These maps show variations in the Earth's magnetic field caused primarily by the uneven distribution of the mineral magnetite in the rocks that make up the upper part of the Earth's crust. The features and patterns of the maps reveal details of subsurface geology including the locations of buried faults, magnetite-bearing rocks, which include many kinds of rocks of interest to mineral exploration and environmental studies, and thickness of surficial sedimentary rocks.
The aeromagnetic data are presented as four different grids that cover the entire state and two additional grids covering parts of the state. These maps are constructed from grids that combine information (see data processing details) collected in 20 separate aeromagnetic surveys conducted between 1952 and 1979. The data from these surveys are of varying quality. The design and specifications (terrain clearance, sampling rates, line spacing, and reduction procedures) varied from survey to survey depending on the purpose of the project and the technology of that time. An index plot gives an overview of the flightline spacing of the original surveys. A data table summarizes the detailed specifications of the surveys.
The six aeromagnetic grids present the data in different ways to give the user maximum flexibility in interpretation. The six grids and grid names are:
A composite grid of data sets having the same grid interval and projection, and having the Definitive International Reference Field (DGRF) removed, but having the original survey elevation retained. Areas of no data surround each separate survey. [utcomp98]
A grid where all surveys have been continued to 12,500 ft barometric elevation (above sea level) and the surveys are blended or merged together. [utah12500]
A grid where the merged data at 12,500 ft (the utah12500 gridded data set) have been converted from level to a datum 1,000 ft above terrain using the xia-m method (Phillips, 1996). [ut1000xia]
A grid where the merged data at 12,500 ft (the utah12500 gridded data set) have been converted from level to a datum 1,000 ft above terrain using the chessboard method and severe filtering (Cordell and others, 1982). [utah1000f3]
A blended or merged grid of surveys C, H, I, and J at the original flight elevation of 9,000 ft barometric elevation. [west9000]
A blended or merged grid of surveys M and O at the original flight elevation of 8,500 ft barometric elevation. [east8500]
Efforts to mathematically merge these 20-plus surveys were hindered by poor-quality data of some surveys and the limited USGS software available in past years. For example, minimal editing of digital flight-line data could be undertaken. Because many of the surveys were flown at a constant, fairly high barometric elevation, we chose to continue each survey to 12,500 ft above sea level. Using this elevation surface permits minimal introduction of spurious anomalies in the data from poor-quality surveys, but it degrades the quality of better surveys. Additionally, several surveys (notably U and V) could not be smoothly added to the merged data grid due to extreme boundary differences.
Two grids covering western and eastern parts of Utah were merged separately at their original flight elevation of 9,000 ft and 8,500 ft respectively. Users working in these areas will benefit from the better resolution of these merged pieces.
The elevation surface of 1,000 ft above terrain is a frequently used standard for aeromagnetic grids. Two grids were prepared from the 12,500-ft merged grid using two different methods of surface continuation: xia-m (Phillips) and chessboard (Cordell and others, 1982; severe filter used)
A composite grid was made by stitching together all 22 surveys (including U and V), using data converted to a common projection, grid interval, and having the DGRF removed, but without continuation to a common surface elevation. Areas of no data surround each survey.
These grids are an interim product. Recent commercial software packages that merge geophysical data have demonstrated the ability to merge poor-quality data such as these in ways that were not available to the authors. We are aware of the mediocre results of our merging efforts, such as visible survey boundaries, poor anomaly resolution, and errors within surveys, and anticipate improving the results using new programs and techniques. However, our results should not be further delayed. We have presented numerous products to compensate for problems in each. To further encourage users (and to reward them for the long wait) we are also releasing all of the original digitized point data as ASCII xyz files for those wishing to process the data themselves.
This project was supported by the Mineral Resource and Geologic Mapping Programs of the USGS. The authors wish to thank our USGS colleagues Anne McCafferty and Richard Saltus for their assistance in preparing this report, and Rob Bracken, Jeffrey Phillips, and Mike Webring for the in-house software used to prepare the aeromagnetic data.
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