Optimization of 2-D Magnetotelluric (MT) Modes Based on Data Dimensionality to Delineate Potential Area for Shale Gas Prospect in Kutai Basin, East Kalimantan, Indonesia

We conducted a study using the magnetotelluric method in the Kutai Basin, which is one of the largest and deepest tertiary sedimentary basin located in the province of East Kalimantan, Indonesia. The Kutai Basin, which is one of the sedimentary basin that is proven to produce hydrocarbons in Indonesia, also has the potential for shale gas with all the complexities of its geological structure. Inversion of 2-D MT can generally be done in three modes with different sensitivity. We perform data processing objectively to obtain the best quality data. We continued our data processing to the inversion process with a range from 80.78% to 97.09% coherency data. We also performed sensitivity skewness calculations to determine the dimensionality of our data. The map of sensitivity skewness is shown for the vertical path A – A’ with direction N – S in our study area. Based on the calculation results, the skewness value below 0.3 is obtained around the frequency 320 - 0.002 Hz, and associated with the 2-D structure while value above 0.3 are obtained around the frequency 0.00198 - 0.00034 Hz at KT34 and KT36 stations. Based on dimensionality calculations, it is concluded that the MT data in the Kutai Basin is dominated by 2-D structural responses, so that the TE + TM (invariant) mode is the best measurement mode for inversion modeling. We also performed calculations to obtain the optimum smoothness factor (tau) using a trade-off curve. Based on the results of the inversion with the optimization of these data parameters, we obtained a subsurface geological structure pattern such as fault and fold structure along the vertical path of A – A’. The low resistivity anomaly is interpreted as a response to the presence of black shale which is part of the Pamaluan Formation. The top of the Pamaluan Formation is estimated at the depth that varies from 2000m to 4000m below the surface along the A – A’ vertical cross-section.

Magma reservoir beneath Azumayama Volcano, NE Japan, as inferred from a three-dimensional electrical resistivity model explored by means of magnetotelluric method

An electrical resistivity model beneath Azumayama Volcano, NE Japan, is explored using magnetotelluric method to probe the magma/hydrothermal fluid distribution. Azumayama is one of the most concerning active volcanoes capable of producing a potential eruption triggered by the 2011 Tohoku-Oki Earthquake. The three-dimensional resistivity model reveals a conductive magma reservoir (< 3 Ωm) at depths of 3–15 km below sea level (bsl). The 67% and 90% confidence intervals of resistivity are 0.2–5 Ωm and 0.02–70 Ωm, respectively, for the magma reservoir. We assumed dacitic melt + rock at a shallow depth of 4 km bsl and andesitic melt + rock at a greater depth of 9 km bsl. The confidence interval of resistivity cannot be explained by using dacitic melt + rock condition at a depth of 4 km bsl. This suggests that very conductive hydrothermal fluids coexist with dacitic melt and rock in the shallow part of the magma reservoir. For the depth of 9 km bsl, the 67% confidence interval of resistivity is interpreted as water-saturated (8.0 weight %) andesitic melt–mafic rock complex with melt volume fractions greater than 4 volume %, while the shear wave velocity requires the fluid and/or melt volume fraction of 6–7 volume % at that depth. Considering the fluid and/or melt volume fraction of 6–7 volume %, the conductive hydrous phase is likewise required to explain the wide range of the 67% confidence interval of resistivity. The Mogi inflation source determined from geodetic data lies on the resistive side near the top boundary of the conductive magma reservoir at a depth of 2.7 or 3.7 km bsl. Assuming that the resistivity of the inflation source region is above the upper bound of the confidence interval of resistivity for the conductive magma reservoir and that the source region is composed of hydrothermal fluid + rock, the resistivity of the source region is explained by a hydrothermal fluid volume fraction below 5 volume %, which is the percolation threshold porosity in an effusive eruption. This indicates that the percolation threshold characterizes the inflation source region.

Understanding Magmatic System of Unzen Volcano (Nagasaki, Southwest Japan) Inferred from Broad-band Magnetotelluric Observation

&lt;p&gt;Unzen Volcano is located in Shimabara Peninsula, Nagasaki, Japan. After 198 years of dormancy, the volcano erupted throughout 1990-1995 and resulted the emergence of new lava dome called Heisei-Shinzan. Following the eruption, numerous studies have been intensively conducted in Unzen volcano to assess the eruption mechanism and the magma plumbing system. Regarding to the magmatic system, the most preferred model is that the primary supply of magma is stored beneath Chijiwa bay. This magma chamber is located about 15 km west of the active dome at vertical depth approximately 15 km, and followed by subordinate shallower magma chambers beneath the volcano (e.g. Nakamura 1995; Kohno et al 2008). Upon the eruption, the magma ascended obliquely towards the summit in east direction (e.g. Umakoshi et al 2001). However, how main magma chamber&amp;#160; and shallower chambers are connected to the summit via oblique pathway is poorly imaged in terms of structure.&lt;br&gt;As widely known, Magnetotelluric method is highly sensitive to low resistivity zone caused by interconnected fluids. Low resistivity zone detected in the volcanic area usually can be interpreted as hydrothermal/magmatic fluid and or magma chamber containing partial melt (e.g. Aizawa et al 2014; Hill et al 2015). Thus, by using broadband Magnetotelluric method, we aim to investigate resistivity structure of Unzen volcano associated with magmatic system and its controlling structure (e.g. pathway and faults).&lt;br&gt;Although the shallow structures around Unzen volcano are estimated by the 2017-2019 campaigns (Triahadini et al 2019; Hashimoto et al 2020), we are unable to image deeper structure around the proposed location of magma chambers and magma pathway. To achieve our goals, during November-December 2020, we installed 35 new sites to cover whole area in Shimabara Peninsula. In total, deployed 99 Magnetotelluric stations covering Unzen volcano and Shimabara Peninsula. On this meeting, we would like to present our resistivity structure derived from all dataset.&lt;/p&gt;