Underground » Strata Control and Windblasts
Understanding the regional to local variability of stress field should be the starting point for mine design and practices because the stresses control the geotechnical behaviour and ground conditions of mine sites. The state of stress in most of the Australian basins is well understood. However, in the latest release of the Australian Stress Map database, the Bowen Basin is represented by only 26 stress data records that show various trends.
Generally, in-situ (virgin) stress measured at any point is the combination relative forces from the tectonic plate-scale to local-scales. Hence, some basins show a uniform stress pattern, meaning that the plate tectonic forces directly control the stress pattern. In contrast, some other basins show variable stress patterns that highlight the role of geology (as smaller scale forces) that are superimposed on the plate tectonic forces. Understanding of this relative contribution is important in geomechanical and geotechnical assessments in a basin.
Our knowledge of the continental pattern of stress in Australia is well understood but relative contribution of large tectonic forces, intermediate intraplate forces, and small-scale forces (due to complex geology) requires further investigation. Hence, this project aims to better investigate the stress pattern of northern Bowen Basin to understand whether the northern Bowen Basin is showing a more N-S trend for the orientation of maximum horizontal stress, i.e., SHmax (similar to north-eastern Australia), or it is more ENE-WSW similar to regional SHmax pattern in central and southern Bowen, and most of Surat Basins.
This project investigated the virgin stress state at a basin scale, looking at both orientation and magnitude, and tests if there are any major stress variability at basin-scale in the northern Bowen Basin. Over 1000 image logs (ATV/OTV) were compiled with overall lengths of ~240km to investigate the in-situ stress pattern of the northern Bowen Basin in the shallower part (<1 km) of the earth crust. A 3D GoCAD-SKUA model of the Nebo Synclinorium was developed and populated by three simplified stratigraphic packages (which defined regionally significant surfaces derived from seismic and drillhole data) to construct a 3D geomechanical-numerical model for the Nebo Synclinorium.
The results revealed a consistent maximum horizontal stress orientation (N18⁰E) across spatial scales in the northern Bowen Basin, which is in contrast with the Southern Bowen, Surat, and NSW's basins, where there are significant rotations of stress (sometimes 60⁰ to 70⁰rotation in 70km) across spatial scales due to geological structures. Numerous very small-scale stress rotations with depth (between 1 and 10 m) in various boreholes in the vicinity of geological structures such as faults and where we see changes in lithology (stiffness contrast) were observed. These variations were mainly observed between coal seams and inter burden strata, which would have impact on designing support in underground mines.
This project resulted in an updated and comprehensive stress map for the northern Bowen Basin, a 3D geological model for the Nebo Synclinorium and a 3D geomechanical model of stress that contains all parameters of stress tensor (at basin-scale) for this region. Output of this project can be used as a tool for he geotechnical and geological analysis of coal mines in the Nebo Synclinorium. It can also be taken to the next level, and be used to improve geotechnical models of mining induced ground movements, reduce the instability issues associated with open cut and underground coal mining, improve gas drainage and groundwater modelling, in particular the communication between fracture zones above longwalls and overlying aquifers.