HYPERSPECTRAL MAPPING OF ALTERATION IN HALEMA UMA U CRATER AND KILAUEA CALDERA

HYPERSPECTRAL MAPPING OF ALTERATION IN HALEMA UMA U CRATER AND KILAUEA CALDERA Geoffrey M. Rehders Department of Geology University of Hawai i at Hilo Hilo, HI 96720 ABSTRACT AVIRIS hyperspectral imagery was used to define and map alteration minerals within Kilauea Caldera. The dominant mineral assemblage was composed of opal and anatase, as identified by X-ray diffraction. The most intense opalization correlates directly with white areas seen on true-color (visible light) images. These areas are surrounded by zones of partially opalized lava surfaces and porous opaline crust. Prior studies had identified opal in the alteration assemblages but it was thought to be a minor component in contrast to sulfates, especially gypsum and anhydrite. While minor amounts of sulfates were identified in samples taken from the area, they were detected in only a few AVIRIS pixels. The silica-rich alteration found at Halema uma u Crater on Kilauea Volcano is formed by acid leaching around the fumaroles and may be a good analogue for deposits found at Gusev Crater on Mars. INTRODUCTION Kilauea Volcano is the most active volcano on the island of Hawai i. Hydrothermal and gas alteration within Kilauea s caldera and Halema uma u crater are the focus of the research. Samples within the area have been previously collected and analyzed using X-ray diffraction (XRD), X-ray fluorescence (XRF) and a Scanning Electron Microscope (SEM) by Bishop (2011) and Desmither (2012). Hydrothermal acid sulfate leaching is believed to be the cause of the alteration of the rocks in and around the crater, such as intense acid sulfate leaching. Almost all the major elements except for Si and Ti have been depleted when compared to the unaltered samples. The elements that were enriched due to being insoluble during leaching were SiO2 and TiO2 creating zones dominantly composed of opal and anatase (figure 1). The goal of this study is to map the distribution of opalrich alteration using AVIRIS hyperspectral imagery. Figure 1. Diagram of the acid sulfate leaching system on Kilauea (from Decker 2013) 79

METHODS Hyperspectral sensors like AVIRIS are instruments that acquire images in many, very narrow, continuous spectral bands throughout the visible, near-ir, mid-ir, and, less commonly, thermal IR portions of the spectrum. These instruments collect 200 or more bands of data from about 0.3 to 3 microns, which represents an effectively continuous reflectance spectrum. The number of spectral bands is far greater than conventional multispectral scanners allowing identification of individual mineral species. Figure 2 shows wavelengths complete reflectance spectra for given minerals in the region occupied by Landsat Band 7 which covers a large range of spectra within one band compared to the low range bands in hyperspectral data. Minerals have unique reflectance patterns for given wavelengths allowing for their identification with an increase in bands which is seen in hyperspectral imagery and lab spectra. The laboratory reflectance patterns of minerals differ from those collected by air- or space- borne imagers that have gaps in data due to atmospheric absorption peaks. (Lillesand et al. 363) Figure 2. Characteristic reflectance and absorption peaks of some common minerals. General regions of alteration can easily be mapped on visible light images such as those from Google Earth shown in Figure 3. Comparison of current alteration patterns shown in orange, with those from 1954, shows a distinct change in spatial distribution through time. While these images are excellent for defining regions of alteration they lack sufficient spectral resolution to define the type of alteration. DeSmither et al. concluded that samples taken around the crater have specific mineralogical signatures and differences using XRF and XRD data. Though this data does show what mineral assemblages are present, this technique does not lend itself well to mapping the overall distribution of minerals. Figure 3. Google Earth satellite image of Halema uma u crater showing current alteration in orange, 1954 alteration patterns in green, and lava flow boundaries in blue. 80

Figure 3 gives a clear idea of where different types of altered compositions are located but it does not give any information about the mineral composition. The composition of the materials around the crater change even within these contacts. To obtain an accurate distribution of mineral assemblages across the entire area would require an unrealistic amount of field sampling. In contrast, modeling the hyperspectral reflectance fingerprints of known minerals in the crater can classify the areas with minimum groundwork. Spectral signatures of field samples will provide ground truth used to classify the AVIRIS data in ENVI. The modeling can help destinguish the compositional makeup of the rocks from the data obtained from AVIRIS. This will allow the idenification of mineral assemblages on the inaccessible walls and floor of the crater and new vent and with comparisons of spectra from similar deposits at Gusev Crater on Mars. Correlating and modeling field reflectance data of minerals and the reflectance data obtained from AVIRIS to classify the mineralogy of the Kilauea area is the focus of the research. Hyperspectral data of the Kilauea caldera has been obtained from the NASA AVIRIS website. AVIRIS (Airborne Visible-Infrared Imaging Spectrometer) was developed by NASA at the Jet Propulsion Laboratory in Pasadena, California. The AVIRIS system (Figure 4) is composed of whiskbroom-scanned linear arrays made of silicon, for visible, and indium-antimonide, for infrared. This system collects hyperspectral data in the range of 400 to 2500 nm in 224 bands at 10nm each. AVIRIS is typically flown 20km above ground level in an ER-2 aircraft. The radiometric resolution of AVIRIS is 16-bit, the system collects data in 12-bits and the data is processed to 16-bit integer values. AVIRIS can also be flown in low-altitude mode which was the case for the data used in this research. Average flight elevation was 13 kft producing 2.2 to 2.4 m pixels. Figure 4. AVIRIS system features Images from three partially overlapping flight lines were previously taken on January 27 th 2007 and cover most of the caldera. The data was downloaded from NASA s AVIRIS website and includes 3 files that cover the caldera with minor holes (f070127t01p00r06, f070127t01p00r20, and f070127t01p00r21). Each image had pixel sizes of 2.2, 2.3, and 2.4 m respectively. The average elevation in the area was 1 km and centered near 19.4/-155.2 (lat/long). The AVIRIS data used is preprocessed to remove fundamental geometric and radiometric errors associated with the motion of the aircraft used during collection. Several types of additional correction must be applied to remove further errors due to atmospheric effects such as absorption and scattering. The corrections were made using FLAASH (Fast Line-of-sight Atmospheric Analysis of Hypercubes) within the data processing program ENVI. FLAASH is a very useful tool for pre-processing. FLAASH corrects atmospheric interference and converts radiance images to surface reflectance. FLAASH uses the MODTRAN4 radiation transfer code in conjunction with standard MODTRAN model atmospheres and aerosol types. Output images included surface reflectance, cloud classification, and water vapor images. Spectral polishing at a 81

width of 9 bands was used in FLAASH which provides artifact suppression. While the images are georeferenced, it is also possible that some additional georectification may have been necessary but no action was taken due to high quality of data. The samples analyzed by Bishop (2011) and Desmither (2012) provide data that can help with correlation of mineral spectra and AVIRIS spectra. The field samples were collected in 2009 and no field spectra were acquired at the time, so correlation of spectra will not be as precise as obtaining spectra in the field at the time of flight. Also because the actual rocks are mixtures of minerals, more data is needed than provided by available spectral library patterns for single minerals. Some of the alteration minerals may not have been previously characterized or may differ greatly from the samples used in other labs. Known samples of previously analyzed samples using XRD were analyzed for reflectance patterns using an ASD Field Spec Pro Jr that collects spectra at.02 micron intervals. Samples of mixtures of alteration minerals, glasses, and ash that are present on the ground were measured to provide data that will help in deconvoluting the AVIRIS spectra. Classification of the AVIRIS data was done using the acquired reflectance patterns from the ASD Field Spec Pro Jr along with spectra from USGS spectral libraries. The focus of the classification was to find alteration sites. These alteration sites are mainly made up of opal, anatase, sulfur, and gypsum and represent fumaroles in the area. Mixtures of the alteration minerals were analyzed using the Field Spec Pro and a spectral library was created. ENVI s Spectral Angle Mapper (SAM) was used for classification under supervised conditions. SAM is a very useful and powerful tool for the classification of AVIRIS images and correlates inputted spectral libraries with pixels on the image. RESULTS Images corrected using FLAASH were noticeably better than the original orthorectified images (Figure 5). Bad bands were removed from the band intervals 106-115 and 152-169 leaving an acceptable 196 bands left for classification. With the data Figure 5. Orthorectified image on left with FLAASH corrected image on right. atmospherically corrected combining the images was the next step. The georeferenced mosaic was used in ENVI for this step seen in Figure 6. As you can see there are apparent gaps (holes) in the data but not too laterally extensive within the alteration areas. Mosaicking was done with no feathering and all images fixed with value 0 being ignored. 82

Figure 6. Mosaic image of Kilauea Caldera Figure 7. Spectral library used in classification The spectral library (Figure 7) was created in the supervised SAM classification and consists of spectra taken from previously analyzed field samples of HM: 1, 19, 23, 24, 31, 33, 36, 45, 54, and 55. Further description is seen in Figure 8. Figure 6. Description of samples used 83

Successful completion of the SAM classification produced the map of mineral assemblages seen in Figure 9. The red and green pixels map the sites of pervasive opalization around active fumaroles coupled with precipitation of native sulfur. The red areas represent opalized basalts that still have recognizable structures such as vesicles preserved in the samples. The green areas have the most intense opalization. These red and green areas were mapped using spectra obtained from samples HM: 24(001), 54(002), 54(007), 31, and 23 which define the most intense alteration in the area. The purple pixels map moderate to weak opalized lava flows and lithic ash surrounding the intense fumarolic alteration. This purple area correlates to downwind weak surface alteration of glassy lava flows by the acidic SO2 plume emanating from Halema uma u Crater. Weak opalization characterized by purple pixels is also found in the northern part of Kilauea Caldera (Figure 10). Much of these alterations correlate with a region of boiling point fumaroles identified on ASTER thermal images by Patrick 2011 showing the heat signatures within the caldera. These heat signatures also correlate to the purple moderate to weak opalization. Figure 7. Spectral Angle Mapper (SAM) classified image of Halema uma u Crater. Red and green show most intense alteration while purple is moderate to weak alterations. 84

Figure 8. Distribution of moderate to weak opal alteration traces from AVIRIS images compare to the ASTER thermal signature and visible alteration mapped by Patrick 2011. DISCUSSION Widespread white altered areas around Halema uma u Crater have been thought to be composed mostly of sulfates such as gypsum with minor amounts of opaline material (Naughton, 1976). Detailed XRD and SEM analysis of field samples demonstrated that most of the alteration found around Halema uma u is opal, and anatase left as a residue from strong acid leaching around active fumaroles (Bishop, 2011, DeSmither, 2012). Similar alteration is found along the 1971 and 1974 fissure vents just SE of Halema uma u. Gases such as sulfur dioxide (SO2) along with boiling groundwater create these conditions for acid leaching alteration and also result in the precipitation of large amounts of sulfur. Mapping of the alteration using spectra acquired from field samples shows an almost 1:1 correspondence between visible white alteration and the presence of complete rock replacement by opal, sulfur, and anatase. These core areas are surrounded by regions of weaker surface alteration and formation of friable opaline crusts. Samples from the intensely altered areas do contain small amounts of gypsum and other sulfates but not in large enough areas to be identified on the AVIRIS images. The alteration patterns are highly irregular and did not produce visible mineral zonation on the spectrally classified images. No central core was found in any of the altered zones. This probably reflects the very uneven distribution of active fumaroles within each of the intensely altered areas, which are probably controlled by irregular fracturing of the volcanic rocks. 85

Weaker opal alteration found in the northern part of Kilauea Caldera differs significantly from the alteration around Halema uma u. While opal is present, it is forming as a precipitate from water-rich boiling point fumaroles that are devoid of sulfur. The surrounding rocks are altered to a reddish, crumbly material with no definable XRD pattern that may be allophane or other amorphous alteration products. This alteration is distributed along the thermal anomalies mapped by Patrick 2010 using ASTER imagery. The fumaroles at Sulfur Banks on the north side of Kilauea Caldera show a signature intermediate between the Halema uma u Crater and Kilauea Caldera alteration suites. While there are small areas that show opal-sulfur type of alteration, it is much less pervasive than that at Halema uma u. CONCLUSION Hyperspectral imagery and data such as AVIRIS is very useful for the mineralogical classification of volcanic alteration features. Correction, processing, and analysis are very time consuming due to high content of data but stand to be very powerful if used correctly. The process of correcting, processing, and analyzing data in ENVI combined with detailed knowledge of the field area can be used to accurately classify mineral assemblages in and around Kilauea Caldera. Maps produced from AVIRIS data clearly identified and mapped alteration found around Halema uma u Crater including inaccessible regions of the crater floor and walls. The most intense alteration associated with white areas are rocks leached of everything but residual silica and titanium leaving an assemblage of opal and anatase along with native sulfur precipitated by the fumaroles. This finger print of intense acid sulfate alteration formed around active volcanoes may be useful in identifying similar environments on other planets. ACKNOWLEDGEMENTS I would like to thank the NASA Space Grant Consortium at the University of Hawai i at Hilo for lending the opportunity to expand my knowledge of remote sensing and analysis of hyperspectral imagery. The knowledge obtained from my research has opened my eyes to opportunities within this field of study and I intend to pursue further education and research. I would like to thank the University of Hawai i at Hilo for allowing me to access much needed computers and programs that helped facilitate my research, without this the research could not have taken place. Also thanks to my mentor Dr. Ken Hon for informing me of this opportunity and helping me to understand the processes taking place in the area of research and Ryan Perroy for much needed help in navigating through the ENVI program. Big thank you to all the HSGC staff that put on great symposiums and made the logistics process a breeze. Again, thanks to all who have helped in this year-long endeavor. 86

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