= Solution
A <gravity anomaly> is measured <gravitational acceleration>, reduced to a stated reference surface and compared with a reference <gravity> field. A local geological interpretation further removes or models the regional background. An absolute negative <Bouguer gravity anomaly> is therefore not itself a local negative <geophysical density contrast>.
Measure relative <gravity> with a calibrated <gravimeter> at stations across the profile, tie it to a base station, repeat base readings to estimate instrumental drift, and measure station coordinates and accurate elevations, for example by <GNSS> and levelling. Record observation times for tide corrections. Subtract the latitude-dependent reference <gravity>, correct instrument drift and Earth tides, add the <free-air correction> for station height, and subtract the attraction of the reference-density <Bouguer slab>. A terrain correction accounts for relief missing from or exceeding that slab. Use a consistent <density> such as $2670\ \mathrm{kg\,m^{-3}}$ for the topographic correction.<Geophysical Data Grids for the Conterminous United States by United States Geological Survey>[Gravity anomaly reduction: free-air, simple-Bouguer and complete-Bouguer corrections]{ref}
The question's reference to the anomaly in panel b is a panel-label slip: the upper plot contains <gravity>, while the lower plot supplies geometry and elevations. A geological slab thickness is the vertical thickness of the anomalous body, not simply its height above sea level.
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