Salahuddin Husein

Transgressive Successions of the Mahakam Delta Province, Indonesia

Abstract Sedimentological and stratigraphic analysis of outcrops and subsurface data in the Mahakam Delta province, combined with concepts of transgressive sedimentary processes derived from a study of the modern Mahakam Delta, indicate that a significant portion of the palaeo-Mahakam Delta succession was deposited during transgressive phases. Some of the transgressive successions resulted from major transgressions and are regionally extensive, but many reflect small, short-lived transgressions within dominantly progradational phases. Two sandstone facies with significant reservoir potential are common within the transgressive successions. Backfilled distributary sandstones occur in outcrop as 10–20 m thick, fining-upward channel sands that become more marine upward and in the subsurface as elongate, coastline-perpendicular sand bodies with a back-stepping stratigraphic architecture and highly variable thickness. Transgressive shoreline sandstones also fine upward, but they are shoreline-parallel, their lateral extent is controlled by the pre-transgression delta morphology and their thickness depends on the rate of relative sea-level rise. Both types of transgressive sandstone can be difficult to distinguish from specific progradational sandstones with wireline log data, especially when they occur within predominantly progradational successions. However, their sand body geometries, volumetrics and connectivity are much different from the stratigraphically adjacent sandstones deposited during progradational phases of deltaic deposition.

Investigation to the Local Site Effects During Earthquake Induced Ground Deformation Using Microtremor Observation in Yogyakarta, Central Java-Indonesia

Local site effects have an enormous influence on the character of ground motion and play an important role in seismic damage to human being made structures. Several powerful earthquakes have struck Yogyakarta, Indonesia during recent years, one of the destructive which was an Mw 6.3 event that occurred on May 27th, 2006 and caused more than 5,700 fatalities and over 37,000 injuries. Following this event, the single observations of microtremors were densely performed at 274 sites in Yogyakarta City. To provide a good coverage of the study area, the single microtremor observations are designed with a grid spacing of 500 m to the main path of the north–south and east–west of the research area to known in detail the configuration soft rocks and contact hard rock. The predominant periods due to horizontal vertical ratio (HVSRs) are in the range of 0.15–4.00 s. The amplification factor (Ag) and the soil natural frequency (Fg) are generally ranging between 0.70–5.56 and 0.4–3.3 Hz according to results derived from HVSR analysis. It is obviously observed that the thickness of local sediments has great influence on amplification characteristics. In general, the local geology effect is more dominant than the topography effect. The level of soil damage due to the local site effects at the ground deformation location was indicated by the vulnerability index (Kg) and the effective shear strain (γ). Actually, vulnerability index (Kg) varied from 0.6 to 51.3 and the effective shear strain (γ) had values ranging from 300 × 10−6 to 25,000 × 10−6 which for γ > 10,000 × 10−6 catastrophic landslide or very large deformation will be occurred. This study illustrates how the vulnerability index and the effective shear strain could be used to make preliminary assessment of the research area deformed during the earthquake.

A Comparison of Maceral and Microlithotype Indices for Interpretation of Coals in the Samarinda Area, Lower Kutai Basin, Indonesia

Strata of the Middle Miocene Balikpapan Formation from the Lower Kutai basin are well exposed in a section near the Samarinda city, East Kalimantan, Indonesia. The succession is characterized by thick sandstone bodies alternating with shales and coal beds. A 250 m thick composite section of exposed sediments (not including the soil interval) was measured, from which 25 coal samples were collected. Petrographic, microlithotype, and maceral analyses were performed in order to determine the depositional environment of the Samarinda coals. In order to assess the development of paleomires, coal facies diagrams were obtained from microlithotype and maceral composition. According to the organic petrologic results, the Samarinda coals represent a highly degraded humodetrinite-rich group deposited from terrestrial into telmatic condition of peat formation with vegetation characteristics of highly degraded woody forest type evolved under alternate oxic to anoxic moor conditions. These formed with intermittent moderate to high flooding as the paleopeat environment shifted from mesotrophic to ombrotrophic.

Estimation of underground structure at Prambanan area, Yogyakarta, Indonesia

In this study, we investigated the underground structure at Prambanan area, Yogyakarta. We performed single observations of microtremor at 124 sites in Prambanan area. The results enabled us to estimate the site-dependent shaking characteristics of earthquake ground motion. We also conducted 2-site bore holes investigation to gain a representative determination of the soil condition of subsurface structures in Prambanan. From the SPT of borehole observations, the prambanan area corresponds to relatively soil condition with Vs ≤ 298 m/s, the predominant periods due to horizontal vertical ratios (HVSRs) are in the range of 0.48 to 1.19 s and the frequency are in the range of 0.95 to 1.92 Hz. By making these observations, we can obtain a relationship between the predominant periods, frequency and distribution of the first layer thickness of the sediment.