Caliper logging of boreholes is commonly conducted to measure variations in borehole diameter over the length of the excavation. Information of borehole diameter, shape and roughness can be used to calculate borehole volume.
The Caliper Tool used for this test consists of 3 or 4 independent spring-loaded caliper arms that continually adjust to the size of the borehole as they are pulled to the surface. These arms are opened and closed using a small motor incorporated in the tool. The tool is lowered into, and lifted from the shaft at a controlled rate using a winch unit. The unit includes a depth encoder, which indicates the depth of the probe and a cable tension reading, which can be used to determine when the probe has reached the bottom of the borehole. The depth and diameter are read by a surface data logger unit interfaced with a PC which promptly displays the data to be adjusted and recorded. Computer software performs the analysis and processes the data resulting in a graph of diameter as a function of depth as shown opposite.
Such graphs provide indications on the presence of cave-ins, collapse or shale swelling and other geological characteristics in the borehole.
