3D data reorientation

 

Necessity of geographic reorientation

The directional rock parameters (in situ stresses, propagation velocity of elastic waves, rock and pore space deformation, strength, pore pressure effectiveness etc.) derived from lab core tests (e.g. RACOS®) are all related initially to a reference line on the core.

 

For most practical interpretations information is required on geographic orientations (azimuth, dip) of the parameter of interests. For this, either orientated core must be taken or the core reference line must subsequently be orientated, together with the parameters related to it.

Methods commonly used for geographic reorientation of cores

Matching of structural features on the core (layer boundaries, visible displacements, crack traces etc.) with corresponding elements on the borehole wall, determined with visible borehole logs.

Matching the known geographical orientation of in situ structures (bedding planes, fractures etc.) with those observable on cores.

Matching of geophysical parameter determined on core samples (wave velocitiy etc.) with those determined with specific borehole logs (sonic log etc.).

Matching the orientation of the rock’s visco-remnant magnetism with that of the recent terrestrial magnetic field.

Visualization of 3D parameter in 2D

For the graphical presentation of the 3D orientations and magnitudes of each of the rock and in situ parameters Schmidt plots are used.

 

Visualization of azimut & dip

Visualization of magnitude, azimut & dip

The Schmidt plot enables the 3D orientations of parameters of interest to be visualized. They are plotted unambiguously on a 2D disc on the basis of their direction (azimuth) in relation to geographical north and their dip in relation to horizontal.

 

Around the outside of the disc is shown the azimuth, beginning at the upper edge with 0° (north) and then increasing clockwise (as in a compass).

 

The values of dip are shown as circles of different radii, with a vertical orientation (dip angle 90°) corresponding to the mid-point of the disc. Parameters with a horizontal orientation (dip angle 0°) are plotted around the outer edge of the disc.

 

The parameter magnitude in each orientation is indicated by the colour.