The space technology and disaster mitigation communities work together to develop efficient methods for disaster prevention, preparedness and relief. Prevention is a long-term phenomenon, which can best be studied with the help of satellites to monitor relevant factors such as changing land use. Preparedness focuses on warnings and forecasts of impending disasters, whether they’re “rapid onset” disasters—the most frequent type—or those that develop slowly, such as drought and famine. Relief occurs after (and sometimes during) an emergency. Important aspects of satellite monitoring involve assessing the damage incurred during a disaster, as well as helping to identify escape routes and locations for temporary shelter.

Sensors and Applications
Although existing satellites weren’t designed solely to observe natural hazards, the variety of spectral bands in today’s visible near infrared (VNIR), shortwave infrared (SWIR), mid-infrared (MIR), thermal infrared (TIR) and Synthetic Aperture Radar (SAR) sensors provides adequate spectral coverage and allows computer enhancement of the data for this purpose. Repetitive or multitemporal coverage allows scientists to study various dynamic phenomena whose changes can be identified over time, including natural hazard events, changing land use patterns, and hydrologic and geologic characteristics.

In disaster management, the aim is to monitor the situation, simulate the complex natural phenomenon as accurately as possible to come up with better hazard predictions, suggest appropriate contingency plans and prepare spatial databases. Remotely sensed data’s inherent characteristics—spatial continuity, uniform accuracy and precision, multitemporal coverage and complete coverage regardless of site location—make satellite imagery invaluable for a wide range of disaster management applications, including:

• Assessing the severity and impact of
damage due to flooding, earthquakes, oil spills and other disasters.
• Planning efficient escape routes from coastal areas during hurricane season.
• Charting quickest routes for ambulances and other assistance to reach victims.
• Locating places for shelter for victims or refugees.
• Calculating population density in disaster-prone areas.
• Identifying hardest-hit disaster areas to provide early warning of potential disasters.
• Performing pre-disaster assessments to facilitate planning for timely evacuation and recovery operations during a crisis.
• Monitoring reconstruction or rehabilitation after a major disaster.
• Developing, maintaining or updating accurate base maps.

Different sensors can provide unique information about Earth’s surface properties. For example, measurements of reflected solar radiation give information on albedo—the fraction of light reflected by a body or surface, thermal sensors measure surface temperature, and microwave sensors measure the planet’s dielectric properties—hence the moisture content of surface soil or snow. The table below reviews the existing land-imaging satellites in orbit. Sensors and their capabilities with reference to disaster mitigation are discussed in the following sections: Earthquakes, Volcanic Eruptions, Tsunamis, Landslides, Hurricanes, Forest Fires and Floods.
 

U.S. Geological Survey (USGS) scientists have operated seismographic stations throughout the world for more than 35 years. For the last few years, in cooperation with the Incorporated Research Institutions for Seismology (IRIS), a consortium of more than 90 universities, USGS has upgraded the system into a state-of-the-art Global Seismographic Network (GSN). The GSN is designed to obtain high-quality digital data that can be readily accessed by users worldwide. For some stations, the data are reported to orbiting satellites, and then to the Internet where information can be viewed via the Web.

Generally, satellite imagery can be used to help identify faults associated with earthquakes. As a result, land use and geological maps can give vital pointers toward potential earthquake zones. Satellite sensors that are active in the visible and near infrared spectral band are useful for such studies. Although many satellite systems collect the required data, Landsat imagery is the most popular for this application because of the comprehensive historical Landsat data archives and the imagery’s cost effectiveness.

Conventionally, aerial remote sensing (airborne radar) would be considered an effective way to delineate unconsolidated deposits sitting on fault zones upon which most of the destruction occurs, as well as to identify areas where an earthquake can trigger landslides. Now such work is being complemented with one-meter-resolution satellite imagery available from commercial companies such as DigitalGlobe, ORBIMAGE and Space Imaging. For example, as shown in the images below, DigitalGlobe’s QuickBird satellite revealed widespread damage to the historic city of Bam, Iran, following a powerful earthquake on Dec. 26, 2003. The earthquake killed more than 50,000 people and destroyed or severely damaged 90 percent of the city’s buildings, including a 2,000-year-old citadel built primarily of mud brick. High-resolution satellite imagery can detect the altered rooflines of buildings that have fully collapsed, thereby assisting authorities with immediate mitigation activities such as search-and-rescue efforts, emergency relief and major infrastructure damage assessment.
 

There are more than 500 active volcanoes around the globe, and about 100 of them erupt every year. Volcano monitoring is important simply because an unexpected awakening can imperil thousands of lives across a wide area. Remote sensing techniques can play an important role by providing vital information with limited fieldwork, thereby saving effort and money. TIR imagery can capture volcanic heat if the spatial resolution is high enough. Also, moderate-resolution panchromatic stereo-pair imagery, due to its 3-D capabilities, helps users find evidence of hazardous activities. An IR pattern of geothermal heat in the vicinity of a volcano indicates thermal activity, which many inactive volcanoes display. Many volcanoes thought to be extinct may have to be reclassified if regular monitoring activities reveal any abnormally high IR emissions from either the summit craters or the flanks. Changes in thermal patterns can be obtained for a volcano only through periodic high-resolution IR imagery, like that of QuickBird. However, temperature and gas emission changes can be monitored with a geostationary satellite at ideal locations identified on the thermal imagery.

The TIR bands of the Advanced Very High Resolution Radiometer (AVHRR) sensor—the primary sensor on National Oceanic and Atmospheric Administration (NOAA) polar-orbiting satellites—can detect volcanic ash, as it has a strong signal difference between channel 4 and channel 5. However, AVHRR’s spatial resolution may not be adequate to detect the dynamic change in volcanic geothermal activity in some situations.

Landsat, SPOT-4 and IRS 1D imagery are valuable for detecting volcanic activity, because their SWIR band is particularly well suited for locating fire hot spots, lava flows and intense volcanic activity. Once alerted by early warning systems, specialists need to monitor levels of volcanic activity continuously so timely precautions can be taken. Imaging sensors can detect hot spots, because they measure the energy emitted from surfaces at temperatures of 220-520 degrees Celsius. Though vulcanologists rely widely on Landsat imagery to obtain this kind of information, SPOT’s resolution and radiometric sensitivity can detect a wider range of temperatures, and its frequent revisit capability enables operational monitoring of a region of interest.

Additionally, research has shown that water surface temperature and area can be measured simultaneously by using all seven spectral bands of Landsat’s Thematic Mapper (TM) sensor. Crater lakes on active volcanoes act as heat and chemical traps, and are amenable to space surveillance as shown by case studies on several volcanoes: Ruapehu (New Zealand), Taal (Philippines), Kawah Ijen and Kelut (Indonesia), Poas (Costa Rica), and Apoyeque and Jiloa (Nicaragua). Also, TM-derived water surface spectral reflectance indicates high concentrations of suspended chemical sediment in the most active crater lakes. Other sensors that have been used to successfully monitor crater lakes include the Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER) sensor aboard NASA’s Terra satellite, as well as the Enhanced Thematic Mapper (ETM) on board Landsat 7.
 

Tsunamis are water waves, or seismic sea waves, caused by large-scale sudden movements of the sea floor due to earthquakes, landslides, volcanic eruptions or man-made explosions. Increasing development along most coastlines poses a corresponding increase in tsunami disaster risk. Tsunamis differ from other earthquake hazards in that they can cause serious damage thousands of kilometers from the causative faults. Once they are generated, they are nearly imperceptible in mid-ocean, where their surface height is less than a meter. They travel at incredible speeds, as much as 900 kilometers/hour, and the distance between wave crests can be as much as 500 kilometers. As the waves approach shallow water, a tsunami’s speed decreases and the energy is transformed into wave height, sometimes reaching as high as 25 meters, but the interval of time between successive waves remains unchanged—usually between 20 and 40 minutes. When tsunamis near a coastline, the sea recedes—often to levels much lower than low tide—and then rises as a giant wave.

The Pacific Tsunami Warning Center provides warnings for Pacific basin teletsunamis (tsunamis that can cause damage far away from their source) to almost every country around the Pacific rim and to most of the Pacific island states. As detailed in Earth Imaging Journal’s March/April issue (see “Help from Above—Tsunami Imagery Aids Relief Efforts,” www.eijournal.com/tsunami.asp), satellite or aerial photography—especially when combined with a good geographic information system (GIS) database of an area—can provide critical information for emergency managers, including damage to structures, transportation and communication links, and other “life-line” infrastructure components.
 

These large-scale low-pressure systems occur throughout the world over zones referred to as “tropical cyclone basins” (www.oas.org/usde). The determination of past hurricane paths for a region can be derived from remotely sensed data from NOAA meteorological satellite sensors. The Tropical Analysis and Forecast Branch of the Tropical Prediction Center (TPC) provides year-round products involving marine forecasts, aviation forecasts and warnings, and surface analyses. The center also provides satellite interpretation and satellite rainfall estimates for the international community. The Technical Support Branch provides support for satellite data processing.

One of the key lessons NASA learned during Hurricane Andrew was that it is critical to select appropriate data and put it together to make informed decisions. Due to the lengthy process required to gather the data, it was suggested that communities not wait until a disaster happens to do so. Imagery is an important aspect of a community’s database. For plotting new data, AVHRR is the best sensor with its 2,940 kilometer swath, twice-a-day coverage and appropriate resolution. The red band is useful for defining daytime clouds and vegetation, while the TIR band is useful for daytime and nighttime cloud observations.

In addition, high-resolution satellite imagery and accurate remote sensing techniques provide a quantifiable data link for defensible forest fire mitigation planning and action, as well as the tactical wildfire planning needs of wildland/urban interface communities. Remote sensing analysis classifies raw high-resolution imagery data into thematic data to identify the species, age and density of trees and various types of groundcover; then managers can use a GIS to analyze and model the data and develop treatment strategies.
 

An image taken following flood recession is useful for assessing damage to buildings and infrastructure. Similarly, post-flood imagery could be an effective tool to evaluate the effect of flooding on coastlines, forests and open space.

A Life-Saving Legacy
Although various satellites and sensors provide numerous possibilities for analyzing imagery data and enabling disaster prediction and mitigation, the search for effective preventive measures continues. Determining the impact of land use on natural disasters and developing the ability to predict them may be remote sensing’s most significant contributions to society during this century.

Author’s Note: I would like to gratefully acknowledge funding from the Institute for Catastrophic Loss Reduction to carry out this work.

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