Determination of Goaf Delamination Horizons and Other Roof Geomechanical Features using Downhole Acoustic Logs

This project has sought to utilise the acoustic scanner log and the sonic log to detect and define features within the overburden of a coal seam which control roof stability and goaf formation. In order to do this it was first necessary to validate the features interpreted from the scanner by comparison to core and other commonly used methods. To determine the usefulness of the sonic log it was compared with historical extensometer data. This work has resulted in the following conclusions:

• interpretation of the acoustic scanner image recognises more than 50% of fractures logged from core, with the advantage of determining the dip and direction of every feature interpreted
• the acoustic scanner can identify additional fractures and lithological features from sections of the borehole with broken or missing core
• planes of weakness are identified by the acoustic scanner which are not recorded by initial core logging, but which are confirmed by later inspection
• recording of bedding breaks from core does not relate well to the definition of weak bedding planes in the acoustic scanner image
• interpreted features matched with fractures identified by core logging provide clearly defined sets which agree with in-situ observations
• breakout identified by the acoustic scanner provides a reliable and cheap indicator of the principal stress direction, and an indication of its magnitude
• the acoustic scanner can be used in isolation or in combination with other geophysical logs to define geomechanical units and horizons which assist in characterising strata behaviour in coal mines
• acoustic scanner images and interpreted data can be displayed in a variety of formats to visually convey the location and nature of geotechnical conditions
• there is no discernible relationship between transit time contrast from sonic logs and height above mining in the intermediate and upper caving zones with respect to propensity for strata separation
• the transit time contrast alone does not provide a reliable indication of potential parting planes that would result in strata separation during goafing
• the sonic velocity can be used quantitatively in the immediate caving zone (up to about 7 times mining height) and a tentative relationship has been proposed for such application

Goafing behaviour above the immediate roof (approx 20m) is governed by a combination of factors, including bed thickness, strength homogeneity, and the juxtaposition of beds of contrasting character, and cannot be predicted solely on the basis of weak partings. However the identification of weak partings from the sonic log and weak beds and a high frequency of fractures or weak bedding planes from the scanner may be useful indicators of potential separation horizons.

Whilst the definition of goaf delamination horizons from the acoustic scanner or the sonic log has not been achieved by this project, considerable knowledge has been gained and demonstrated on the reliability, cost effectiveness, and contribution of these tools to the understanding and management of coal seam roof and goaf conditions.

To successfully define those horizons which contribute to the caving behaviour at any mine site significantly more data needs to be collected. The combination of suitably located and monitored extensometer installations in boreholes with acoustic scanner logs of the borehole will considerably assist the further understanding of this behaviour. The combination of various tools and methods may be required to ultimately predict the primary features which influence goaf formation.