GRAVITY METHOD: ENVIRONMENTAL AND ENGINEERING APPLICATIONS Kevin Mickus Department of Geosciences, Southwest Missouri State University, Springfield, MO 65804; klm983f@.smsu.edu OVERVIEW The gravity method is a nondestructive geophysical technique that measures differences in the earth’s gravitational field at specific locations. It has found numerous applications in engineering and environmental studies including locating voids and karst features, buried stream valleys, water table levels and the determination of soil layer thickness. The success of the gravity method depends on the different earth materials having different bulk densities (mass) that produce variations in the measured gravitational field. These variations can then be interpreted by a variety of analytical and computers methods to determine the depth, geometry and density the causes the gravity field variations. Gravity data in engineering and environmental applications should be collected in a grid or along a profile with stations spacing 5 meters or less. In addition, gravity station elevations must be determined to within 0.2 meters. Using the highly precise locations and elevations plus all other quantifiable disturbing effects, the data are processed to remove all these predictable effects. The most commonly used processed data are known as Bouguer gravity anomalies, measured in mGal. The interpretation of Bouguer gravity anomalies ranges from just manually inspecting the grid or profiles for variations in the gravitational field to more complex methods that involves separating the gravity anomaly due to an object of interest from some sort of regional gravity field. To perform the later, there are several manual and computer techniques including graphical smoothing and polynomial surface fitting. The interpretation of separated (residual) gravity anomalies commonly involves creating a model of the subsurface density variations to infer a geological cross-section. These models can be determined using a variety of methods ranging from analytical solutions due to simple geometries (e.g., sphere) to complex three- dimensional computer models. INTRODUCTION The gravity method involves measuring the earth’s gravitational field at specific locations on the earth’s surface to determine the location of subsurface density variations. The gravity method works when buried objects have different masses, which are caused by the object having a greater or lesser density than the surrounding material. However, the earth’s gravitational field measured at the earth’s surface is affected also by topographic changes, the earth’s shape and rotation, and earth tides. These factors must be removed before interpreting gravity data for subsurface features. The final form of the processed gravity data can be used in many types of engineering and environmental problems, including determining the thickness of the surface or near-surface soil layer, changes in water table levels, and the detection of buried tunnels, caves, sinkholes and near-surface faults. Relatively new applications include four-dimensional (4-D) gravity, where temporal variations of the gravitational field can used to determine variations in the water table (Mokkapati, 1995; Hare et al., 1999) and changing of subsidence levels in sinkholes (Rybakov et al., 2001). Table 1 lists the main uses of the gravity method in engineering and environmental studies. The gravity method can be a relatively easy geophysical technique to perform and interpret. It requires only simple but precise data processing, and for detailed studies the determination of a station’s elevation is the most difficult and time-consuming aspect. The technique has good depth penetration when compared to ground penetrating radar, high frequency electromagnetics and DC-resistivity techniques and is not affected by the high conductivity values of near-surface clay rich soils. Additionally, lateral boundaries of subsurface features can be easily obtained especially through the measurement of the derivatives of the gravitational field. The main drawback is the ambiguity of the interpretation of the anomalies. This means that a given gravity anomaly can be caused by numerous source bodies. An accurate determination of the source usually requires outside geophysical or geological information. The use of the gravity data is relatively straightforward as can be seen in the following summary of the fundamentals of the gravity method as applied to engineering and environmental studies including overviews Christopher Taylor Sotelo, E.S.P.M. Graduate, X-Reg Pre-Law Student/Researcher Department of Environmental Science, Policy & Management, College of Natural Resources, University of California, Berkeley cs.taylorsotelo@berkeley.edu