Gravity
The gravity method is an ideal method for providing useful information about subsurface geology in a relatively inexpensive and non-invasive manner. It is known that higher-density bodies exert a greater degree of gravity than bodies of lesser density. Gravimetric surveys take advantage of this property by measuring differences in gravity forces exerted by various earth materials. Gravimetric methods are utilized for finding ore bodies and buried (hidden) geologic structure, and anything else in which a sufficient contrast is indicative.
Gravimeters employ an internal elastic system that senses extremely small differences in accelerations due to gravity caused by earth materials, compared with an internal proof-mass. Differences as small as a few microgals can be read over a range of thousands of milligals (1 gal = 1 cm/sec/sec), making the equipment suitable for differentiating between a wide range of densities at considerable depth.
Gravity surveying aids in determining the depth to bedrock, the overall basin geometry within and surrounding a site, and relative lateral changes in bedrock densities. Lateral differences in gravitational attraction are caused by contrasting densities of geologic media, such as alluvium versus bedrock, along with other influences; these external influences are accounted for in processing. The information gained from the survey determines relative changes in bedrock character over a defined investigation area. Gravity surveying can also define the locations of possible buried geologic structures.
Residual Bouguer Anomaly map showing that gravity is higher over mountains (red hues) and basin fill is lower in gravity (blue hues).
The following figure shows an example of a 2-Dimensional gravity model that uses the acquired gravity data and converts it into a cross-section of the subsurface. These models help guide the decision making processes related to water resources, drilling programs, and geothermal exploration to name a few. In this figure the gray polygon represents bedrock, blue polygon represents saturated alluvium and the thin yellow polygon represents unsaturated alluvium.
2-Dimensional Gravity model results in a typical Basin and Range environment.
The figure below shows a 3-Dimensional model that is constructed and delivered to the end user which is generally a hydrology firm that will use the depth to bedrock values as an input parameter to their groundwater modeling. In this figure, red hues represent where the elevation of bedrock is highest and the converse for the blue hues. The black lines indicate the location of the gravity models.
3-Dimensional Gravity model results showing elevation of bedrock relative to mean sea level.