Digital imaging of geotechnical boreholes: benefits and risks

In recent years, the use of borehole televiewers has become common in geotechnical site investigations in Australia, particularly those involving engineering structures in rock. Borehole televiewers can rapidly and efficiently generate vast lengths of high resolution digital imagery of borehole walls and borehole survey data. These extraordinary geo-referenced images of the subsurface present several major advantages for ascertaining in situ geotechnical information over conventional geotechnical core logging. The mystery of core loss zones can be resolved. Rock fabric directions do not require relatively unreliable oriented core measurements. Deviated boreholes can now be correctly placed in 3D ground models. Borehole principal stress directions can be assessed and the painstaking task of logging rock defects at mm scale can be done patiently onscreen. As with any ground test procedure, there are potential bad practices, hidden sources of error and some drawbacks in the results of televiewer data. Poor results can be due to the wrong choice of tools, overlooking of tool calibration checks, and lack of care with tool centralisation. Major sources of inaccuracies can arise from neglecting corrections of borehole diameter and trajectory variations. Quality issues and erroneous readings of borehole geometry aside, the televiewer images present factual and unbiased depictions of the boreholes. Yet analysis of the images to extract information of geotechnical interest can be subjective, requires input from an engineering geologist or geotechnical engineer and verification with rock core data.