and U.S. Army
Earth Science Applications, National Training Center, Fort Irwin

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Geophysical Maps

Geophysical methods allow us to "see'"into the Earth and thereby learn about certain aspects of the rocks at depth. This can be useful for helping to describe concealed geologic structures, such as faults, granite bodies, and sedimentary basins. Two kinds of geophysical studies are underway at Fort Irwin . . .

Magnetic studies

Some rocks are more magnetic than others. The rocks within a young lava flow, for example, usually have more magnetic minerals than typical sedimentary rocks. By measuring the magnetic field just above the earth, we can learn something about the contacts between such rocks based on their magnetic properties.

A "magnetic survey" was conducted at Fort Irwin during 1994 and 1995. The survey was conducted by flying an airplane back and forth approximately 1000 feet above ground along parallel lines spaced about 1/3 mile apart. The airplane carried a "magnetometer", an instrument for measuring the intensity of the earth's magnetic field. These measurements were then processed on a computer to produce a "magnetic anomaly map". Such maps show areas with unusually high or unusually low magnetic fields. At Fort Irwin, magnetic anomaly maps are particularly useful for learning about faults and plutons that underlie the area. Note, for example, the different patterns of anomalies to the north and south of the Coyote Canyon fault. By carefully studying these patterns, we can learn about the dip and extent of the Coyote Canyon fault beneath the earth's suface. Also notice the bright red, oval-shaped feature just west of label "G". This magnetic anomaly is underlain by highly magnetic rocks, probably a combination of gabbro (a kind of igneous rock) and skarn (iron-rich deposits in sedimentary rocks that surround the gabbro). Both the gabbro and skarn have many iron minerals that contribute to the magnetic anomaly here.

Gravity studies

Rocks also have variable densities. For example, a volume of unconsolidated sedimentary deposits has less mass andproduces a smaller gravitational attraction than the same volume of limestone or dolomite. Sedimentary basins, therefore, are often reflected in the earth's gravity field as regions of relatively low gravitational attraction. These highs and lows are very small, on the order of 0.001 percent of the earth's gravity field. By measuring the earth's gravitational attraction at many points above the ground, we can produce maps of these "gravity anomalies" and thereby learn about the distribution of rocks below the ground.

This map shows the regional gravity at Fort Irwin and surrounding areas. The dashed lines outline deep sedimentary basins (thicker than 500 meters deep) predicted to exist on the basis of these gravity measurements and geologic mapping. The three hatched areas, on the other hand, indicate actual basins at Fort Irwin. Notice that none of the Fort Irwin basins are very deep. Indeed, there are no deep basins within 15 km of Fort Irwin, a factor that could affect future development of the military base.

The U.S. Geological Survey is conducting detailed gravity studies at Fort Irwin. These measurements will be analyzed in conjunction with detailed geologic mapping and drill information to determine the depth, shape, and extent of sedimentary deposits near the military base. An understanding of the size and nature of the sedimentary basins at Fort Irwin is critically needed for future land-use planning in the area.

Detail of aeromagnetic map of the Fort Irwin Basin area

The accompanying map shows detailed aeromagnetic data from Fort Irwin Basin in the area where most housing and administrative installations are located. On this map, the relative strength of the magnetic field is indicated by blue contours. These are plotted on top of geologic, topographic, and tectonic information. The colored areas on the map indicate the kinds of rocks exposed at the surface: pink and green are granite bodies, orange is volcanic rock, and yellows are unconcolidated sedimentary deposits. The gray contours indicate topographic elevations, and gray grid lines represent section lines (each gray box is one mile wide). Heavy black lines show the principal identified faults, and red lines show some magnetic lineaments (linear steep gradients) that may represent unidentified faults. Letters on the map identify specific locations: FIB=Fort Irwin Basin; BL=Bicycle Lake; LWL=Langford Well Lake; TM=Tiefort Mountains.

At this map scale, the strongly magnetic rocks in the subsurface show up as closely packed blue contours. As discussed earlier, these highly magnetic rocks are probably iron skarn and gabbro, both of which contain many magnetic iron-rich minerals. Gabbros and skarns are exposed at the surface in a couple of places, but mostly are concealed by other kinds of rocks. Note that the magnetic field over sedimentary basins (such as in the lower right part of the map) tends to be smooth because sedimentary deposits are relatively nonmagnetic. Some granites are relatively nonmagnetic too and also produce smooth magnetic contours. For example, look at the western part of the Tiefort Mountains (shown in pink) and the green area at the southwest edge of the map. These areas are underlain by granite containing very few iron-bearing minerals.

Linear gradients in the magnetic field match mapped faults in several places, but not in others. Mapped faults without a coinciding magnetic lineament can be explained in several ways. The fault may have a very small displacement, or the fault may have a large displacement which has brought rocks together on each side of the fault with similar magnetic properties. In either case, the fault would be difficult to resolve in the magnetic contours. Of greater interest are those magnetic lineaments that don't overlie mapped faults. These lineaments may indicate undiscovered faults in the subsurface, and such areas are targets for detailed geologic and geophysical investigations.

More information about geophysical studies in the western United States