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Software

   

 

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

  • Gravity data with corrections applied (i.e. instrument drift, tidal, "free air", topography)

  • Vertical depth of the gravity stations

  • Horizontal coordinates of each station in relation to the well position

  • Density log

Data can be imported from ASCII files or edited manually.

Data processing

From the input data Hole-o-Grav can calculate

  • Apparent density (see appendix)

  • 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

  • Contoured calculated gravity on arbitrary vertical or horizontal sections through the model

  • 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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