Description of Data These data represent over 40 years of gravity surveying in the state of Minnesota. This release is the first significant revision of the Minnesota gravity database since the compilation by Chandler and Schaap (1991), that the reader is referred to for information on the older data. Since that compilation, approximately 2000 stations have been added from data used to support a variety of projects for the Minnesota Geological Survey (MGS). Additionally, archival data not available for the earlier compilation are now included. The newly added data includes stations on profiles that are spaced at 0.25 mile (0.4 kilometers) intervals along selected corridors, to areas where stations are spaced at 0.5 to 1.0 mile (0.8 to 1.6 kilometers) intervals along available roads and trails. Elevation control for the new data is often based on spot elevations that are shown on 1:24,000 scale USGS topographic maps. Some elevations, particularly in areas of low relief were extrapolated from topographic contours. Occasionally elevations were surveyed by transit, and more recently Global Positioning System (GPS) readings and altimeter data. The newer stations are therefore located to accuracy of 50 meters or less, equal to or smaller than 0.1 inch on a 1:24,000 scale topographic map. Station locations for some of the older data may not be accurate to better than +/- 200 meters. Most of the new measurements were taken with a Lacoste and Romberg gravity meter (#G-320) with a stated reading precision of 0.01 milligals. As a result of various error sources gravity values are probably precise to +/- 0.10 milligals, although errors up to +/- 0.3 milligals may occur locally. All data have been reduced using the Geodetic Reference System 1967 (International Association of Geodesy, 1971), assuming a sea level datum and a Bouguer reduction density of 2.67 grams/cc. Corrections are made for earth curvature, but owing to the generally low topographic relief across most of Minnesota, no terrain corrections have been made. Description of Gravity File The principal fact gravity data is provided in a database (dbf) format in the file named gravity.dbf. The current file contains 57905 records. Abbreviated field names in the dbf file are defined as follows: Lat_d_m_dm--Latitude in degrees-minutes-decimal minutes Lon_d_m_dm--Longitude in degrees-minutes-decimal minutes Utme27--UTM projection, NAD27, easting Utmn27--UTM projection, NAD27, northing Elev_ft--elevation in feet above mean sea level Abs_g_mgal--absolute gravity values, milligals F_air_mgal--free air gravity values, milligals Sboug_mgal--simple Bouguer gravity values, milligals Data_code--date code to search on, approximate date of data collection, see below for more details Sta_id--station identification when available Utme_n83--UTM projection, NAD83, easting Utmn_n83--UTM projection, NAD83, northing Explanation of the Data_code that designates the source of the data and the approximate time of acquisition: Part I. Previously existing data (described in more detail by Chandler and Schaap, 1991) 1950-Data acquired in the 1950�s through the mid 1960�s, extracted from a defunct compilation by C. Craddock for the Minnesota Geological Survey. 1956- Data acquired in 1956 by G. Durfee for U. S. Steel Corporation (Virginia-Eveleth area, northeastern Minnesota). 1960-Data acquired during the late 1960�s for the Minnesota Geological Survey, under supervision of R. J. Ikola (mainly in northeastern, northwestern and southwestern Minnesota). 1970- Data, acquired during 1970-1980 under supervision of L. D. McGinnis and C. P. Ervin, Northern Illinois University for the Minnesota Geological Survey (most parts of the state, excluding parts of northeastern Minnesota). Note: several minor errors were detected in these data during more recent work in Rock County (Data_code 1993) and Mower County (Data_code 1995), and subsequently have been corrected. 1983a-Data acquired in 1983 for the Minnesota Geological Survey under supervision of V. W. Chandler (International Falls area, northern Minnesota). 1984- Data acquired in 1984 for the Minnesota Geological Survey under supervision of V. W. Chandler (Hibbing area). 1985- Data acquired in 1985 for the Minnesota Geological Survey under supervision of V. W. Chandler (Brainerd, International Falls, New Ulm and St. Cloud areas). 1987-Data acquired by 1987 by the United States Geological Survey and the Geological Survey of Canada (Roseau-International Falls area). 1989a-Data acquired in 1989 for the Minnesota Geological Survey under supervision of V. W. Chandler (Lake of the Woods County). Part II. Newly Added Data (Source of support and pertinent references included) 1979-Data acquired in 1979-80 for the Minnesota Geological Survey under supervision of V. W. Chandler, consisting of profile and area coverage in the Cedar Mountain Area, southwestern Minnesota (Beltrame and others, 1982). Conducted with support from the Minnesota Legislature as granted by the Aeromagnetic Survey Program of the Legislative Commission on Minnesota Resources. 1983b-Data acquired in 1983 for the Minnesota Geological Survey under supervision of V. W. Chandler, consisting of profile coverage in the Babbitt-Isabella and Fairbanks-Sullivan Lake areas, northeastern Minnesota). Conducted with support from the Division of Minerals of the Minnesota Department of Natural Resources. 1989b-Data acquired in 1989 for the Minnesota Geological Survey under supervision of V. W. Chandler; consisting of area coverage in Rock County, southwestern Minnesota (Chandler 1994 and 1997). Conducted with support from the Minnesota Legislature through the State Special appropriation for the MGS. 1990-Data acquired in 1990 for the Minnesota Geological Survey under supervision of V. W. Chandler, consisting of profile coverage the Finland area, northeastern Minnesota, Miller and others, 1990 and area coverage in the Effie-Coon Lake area, northeastern Minnesota. Conducted with Support from the Minnesota Legislature as granted by the Minnesota Minerals Coordinating Committee and from the U. S. Geological Survey as part of their Conterminous United States Minerals Assessment Program. 1992- Data acquired in 1992 for the Minnesota Geological Survey under supervision of V. W. Chandler; consisting of profile coverage in the Duluth area, northeastern Minnesota. Conducted with support from the Minnesota Legislature, as granted by the Minnesota Minerals Coordinating Committee. 1993- Data acquired in 1993 for the Minnesota Geological Survey under supervision of V. W. Chandler; consisting of area coverage in Rock County, southwestern Minnesota (Chandler, 1994 and 1997). Conducted with support from the County Geologic Assessment/Regional Hydrologic Atlas Program of the MGS and the Division of Waters of the Minnesota Department of Natural Resources. 1994- Data acquired in 1994 for the Minnesota Geological Survey under supervision of V. W. Chandler; consisting of area coverage in Rock County, southwestern Minnesota (Chandler, 1994 and 1997). Conducted with support from the County Geologic Assessment/Regional Hydrologic Atlas Program of the MGS and the Division of Waters of the Minnesota Department of Natural Resources. 1995- Data acquired for the Minnesota Geological Survey under supervision of V. W. Chandler, consisting of area coverage in Mower County, southeastern Minnesota (Chandler, 2000). Conducted with support from the County Geologic Assessment/Regional Hydrologic Atlas Program of the MGS and the Division of Waters of the Minnesota Department of Natural Resources. 1996- Data acquired in 1996 for the Minnesota Geological Survey under supervision of V. W. Chandler; consisting of profile and area coverage in the Allen-Virginia area, northeastern Minnesota (Chandler and Lively, 1997; Chandler and others, 2003). Conducted with support from the Minnesota Legislature, as granted by the Minnesota Minerals Coordinating Committee. 1999-Data acquired for the Minnesota Geological Survey under supervision of V. W. Chandler, consisting of area coverage in Pine County, east-central Minnesota (Bauer, 2001a) area coverage in Wabasha County, southeastern Minnesota (Bauer, 2001b), and area coverage in St. Louis County, northeastern Minnesota. Conducted with support from the County Geologic Assessment/Regional Hydrologic Atlas Program of the MGS and the Division of Waters of the Minnesota Department of Natural Resources, and from Minnesota Legislature through the State Special appropriation for the MGS. 2000- Data acquired in 2000 for the Minnesota Geological Survey under supervision of V. W. Chandler; consisting of area coverage in the Eagles Nest and Hibbing areas, northeastern Minnesota. Conducted with support from the Minnesota Legislature, as granted by the Minnesota Minerals Coordinating Committee and the Legislative Commission on Minnesota Resources. 2002- Data acquired in 2002 for the Minnesota Geological Survey under supervision of V. W. Chandler, consisting of area and profile coverage in the Two Harbors-Castle Danger area of northeastern Minnesota, the Warroad area of northwestern Minnesota, the Cook area of northeastern Minnesota and the Greenwood Lake area of northeastern Minnesota. Conducted with support from the Minnesota Legislature, as granted by the Minnesota Minerals Coordinating Committee and from the United State Geological Survey through the STATEMAP program. Description of Gravity Data Grids Data Collection and Compilation: The two gravity data grids included in this distribution are based on over 40 years of gravity surveying in the state of Minnesota. These grids and the principal fact gravity data on this page represent the first significant revision of the Minnesota gravity database since the compilation by Chandler and Schaap (1991). Since that compilation, approximately 2000 stations have been added as data collected in support a variety of projects related to bedrock mapping by the Minnesota Geological Survey (MGS). In addition, archival data not available for the earlier compilation are now included. The post-1991 data includes profiles with gravity measurements collected at 0.25 mile (0.4 kilometers) intervals along selected corridors, to area surveys with stations spaced at 0.5 to 1.0 mile (0.8 to 1.6 kilometers) intervals along available roads and trails. Horizontal locations were usually determined by locating a point on 1:24,000 scale USGS topographic maps. These points were later digitized and combined into a database. Vertical control for the newer data was usually obtained from spot elevations shown on 1:24,000 scale USGS topographic maps. Occasionally, locations and elevations in areas of low relief were extrapolated from topographic contours. Small groups of locations and elevations were obtained by transit surveys tied into USGS benchmarks, and since 2000, Global Positioning System (GPs) readings have been used for point location with elevations determined from a built-in altimeter data. These stations therefore have horizontal accuracy's +/-50 meters down to +/-10 meters or less. Station locations for some of the pre-1991 data may not be accurate to better than +/- 200 meters. Most of the post-1991 measurements were taken with a Lacoste and Romberg gravity meter (#G-320) with a stated reading precision of 0.01 milligals. As a result of various error sources, gravity values are probably precise to +/- 0.10 milligals, although errors up to +/- 0.3 milligals may occur locally. All data have been reduced using the Geodetic Reference System 1967 (International Association of Geodesy, 1971), assuming a sea level datum and a Bouguer reduction density of 2.67 grams/cc. Corrections were made for earth curvature, but because of the generally low topographic relief across most of Minnesota, no terrain corrections have been made. Description of the available gravity grids: The gravity data were gridded and processed using the potential field software of the U. S. Geological Survey (Phillips, 1997). A minimum curvature program (program MINC) was used to create a grid of Bouguer gravity anomaly values; this grid has 463 rows, 404 columns, a grid spacing of 1500 meters, and a southwest origin at easting 170,500 meters and northing 4,790,000 meters (UTM coordinates, Zone 15, NAD27). In order to sharpen the gravity signature of sources at or near the bedrock surface, new grid values were obtained using the second vertical derivative data calculated from the original Bouguer anomaly grid. To eliminate the minor effects of noise, the derivative gravity data were slightly smoothed by upward continuation to a level of 2,000 meters above the surface. The Bouguer anomaly and the smoothed, second vertical derivative of gravity grids were re-projected to UTM, zone 15 (NAD83,) datum currently in use at MGS. These are posted as files ggd.zip and ggd2d2.zip, respectively. Each of these grids have 3074 rows, and 2680 columns, a grid spacing of 213.36 meters, and a southwestern origin located at an easting of 189,769.572 meters and northing of 4,816,380.742 (assume lower left cell corner for Arc-based applications). In using the gravity data grids the user must be aware that the data has been considerably over-gridded relative to actual data control. In most parts of the state the average spacing between gravity stations is actually 1.61 kilometers or wider. The tight grid interval is used here to create grids that register with the magnetic grids, which is useful for a variety of graphic and GIS applied studies that use both types of data. It is recommended that the original gravity station control be presented with any images or maps of the gravity grids; these locations can be obtained from the principal fact gravity data herein. References Cited Bauer, E. J., 2001a, Data-base map, in Geologic Atlas of Pine County, Minnesota, T. J. Boerboom, Project Manager, Minnesota Geological Survey Map C-13, Scale 1:100,000, 7 Plates. Bauer, E. J., 2001b, Database map, in Geologic Atlas of Wabasha County, Minnesota, A. C. Runkel, Project Manager, Minnesota Geological Survey Map C-14, Scale 1:100,000, 7 Plates. Beltrame, R. C., Chandler, V. W., and Gulbranson, B. L., 1982, Geophysical investigation of the Cedar Mountain Complex, Redwood County, Minnesota: Minnesota Geological Survey Reports of Investigations 27, 20 p. Chandler, V. W., and Schaap, B. D., 1991, Bouguer gravity anomaly map of Minnesota: Minnesota Geological Survey State Map Series Map S-16, scale 1:500,000. Chandler, V. W., 1997, Revised gravity investigation for potential ground water resources in northwestern Rock County, Minnesota: in Patterson, C. J., (ed.), Contributions to the Quaternary geology of southwestern Minnesota: Minnesota Geological Survey Report of investigations 47, p. 46 - 57. Chandler, V. W., 2000, An investigation of bedrock surface topography using the gravity-geologic method, Mower County, Minnesota: in Mossler, J. H., Contributions to the geology of Mower County, Minnesota, Minnesota Geological Survey Report of Investigations 50, p. 83-102. Chandler, V. W., and Lively R. S., 1998, Gravity and magnetic modeling of the Duluth Complex in the Allen 7.5 minute quadrangle, St. Louis County, Minnesota: Minnesota Geological Survey Miscellaneous map M-90, Scale 1:24000. Chandler, V. W., Jirsa, M. A., and Lively R. S., 2003, Gravity and magnetic studies in the Virginia Horn area, in Jirsa, M. A., and Morey G. B. eds., Contributions to the geology of the Virginia Horn area, St. Louis County, Minnesota: Minnesota Geological Survey Report of Investigations 53. International Association of Geodesy, 1971, Geodetic Reference System 1967: Special Publication 3, 115 p. Miller, J.D., Schaap, B.D., and Chandler, V.W., 1990, The Sonju Lake intrusion and associated Keweenawan rocks: Geochemical and geophysical evidence of petrogenetic relationships [abs.]: Institute on Lake Superior Geology, 36 Annual Meeting, Thunder Bay, Ontario, p. 66-67.