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  • Gravity inversion using block model 2

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GRABLOX2 can be used for both forward and inverse modeling (inversion). Gravity field (gz) and either the vertical gravity gradient (gzz) or seven gravity gradients (tensor elements) (gxx, gxy, gxz gyy, gyz, gzz & guv=(gxx-gyy)/2 curvature component) 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.

Screen dump of the GRABLOX2 program

GRABLOX2 can be run on a PC with Microsoft Windows operating system and a graphics display with a screen size of at least 1280x1024 pixels. In forward modelling the computer memory and CPU requirements are usually not critical factors, because the program uses dynamic memory allocation and the computations are quite simple. However, models with hundreds of thousands of elements may take several hours to compute even on a modern computer. Likewise, inversion of block models with tens of thousands of elements may not be practical because of the increased computation time. The size of continuous memory (1 GB) that is allocated for the sensitivity matrix restricts the usage of the 32 bit version. The 64 bit version of GRABLOX2 that can only be run under 64 bit Windows does not have this memory restriction. Furthermore, the 64 bit version uses OpenMP parallelisations and therefore runs faster on modern PC's with multiple processor cores. GRABLOX2 has simple graphical user interface (GUI) that can be used to modify the parameter values, to handle file input and output, and to visualize the gravity data and the model. The user interface and graphics are based on the DISLIN graphics library (http://www.dislin.de).

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