Changes

==Data reduction==

==Data reduction==

Corrections are applied to the field observations for instrument drift, tidal effects of the varying position of the sun and moon, the rotation and oblateness of the earth, variations in station elevation, water depths, vehicular motion (for sea and airborne surveys), and terrain. The result is called the Bouguer anomaly, which may be plotted on profiles and contoured on maps<ref name=pt07r11>Erwin, C. P., 1977, Theory of the Bouguer anomaly: Geophysics, v. 42, p. 1468., 10., 1190/1., 1440807</ref><ref name=pt07r32>LaFehr, T. R., 1991, Standardization in gravity reduction: Geophysics, v. 56, p. 1170., 10., 1190/1., 1443137</ref>.

Corrections are applied to the field observations for instrument drift, tidal effects of the varying position of the sun and moon, the rotation and oblateness of the earth, variations in station elevation, water depths, vehicular motion (for sea and airborne surveys), and terrain. The result is called the Bouguer anomaly, which may be plotted on profiles and contoured on maps.<ref name=pt07r11>Erwin, C. P., 1977, Theory of the Bouguer anomaly: Geophysics, v. 42, p. 1468., 10., 1190/1., 1440807</ref><ref name=pt07r32>LaFehr, T. R., 1991, Standardization in gravity reduction: Geophysics, v. 56, p. 1170., 10., 1190/1., 1443137</ref>

 

[[file:the-gravity-method_fig1.png|thumb|{{figure number|1}}Example of observed, regional, and residual Bouguer anomalies.]]

==Interpretation==

==Interpretation==

The Bouguer anomaly is usually separated into components arising from the geological features of interest (residuals) and other components (often termed the ''regional field''<ref name=pt07r40>Nettleton, L. L., 1954, Regionals, residuals, and structures: Geophysics, v. 19, p. 1., 10., 1190/1., 1437966</ref>. The residual Bouguer anomaly may then be interpreted in terms of the most probable geological causes. An example is shown in Figure 1. Regional fields may or may not be simple, depending upon their sources.

The Bouguer anomaly is usually separated into components arising from the geological features of interest (residuals) and other components (often termed the ''regional field''<ref name=pt07r40>Nettleton, L. L., 1954, Regionals, residuals, and structures: Geophysics, v. 19, p. 1., 10., 1190/1., 1437966</ref>. The residual Bouguer anomaly may then be interpreted in terms of the most probable geological causes. An example is shown in [[:file:the-gravity-method_fig1.png|Figure 1]]. Regional fields may or may not be simple, depending upon their sources.

 

Two general approaches can be taken toward the interpretation of the residual anomaly: forward and inverse calculations. In the ''forward calculation'', the geological source is assumed or approximated from well, seismic, or geological data and the resulting anomaly is compared with the residual Bouguer anomaly. Differences between the calculated and residual anomalies may suggest required changes in the geological model or in the anomaly separation. In the ''inverse calculation'', reasonable limits are placed on the rock densities, depth, or dimensions of the proposed geological source and the residual anomaly is “inverted” to find a possible geological solution. Inversion is nonunique, however, and the solution cannot be considered final without corroborating information.

Two general approaches can be taken toward the interpretation of the residual anomaly: forward and inverse calculations. In the ''forward calculation'', the geological source is assumed or approximated from well, seismic, or geological data and the resulting anomaly is compared with the residual Bouguer anomaly. Differences between the calculated and residual anomalies may suggest required changes in the geological model or in the anomaly separation. In the ''inverse calculation'', reasonable limits are placed on the rock densities, depth, or dimensions of the proposed geological source and the residual anomaly is “inverted” to find a possible geological solution. Inversion is nonunique, however, and the solution cannot be considered final without corroborating information.