Magnetic Data

123°45' 40' 35' 30' 25' 20' 15' 10' 05' 123°00' 55' 50' 45' 40' 35' 30' 25' 20' 15' 122°10'
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Raw Magnetic Data Map
Raw Scale Bar

Arc Explorer
Themes visible in the above map: MAGRAW (Image), LATITUDE/LONGITUDE and COASTLINE. The theme which presents the legend for MAGRAW (Image) is MAGRWBAR (Image). When the magnetic data is viewed in Arc Explorer, please note that this theme is an image and therefore not able to be queried. Magnetic values are given in the legend presented above.

Processed Magentic Data Map
Processed Scale Bar
Arc Explorer
Themes visible in the above map: MAGPROC (Image), LATITUDE/LONGITUDE and COASTLINE. The theme which presents the legend for MAGPROC (Image) is MAGPRBAR (Image).When the magnetic data is viewed in Arc Explorer, please note that this theme is an image and therefore not able to be queried. However, the MAGNETICS CONTOURS theme may be queried. The contour values are contained in the CONTOUR field.

Aeromagnetic anomalies of the eastern Juan de Fuca Strait
Richard Blakely and Carmel Lowe

The Raw Magnetic data map (top) shows the relative intensity of the magnetic field as measured near the earth's surface. The colors on the maps reflect the variable magnetic properties of rocks in the upper crust, which in turn reflect subsurface lithology. Volcanic and ultramafic rocks, for example, are often strongly magnetic and produce high-amplitude magnetic fields when measured near the earth's surface. Glacial and fluvial deposits, on the other hand, are only weakly magnetic and produce subdued patterns of magnetic anomalies.

These maps are based on five airborne magnetic surveys, flown at different times and with different specifications. Most of the region (the area east of longitude 123°W) is represented by a high-resolution aeromagnetic survey flown in 1997 by the U.S. Geological Survey over the U.S. part of the Puget Lowland (Blakely and others, 1999). These data were collected at an altitude of 245 m (800 ft) above terrain along north-south flight-lines spaced 400 m (0.25 mi) apart. The region west of 123°W longitude is covered by four separate surveys, three flown in 1976 by the Canadian Geological Survey and one flown in 1974 by the U.S. Geological Survey (1974). All of the western aeromagnetic surveys were flown along north-south lines but at different constant altitudes ranging from 305 m (1000 ft) to 1370 m (4500 ft), and at different line spacings, ranging from 1200 m (0.75 mi) to 1600 m (1 mi).

To produce the Raw Magnetic Data map, the original digital data from each survey were projected to a common rectangular grid. The five gridded surveys were merged digitally, allowing no changes to the 1997 Puget Lowland survey and allowing only first-order baseline changes to the other four surveys. No attempt was made to continue the surveys to a uniform altitude. Due to the disparate flight specifications, certain artifacts are apparent along survey sutures, notably along meridian 123°15'W.

The most magnetic rocks in this part of the Puget Lowland are Eocene basalts of the Crescent Formation and other mafic rocks of similar age, which crop out in the western and southwestern parts of the map, and pre-Tertiary ultramafic rocks located in the northeastern part of the map. Linear alignments of anomalies and anomaly gradients sometimes indicate important crustal structures. The linear east-west gradient extending from the eastern edge of the map to about longitude 123°15'W, for example, is associated with the western part of the Devils Mountain fault, a regionally important fault that juxtaposes highly magnetic ultramafic rocks north of the fault against less magnetic Tertiary rocks to the south (e.g., Whetten and others, 1980; Johnson and others, 2000). West of longitude 123°15'W, the Devils Mountain fault merges with the Leech River fault, which also is apparent in the magnetic data.

The Processed Magnetic Data map shows the aeromagnetic data digitally processed in order to emphasize shallow magnetic sources. The original merged data (the Raw Magnetic Data map) were continued to a higher flight surface 100 m above the original flight surface. Subtracting this "regional field" from the original data tends to suppress anomalies originating at deep levels relative to those generated by shallower sources. Thus, the high-amplitude anomalies at the western edge of the Raw Magnetic Data map are suppressed in the Processed Magnetic Data map, while anomalies caused by shallow sources, like the anomalies associated with the Devils Mountain fault, are emphasized. This presentation is especially useful in identifying various linear magnetic contacts, some of which reflect faults in the shallow subsurface. Note that this technique also emphasizes "noise" in the original data. Flight-line artifacts in the original data, for example, cause the north-south striation in the western part of the map.

References

Blakely, R.J., Wells, R.E., and Weaver, C.S., 1999, Puget Sound aeromagnetic maps and data: U.S. Geological Survey Open-File Report 99-514.

Johnson, S.Y., Dadisman, S.V., Mosher, D.C., Blakely, R.J., and Childs, J.R., 2000, Active tectonics of the Devils Mountain fault and related structures, northern Puget Lowland and eastern Juan de Fuca Strait region, Pacific Northwest, in review.

Whetten, J.T., Zartman, R.E., Blakely, R.J., and Jones, D.L., 1980, Allochthonous Jurassic ophiolite in northwest Washington: Geological Society of America Bulletin, v. 91, p. 359-368.

U.S. Geological Survey, 1974, Aeromagnetic map of part of the Puget Sound area, Washington: U.S. Geological Survey Open-File Report 74-1106, scale 1:125,000.

 

Reference citation:
Blakely, R., and Lowe, C., 2000. Aeromagnetic anomalies of the eastern Juan de Fuca Strait region,
in: Mosher, D.C. and Johnson, S.Y. (Eds.), Rathwell, G.J., Kung, R.B., and Rhea, S.B. (Compilers), Neotectonics of the eastern Juan de Fuca Strait; a digital geological and geophysical atlas. Geological Survey of Canada Open File Report 3931

123°45' 40' 35' 30' 25' 20' 15' 10' 05' 123°00' 55' 50' 45' 40' 35' 30' 25' 20' 15' 122°10'
48°30'
25'
20'
15'
10'
05'
48°00'
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