Abstract
The Banyumas Basin is a tertiary sedimentary basin located in southern Central Java, Indonesia. Due to the presence of volcanic deposits, 2-D seismic reflection methods cannot provide a good estimation of the sediment thickness and the subsurface geology structure in this area. In this study, the passive seismic tomography (PST) method was applied to image the 3-D subsurface Vp, Vs, and Vp/Vs ratio. We used 70 seismograph borehole stations with a recording duration of 177 days. A total of 354 events with 9, 370 P and 9, 368 S phases were used as input for tomographic inversion. The checkshot data of a 4, 400-meter deep exploration well (Jati-1) located within the seismic network were used to constrain the shallow crustal layer of the initial 1-D velocity model. The model resolution of the tomographic inversions was assessed using the checkerboard resolution test (CRT), the diagonal resolution element (DRE), and the derivative weight sum (DWS). Using the obtained Vp, Vs, and Vp/Vs ratio, we were able to sharpen details of the geological structures within the basin from previous geological studies, and a fault could be well-imaged at a depth of 4 km. We interpreted this as the main dextral strike-slip fault that controls the pull apart process of the Banyumas Basin. The thickness of the sediment layers, as well as its layering, were also could be well determined. We found prominent features of the velocity contrast that aligned very well with the boundary between the Halang and Rambatan formations as observed in the Jati-1 well data. Furthermore, an anticline structure, which is a potential structural trap for the petroleum system in the Banyumas Basin, was also well imaged. This was made possible due to the dense borehole seismographic stations which were deployed in the study area.
Highlights
Based on the sedimentary basin map of Indonesia (Indonesia Geology Agency, 2009), there are at least 13 sedimentary basins on Java Island; eight of these are categorized as intermontane arc basins and are located in the southern part of Java Island
A resolution test was performed using the checkerboard resolution test (CRT), the diagonal resolution element (DRE), and the derivative weight sum (DWS), using a grid node with the same cell size, which we used for tomography inversion; as mentioned in Section Travel Time Tomography Inversion to Determine 3-D Vp, Vs, and Vp/Vs Ratio
Travel time tomography inversion results are presented in vertical and horizontal cross sections of the Vp structure, and in horizontal cross sections of Vs structure and Vp/Vs ratio, which will be covered in the following discussion
Summary
Based on the sedimentary basin map of Indonesia (Indonesia Geology Agency, 2009), there are at least 13 sedimentary basins on Java Island; eight of these are categorized as intermontane arc basins and are located in the southern part of Java Island. A resolution test was performed using the checkerboard resolution test (CRT), the diagonal resolution element (DRE), and the derivative weight sum (DWS), using a grid node with the same cell size, which we used for tomography inversion; as mentioned in Section Travel Time Tomography Inversion to FIGURE 6 | Previous studies available for the study area were used for comparison with our horizontal cross section results: (A) A residual gravity anomaly (modified from Hidayat et al, 2020), (B) the regional Miocene geological structure configuration of the southern part of Central Java (modified from Muchsin et al, 2002), and (C) the complete Bouger anomaly map (modified from Hidayat et al, 2020). In the vertical sections, we expect to reveal the anticline structure that could be a potential structure trap for the Banyumas Basin
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