Application of the VLF-EM method with EEMD to the study of a mud volcano in southern Taiwan

A three-year geophysical investigation was carried out in the Wu-Shan-Ding mud volcano nature reserve in southwest Taiwan. This study utilized the very low-frequency electromagnetic (VLF-EM) method with a recent data processing technique, ensemble empirical mode decomposition (EEMD), to investigate the subsurface structure of the mud volcanic site through the single profile modeling and the surface 2D contour analysis. With the help of this new data processing technique, the signal of VLF-EM data was enhanced to an extent that a regularized inversion with finite-element forward modeling scheme can be applied to map the subsurface resistivity image of a single profile with satisfactory accuracy. This study successfully predicted the evolving of a gryphon, and proposed a model showing that the mechanism of fissure eruptions is more appropriate than a point source model for the shallow structure of the Wu-Shan-Ding mud volcano. The results also indicate that the Chi-shan fault is still active and that it traverses the study area with a nearly vertical up-thrown angle.