Measurement of Seismic Wave Velocities on Soft-Sedimentary Rocks from Horonobe Under Confining Pressure

Ogawa, Toyokazu*; Matsui, Hiroya 

Horonobe Underground Research Center has been conducting researches and developments into geological disposal of high-level radioactive wastes in sedimentary rocks. In the 2003 borehole investigation programmes, 3 boreholes (HDB-6, 7, and 8) were drilled in order to acquire geoscientific information. This technical report presents the results of elastic wave velocity measurements under confining pressure, carried out on the rock cores extracted from BH HDB-6. The main observations which became clear from the laboratory tests are as follows: 1. the basic physical properties of the rock specimens are consistent with the existing measured values. Moreover, when the wet sample is dried, although P wave velocity at first shows a downward tendency, it increases gradually and approaches the dry velocity as the degree of saturation decreases. Therefore, it is felt that the sample is uniformly dried across the cross-sectional area. ; 2. It appears that the elastic wave velocities of the Horonobe sedimentary rocks are dependent upon pressure. However shear rigidity remains unchanged, even if the saturation conditions of pores change (from wet to dry conditions). In the study of anisotropy of velocities, horizontal velocity seems larger than the vertical one. Although there was no remarkable influence on elastic wave velocities when the loading and unloading of confining pressure were repeated, a certain amount of hysteresis could be observed in porosity. Continuous investigation is required. ; 3. Measured wet velocity may practically be used in evaluating in-situ velocity measurement results.