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Hole-o-Grav
Interpreting
Borehole Gravity Data
Hole-o-Grav
is a new program for the interpretation of gravity data. Like other
programs it can be used for surface data but it also has special
features for the interpretation of borehole gravity data. The
basic concepts of borehole gravity are explained in an appendix to this
paper.
Hole-o-Grav
can handle an unlimited number of boreholes in one model. For each well
the coordinates and the elevation are reqiured.
Input data
Hole-o-Grav can handle the
following input data
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Gravity data with
corrections applied (i.e. instrument drift, tidal, "free air",
topography)
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Vertical depth of
the gravity stations
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Horizontal
coordinates of each station in relation to the well position
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Density log
Data can be imported from ASCII
files or edited manually.
Data processing
From the input data
Hole-o-Grav
can calculate
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Apparent density
(see appendix)
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Borehole Bouguer
Anomaly
-
Density profile
averaged over the station intervals of the borehole gravity meter
-
Automatic separation
of the gravity into two parts, representing the influence of 1-D
(“layered earth”) and 3-D structures
Model calculations
The model gravity data are
processed in the same way as the observed data so that a direct
comparison between them is possible and the model fit can be checked
easily.
Coming soon
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Contoured calculated gravity on arbitrary vertical or horizontal
sections through the model
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Processing and interpretation of borehole magnetic data
The following figures created with Hole-o-Grav show the apparent
density and the BHBA for a large geological model. Please note the
minimum in the fourth column (3-D anomaly) of the apparent density plot
at about 115 m. It is created by a low density zone offside the well.
The fact that in the calculated data the minum is less strong than in
the obserevd data shows that in the model the density of this zone is
too high.
In the fourth column of the BHBA curves the minimum is replaced
by a typical undulation. Please note also that the BHBA still has a
negative slope which is equivalent to the positive values in the 3-D
part of the apparent densitty (except for the minimum). This is caused
by a large crystalline body several hundred meters away from the well.
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