Apr 11: SEG 2010 Middle East & Africa Honorary Lecture - Deep Gravity – Long Wavelengths and Measured Moho

In most interpretations of gravity data, the long wavelength gravity signal (~100 km) is ignored and assigned to the dust bin of the “regional anomaly.” If the long wavelengths are considered at all, they are generally “cheaply and cheerfully” removed by inverting topography in a simplistic isostatic correction that assumes a Moho that mirrors the topography. Using the isostatic gravity anomaly often clarifies near-surface geological features of interest. However, this simplistic method has serious flaws for larger-scale features. From several data compilations, it is apparent that the Moho does not mirror the surface topography and that the uppermost mantle may have significant density variations, resulting in gravity anomalies of similar wavelengths. Thus, simplistic isostatic corrections are naïve at best and at worst can lead to serious misinterpretations for mid- to large-scale gravity modeling. With the advent of 3D gravity modeling, it is now becoming possible to model large-scale targets and to consider these regional effects in more detail. As a result of the Kaapvaal Project, we now have unprecedented coverage of the crustal thickness and details of the seismic velocity variations in the lithosphere beneath southern Africa. By using these data, we can calculate the contribution of each component to the overall gravity field. Keeping in mind that a large sedimentary basin such as the Karoo, or an important PGE deposit such as the Bushveld complex, may extend well over 400 km laterally, full and proper account of the Moho and upper mantle density variations may significantly alter the modeling result and lead to possible targets.