Comparison of electronic and mechanical Schmidt hammers in the context of exposure-age dating: are Q- and R-values interconvertible?

摘要

Application of Schmidt-hammer exposure-age dating (SHD) to landforms has substantially increased in recent years. The original mechanical Schmidt hammer records R-(rebound) values. Although the newly introduced electronic Schmidt hammer (SilverSchmidt) facilitates greatly improved data processing, it measures surface hardness differently, recording Q-(velocity) values that are not a priori interconvertible with R-values. This study is the first to compare the performance of both instruments in the context of field-based exposure-age dating with a particular focus on the interconvertibility of R-values and Q-values. The study was conducted on glacially polished pyroxene-granulite gneiss, Jotunheimen, southern Norway. Results indicate that mean Q-values are consistently 8-10 units higher than mean R-values over the range of values normally encountered in the application of SHD to glacial and periglacial landforms. A convenient conversion factor of ±10 units may, therefore, be appropriate for all but the softest rock types close to the technical resolution of the instruments. The electronic Schmidt hammer should therefore be regarded as a useful complement and potential replacement for the mechanical Schmidt hammer. Conversion of published R-values data to Q-values requires, however, careful control and documentation of instrument calibration.