Gravity and Magnetics Research

Gravity

ONLINE GRAVITY DATA SET for the lower 48 states

PACES has played a significant role in the compilation of gravity data in southwestern North America. This effort has been conducted in close cooperation with the U. S. Geological Survey, the National Geospatial-Intelligence Agency (formerly the National Image and Mapping Agency), National Oceanic and Atmospheric Administration, industry and academic colleagues. Together, we have developed the new GeoNet gravity database portal that provides access to our new database for the U. S. (GeoNet). The U. S. Geological Survey Open File report that describes this effort is available at this link: Open-File Report 02-463. The version now available is a major update to existing databases and is now also terrain corrected. This effort is also a key part of our NSF-supported projects to develop a geoscience cyberinfrastructure (GEON and SWGeoNet), and also features a tutorial "Getting Started: Using and Understanding Gravity Data".

The Data: The new terrain corrected United States gravity database is available through the GeoNet link below. The terrain corrections were calculated by Mike Webring of the U. S. Geological Survey using a digital elevation model and a technique based on the approach of Donald Plouff. The reduction of these data will be updated this year with modern geodetic datums and a higher precision digital elevation model. In the present version, latitude and longitude values are referenced to NAD27 (North American Datum 27; horizontal datum) and elevation values in meter are referenced to NGVD29 (Vertical datum).

Base Stations: Our effort also includes archiving gravity base station information for the United States. This element of the portal includes the ability to download base station data and diagrams provided by Dan Winester of the National Geodetic Survey who has made a major effort to insure its accuracy and current. This information is based on a base station database maintained by the National Geospatial-Intelligence Agency. However, the two databases are not identical and a map of the NGA stations is provided via the link below. Information about specific stations can be obtained via the NGA website. This information is available from the links below:

Gravity Base Stations - Data and Diagrams

NORTH AMERICAN GRAVITY DATABASE EFFORT:

The obvious next step after the U. S. gravity database and North American Magnetic Map efforts is an improved regional gravity database for North America. Thus, an international effort by governmental agencies, universities, professional organizations, and private industry is underway to update the publicly available North American Gravity Database. The current database that was released roughly two decades ago needs revision to improve its overall quality, coverage, observation density, and versatility. Considerable data have been made available in the intervening period and improvements are possible in the calculation of gravity anomalies by taking advantage of available terrain and geodetic models and high-speed data processing procedures and facilities. Data will be made available in a web-based system as part of the U. S. Geoinformatics program and through other governmental agencies.

The database will have a comprehensive menu making it useful for those with differing scientific interests and backgrounds. The user will be able to select desired corrections to the gravity data, units used, datums employed, and type of gravity anomaly and retrieve information on the predicted errors in the data. The default gravity anomalies (in milligals) of the database based on internationally accepted datums and constants will be useful for geological studies and most geophysical investigations. Database fields and formats will accommodate the increasingly available high-resolution, airborne, satellite, marine, and gradient gravity data and will be updated as additional data are obtained and improvements are made in data processing. The database will be updated as additional data are obtained and improvements are made in data processing.

Click on the link below to see an initial report from the Standards /Format Working Group, of the North American Gravity Database Committee. Standards/Format Working Group Final Report

Magnetics

Magnetic Anomaly Map of North America Project

A cooperative effort between Canada, the U.S. and Mexico has produced a very nice magnetic map and database for North America. The main goal of our effort is to simply make these data available in a more convenient form.

Aeromagnetic data is a fundamental data layer for integrated studies in which other spatial data such as gravity, Landsat images, digital elevation models, geologic maps, and seismic tomography form layers. The U. S. Geological Survey has prepared a brief document (Introduction to Potential Fields: Magnetics) for those interesting in learning more about this technique.

The UTEP group has worked to aid efforts to compile aeromagnetic data in southwestern North America. This effort has been conducted in close cooperation with the U. S. Geological Survey and Mexican colleagues. There is has been a concerted effort on the part of the U. S. Geological Survey to construct an aeromagnetic compilation for the U. S., which was, in turn, used in a companion effort to compile a database for North America. We have sent all of our data to the U. S. Geological Survey for use in these compilations, which are now complete. To quote from their website, "The data sets that make up the current U.S. national data base (more than 1,000 individual surveys, including offshore data to 320 km) will be standardized to a consistent ASCII format".
This project is described in detail at the North American Magnetic Map project website at: http://crustal.usgs.gov/namad/, and booklet describing the project and a page-sized copy of the map can also be downloaded at this site.

The digital grids that were prepared are described at http://pubs.usgs.gov/of/2002/ofr-02-414/ and can downloaded at that site.

In addition, individual compilations for many states and for numerous individual separate surveys can be found at http://crustal.usgs.gov/projects/namad/us_digital_data.html.
Those interested in a relatively small area that is solely contained within a single state or an area covered by a specific survey, should consider using these data directly in order to obtain maximum resolution. These data are distributed in various grid formats.

ONLINE MAGNETIC DATA SET

As part of the GEON effort, we have prepared a version of the North American Magnetic Map grid that is searchable by latitude and longitude. Thus individuals wanted to use these valuable data can easily download only the section of the grid of interest to them in a simple ASCII format. In order to make the gird searchable, we attached a latitude and longitude (NAD27) to each data point. Thus, the data are basically individual points like our accompanying gravity data set. This approach makes it easy to make matching gravity and magnetic maps but does require that the user regrid the data make a contour map.

The data set for Texas and environs

The only product that we have produced that is not readily available from the U. S. Geological Survey is an edited and carefully merged version of the original flight line data for the National Uranium Resource Evaluation (NURE) program for the State of Texas and adjacent areas. These data are idea for computer modeling and analytical calculations such as depth to basement estimation efforts, because they have not been interpolated as part of a gridding algorithm. This data set is available by simply requesting it via email to:
(Randy Keller; keller@utep.edu).

Future efforts

There is an initiative underway to improve the U. S. aeromagnetic database via a high altitude aeromagnetic survey. Magnetic anomalies reveal much about the composition, structural architecture, and even thermal regime of a region. Thus, a high quality aeromagnetic data set is a fundamental data layer for integrated studies in which other spatial data such as gravity, Landsat images, digital elevation models, geologic maps, and seismic tomography form layers. The need for a much-improved aeromagnetic database for North America has been recognized for many years, and legacy low-altitude aeromagnetic data have just been complied into a data set for North America. However, there is a strong need for a high-altitude survey to both serve as a mechanism to stitch the low-altitude data together properly and to provide long wavelength data that are of considerable significance and utility in their own right. Recently a broad cross-section of the geomagnetic community held a workshop because of an exciting and cost-effective opportunity to collect invaluable magnetic anomaly data during a mission over the U.S. in the near future (Open-File Report 2002-366).