00ReadMe.txt 2006 Compilation of Alaska Gravity Data and Historical Reports, USGS Data Series 264 By Richard W. Saltus, Philip J. Brown II, Robert L. Morin, Patricia L. Hill CD-ROM implementation and design by Philip J. Brown II U.S. DEPARTMENT OF THE INTERIOR DIRK KEMPTHORNE, Secretary U.S. GEOLOGICAL SURVEY Mark D. Myers, Director Distributed by: U.S. Geological Survey Information Services Box 25286, Building 810 Denver Federal Center Denver, CO 80225-0086 Call 1-888-ASK-USGS *********************************************************************** TABLE OF CONTENTS *********************************************************************** - Quick Start - Overview - Introduction - Getting Started - System Requirements - CD Organization - Authorship Credit - Contact Information - Disclaimer *********************************************************************** QUICK START *********************************************************************** This USGS Data Series report is presented in Adobe Portable Document Format (PDF). Adobe® Acrobat Reader® version 5.0 or later is required to view it. A copy of Adobe Acrobat Reader is included in the CD archive, or if you wish to download the latest version of Adobe Reader free of charge, go to http://www.adobe.com/products/acrobat. The document “main.pdf” contains numerous “clickable” hot links to ancillary descriptive files, historical reports, gravity station descriptions, gravity data, gravity maps and reference compilations. To install the free Geosoft® Oasis montaj® viewer software and corresponding data reduction algorithms, this report must be installed on your computer. Refer to the “Getting Started” section of this README file for installation instructions. *********************************************************************** OVERVIEW *********************************************************************** Gravity anomalies provide fundamental geophysical information about Earth structure and dynamics. To increase geologic and geodynamic understanding of Alaska, the USGS has collected and processed Alaska gravity data for the past 50 years. This USGS Data Series report introduces and describes an integrated, state-wide gravity database and provides accompanying gravity calculation tools to assist in its application. Additional information includes gravity base station descriptions and digital scans of historical USGS reports. The gravity calculation tools enable the user to reduce new gravity data in a consistent manner for combination with the existing database. This database has sufficient resolution to define the regional gravity anomalies of Alaska. Lack of local isostatic compensation complicates interpretation of regional anomalies in both southern and northern Alaska. However, when filtered appropriately, the Alaska gravity data show regional features with geologic significance. These features include gravity lows caused by low-density rocks of Cenozoic basins, flysch belts, and felsic intrusions, as well as many gravity highs associated with high-density mafic and ultramafic complexes. *********************************************************************** INTRODUCTION *********************************************************************** Mapping of gravity anomalies is an important tool for geologic and tectonic reconnaissance. Regional gravity lows can delineate basins that may be prospective for hydrocarbons. Gravity highs can assist in mapping mafic/ ultramafic rock units that may be important to mineral exploration. Overall gravity anomaly trends and features may provide useful clues to tectonic structure, including the location of major faults. Gravity data are also critical to construction of accurate geoid maps. The geoid, in turn, is required for the determination of orthometric elevations from geodetic positioning systems(GPS). Because of this utility, the USGS has collected and processed extensive gravity data for Alaska. Despite some 50 years of effort, the spatial coverage of Alaska gravity data is incomplete and irregular. This reflects the logistical difficulties and high costs involved in moving about the State. However, the existing gravity station network is sufficient to define regional gravity anomalies with wavelengths greater than 25 km State-wide, as well as more detailed local anomalies along transportation corridors and in well-explored regions. This report describes the integrated USGS Alaska gravity database and provides background information to assist users of the data. Because of the arcane, sometimes confusing, and occasionally inconsistent procedures and practices of gravity data reduction, it can be difficult to satisfactorily merge data collected by different researchers. A primary purpose of this database and the accompanying information is to document the USGS gravity data and make it easier to successfully use or integrate with other gravity data. Although years of effort have gone into collecting and processing the Alaska gravity data, errors undoubtedly remain within the database. The primary sources of error likely relate to uncertainties in the determination of elevation at the gravity measurement sites. Users of these data are invited to communicate with the USGS about any errors or problems encountered and to submit additional data for inclusion in the database. ---------------------------------------------------------------------- Brief history of USGS gravity data collection in Alaska David F. Barnes and his USGS colleagues began systematic efforts to construct a regional gravity map and database for Alaska starting in the late 1950s. Initial efforts were concentrated on building a network of base stations tied to previous measurements, including those by Edward Thiel and his colleagues (Thiel and others, 1958) and by Rice (1958) using a pendulum system. The early reconnaissance gravity work concentrated on available transportation corridors, including many river traverses using small boats. Unequivocally establishing horizontal map position was often difficult on these river traverses. Anecdotal reports indicate that at least one entire traverse resulted in no data collection because it was impossible to reliably establish map position at any point along the way. Special procedures were developed for dealing with the vagaries of gravity data collection in Alaska (Barnes, 1972). In particular, multiple methods of obtaining elevation estimates (for example, from available maps, photogrammetry, or altimetry) were required because of limited map control (many areas covered only by 1:250,000-scale maps with 100- or 200-foot elevation contours). Reduction of field measurements to produce calculated Bouguer anomalies involved considerablejudgment, particularly for estimating and eliminating gravity meter drift and for choosing the best elevation estimate for the site. Barnes (1972) estimates 0.05 mGal as the precision of observed gravity values and most base station values. He defines an accuracy code (for Bouguer anomalies) that varies from 0.01 to >5.0 mGal, but states that most surveys “based on reasonable altimetry” are accurate to 1.0 mGal. By 1975 Barnes and his colleagues had completed a first draft of a State-wide Bouguer gravity anomaly map, and he published an abstract outlining the gross features of the Alaska gravity field (Barnes, 1975). He pointed out arcuate gravity highs, some exceeding 1,000 km in length, and postulated that they reflected thrust belts and associated vertical tectonic displacements. The large strike-slip systems of Alaska did not appear to have obvious gravity expression apart from local basins developed along them. Many gravity lows were interpreted as Cenozoic basins, including Cook Inlet, the largest, and many smaller basins in interior Alaska. The magnitude of regional gravity lows precluded crustal thickness much in excess of 50 km, but Barnes pointed out that active tectonics likely limited the ability to assign an empirical relationship between gravity and crustal thickness in parts of the State. In 1976 and 1977 Barnes published contour maps of Bouguer gravity (Barnes, 1976a; 1977) based on about 30,000 land measurements and 40,000 km of ship traverses over the adjacent continental shelves. These maps include approximately 15,000 gravity measurements collected directly by the USGS during the previous 15 years. In the map text (Barnes, 1976a) he cited support from the Division of Geological and Geophysical Surveys of the Alaska State Department of Natural Resources, the U.S. Army Topographic Command, and the Defense Mapping Agency. He also cites logistical support from the Office of Naval Research for river traverses in the state. Between 1977 and 1994 significant additions to the USGS gravity database came from exploration work in the National Petroleum Reserve Alaska (NPRA; Gryc, 1988) and from USGS Wilderness studies and Alaska Mineral Resource Appraisal Program (AMRAP; Grybeck, 1991) studies. In 1994 Barnes and several colleagues produced an updated Alaska gravity map for publication in the Alaska Decade of North American Geology (DNAG) volume (Barnes and others, 1994). Since 1994 some additional data have been added to the database and primary USGS responsibility for its maintenance has passed to Robert L. Morin. Addition of new data, as well as revision and editing of older data, continues as part of the USGS Surveys and Analysis Program, Alaska, under the overall direction of Frederic H. Wilson. *********************************************************************** GETTING STARTED *********************************************************************** PDF FILE INSTALLATION Download the CD archive. Open the ZIP file and extract the files to your computer. Install the report files by (1) double-clicking “setup.exe” in the root directory of this CD-ROM archive or (2) selecting the “Install Report” option on the autorun menu. Once you have installed the report files, you can launch the report's navigational menu by (1) opening the main.pdf file in the “Alaska Gravity Compilation” directory of the computer hard drive used for the installation, or (2) double-clicking the USGS Alaska Gravity Compilation icon on your desktop, or (3) selecting “Alaska Gravity Compilation” on your start menu. ---------------------------------------------------------------------- USGS GEOSOFT OASIS MONTAJ GX GRAVITY REDUCTION TOOLKIT INSTALLATION To work with the Alaska gravity database and the supplied USGS gravity reduction tools, the free Geosoft Oasis montaj interface or a full version of Oasis montaj must first be installed on your computer. You can install the free Oasis montaj interface by using the link provided under Gravity Data Reduction Tools on the opening page of the file main.pdf. Note that for this option to work, Oasis montaj must have been selected as part of the installation when running setup.exe. You can also install the free Oasis montaj interface by launching "Oasismontaj_5.1.8.exe" found in the Oasis_Montaj directory of the 2006 Compilation of Alaska Gravity Data and Historical Reports CD-ROM. After installing the Oasis montaj free interface, the gx_setup.exe program must be run to plug the USGS gravity toolkit GX into the Oasis montaj viewer. This can be done by launching "gx_setup.exe" found in the "USGS_GX" subdirectory of the "Alaska Gravity Compilation" directory or by clicking the link "Gravity Data Reduction Tools" on the opening page of the main.pdf file and clicking the link "Install Oasis montaj free interface". NOTE: Oasis montaj MUST be installed on your computer before plugging the USGS gravity GX tools into the Oasis montaj viewer. Also note that you must select the path to the Oasis montaj parent directory during the installation of this toolkit. Make sure the drive and directory path used for the installation match the drive and directory path of the Oasis montaj parent directory. A typical path would be C:\Oasismontaj. Additionally, these GX tools have only been tested with Oasis versions older than 6.0. ---------------------------------------------------------------------- LAUNCHING GEOSOFT OASIS MONTAJ VIEWER AND USING THE GRAVITY TOOLKIT Once the toolkit is installed, click on "Data Files" on the opening page of the file “main.pdf”, then on the link "Open Alaska 2006 gravity database in Oasis montaj." This link will work only if Oasis montaj is installed in C:\Oasismontaj. If this is not the case, launch Oasis montaj and open the database file “AlaskaGravity.gws”, which was placed in the Oasis montaj root directory, in the subdirectory “AkgravData”, during the USGS gravity toolkit installation. Additionally, all files for the USGS GX gravity toolkit installation can be found in uncompressed format on the 2004 Compilation of Alaska Gravity Data and Historical Reports CD-ROM in the “USGS_GX directory”. If for any reason this installation does not work, these files may be copied manually into the correct Oasis montaj subdirectories. This method should also work for Oasis montaj versions 6.0 and newer. *********************************************************************** System Requirements *********************************************************************** - Microsoft® Windows® 95 OSR 2.0, Windows 98 Second Edition, Windows Millennium Edition, Windows NT 4.0 with Service Pack 5 or 6 (Service Pack 6 recommended), Windows 2000, or Windows XP - 16 MB of RAM (32 recommended) - 500 MB of available hard disk space *********************************************************************** CD ORGAINIZATION *********************************************************************** CONTENTS OF THIS USGS DATA SERIES REPORT This initial release of the integrated Alaska gravity database includes the following components: A. Report, including historical information, datum description, instructions for use of the gravity reduction tools, and discussion of regional gravity anomalies. B. Gravity data files C. Gravity base station descriptions D. Reference lists for Alaska gravity study reports E. Historical reports (scanned from original paper copies) F. Gravity data reduction software tools (based on the Oasis montaj freeware interface) G. Digital gravity maps *********************************************************************** AUTHORSHIP CREDIT AND ACKNOWLEDGMENTS *********************************************************************** This README text file: Philip J. Brown II Data Series Introduction: Richard Saltus and Robert Morin Gravity Data Compilation: Robert Morin, Richard Saltus, and Philip J. Brown II Gravity Base Station Description Compilation: Richard Saltus and Philip J. Brown II Historical Report Compilation: Richard Saltus and Philip J. Brown II Gravity Surveys Performed Compilation: David Barnes Periodical and Open-File Report Reference Compilation: Patricia L. Hill Regional Gravity Map Development: Richard Saltus Geosoft Oasis montaj GX data reduction program development: Northwest Geophysical Associates, Inc., and Philip J. Brown II CD-ROM implementation and design: Philip J. Brown II Acrobat PDF implementation and design: Philip J. Brown II Metadata: Richard Saltus and Philip J. Brown II ACKNOWLEDGMENTS The authors would like to acknowledge the Alaska Surveys and Analysis Program for funding this project. We would also like to acknowledge David Barnes. A special thanks to all working within the Denver Publishing Service Center of the USGS for editing and publishing this report, as well as John Meyer of the Alaska Division of Oil and Gas and Joe Duval of the USGS for their review of this report. *********************************************************************** CONTACT INFORMATION *********************************************************************** Inquiries about this CD-ROM should be addressed to: Philip J. Brown II U.S. Geological Survey Box 25046, Mail Stop 964 Denver Federal Center Denver, CO 80225-0046 Email: pbrown@usgs.gov Inquiries about the U.S. Geological Survey Crustal Imaging and Characterization Team can be addressed to Victor Labson. *********************************************************************** DISCLAIMER *********************************************************************** This Compact Disc-Read Only Memory (CD-ROM) DS-264 publication was prepared by an agency of the U.S. Government and has been approved for release and publication by the Director of the USGS. Although this report has been subjected to rigorous review and is substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. Furthermore, it is released on condition that neither the USGS nor the U.S. Government may be held liable for any damages resulting from its authorized or unauthorized use. Oasis montaj software is the intellectual property of Geosoft and is used here with their permission. Other than those rights granted to the USGS and third parties to assist them in using the product (any venue), Geosoft reserves all rights in their software. Reference therein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S Government or any agency thereof. Although all data and software published on this CD-ROM are used by the U.S. Geological Survey, no warranty, expressed or implied, is made by the U.S. Geological Survey as to the accuracy of the data and related materials and (or) the functioning of the software. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the U.S. Geological Survey in the use of the data, software, or related materials.