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and time limit the dimension and discretization of the study volume in practice. Note that the
same concerns the gravity inversion of a single profile data, although much finer
discretization is allowed for 2.5-D models. A rule-of-thumb states that if the forward
computation takes longer than 30 seconds then the inversion must be run overnight.
6.2
Data preparation
First of all, one needs to define the gravity data for the inversion. In other words, one needs to
create a column formatted data file (*.DAT) and check its format, dimensions (m, km, ml or
ft), and units (mgal, Eo). The data can be evenly discretized or irregularly spaced, and it can
include topography or elevation values. Remember, however, that one must choose the study
area and the data sampling based on the practical limits of the inversion. For example, the
inversion of 3030 data points using a 3-D block model with 10 layers leads to a quite
feasible problem of dimension MN = 900 data points  9000 blocks.
Pre-processing (filtering, levelling. corrections) should be made to the data beforehand.
Although it is not required, the gridded data should be stored as continuous lines of x
coordinate (from West to East) for each y coordinate (from South to North). If the data are
measured along lines that are not aligned with main geographical coordinate axes, the
coordinates should be rotated so that the new data area coincides with the coordinate axes of
the model. If the data coordinates cannot be rotated the model must be made larger than data
area and a certain amount of the blocks will be wasted in the corners of the model. In that
case, one should hide the blocks outside the data area with the BLOXER program.
Note that data from a single profile can be interpreted using a 2.5-D model (see chapter 7.2).
In that case, the data should be defined so that the profile is aligned with the x axis or y axis
so that an initial 2.5D model perpendicular to the profile can be constructed (Compute Default
menu item). Note, however, that it is possible to interpret single profile data using a fully 3D
block model, although that would be somewhat ambitious.
Secondly, one needs to define the regional field if such data are available. The regional field
can be included in the gravity data file or read from a separate file. In the latter case, the data
should have exactly the same x and y coordinates as the gravity data. In practice, however, the
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