The purpose of this fieldwork was to collect spectra—measurements of reflected sunlight—from the various rock types and fault materials along the Big Sur coast with a field spectrometer.
Hyperspectral Data Analysis for Mapping Coastal Landslide Hazards Along the Big Sur Coast
By Cheryl Hapke and Krystal Green
Researchers from the U.S. Geological Survey (USGS)'s Coastal and Marine Geology Program and the University of California, Santa Cruz (UCSC)'s Earth Sciences Department joined together in late July to collect field data along the Big Sur coast. The team consisted of Cheryl Hapke and Krystal Green (USGS) and Ty Kennedy-Bowdoin, a UCSC graduate student. The purpose of the fieldwork was to collect spectra—measurements of reflected sunlight—from the various rock types and fault materials along the coast with a field spectrometer.
This research, which is part of the Coastal Landslides project, involves the processing and analysis of hyperspectral remote-sensing data collected via aircraft (by HyVista Corp.) in July 2003 along the Big Sur coast. Hyperspectral data provide a digital image of surface reflectance from more than 100 different spectral channels—discrete parts of the electromagnetic spectrum—ranging from the visible near-infrared (wavelength, 0.4-1.9 µm) to the short-wave infrared (wavelength, 2.0-2.5 µm). In comparison, standard aerial photography has three channels: red, green, and blue (all in the visible region; wavelength, 0.4-0.7 µm). Because of the large number of channels, hyperspectral data are ideal for mapping the composition of geologic materials on the basis of their spectral signatures, and are especially good for detecting alteration minerals and providing high-resolution information about the chemistry and spatial distribution of minerals.
The geology of the Big Sur area is highly complex, consisting of folded and faulted metamorphic rocks of the Sur complex and highly sheared metasedimentary and metavolcanic rocks of the Franciscan Complex. Because the Big Sur coast is remote and in many areas inaccessible, detailed geologic mapping is difficult, and so the geologic maps that do exist for the area are somewhat generalized. A focus of our current research is to better understand the processes and hazards related to large coastal landslides that are common along this stretch of coast. Landslide occurrence is related to the underlying geology, specifically to variations in the strength of the rocks, which is a function of the rock type, the degree of weathering, and the presence (or absence) of such geologic structures as joints, faults, and shear zones.
The objective of our hyperspectral analysis is to refine existing geologic maps and to identify and map the spatial distribution of previously unmapped or unknown faults and shear zones through the detection of alteration minerals. Although existing spectral-map libraries can be used to identify minerals, the spectra of a particular mineral can vary depending on the specific host rock. Therefore, collection of spectral data in the field is crucial to ground-truth the remotely sensed data.
From July 26 to 29, Cheryl, Krystal, and Ty collected numerous rock samples and recorded the spectra of the various rock types and fault materials along the Big Sur coast, using a field spectrometer. The field spectrometer, which belongs to the Naval Postgraduate School and is on loan to the UCSC Earth Sciences Department, weighs about 50 lb and is carried in a backpack. A hand-held sensor records the reflectance of sunlight off the surface of rock outcrops, which is output directly into dedicated software in a laptop computer. Rock and sand samples were also collected at the outcrops so that additional analyses (measurement of rock strength and X-ray diffraction for clay identification) can be conducted. Initial processing has already identified mizzonite, chlorite, and antigorite, alteration minerals common in metamorphic rocks.
The research team stayed at the Landels-Hill Big Creek Reserve's Whale Point Research Cabin, about 80 km south of Monterey. The cabin, perched on top of a high ridge, has all the necessary amenities for fieldwork, including electricity for recharging the field equipment, kitchen facilities, and a hot (outdoor!) shower. The Landels-Hill Big Creek Reserve, one of more than 30 wildland sites in the University of California's Natural Reserve System and one of four such reserves managed by UCSC, is guarding against an epidemic of sudden oak death. As a result, upon returning from the field each day, we had to sweep the vehicles clean and disinfect the bottoms of our shoes before entering the reserve.
The data collection was highly successful, and future work will include incorporating the results of the hyperspectral analysis into current landslide-distribution maps. The spectra collected in the field will provide additional data to expand existing spectral libraries for numerous applications, including an important goal of our project: helping the California Department of Transportation (Caltrans) best manage the scenic Coast Highway (California Highway 1) while protecting the valuable natural resources of the Monterey Bay National Marine Sanctuary.