Development of an electronic probe for rock mass characterisation

Abstract

While considerable technological advances have been made in different fields related to both soil and rock mechanics, there is a dearth of low-cost geophysical tools aimed at rock mass characterisation through pre-drilled small-diameter boreholes (as small as 60 mm). A literature review was conducted to understand the tools and methodologies currently in use for analysis. This was supplemented by site visits and hands-on experience to help familiarisation with borehole drilling operations, discontinuity logging in a soil and rock testing laboratory, rock mass discontinuity surveying and the use of downhole televiewers. While techniques and commercially-available tools exist and have previously been used locally, these do not provide an all-in-one solution, particularly because of their relatively large physical size and high cost. After establishing a set of design requirements, a small-diameter borehole 3D-scanning system was proposed. At its core, the system uses a miniature single point IR-ToF distance sensor that measures distance to the internal surface of the borehole as it rotates and moves along the bore axis. The data acquired is then processed to generate a 3D point cloud model of the borehole and its discontinuities (fissures). Processing includes the orientation of the point cloud data according to data provided by an orientation sensor. The distance sensor and orientation sensor were both selected and characterised through a series of experiments using hardware and software tools developed specifically for this purpose. The selected distance sensor was used to 3D-scan a physical model that emulated a segment of a fissured borehole. After validating the proposed system, a prototype instrument consisting of a downhole probe, a probe deployment system, and a ground station, was designed and built to test the technology in the intended operating environment. Each unit is made up of various mechanical and electrical parts such as electronic circuit boards, along with the necessary firmware. Field testing confirmed that the developed instrument was capable of obtaining the data needed to faithfully model a borehole and its discontinuities. Further testing is required to continue characterising the instrument in different operating conditions. Hardware and software modifications are also needed to address certain limitations that were identified during field testing.