Maximal Lyapunov exponent variations of volcanic tremor recorded during explosive and effusive activity at Mt Semeru volcano, Indonesia

K. I. Konstantinou, C. A. Perwita, S. Maryanto, Surono, A. Budianto and M. Hendrasto

We analyze 25 episodes of volcanic tremor recorded from 22 November until 31 December 2009 at Mt Semeru volcano in order to investigate their spectral and dynamical properties. The overtone frequencies for most of the tremor events indicate a pattern of period-doubling, which is one possible route that can lead a system to chaotic behavior. Exponential divergence of the phase space orbits is a strong indicator of chaos and was quantified by estimating the maximal Lyapunov exponent (MLE) for all tremor events. MLEs were found to vary linearly with the number of frequency overtones present in the tremor signals. This implies that the tremor source at Semeru fluctuates between a quasi-periodic state with few overtone frequencies (2–3) and small MLEs (~0.013), and a chaotic one with more overtones (up to 8) and larger MLEs (up to 0.039). These results agree well with the tremor generation model suggested previously by Julian (1994), which describes wall oscillations of a crack excited by unsteady fluid flow. In this model, as fluid pressure increases, a period-doubling cascade leads to numerous new frequencies and a chaotic tremor signal. The temporal variation of MLEs exhibited significant fluctuations from 23 until 31 December when the eruptive activity shifted from explosive to effusive. Such a situation may reflect variable
fluid pressure conditions inside the conduit, where at first magma is accumulated and subsequently is erupted, releasing the buildup of pressure. Our results give further evidence for the role of nonlinear deterministic processes in generating volcanic tremor and call for similar investigations to be conducted in other volcanoes.


Analysis of Zero Offset Vertical Seismic Profiling Data Processing To Evaluate the Oil and Gas Reservoir in Well “X”, Murzuk Basin, Soutwest Libya
Abdel Razak Saad Mohamed, Adi Susilo, Sukir Maryanto

The Murzuq Basin, SW Libya, is one of the high potential area of oil and gas on the North African Saharan Platform. The Mamuniyat Formation in Well ―X‖ of Murzuq Basin is the primary reservoir target in our study. The present study in Well ―X‖ is base on well log data and Zero Offset Vertical Seismic Profiling (ZVSP). This research focuses in ZVSP data processing. The processing of ZVSP includes wavefield separation and corridor stacking. The wavefield separation is done by using a frequency-wave number (F-K filter) and Median filters which intended to isolate the ZVSP data into the upgoing and the downgoing wavefields. Meanwhile, the well logs of interest are: Gamma Ray Log, Acoustic Log, Bulk Density Log, P-wave Velocity Log, and Caliper Log. Base on the ZVSP Data processing and correlation with well log analysis shows that the sandstone at Mamuniyat formation in Well ―X‖, Murzuq Basin, Soutwest Libya contains significant accumulation of oil and gas. At the top of the Mamuniyat formation there is some crossover between the Gamma Ray, density, P-wave velocity, and Acoustic Impedance, possibly indicating the presence of gas. This crossover is very thin (~ 52 ft). This well also show a much thicker (~148 ft) at the top of the Lower Mamuniyat formation until buttom of Melez Shuqran formation, possibly indicating the saturated with gas, oil, and water. In addition, base on analysis of reservoir connectivity in Mamuniyat formation the sandstone reservoir in Upper, Middle and lower Mamuniyat, and even Melez Shuqran are becoming thicker from SE to NW.

Keywords: Zero Offset Vertical Seismic Profiling, Mamuniyat Formation, Murzuq Basin, Libya.