Frequency-dependent S-wave splitting parameters analysis: A case study from azimuthal PS data, Sanhu area of the Qaidam Basin, China

Abstract

The Sanhu area, which is located in the eastern Qaidam Basin in China and formed of low-amplitude anticlinal and lithologic traps, is a favorable area for biogas exploration. The fracture systems in this area are characterized in detail with the use of frequency-dependent S-wave splitting parameters, which are sensitive to the size of the fractures. The results indicate that the delay time between slow and fast S-waves decreases rapidly with increasing frequency between 5 and 30 Hz, and then it slowly decreases to a stationary value at high frequencies. Moreover, the frequency-dependent delay times suggest that fractures of different scale have different 2D density distribution. The frequency-dependent orientation of the fractures suggests that large-scale fractures, which correspond to a low-frequency band (5–11 Hz), are oriented at approximately N48°E and have small random disturbances. The mesoscale fractures that correspond to the dominant frequency band (12–36 Hz) are oriented along approximately N54°E in the northeastern region and N45°E over the remaining area. As expected, the average fracture orientation and delay time of the dominant frequency band are consistent with previous results from conventional S-wave splitting analysis in the time domain, but the frequency-dependent fracture orientation and delay time indicate finer heterogeneity and spatial anomalies. In summary, the results show the potential for accurately characterizing fracture systems using frequency-dependent S-wave splitting parameters.