The GRABLOX2 program computes the gravity field and the gravity gradients of a 3-D block model which consists of a large rectangular block divided into smaller brick-like elements the size of which can vary. Each minor block is assigned individual density value. For more information about 3-D block models, please, see the documentation of the BLOXER program, which is used to visualize and to maintain the 3-D block models.
GRABLOX2 can be used for both forward and inverse modeling (inversion). Gravity field (gz) and optionally either the vertical gravity gradient (gzz) or seven gravity gradients (tensor elements) (gxx, gxy, gxz gyy, gyz, gzz and guv by Falcon) can be modelled. The inversion optimizes the density of the blocks so that the difference between the measured and the computed gravity (and gradient) data gets minimized. The optimization is based on linearized inversion. The unconstrained inversion uses singular value decomposition (SVD) with adaptive damping. The constrained inversion utilizes Occam's method where the roughness of the model is minimized together with data misfit. The coefficients of the base anomaly, which is represented by a second degree polynomial can be optimized separately for the gravity and gradient data. Density of the blocks can be fixed (and weighted) based on a priori information (e.g., petrophysical or drill-hole data). Gradient data can be used together with gravity data in the inversion, but gradient data cannot be inverted alone. After density inversion the distribution of the density variations inside the resulting block model can be used in geological interpretation.