Alford rotation

1. n. [Formation Evaluation]
A processing technique to project formation shear data recorded in any two orthogonal directions into the fast and slow shear directions in the presence of shear-wave anisotropy. In the sonic logging application, a dipole transmitter excites a flexural mode that is recorded at one set of receivers that is in-line with the dipole and other receivers that are 90o out of line (the cross-dipole component). A similar recording is made of the wave from a second dipole transmitter, mounted orthogonally to the first. The flexural-wave velocity is closely related to the formation shear velocity, particularly at low frequencies and in hard formations. Using all four waveforms, the Alford rotation is used to determine the speed and direction of the fast and the slow shear wave. Reference: Alford RM: "Shear Data in the Presence of Azimuthal Anisotropy: Dilley, Texas," Expanded Abstracts, 56th SEG Annual International Meeting and Exposition, Houston, Texas, USA, November 2-6, 1986, Paper S9.6