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Gridded geophysical data sets for the conterminous United States are available on CD-ROM from the U.S. Geological Survey (USGS). The data sets include magnetic anomalies; Bouguer gravity anomalies; isostatic residual gravity anomalies; uranium, thorium, and potassium gamma-ray measurements; and topography with and without bathymetry. The magnetic anomaly and gamma-ray data sets are gridded at a 2- kilometer grid interval; the gravity anomaly and topographic-bathymetric data sets are gridded at a 4-kilometer grid interval. The gridded data are provided in both ASCII and PC-binary formats. Potential-field geophysical software, included with the data sets, can be used to display, process, and interpret the gridded data.
Magnetic anomalies are produced by variations in the distribution of iron minerals, usually magnetite, in the rocks of the Earth’s crust. Igneous and metamorphic crystalline rocks can be very magnetic. By comparison, sedimentary rocks are usually nonmagnetic. Magnetic anomalies therefore provide a way of mapping exposed and buried crystalline rocks.
Gravity anomalies are produced by density variations within the rocks of the Earth’s crust and upper mantle. Mapping of these density variations is the primary use of gravity anomalies. Gravity measurements made on the surface of the Earth must be corrected in various ways before they can be made into an anomaly map.
The free-air correction adds to the measured gravity the predicted difference in gravity between sea level and the elevation of the observation. The simple-Bouguer correction subtracts the predicted effect of the mass between the observation and sea level, using a uniform slab of constant density to model the mass. The complete-Bouguer correction adds the predicted effect of constant-density topography within 166.7 km of the measurement location. A gravity reference field is subtracted from the corrected measurements to produce the free-air, simple-Bouguer, or complete-Bouguer anomaly.
Figure – Color shaded-relief map showing the complete-Bouguer gravity anomaly data for the conterminous United States (onshore) and free-air gravity anomaly data offshore.
|The Earth’s gravity field is depicted in two principal ways: gravity anomaly maps and maps of the Earth’s geoid.
Gravity anomaly maps (see globe below) show how much the Earth’s actual gravity field differs from the gravity field of a uniform, featureless Earth surface. The anomalies highlight variations in the strength of the gravitational force over the surface of the Earth. Gravity anomalies are often due to unusual concentrations of mass in a region. For example, the presence of mountain ranges will usually cause the gravitational force to be more than it would be on a featureless planet — positive gravity anomaly. Conversely, the presence of ocean trenches or even the depression of the landmass that was caused by the presence of glaciers millennia ago can cause negative gravity anomalies.
The geoid is a hypothetical Earth surface that represents the mean sea level in the absence of winds, currents, and most tides. The geoid is a useful reference surface. It defines the horizontal everywhere and gravity acts perpendicular to it. A carpenter’s level aligns itself along the geoid and a carpenter’s plumb bob points down the vertical or perpendicular to the geoid. Water will not flow in aqueducts if the pipes are perfectly aligned along the geoid. Surveyors use knowledge of the geoid and the horizontal when they lay out highways and boundaries.