Tom Wilson, Department of Geology and Geography Environmental and Exploration Geophysics II tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV Gravity: In-class Problems Overview

Tom Wilson, Department of Geology and Geography What is the radius of the smallest equidimensional void (such as a chamber in a cave - think of it more simply as an isolated spherical void) that can be detected by a gravity survey for which the Bouguer gravity values have an accuracy of 0.05 mG? Assume the voids are in limestone and are air-filled (i.e. density contrast, , = 2.7gm/cm 3 ) and that the void centers are never closer to the surface than 100m. i.e. z ≥ 100m Gravity reminders: some of those in-class problems

Tom Wilson, Department of Geology and Geography Basic formula are available for the simple geometrical objects. In this case we use those for the sphere. Let gmax = 0.1 We reasoned that g anom shouldbe at least 0.1 mGal; that Z would be at least 100m, and  = gm/cm 3 or 1.7gm/cm 3

Tom Wilson, Department of Geology and Geography Determine their depths Use a diagnostic position Assume a minimum value of 0 A. C. B. Anomalies associated with buried equidimensional objects -

Tom Wilson, Department of Geology and Geography In this in-class/take home problem determine whether the anomaly below is produced by a sphere of a cylinder

Tom Wilson, Department of Geology and Geography What’s the station elevation? What’s the average elevation in Sector 1? What’s the relative difference between the station elevation and the average elevation of sector 1?

Tom Wilson, Department of Geology and Geography (0.03mG)0.0279mG Determine the average elevation, relative elevation and T for all 8 sectors in the ring. Add these contributions to determine the total contribution of the F-ring to the terrain correction at this location. We will also consider the F-ring contribution if the replacement density of 2.67 gm/cm 3 is used instead of 2 gm/cm 3 and the result obtained using the ring equation. What did you get?

Tom Wilson, Department of Geology and Geography Equation 6-30 What would the answer be if the replacement density were 2.67gm/cm 3

Tom Wilson, Department of Geology and Geography The residual anomaly is identified by marking the intersections of the extended regional field with the actual anomaly and labeling them with the value of the actual anomaly relative to the extended regional field After labeling all intersections with the relative (or residual ) values, you can contour these values to obtain a map of the residual feature ?

Tom Wilson, Department of Geology and Geography begin preparing your magnetics paper summaries Look over the initial gravity modeling effort that is combined with the magnetics lab. read chapter 7