Dana E. Peterson Carol A. Finn 20200626 tabular digital data, jpg photos, geotiff, and zip shape files Denver, CO U.S. Geological Survey Additional information about Originator: Peterson, D.E. https://orcid.org/0000-0002-1941-265X; Finn, C.A., https://orcid.org/0000-0002-6178-0405. Suggested Citation: Peterson, D.E., and Finn, C.A., 2020, Bouguer gravity and magnetic susceptibility measurements at Iliamna Volcano, Alaska 2019: U.S. Geological Survey data release, https://doi.org/10.5066/P90Q3ZQ8. https://doi.org/10.5066/P90Q3ZQ8 Gravity data were collected in August of 2019 at 21 sites on and around Iliamna Volcano and Anchorage, Alaska. Measurements were taken with a Lacoste & Romberg G-161 meter and reduced to obtain the complete Bouguer anomaly. A total of 39 magnetic susceptibility measurements were taken at 13 locations using a ZH Instruments SM30 susceptibility meter. This data release includes susceptibility measurements, processed gravity data, shapefiles with field locations, and site photos. These data were collected to support subsurface and surface alteration and geologic mapping and slope stability modeling at Iliamna Volcano. Blakely, R.J., 1995, Potential Theory in Gravity and Magnetic Applications:Cambridge, Cambridge University Press, 441 p. DGGS Staff, 2013, Elevation Datasets of Alaska: Alaska Division of Geological & Geophysical Surveys Digital Data Series 4, http://maps.dggs.alaska.gov/elevationdata/. http://doi.org/10.14509/25239 International Association of Geodesy, 1971, Geodetic reference system 1967: International Association of Geodesy Special Publication No. 3, 116 p. Kampes, B., Blaskovich, M., Reis, J. J., Sanford, M., & Morgan, K., 2011, Fugro GEOSar airborne dual-band IFSAR DTM processing: In Proceedings of the ASPRS 2011 Annual Conference, Milwaukee, WI, USA, p. 1-5. McCubbine, J., Tontini, F. C., Stagpoole, V., Smith, E., & O’Brien, G., 2018, Gsolve, a Python computer program with a graphical user interface to transform relative gravity survey measurements to absolute gravity values and gravity anomalies: SoftwareX, v. 7, p. 129-137. Morelli, C., ed., 1974, the International gravity standardization net 1971: International Association of Geodesy Special Publication no.4, 194 p. Nagy, D., 1966, The prism method for terrain corrections using digital computers: Pure and Applied Geophysics, v. 63, no. 1, p. 31-39. Saltus, R. W., Brown II, P. J., Morin, R. L., & Hill, P. L., 2008, 2006 compilation of Alaska gravity data and historical reports: U.S. Geological Survey Digital Series 264, CD-ROM. 20190814 20190822 ground condition Complete None planned -153.2360 -149.8039 61.1882 59.9530 ISO 19115 Topic Category geoscientificInformation USGS Thesaurus geophysics volcanology isostatic anomaly bouguer anomaly gravimeter measurement magnetic susceptibility free-air anomaly gravitational field (earth) None Geology, Geophysics, and Geochemistry Science Center GGGSC USGS Volcano Hazards Program U.S. Geological Survey USGS Mineral Resources Program MRP USGS Metadata Identifier USGS:5ea31dcd82cefae35a19d24c Geographic Names Information System (GNIS) Iliamna Volcano Kenai Peninsula Cook Inlet Lake Clark National Preserve Common geographic areas Iliamna Alaska none Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although these data have been processed successfully on a computer system at the U.S. Geological Survey (USGS), no warranty expressed or implied is made regarding the display or utility of the data for other purposes, nor on all computer systems, nor shall the act of distribution constitute any such warranty. The USGS or the U.S. Government shall not be held liable for improper or incorrect use of the data described and/or contained herein Dana E Peterson ROCKY MOUNTAIN REGION: GEOL&GEOPHYSICS&GEOCHEM SC Geophysicist mailing and physical

Box 25046, Mail Stop 964, W 6th Ave Kipling St

Lakewood CO 80225 US 303-236-1413 dpeterson@usgs.gov Alaska Volcano Observatory External factors, such as wind, which may have impacted gravity measurements were recorded in the field. Processed gravity measurements were compared to a regional survey (Saltus et al., 2006) to qualitatively assess large scale anomaly locations and values. Data were loaded in Geosoft Oasis montaj software to visually inspect for reasonableness and spatial consistency. Data set is considered complete for the information presented, as described in the abstract. Users are advised to read the rest of the metadata record carefully for additional details. No formal positional accuracy tests were conducted No formal positional accuracy tests were conducted Magnetic Susceptibility Data Collection: Susceptibility data were collected using a ZH Instruments SM-30 magnetic susceptibility meter. When possible, multiple measurements were taken for each rock type at each location. Locations were collected using the Collector for ArcGIS App on an iPad, and the iPad's internal GPS was used to record latitude and longitude. Location accuracy is 5 - 10 meters. Gravity Data Collection: Relative gravity data were collected using a Lacoste & Romberg G-161 gravity meter. When possible, three or more measurements were taken at each location to check for consistency. Daily loops were started and ended at a base station outside of our lodging on Snug Harbor and measurements were taken at an absolute gravity station located at the Alaska Pacific University in Anchorage, Alaska at the beginning and end of the field campaign. Measurements were recorded using the Collector for ArcGIS App on an iPad, and the iPad's internal GPS was used to record latitude and longitude. Location accuracy is 5 - 10 meters. Elevations were later sampled from a digital elevation model derived from Alaska Statewide Digital Mapping Initiative with 5.6 m horizontal resolution and 1.85 m vertical accuracy (Kampes et al., 2011). 201908 Gravity Processing: Conversion to milligals is made using factory calibration constants and factors. Observed gravity values are based on an assumed linear drift between successive base readings. Theoretical gravity at sea level is based on the Geodetic Reference System 1967 (International Association of Geodesy, 1971) for the shape of the spheroid. The datum for the observed gravity is the International Gravity Standardization Net 1971 (IGSN 71) (Morelli, 1974). Observed gravity values are calculated by adding earth tide and meter drift corrections to the milligal equivalent meter readings using Gsolve (McCubbine et al., 2018). Free-air anomalies are calculated by subtracting the theoretical gravity from the observed gravity and adding the free-air correction, using standard formulas (Blakely, 1995). Simple Bouguer anomalies are calculated by subtracting the Bouguer correction, which accounts for the attraction of rocks between the station and sea level using a rock density of 2.67 g/cm3 from the free air anomaly (e.g., Blakely, 1995). 201910 Terrain Corrections: Terrain corrections were calculated using Gsolve, a software program developed by McCubbine and others (2018) which is based on the Nagy prism method (Nagy, 1966). Terrain correction calculations in GSolve work by estimating the gravity effect of the topography in a series of annuli around the observation site. The user imports the appropriate topography (or bathymetry) grid and observation location data into Gsolve, then selects the density value and inner and outer radius for the terrain calculation. Four zones were used to correct for terrain with a density of 2.67 g/cm3. The first zone correction is based on 5-m digital elevation data and extends from 2-170 m away from the gravity station. The second zone is based on 10-m digital elevation data and extends from 170-2610 m away from the station. The third zone is based on 40-m digital elevation data and extends from 2610-21900 m away from the station. The fourth zone is based on 1173-m digital elevation data and extends from 21900-167000 m away from the station. Terrain correction calculations over water bodies require bathymetry and water surface elevation grids for each set of specified radii. Bathymetry grids need to be in elevation (i.e., negative numbers below sea level) and water surface grids are the same extent as the bathymetry grid with cell values at the elevation of the water body (0.0 for sea level). It is also important that topography grids do not extend over the water bodies if a correction is being made for the bathymetry (i.e., “no data” in cells covered by bathymetry grids), otherwise corrections will be made twice for the same area. Equally, bathymetry and water surface grids should have “no data” in cells covered by topography grids. Calculating the terrain corrections over water bodies is a two-step process; calculate the correction for the water density then the correction for the ‘air gap’ between water surface and the observation elevation. In the oceans a density of 1.64 g/cm3 is used for bathymetry grids and a density of 1.03 g/cm3 is used for the water surface grids. This gives a correction of 1.64 below the water surface and 2.67 above the water surface. Three zones were used for the water and air gap corrections. The first zone uses bathymetry and water surface grids with 10-m resolution which extend from 2-2610 m away from the station. The second zone is based on 40 m resolution data which extends from 2610-21900 m away from the station. The third zone is based on 1173-m resolution data which extends from 21900-167000 m away from the station. 201910 Complete Bouguer anomalies were calculated by adding all the terrain corrections from Step 3 to the simple Bouguer anomaly (e.g., Blakely, 1995). 201910 GEOTIFF Format: The bouguer anomaly grid was exported from Geosoft Oasis montaj as a .gxf file and then projected to UTM 5N (WGS84) and saved as a GeoTIFF in ArcMap 10.7 202006 Point Point 1.0E-4 1.0E-4 Decimal degrees WGS_1984 Geodetic Reference System 1980 6378137.000000 298.257222 IliamnaAK2019_gravity_locations.shp Locations of gravity measurements U.S. Geological Survey Lat Site latitude, WGS84, Decimal Degrees U.S. Geological Survey 59.9496 61.1882 decimal degrees Lon Site longitude, WGS84, Decimal Degrees U.S. Geological Survey -153.1697 -149.8041 decimal degrees SiteID Measurement site ID U.S. Geological Survey Unique identifier assigned to survey station IliamnaAK2019_susceptibility_locations.shp Locations of magnetic susceptibility measurements U.S. Geological Survey SiteID Magnetic susceptibility measurement station ID U.S. Geological Survey Unique identifier assigned to survey station Lat Site latitude, WGS84, Decimal Degrees U.S. Geological Survey 59.9530 60.0651 decimal degrees Lon Site longitude, WGS84, Decimal Degrees U.S. Geological Survey -153.2360 -152.6874 decimal degrees IliamnaAK2019_susceptibility_measurements.csv Magnetic susceptibility measurements U.S. Geological Survey SiteID Measurement location U.S. Geological Survey Unique identifier assigned to survey station Lat Site location longitude, WGS84, Decimal Degrees U.S. Geological Survey 59.9530 60.0651 decimal degrees Lon Site location longitude, WGS84, Decimal Degrees U.S. Geological Survey -153.2360 -152.6874 decimal degrees Rock_Type Type of rock on which magnetic susceptibility was measured. U.S. Geological Survey Qualitative assessment of rock type determined by visual inspection in the field. Measured_Susceptibility_x10-3_SI A numeric identifier for magnetic susceptibility of rock at sample location. U.S. Geological Survey -0.013 55.000 x10-3 (SI) IliamnaAK2019_gravity_site_photos.zip Zip file containing photos of the gravity measurement sites and readings. The photos are available for download here: https://www.sciencebase.gov/catalog/item/5ea31dcd82cefae35a19d24c U.S. Geological Survey IliamnaAK2019_gravity_processed.csv CSV table of processed gravity measurements U.S. Geological Survey SiteID Gravity station identifier U.S. Geological Survey Gravity station identifier Latitude_WGS84 latitude, WGS84, Decimal Degrees U.S. Geological Survey 59.9496 60.1091 decimal degree Longitude_WGS84 longitude, WGS84, Decimal Degrees U.S. Geological Survey -153.1697 -152.5802 decimal degree elev_NAVD88 elevation in meters, NAVD88 datum U.S. Geological Survey 4.66 1897.90 meters ObsGrav observed gravity, in milligals U.S. Geological Survey 981495.11 981900.71 milligal TheoGrav theoretical gravity, in milligals U.S. Geological Survey 981913.07 981925.59 milligal FAC free-air correction in milligals, in milligals U.S. Geological Survey 1.44 585.69 milligal BSC Bouguer slab correction in milligals, assumes density of 2.67 g/cm3 U.S. Geological Survey 0.52 212.37 milligal FAA free-air anomaly, in milligals U.S. Geological Survey -30.37 162.38 milligal SBA simple Bouguer anomaly in milligals, assumes density of 2.67 g/cm3 U.S. Geological Survey -50.00 -19.07 milligal TCL[i] terrain correction using elevation grids over land with outer radius [i] and inner radius as defined in processing step 3, assumes density of 2.67 g/cm3, in milligals U.S. Geological Survey 0.01 18.68 milligal TCB[i] terrain correction using bathymetry grids with outer radius [i] and inner radius as defined in processing step 3, assumes density of 1.64 g/cm3, in milligals U.S. Geological Survey 0.00 1.52 milligal TCA[i] terrain correction using water surface grids with outer radius [i] and inner radius as defined in processing step 3, assumes density of 1.03 g/cm3, in milligals U.S. Geological Survey 0.00 0.92 milligal CBA complete Bouguer anomaly, assumes density of 2.67 g/cm3, in milligals U.S. Geological Survey -29.48 -2.07 milligal Bouguer Anomaly Data - GEOTIFF file - Iliamna_AK2019_BA_nomap.tif This is a geotiff format file of the Bouguer anomaly for each pixel in the area covering the local gravity survey area of Iliamna Volcano, Alaska. The raster values (-29.48 to -2.07) are based on the CSV file, IliamnaAK2019_gravity_processed.csv. The file is available for download here: https://www.sciencebase.gov/catalog/item/5ea31dcd82cefae35a19d24c. Bouguer Anomaly Data- GEOTIFF world file - Iliamna_AK2019_BA_nomap.tfw This is a geotiff world file containing the location information associated with the *.tif file of the same name. Bouguer Anomaly Data- - colormap file - - Iliamna_AK2019_BA_nomap.tif-ColorRamp.SLD This is a text file containing the RGB color information that can be used to symbolize the *.tif file of the same name. Bouguer Anomaly Map - GEOTIFF file - Iliamna_AK2019_BA.tif This is a geotiff format file with map background of the Bouguer anomaly in the area covering the local gravity survey area of Iliamna Volcano, Alaska. The file is available for download here: https://www.sciencebase.gov/catalog/item/5ea31dcd82cefae35a19d24c. U.S. Geological Survey GS ScienceBase mailing address

Denver Federal Center, Building 810, Mail Stop 302

Denver CO 80225 United States 1-888-275-8747 sciencebase@usgs.gov Unless otherwise stated, all data, metadata and related materials are considered to satisfy the quality standards relative to the purpose for which the data were collected. Although these data and associated metadata have been reviewed for accuracy and completeness and approved for release by the U.S. Geological Survey (USGS), no warranty expressed or implied is made regarding the display or utility of the data for other purposes, nor on all computer systems, nor shall the act of distribution constitute any such warranty. 20200821 Dana E Peterson ROCKY MOUNTAIN REGION: GEOL&GEOPHYSICS&GEOCHEM SC Geophysicist mailing and physical

Box 25046, Mail Stop 964, W 6th Ave Kipling St

Lakewood CO 80225 US 303-236-1413 dpeterson@usgs.gov Content Standard for Digital Geospatial Metadata FGDC-STD-001-1998