Comparison of Discrete and Voxel 3-D Gravity Inversions Over Karst-Related Gravity Anomalies On The Coastal Plain of North-West Malaysia

A negative gravity anomaly of amplitude 12 μm/sec2 and diameter 400 metres on the coastal<br>plain of north-west Peninsula Malaysia has been modelled as due to cavities in buried limestone<br>bedrock. A model of equi-dimensional cavities using spheres does not reproduce the sharpness of the<br>measured anomaly but a model using ellipsoids is successful. The value of the expected density contrast<br>is the only constraint required to develop these parametric models. Maximum-smoothness voxel<br>inversions are poorly suited to the binary density distribution of cavity models, but when guided by<br>earlier parametric inversion results the voxel inversions enable the testing and development of more<br>complex source shapes. For the deeper equi-dimensional cavity model the computed field is only weakly<br>sensitive to source shape but for the shallower ellipsoid model the voxel inversion may provide a valid<br>improvement in representing the shape of the source body.<br>The anomalies can also be modelled as due to changes in overburden thickness. Parametric and<br>voxel models of basement topography are similar to each other and differ significantly from simple<br>Bouguer slab predictions establishing the importance of 3-D modelling even for such a simple<br>geological model. There appears to be no inherent superiority of either parametric or voxel inversions<br>provided each has the versatility to select models according to guiding geological concepts.