Publications

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, q), X: distance of measuring point from a reference origin, and q: 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 (qｃ) along a dip line of survey. qｃ varies from 0.7 to 3°, and is mostly 0.7° to 1.2°. 1,000 mSS in water-depth approximately corresponds to the qｃ, and the contour line of the depth virtually indicates the boundary between the upper and lower slopes.Actual drilling operations confirmed the predrilling diagnosis of the sediment stability in this study.