Preliminary evaluation of the rock mass permeability of a granite site based on sonic logging

Abstract

It is of great value to reasonably obtain the permeability characteristic value of a site’s bedrock, especially for the deep rock mass space excavation, site layout design, and subsequent effective management of some major engineering sites. Previous studies have found that the permeability of water-bearing media decreases with depth, and the compressional wave velocity can be used to estimate the permeability coefficient. Taking a preselected granite site as an example, we used deep borehole acoustic logging technology to obtain the longitudinal wave velocity data at different depths. The empirical relationship between the longitudinal wave velocity and the permeability coefficient in the granite field was further used to obtain the permeability coefficient at different depths of a borehole in the study area; we then compared these data with in situ borehole water pressure test data and found that the empirical relationship between the longitudinal wave velocity values and the permeability coefficient values at different depths cannot be directly applied. For example, the permeability coefficient increased sharply at –405 m to –425 m, instead of continuing to decrease according to the previous law, the measured value was 1 orders of magnitude larger than the empirical value. Through comprehensive analysis of the borehole acoustic testing of the longitudinal wave velocity changes and the corresponding core data, it can be preliminarily determined that the permeability characteristics of the deep rock mass in this area are obviously influenced by the geological structure.