G. Randy Keller
Department of Geological Sciences
Pan American Center for Earth and Environmental Studies
University of Texas at El Paso, El Paso, Texas, USA
Studies of the Earth's gravity and magnetic field (and those of other planetary bodies) are prime examples of modern applications of classical Newtonian physics. There are applications that use knowledge of the Earth's gravity field to study topics such as the details of the Earth's shape (geodesy), predicting the orbits of satellites and the trajectories of missiles, and determining the Earth's mass and moment of inertia. However, the gravity studies addressed here are those that involve geophysical mapping and interpretation of features in the Earth's lithosphere (the relatively rigid outer shell that extends to depths of ~100 km beneath the surface). In fact, the emphasis is on the upper crust of the Earth that extends to depths of about 20 km, because it is this region where gravity and magnetic data can best help delineate geologic features related to natural hazards (faults, volcanoes, landslides), natural resources (water, oil, gas, minerals, geothermal energy), and tectonic events such as the formation of mountain belts. Such studies provide elegantly straightforward demonstrations of the applicability classical physics and digital processing to the solution of a variety of geological problems. These problems vary in scale from very local investigations of features such as faults to regional studies of the structure of tectonic plates.
A big advantage of using gravity data is that a considerable amount of regional data is freely available through universities and governmental agencies. As more detailed data are needed, commercial firms can provide this product in many areas of the world. Finally, relative to most geophysical techniques, the acquisition of land gravity data is very cost effective. The determination of the precise location of the instrument is in fact more complicated than making the actual measurement. Marine and airborne gravity measurements are also common but require complex instrumentation and thus greater costs.Applications
As mentioned above, gravity data is widely available and relatively straight forward to gather, process, and interpret. A particularly nice aspect of gravity techniques is that the instrumentation and interpretative approaches employed are mostly independent of the scale of the investigation. Thus, these techniques can be employed in a wide variety of applications. In addition, images of gravity anomalies are ideal candidates for layers in a Geographic Information System (GIS), and typical image processing software provides numerous mechanisms to merge gravity anomaly data with data sets such as Landsat images.
The regional geophysics section of the Geological Survey of Canada and the western and central regions of the U. S. Geological Survey maintain web sites that include nice case histories demonstrating applications of gravity and magnetic techniques as well as data sets and free software.
A good example of the utility of gravity anomalies is their use to delineate the geometry and lateral extent of basins that contain ground water resources. The sedimentary rocks that fill a basin have low densities and thus produce a negative gravity anomaly that is most intense where the basin is deepest. Gravity modeling is used to provide quantitative estimates of the depth of the basin and thus the extent of the water resource.
In the search for petroleum, gravity data are used in conjunction with other geophysical data to map out the extent and geometry of subsurface structures that form traps for the migrating fluids. For example, salt is a low density substance whose tendency to flow often creates traps. Gravity data have been used to detect and delineate salt bodies from the very early days of geophysical exploration to the present.
Gravity data can often delineate ore bodies in the exploration for mineral resources. Maps of gravity anomalies also reveal structures and trends that may control the location of ore bodies even when the bodies themselves produce little or no gravity anomaly.
Studies of geologic hazards often rely on the use of gravity anomalies to detect faults and evaluate their size. Gravity anomalies also can be used to help study the internal plumbing of volcanoes.
The intent here is to introduce only those terms and concepts necessary to understand the basics of gravity and magnetic techniques. The Gravity and Magnetics Committee of the Society of Exploration Geophysicists maintains a web site that includes a Dictionary of Terms, and a link to a Glossary that is maintained by the Integrated Geophysics Corporation.