WASET
    Ernst D. Schmitter,  Signals from the Rocks.   journal   = {International Journal of Geotechnical and Geological Engineering}, [online]. World Academy of Science, Engineering and Technology.
    January 2008, vol. 13(1). 1 - 4
    [viewed 23 April 2024]. Available from: https://publications.waset.org/pdf/3036.