Abstract
This study shows a map depicted over a limited time, which indicates an area of stability of surface and shallow sediments in a deepwater area. The map was a geo-engineering reference in evaluating and selecting drilling sites in deepwater. The area of study was located in the shelf-edge and upper slope off Northern Territory, Australia through the eastern part of the Timor Trough, West Arafura Sea, Indonesia. Plastic deformations are represented by various kinds of sea-bottom topography in the upper slope through the trough (water depths: ca. 400 to 2,000 mSS). Stability of the sediments is the subject critical for sea-bottom facilities in operations of drilling and facilityconstruction in the deepwater area. A simple, quick-look method to identify an area of unstable sediments at the sea-bottom and shallow depths was proposed and examined. Sea-bottom topographic irregularity is eyeballed and semiquantitatively measured mainly on 2D seismic images with an aid of a workstation for seismics. A mechanical model of bending plastic-rigid plates was applied to insure objectiveness in the semiintuitive analysis. Bending rigidity is a key in this analysis. The differential profile was defined to estimate the sea-bottom-topographic irregularity. Cross plots of (X, θ); X: distance of measuring point from a reference origin, and θ: bending angle with reference to the horizontal datum, of plastic deformation of surface and near surface sediments in the area. The deformation begins at a critical angle (θc) along a dip line of survey. θc varies from 0.7 to 3°, and is mostly 0.7° to 1.2°. 1,000 mSS in water-depth approximately corresponds to the θc, and the contour line of the depth virtually * INPEX Corporation, Tokyo ** Advanced Geomechanics *** INPEX Corporation, Jakarta indicates the boundary between the upper and lower slopes. Actual drilling operations confirmed the predrilling diagnosis of the sediment stability in this study.
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