Various geophysical methods have been applied to the survey of the lava tunnel of Manjanggul in Cheju Island to study the effectiveness of each method in investigating underground tunnels. The surveys employing gravity, magnetic, electrial, AMT and VLF methods were carried out along seven profiles across the Manjanggul; especially, all the five methods were used on one representative profile. Several aspects of different methods pertinent to their use in investigation of underground tunnels have been noted. The electrical method employing the dipole-dipole array appeared to be the most effective one among five methods. Therefore, we have tested the electrical method more carefully by using various electrode spacings, and obtained successful resistivity sections showinf the existence of lava tunnels. The gravity method provided relatively sucessful responses associated with the tunnel although the gravity readings were contaminated by wind blowing durinf the survey. The gravity data were also useful for the quantitative modeling study. The magnetic data were also sucessful in delineating the tunnel qualitatively. The AMT data were not sucessful because the used frequency band was not appropriate in detecting very shallow target. The VLF data were severely influenced by the neighboring noise sources such as power lines and were not sucessful in detecting the tunnel responses. The comprehensice result of electrical, gravity and magnetic surveys suggests that undiscovered lava tunnels may exist adjacent to the Manjanggul.
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