Use of the Acoustic Energy Meter (AEM) for rapid geomorphological field assessment of rock hardness: comparison with Schmidt hammer R -value

Abstract

Rapid, field-based assessments of rock hardness are required in a broad range of geomorphological investigations where rock intact strength is important. Several different methods are now available for taking such measurements, in particular the Schmidt hammer, which has seen increasing use in geomorphology in recent decades. This is despite caution from within the engineering literature regarding choice of Schmidt hammer type, normalization of rebound (R-) values, surface micro-roughness, weathering degree and moisture content, and data reduction/analysis procedures. We present a pilot study of the use of an Acoustic Energy Meter (AEM), originally produced, tested and developed within the field of underground mining engineering as a rapid measure of rock surface hardness, and compare it with results from a mechanical N-Type Schmidt hammer. We assess its capabilities across six lithological study sites in southeast Queensland, Australia, in the Greater Brisbane area. Each rock exposure has been recently exposed in the 20th/21st century. Using a 'paired' sampling approach, the AEM G-value shows an inverse relationship with Schmidt hammer R-value. While both devices show variability with lithology, the AEM G-values show less scatter than the Schmidt hammer. We conclude that each device can contribute to useful rock hardness testing in geomorphological research, but the AEM requires further field testing in a range of environments, and in particular on older and naturally-exposed rock surfaces. Future evaluations can extend this pilot study by focusing on sampling procedures, energy sources, and data reduction protocols, within the framework of a comparison study with other rock hardness testing apparatus.