Version 2.1a (c) 2014 by Markku Pirttijärvi

GRABLOX2 computes the gravity field and the gravity gradients of a 3-D b= lock model which consists of a large super block divided into smaller brick= -like elements the size of which can vary. Each minor block is assigned ind= ividual density value and the gravity effect is obtained as a superposition= of all volume elements. For more information about 3-D block models, pleas= e, see the documentation of the BLOXER program, which is used to visualize = and to maintain 3-D block models.

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= =3D(gxx-gyy)/2 curvature component) can be modelled. The inversion optimize= s 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 inv= ersion utilizes Occam's method where the roughness of the model is minimize= d 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 weig= hted) based on a priori information (e.g., petrophysical or drill-hole data= ). Gradient data can be used together with gravity data in the inversion, b= ut gradient data cannot be inverted alone. After density inversion the dist= ribution of the density variations inside the resulting block model can be = used in geological interpretation.

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 forw= ard modelling the computer memory and CPU requirements are usually not crit= ical factors, because the program uses dynamic memory allocation and the co= mputations are quite simple. However, models with hundreds of thousands of = elements may take several hours to compute even on a modern computer. Likew= ise, inversion of block models with tens of thousands of elements may not b= e practical because of the increased computation time. The size of continuo= us 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. Furtherm= ore, the 64 bit version uses OpenMP parallelisations and therefore runs fas= ter on modern PC's with multiple processor cores. GRABLOX2 has simple graph= ical 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 lib= rary (http://www.dislin.de).

Download the GRABLOX2.ZIP pac= kage (3.4 MB).

The program requires either the 32 bit stand-alone version GRABLOX2.EXE = or the 64 bit version GRABLOX64.EXE and the LIBIOMP5MD.DLL dynamic link lib= rary for OpenMP parallel computations. The distribution file (GRABLOX2.ZIP)= also contains a short description file (_README.TXT), user's manual (GRABL= OX2_MANU.PDF) in PDF format, and some example data (*.DAT & *.GAT) and = model files (*.INP & *.BLX). To install the program simply decompress t= he distribution ZIP file somewhere on a hard disk and a new folder GRABLOX2= appears. When running the program over local network, one should provide t= he network drive with a logical drive letter (e.g., Explorer/Tools/ Map net= work drive=E2=80=A6).

User's guide (Grablox2_m= anu.pdf) (1.9 MB).

Since I've not been able to make a paper on this subject, please, refer = to this web-page and program manual if possible. Before I manage to prepare= better examples, please, use the following workshop handout (2.25 MB) as a starting point to your= own experiments. Here is also a small pagage of Grablox examples including 2.5D and overburden= models.

Note that GRABLOX2 does not include block height inversion. Use GRABLOX = 1.6 for two-layer inversions.

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