Petroleum exploration in sub-volcanic area always poses an inevitable challenge. Active seismic exploration method fails to obtain reliable imaging of the sediment beneath volcanic formation due to massive attenuation. This issue has been a long-standing problem in onshore seismic activity in Indonesia, especially in areas where volcanic formations present above the sedimentary formation of interest. To address this issue, we propose an alternative method utilizing a passive seismic approach to obtain reliable subsurface information. This paper discusses our experience in employing ambient noise tomography to evaluate the sedimentary structure beneath the volcanic area in Southern Malang, East Java. The passive seismic network deploying 70 seismometers were installed in a relatively regular grid. With the maximum offset between two furthest stations was 44.5km, we can capture the maximum wavelength of 15 km which is associated with the minimum frequency as low as 0.08 Hz to be used in the inversion. In principle, the seismometers record the coherent seismic noise coming from the atmospheric activity, sea wave, or industrial activity in the surface. Cross correlation between signal received in each station and their continuous stacking yields useful signals to reveal the dispersion curve which can produce the subsurface velocity profile through an inversion technique. From the inversion result we obtain the subsurface s-wave velocity structure down to a depth of 6 km. Higher s-wave velocity structure on the shallow depth in the northern area of the survey confirms the presence of the thick volcanic sediment situated near volcanic mountain. Towards the southern area we observe a slower s-wave velocity profile that indicates the thinning of volcanic formation. Although the method has successfully delivered a reliable s-wave structure over an entire survey area, its resolution is limited due to large spacing between stations. We suggest deploying denser stations to improve the velocity resolution.
Crustal and uppermost mantle S-wave velocity structure beneath the Japanese islands from seismic ambient noise tomography