Survey Table Options

To view drillhole desurveying options, double-click on the survey table to open it, then click on Compatibility. There are two options for changing the way survey table data is handled in Leapfrog Geo:

When the survey table is imported, Leapfrog Geo sets the Negative survey dip points down value according to the data in the imported table. When the majority of the dip data in the table is positive, Leapfrog Geo assumes all these drillholes will point down and leaves the field Negative survey dip points down disabled. When most of the values are negative, the field is enabled. If you are going to change the automatically set value of this field, consider carefully the implications if there is a mix of drillholes pointing down and up.

When drillhole data is imported, the Desurveying method applied is Leapfrog spherical arc approximation. The Balanced tangent algorithm option is compatible with Vulcan and Minesight. These options are described in more detail below.

Drillhole desurveying computes the geometry of a drillhole in three-dimensional space based on the data contained in the survey table.

Under ideal conditions, the drillhole path follows the original dip and azimuth established at the top of the drillhole. Usually, though, the path deflects away from the original direction as a result of layering in the rock, variation in the hardness of the layers and the angle of the drill bit relative to those layers. The drill bit will be able to penetrate softer layers more easily than harder layers, resulting in a preferential direction of drill bit deviation.

There are a number of paths a drillhole could take through available survey measurements, but the physical constraints imposed by drilling are more likely to produce smoother paths. Selecting the desurveying method that gives the best likely approximation of the actual path of the drillhole will ensure that subsequent modelling is as accurate as possible.

Leapfrog Geo implements three algorithms for desurveying drillholes.

Spherical Arc Approximation

The default algorithm used in Leapfrog Geo is spherical arc approximation, which is sometimes referred to as the minimum curvature algorithm. Downhole distances are desurveyed exactly as distances along a circular arc:

The algorithm matches the survey at the starting and end positions exactly and the curvature is constant between these two measurements. At the survey points, the direction remains continuous and, therefore, there are no unrealistic sharp changes in direction.

If you wish to use spherical arc approximation, there is no need to change any settings.

Raw Tangent

The raw tangent algorithm assumes the drillhole maintains the direction given by the last survey measurement until the next new measurement is reached:

This implies that the drillhole makes sharp jumps in direction whenever a measurement is taken, which is unlikely, except when the drillhole is being used to define a trench.

To use the raw tangent algorithm for a drillhole, double-click on the collar table in the project tree. Tick the trench box for the drillholes you wish to desurvey using the raw tangent algorithm.

Balanced Tangent

The balanced tangent algorithm uses straight lines but attempts to improve the accuracy of the raw tangent algorithm by assigning equal weights to the starting and end survey measurements:

It is an improvement on the raw tangent algorithm but still suffers from an unrealistic discontinuity in the drillhole path. It is, however, a better approximation of the overall drillhole path and is reasonably accurate when the spacing between measurements is small.

To use the balanced tangent algorithm, double-click on the survey table in the project tree. Click on Compatibility tab and change the Desurveying method.