Oriented Drill Cores

Tobias Stephan

2025-11-22

This vignette describes how convert alpha-beta (and gamma) measurements from oriented drill cores into geological planes and lines using the drillcore_transformation() function of the {structr} package.

library(structr)

Orientations in drill-cores are usually given by α and β angles (lineations on a plane additionally have a γ angle) which describe orientations with respect to the drill orientation. To convert these angles from the “drillcore coordinate reference system” to our geographical reference system, you can use the function drillcore_transformation().

This function calculates the orientation of a plane or line from internal core angles (α, β, and γ) of oriented drill cores, using the the azimuth of drill core axis orientation (azi given in degrees, measured clockwise from North), and the inclination of drill core axis (inc in degrees, measured anticlockwise from the horizontal plane).

azi <- 225
inc <- 45

Note that negative values for the inclination indicate downward direction

alpha and beta are the internal core angles alpha and beta, respectively, measured in degrees.

drillcore_transformation(azi, inc, alpha = 60, beta = 320)
#> Plane object (n = 1):
#> dip_direction           dip 
#>      25.00392      70.02959

The function returns a spherical objects. Since only alpha and beta angles are specified, the output is a "plane" object.

For several alpha and beta angles:

planes_AB <- drillcore_transformation(azi, inc, alpha = c(60, 45), beta = c(320, 220))

The orientations can be plotted in a equal-area (lower hemisphere) projection:

# initialize plot:
stereoplot()

# plot the core axis' azimuth and inclination)
points(Line(azi, inc))
text(Line(azi, inc), lab = "core-axis", pos = 1, font = 3)

# plot the plane orientations as poles...
points(planes_AB, col = c("#B63679FF", "#FB8861FF"))
text(planes_AB, lab = c("A", "B"), col = c("#B63679FF", "#FB8861FF"), pos = 1, font = 3)

# ... and as great circles
lines(planes_AB, col = c("#B63679FF", "#FB8861FF"))

References

Stigsson, M., & Munier, R. (2013). Orientation uncertainty goes bananas: An algorithm to visualise the uncertainty sample space on stereonets for oriented objects measured in boreholes. Computers and Geosciences, 56, 56–61. https://doi.org/10.1016/j.cageo.2013.03.001