animate and inanimate objects, solids, liquids and gases all absorb certain wavelengths from the sun and reflect or emit others depending on their molecular structures or structural arrays. Since no two entities are the same, the wavelengths they reflect or emit are not the same to include their ability to retain heat. It is the differences or spectral signatures (fingerprints) of dissimilar physical characteristics at sites that hyperspectral imaging scanners can collect for analysis and assessment.

The data that is collected from a site or sites is stored on a data tape and pre-processed by computer to account for aircraft movement and atmospheric conditions. Global positioning information is also included for geography location. The data is then ready for additional processing using imagery analysis software that is commercially available.

Customized waveform algorithms are applied to the data to produce color pictorials (anomaly maps) that can show the site assessor potential areas of contamination, or it can show the engineering companies the location and distribution of wells.

Vegetation such as trees, brush, and grasses that show "obvious" signs of visual distress can alert the site assessor to the potential for contamination. Areas that do not show obvious visual distress go unnoticed except to a digital airborne imaging spectrometer. When leaves from vegetation are exposed to sunlight, they reflect certain wavelength signatures or fingerprints. The reflectance spectra of these fingerprints depend on whether the vegetation is normal (healthy) or under some kind of physiological distress from, for example, heavy metals. The reason is that leaves contain colorant pigments such as chlorophyll a, chlorophyll b, B-carotenes and luteins. Each pigment has its own unique spectrum. When vegetation is under distress, the concentrations and ratios of these pigments will vary in a predicable manner. The variations in these changes are measured and assessed by hyperspectral imaging methodology. In essence, contaminants and the movement of contaminants over terrain leave a subtle footprint in vegetative growth that is obvious to the digital imaging scanner.

The following serves to illustrate the method that would be used to gather remote sensing data on a site. The site for assessment is a one square mile area located in an established industrial park. The terrain and vegetation is variable, and a stream traverses the property. The remote sensing equipment used is a DAIS 3715 scanner that contains 37 detectors to include two highly sensitive thermal detectors. The instrument is mounted in a highly modified twin engine aircraft. The plane will over-fly the site at about 90 knots and 1000 ft above ground level to achieve the optimum spatial and spectral data for analysis.

The pilot and/or staff divide not only the site to be investigated but also adjoining sites into a series of overlapping flight lines while taking into consideration sun angle, maximum reflectance, wind direction and other variables. As the plane is flying, the remote sensor's scanner sweeps the ground below storing terrain and geography information on a data tape that is later corrected for aircraft movement and atmosphere. The data in the tape is in the form of a digital array, square meter by square meter, of the terrain (sites) below.

The cost for using remote sensing services will vary on a case by case basis depending on the needs of the end user and the distance the site or sites to be investigated are from the home base of the aircraft. Four factors are considered.

• Level of Data Analysis: There are nine levels of analysis applied to hyperspectral remote sensing data. Level 2 is the lowest level that includes the data tape record pre-processed for aircraft movement and atmosphere. There is a standard charge for pre-processing by gigabyte with one gigabyte as the minimum. Levels 3 and above involve computer analysis to product pictorial products for anomaly maps, well locations, etc. The analytical cost of these levels depends on the requirements of the end user.
• Flight Time: Aircraft charges are determined by flight hours needed to fly to an area, over-fly the area, and return to home base.
• Overhead: Overhead charges are based on day charges and per-diem for aircraft personnel as well as miscellaneous expenses. Generally, these will not apply for round trip missions.
• Logistics: One way missions that require basing the aircraft at another location or overnight stays for aircraft personnel affect the cost due to overhead.

In those cases where home base of the aircraft allowed for a round trip mission of the aircraft, the cost for a "single site" assessment by an environmental engineering firm would range from about $4000 to 6000/site. If, however, there were "ten sites" to be assessed in relative close proximity by the same engineering company, the cost would then be about $400 to 600/site. The reason is that one hour of flight time can cover several square miles of terrain and one gigabyte can cover several square miles of data accumulation. Because overhead and logistics are not a concern in a roundtrip mission -- only levels of analysis and flight time -- many sites for assessment can be over-flown for the same cost as one site. The cost of a one-way mission for a site/sites would have to be considered on a case by case basis.

When a Phase II-III audit is required resulting from a Phase I audit, remote sensing technology can be of great economic advantage to stakeholders such as insurance companies, financial institutions, real estate companies, etc. Specialized computer programs can analyze the digital data and assign the most efficient and effective placement for wells to be drilled and monitored. This can reduce the numbers of wells needed on a site which will reduce the engineering cost associated with the initial stage of a Phase III audit. After the initial base line map is established, the site can be economically monitored with remote sensing on a routine basis to determine if wells need to be moved or eliminated.

Phase I audits are required by insurance companies, financial institution, real estate companies and other stakeholders to determine from documentary research and visual observation if a site is free of potential contamination, and, therefore liability, at the time the audit was conducted. The audit, however, does not tell the stakeholder whether the site is "actually" contaminated, or whether adjoining properties are "actually" contaminated or whether the property was or was not "actually" contaminated several years later. One advantage of remote sensing data analysis is that it can determine if a site or adjoining sites are "actually" contaminated in the present or in the future. The other advantage is that the data tape serves as a permanent legal record of a site or adjoining sites which can be referenced if future liabilities were to occur.

A real estate development project would quickly lose value if at a later time a large illegal chemical dump was accidentally discovered in close proximity. This event did occur in South Carolina. The illegal dumpsite, which contained over 3000 drums of hazardous chemicals, was found on a 65-acre farm. Digital airborne imaging technology would have discovered the presence of the dumpsite before several developers had invested capital.

Remote sensing technology has evolved over the years to provide more and more detailed and accurate information about the terrain on the earth's surface. Property transactions have also evolved over the years and in a similar manner. Insurance companies, financial institutions, real estate developers and other stakeholders also require more and more information about the terrain on the earth's surface. Remote sensing technology and property assessment requirements are complementary pursuits intended to supplement documentary research, aerial photography, and ground surveillance. In this way, environmental engineering firms are provided the maximum in reliable, repeatable and achievable technology. This creates results for their clients who will permit the most cost efficient method to avoid liability, and to demonstrate due diligence in seeking environmentally effective solution to existing or new mandates under Resource Conservation Recovery Act (RCRA) and/or the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA), which is also know as Superfund.

e-sources

1. Introduction to Hyperspectral Imaging -- www.microimages.com

2. USGS Spectroscopy Lab -- speclab.cr.usgs.gov

3. Hyperspectral Sensors -- www.techexpo.com

4. Fourteenth International Conference on Applied Geologic Remote Sensing -- www.erim-int.com

5. Bioindicators of Forest Sustainability -- Search: Forest Research Information Paper No. 137

6. General Information -- Search: hyperspectral imaging

This article originally appeared in the 04/01/2002 issue of Environmental Protection.