INTRODUCTION

The magnetic-anomaly map in this report was produced to provide a framework for geologic and geophysical interpretations at both local and regional scales. The map is compatible in scale with gravity maps of Idaho by Bankey and Kleinkopf (1988). Additionally, this compilation augments a regional compilation of magnetic data that covers most of Idaho and southwest Montana (McCafferty, 1992; see also gravity maps of Bankey, 1992, and a topographic map of Cady, 1992).

The state of Idaho is covered by a collection of approximately 60 aeromagnetic surveys that were flown over the course of three decades. 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. Interpretation and analysis of anomalies over areas that are covered by more than one survey is difficult unless the surveys can be merged and represented at a common datum and scale. The work presented here has synthesized data from these various surveys in order to produce a magnetic anomaly map of Idaho that allows for a continuous representation of the magnetic field and facilitates analysis and interpretation of anomalies across survey boundaries.

MERGED AEROMAGNETIC ANOMALY MAP

Previous Work

An aeromagnetic map of Idaho was published by Zietz and others (1978). The compilation of magnetic surveys in this report supercedes the previous magnetic anomaly map of Idaho for the following reasons; 1) The compilation presented here includes higher quality, more detailed data obtained from surveys either not available at the time or not included in the earlier compilation. 2) Surveys with flight line spacing of 4.8 km or less cover the majority of the state and were used for this compilation. This spacing is still far from the ideal, but a significant improvement over the prior compilation that used data from regional surveys flown with flight lines at 8-km spacing for the majority of the state. 3) Advancements in computer technology and analytical methods over the past decade has allowed the compilation of data in this report to be performed digitally, allowing for a more mathematically rigorous merging process (explained below).

General

The merged aeromagnetic anomaly map shows changes in the earth's magnetic field as a result of variations in the magnetic-mineral content of near surface rocks. In general, sedimentary rocks have little or no magnetic minerals and do not produce anomalies. Therefore, the anomaly map primarily reflects lithologic and structural changes related to magnetic properties of crystalline and volcanic rocks that are likely to contain enough magnetic minerals (principally the mineral magnetite) to produce anomalies.

Map Compilation

The map was compiled from a synthesis of digital data acquired from 56 separate aeromagnetic surveys flown at different times with varied flight elevations, flight-line spacings, and data-reduction procedures. Figures 1a, 1b, 1c, and table 1 illustrate and describe the surveys used in the map compilation. Flight-line elevations ranged from 120 m above terrain to 4,300 m barometric (400 ft to 14,000 ft) ; flight-line spacings ranged from 0.8 km to 9.6 km (0.5 mi to 6 mi). Survey aerial coverage ranged from a 15' x 15' quadrangle to a 1o x 2o quadrangle. The data were projected onto a Cartesian coordinate system using a Lambert projection with standard parallels of 33oN. and 45oN., a central meridian of 114oW., and a base latitude of 0oN.

Each survey was interpolated to a square grid using a minimum-curvature algorithm (Webring, 1981); grid spacing was typically 1/4 to 1/3 the original flight-line spacing. The magnetic-anomaly grid [total field intensity minus the Definitive International Geomagnetic Reference Field: (DGRF)] was calculated (Sweeney, 1990) for the appropriate time of year and elevation of the original survey. If an obsolete regional field other than the DGRF had been removed, as was the case with much of the digitized data, the outdated geomagnetic reference field was added back and the DGRF was subtracted from the grid.

An elevation of 305 m (1,000 ft) above terrain was selected as the reduction datum level in order to be compatible with adjacent regional compilations in Nevada (Hildenbrand and Kucks, 1988a, 1988b) and the Basin and Range province (Hildenbrand and others, 1983), Utah (Bankey and others, 1998) and a regional compilation covering most of Idaho and southwest Montana (McCafferty, 1992). Surveys flown in draped mode (constant elevation above terrain) above or below this datum level were analytically continued upward or downward (Hildenbrand, 1983) so that the data would be consistent with adjacent surveys. For surveys flown at a constant barometric elevation, the related data were analytically continued to the draped surface of 305 m above ground using the method of Cordell (1985). If the survey's data had to be continued more than two grid intervals downward, the data were regridded to a coarser interval prior to continuation to minimize short-wavelength noise enhanced by the method. After reducing the data to a common level, each survey was regridded to a 1-km interval and merged to adjoining surveys using a cubic-spline method (Cordell and others, 1992).

Every attempt was made to acquire the data in digital form. Most of the available digital data were obtained from aeromagnetic surveys flown by the U.S. Geological Survey (USGS), flown on contract with the USGS, or were obtained from other federal agencies and state universities. Much of the pre-1975 data are available only on hand-contoured maps and had to be digitized. These maps were digitized along flight-line/contour-line intersections, which is considered to be the most accurate method of recovering the original data. Table 1 specifies availability of digital data.

The entire study area is covered by aeromagnetic data collected as part of the National Uranium Resource Evaluation (NURE) program of the U.S. Department of Energy. These data are available in digital form and provided the framework for the map compilation. However, because magnetic surveying was not the primary objective in the design of the NURE surveys, these data are subject to certain limitations. Although the NURE surveys were flown at elevations close to the reduction datum level, the spacing between flight lines ranged from 4.8 km to 9.6 km, with the exception of part of the Challis 1o x 2o quadrangle, which was flown at 1.6 km flight line spacing. The wide spacing between flight lines flown at low altitudes over surface rock units having high magnetizations (basalts of the Snake River Plain for example) causes anomalies with short spacial wavelengths to be elongated between flight lines, producing lineations perpendicular to the flight-line direction and 'pearl string' anomalies along the flight line. This problem was especially severe over the Snake River Plain and Columbia Plateau basalt fields. Consequently, data from surveys other than NURE were incorporated into the framework of NURE surveys wherever possible.

Limitations

The aeromagnetic map contains a diversity of magnetic-anomaly trends, textures, and patterns. The majority of the magnetic features can be attributed to the wide range of geologic terranes contained within the study area. However, some variations can be attributed to inconsistent specifications of the magnetic surveys. The boundaries of the individual surveys are shown on map A in order to highlight magnetic anomalies that may be associated with varying data types and flight-line specifications rather than anomalies associated with geologic sources.

The majority of gradients in the study area reflect lateral changes in the magnetic properties and depths of crystalline and volcanic rocks; however, a few gradients located at survey edges may be artifacts of survey boundaries. Unfortunately, gradients coincident with survey boundaries were unavoidable in some cases due to very different survey specifications and data-reduction steps. Smooth boundaries across adjacent surveys were a goal, but not at the expense of having to further filter an entire survey to obtain that goal.
 
 

ACKNOWLEDGMENTS

The authors wish to thank Esther Sandoval for digitizing and computer support.
 
 

REFERENCES CITED

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