Azimuthal resistivity

Azimuthal resistivity surveys are designed to measure anisotropy caused by near-vertical dipping structure (due to bedding plane or to fracturing). The idea is to record resistivity at one location for a range of angular orientations of the array. In other words the electrode array is rotated about a central point.

Data (apparent resistivities) are generally plotted on a rose diagram with resistivity increasing outwards along the radius, and the angular position depending on the orientation of the array.

The method works well when there is little or no overburden, but the results tend to rotate as overburden thickens. Eventually the rose diagram appears to be 90 degrees off what would be expected when the direction of anisotropy in rocks below overburden is known. The effect appears to have been described well enough that results can be interpreted if overburden thickness is known. See Sandberg and Jagel, 1996, Jansen and Taylor, 1996, and Carlson et al, 1996, all in the SAGEEP '96 proceedings.

The example shown here is from Sandberg and Jagel (1996). It indicates good correlation between the fracture orientation (and hence, hydraulic transmissivity) and apparent resistivity, which is higher in line with fractures, as predicted when the "anisotropy paradox" is accounted for.

Reference

• Sauck and Zabik 1992 Azimuthal resistivity techniques and the directional variations of hydraulic conductivity in glacial sediments, Proceedings, Symposium on the Application of Geophysics for Engineering and Environmental Problems (SAGEEP) 1992, p 197.
• In the proceedings of the Symposium on the Application of Geophysics for Engineering and Environmental Problems (SAGEEP) 1996, there are three relevant papers between pages 31 and 61:
  • Sandberg and Jagel, 1996,
  • Jansen and Taylor, 1996, and
  • Carlson et al, 199