Environmental Mapping of the World Trade Center Area with Imaging

Chapter 4

Environmental Mapping of the World Trade Center Area with Imaging Spectroscopy after the September 11,2001 Attack The Airborne Visible/Infrared Imaging Spectrometer Mapping 1

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Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: November 15, 2005 | doi: 10.1021/bk-2006-0919.ch004

Roger N. Clark , Gregg A, Swayze, Todd M. Hoefen, Robert O. Green , K. Eric Livo , Gregory P. Meeker , Stephen J. Sutley , Geoffrey S. Plumlee , Betina Pavri , Chuck Sarture , Joe Boardman, Isabelle K. Brownfield , and Laurie C. Morath 2

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Denver Microbeam Laboratory, U.S. Geological Survey, MS 964, Box 25046 Denver Federal Center, Denver, CO 80225 Jet Propulsion Laboratory, 400 Oak Grove Drive, Pasadena, CA 91109 Analytical Imaging and Geophysics LLC, 4450 Arapahoe Avenue, Suite 100, Boulder, CO 80303

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The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) was flown over the World Trade Center area on September 16, 18, 22, and 23, 2001. The data were used to map the WTC debris plume and its contents, including the spectral signatures of asbestiform minerals. Samples were collected and used as ground truth for the AVARIS mapping. A number of thermal hot spots were observed with temperatures greater than 700° C. The extent and temperatures of the fires were mapped as a function of time. By September 23, most of the fires observed by AVIRIS had been eliminated or reduced in intensity. The mineral absorption features mapped by AVARIS only indicated the presence of serpentine mineralogy and not if the serpentine has asbestiform.

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U.S. government work. Published 2006 American Chemical Society

Gaffney and Marley; Urban Aerosols and Their Impacts ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction Spectroscopy is a tool that detects chemical bonds in molecules (solid, liquid or gas) through absorption (or emission) features in the spectrum of the material. Imaging spectroscopy obtains a spectrum for every spatial pixel in an image format. The Airborne Visible / Infrared Imaging Spectrometer (AVIRIS), a hyperspectral remote sensing instrument, was flown by the Jet Propulsion Laboratory (JPL/NASA) over the World Trade Center (WTC) area on September 16, 18, 22, and 23, 2001. The AVIRIS sensor obtains an ultraviolet to nearinfrared (0.37 to 2.50 jim) spectrum for each pixel in a spatial array. If materials are present in sufficient abundance and are spectrally active in the AVIRIS ultraviolet to near-infrared spectral region, their spatial locations can be mapped in far greater detail than traditional ground sampling methods. A single AVIRIS flight can measure spectra for approximately 25 million sample locations. The purpose for this study was to provide data to map materials that characterize the environment around the WTC, including asbestos, which was feared by first responders to have been potentially spread by the collapse of the WTC. The U . S. Geological Survey (USGS) effort was begun in response to requests from other Federal agencies. In responding to this request, we mobilized a 2-person team (authors T.M. Hoefen and G.A. Swayze) to the WTC site to obtain calibration data in order to correct the AVIRIS data to surface reflectance and collect samples to provide ground truth to verify the AVIRIS mapping analyses (see Chapter 3). Samples were collected of dusts and air fall debris from more than 34 localities within a 1-km radius of the WTC on the evenings of September 17 and 18, 2001. This sample set provided the needed ground truth for the AVIRIS data as well as samples for more detailed laboratory analyses. Of the samples collected, all but two were outdoor samples. Two samples were collected from indoor locations that were presumably not affected by rainfall (there was a rainstorm on September 14). Two samples of material coating a steel beam in the WTC debris were also collected. The sample set and the spectroscopic and mineralogic analyses are presented in Chapter 3 while the geochemistry of the samples is presented in Chapter 12 and the microbeam analyses is presented in Chapter 5 (7). The USGS ground crew also carried out on-the-ground reflectance spectroscopy measurements during daylight hours to field calibrate the AVIRIS remote sensing data. Radiance calibration and rectification of the AVIRIS data were done at JPL/NASA. Surface reflectance calibration, spectral mapping, and interpretation were done at the USGS Imaging Spectroscopy Lab in Denver. The dust/debris and beam-insulation samples were analyzed for a variety of mineralogical and chemical parameters using Reflectance Spectroscopy (RS) and


68 X-Ray Diffraction (XRD) (see Chapter 3), Scanning Electron Microscopy (SEM) (Chapter 5), (/) chemical analysis, and chemical leach test techniques (Chapter 12) in USGS laboratories in Denver, Colorado.


Imaging Spectroscopy Data Analysis The AVIRIS instrument measures upwelling spectral radiance in the visible through near-infrared. The instrument has 224 spectral channels (bands) with wavelengths from 0.37 to 2.5 |im (micrometers) with sufficient spectral resolution to characterize diagnostic spectral features in materials (2, 3). AVIRIS was flown at 2 altitudes in the WTC area for this study to give pixel sizes of approximately 2 and 4 meters. Reported here are the higher resolution 2-meter/pixel data. Because AVIRIS measures reflected sunlight, it generally cannot detect materials deeper than can be seen with the human eye. For most solid materials this optical penetration is measured in millimeters. The AVIRIS instrument was flown by NASA/JPL over the WTC area on Sept. 16,18, 22, and 23, 2001, after the attack on the WTC. Collection of data on Sept. 18 and 22 was hampered by clouds, whereas the 16 and 23 were clear. The 13 gigabytes of September 16th data were sent to the USGS in Denver on September 17-18th. Atmospheric absorptions, instrument response and the solar spectrum were removed using ground calibration that employs spectra of large, uniform areas in New Jersey outside the WTC debris zone. The calibration used a theoretical model of atmospheric transmittance followed by residual corrections using areas whose reflectance properties were measured by the ground crew. The specific methodology is presented elsewhere (4). While several calibration sites were initially tried, the best site was found to be the top level of a concrete parking garage in New Jersey (5). Because AVIRIS data were delivered to our laboratories less than 24 hours from acquisition, we used the preliminary radiative transfer model calibration to explore calibration sites. Portions of the parking garage site had undesirable organic absorptions, presumably due to oil from cars, even though there was no visual clue to its presence. Therefore, the field crew was guided in real time via cell phone to the best, spectrally neutral, areas of the site. This was accomplished by extracting spectra from the AVIRIS preliminary calibrated data (from our lab in Denver) for pixels in and around the ground crew, interpreting composition and informing the ground crew what the composition was at and around their location. They were then directed to a spectrally neutral site that allowed for a more precise calibration free of residual and anomalous absorption features. The field crew sent data via the internet to our labs where the data were reduced and information fed back to the ground crew. Several iterations resulted in an th


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excellent calibration. Normally, such a calibration takes a month or more, but the near real-time feedback reduced calibration time to about 2 days. The reflectance calibration spectra and locations are given elsewhere (7) Once calibrated, the data were analyzed for the presence of specific spectral features to identify materials expressed in the spectrum at each pixel in the image. The methods employed, from calibration to imaging spectroscopy analysis (Tetracorder analysis system), to verification of results follow the procedures and methods established previously (6-9). Characterization of reflectance spectra of asbestiform minerals of concern in the WTC debris can be found in Chapter 3 (5, 9, 10).

Thermal Hot Spots Results of the AVIRIS remote sensing data analysis and interpretations show the distribution and intensity of thermal hot spots in the area in and around the World Trade Center on September 16, 18, and 23, 2001. Plate 1 shows false color images of the core affected area around the WTC extending from 5 to 12. days after the collapse. Hot spots appear orange and yellow in the images. Dozens of hot spots are seen on September 16 and 18 . The image on the 18 appears dark because of clouds which blocked the sunlight but not the light emitted by the fires. Analysis of the data indicates temperatures greater than 700° C. Over 3 dozen hot spots are identified in the core zone. By September 23, only 4, or possibly 5, hot spots are apparent in the image, with temperatures cooler than those on September 16. There are other red/orange spots in Plate 1 in the area south of the World Trade Center zone. These areas are hot spots from chimneys or heating exhaust vents and are normal and do not represent other uncontrolled fires. Plate 1 also shows vegetated areas as green. Water appears blue, and the smoke from the fires appears as a light blue haze. White and lighter blue areas are rooftops, roads, and concrete as well as dust and debris from the collapsed buildings. Dust, probably more than a few millimeters thick (the optical depth), appears in shades of brown around the core WTC area on the 16 . The key in Plate 1 (lower right) shows the hot spot locations listed in Table I. Data collected on the 16th were processed, interpreted and released to emergency response teams on the September 18, 2001. On September 18 fire fighters changed from a rescue to a recovery operation and our field team observed increased water being put on the fires. The images show significant thermal hot spots on September 16 and 18 (see Table I), but by September 23 most of the hot spots had cooled or thefireshad been put out. On the September 16, 2001 image (Plate 1), large areas around the WTC show brownish colors, indicating the debris. On September 20,2001 there was th

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Plate 1. False color images of the core affected area around the WTC extending from 5 to 12 days after the collapse. Hot spots appear orange and yellow. The key shown at the right corresponds to the hot spot locations listed in Table 1. (See page 4 of color inserts.)


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Table I. Temperature and Size of the Thermal Hot Sspots Detected by AVIRIS Remote Sensing on September 16,18 and 23,2001. Sept. 18 Sept. 23 Sept. 16 T A A Hot Lat Lon T Max T A Spot Area A

Environmental Mapping of the World Trade Center Area with Imaging

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