Determination of properties of hydrothermally altered rocks using a Schmidt hammer (Bolshoy Semyachik, Kamchatka peninsula)

In the course of field work on Bolshoy Semyachik in 2020, 14 samples of varying degrees of alteration were taken on two thermal fields (Verhnee thermal field on Burlyashchiy volcano and thermal field of the northern crater of Central Semyachik). Samples included rocks of all stages of alteration from unaltered andesite and basaltic andesite, to completely transformed opalites. Samples were taken from outcrops in various parts of the fields and beyond. Seven samples were taken from each thermal field, and a series of cylinders were prepared from each sample partly in laboratory and partly in field conditions, which were used to measure properties. Unaltered samples from the Verhnee thermal field of Burlyashchy volcano are represented by basaltic andesites, and from Central Semyachik, by andesites. In order to determine the correlation, the properties of 123 cylinders were studied in laboratory conditions, and each of the 14 outcrops was examined with a Schmidt hammer.

In this study Schmidt hammer RGK SK-60 was used (type N). For each sample, 20 measurements were made on each outcrop, which made it possible to calculate the standard values of the elastic rebound in accordance with the methods set in ISRM. At the same time all the prepared cylinders were tested in the laboratory using standard methods to define different physical and mechanical properties.

In the most obvious way, the rebound height turns out to be related to the porosity of the opalized rocks and the dependence turns out to be linear. And porosity determines the indicators of a variety of other physical and mechanical properties. Due to that fact there is also a close relationship between the rebound height and such properties as density, water absorption, velocities of elastic waves, compressive and tensile strength etc.

As a result of the studies carried out, it was found that the height of the elastic rebound, obtained with the Schmidt hammer, is closely related to the physical and mechanical properties of effusive hydrothermally altered rocks. Moreover, the connection turns out to be closer in the rocks of the greatest degree of change, due to the fact that in the process of opalization, due to acid leaching and partial deposition of secondary minerals, the original composition and structure of the rocks completely changes and becomes more similar.

Based on the obtained regularities, it can be concluded that in conditions of strong variability of the geological structure, which occurs in thermal fields characterized by the discharge of acidic thermal waters, the use of the Schmidt hammer as an indirect method for determining the indicators of physical and mechanical properties is very expedient. The revealed interrelationships of the elastic rebound height (R_N) with physical and mechanical properties  make it possible to reliably assess the properties of rocks of varying degrees of alteration even in the field.