The simplest type of Bouguer correction to gravity data is the slab correction which removes the gravitational effect of an infinite horizontal mass between the station elevation and an assumed datum. In many areas, that underlying mass is not a horizontal slab but has an undulating upper surface, or topography. For land or sea bottom surveys, the result is that the part of the mass above the meter elevation exerts an upward pull on the gravity meter and the part lower than the meter is over-corrected by the slab-type correction. That distorts the desired gravity generated by a deeper geologic source body. For shipborne marine gravity surveys, the thickness of the water layer can be quite variable and its effect cannot be corrected with a simple slab correction. Sometimes the undulations are severe and the simple Bouguer-corrected gravity is grossly inaccurate. That part of the problem can be mollified by computing and applying a 2D terrain correction to the Bouguer value along a profile.
However, in the real world, terrain/topographic undulations occur in three dimensions, so the mass variations from off-line topography still affect gravity values. The solution is to apply a Three-Dimensional (3D) Bouguer correction. A modern 3D Bouguer Correction combines the simple Bouguer slab correction with a terrain correction computed directly from a Digital Elevation Model (DEM) rather than from ring zone (Hammer, 1939) estimates made during field acquisition or later in processing. The DEM process provides an efficient automated approach to making 3D Bouguer corrections, but does have a weakness unless the gravity stations are properly located. That weakness can come from the grid spacing of the DEM: if the topography variations have a shorter wavelength than the DEM grid, the terrain corrections may be inadequate. If gravity stations cannot be placed adequately far from sharp topographic features such as cliffs or gullies, the meter operator must estimate topography variations between the station location and closest DEM value. The 3D Bouguer correction is applicable for gravity surveys both onshore and offshore. The main consideration will be selection of the density to use for calculating the correction, and that selection will be guided by local geology and the exploration objective.
After completing this module, you’ll be able to:
- Describe a 3D Bouguer Correction.
- Determine when a 3D Bouguer Correction should be applied.
- Determine how extensive it should be.
- Determine what type of data and constraints are required.
- Explain how a 3D Bouguer Correction can benefit your exploration objective through improved delineation of geologic structure and lithology.