Application of downhole seismic refraction test to the rocks and sediments of the Maltese islands

Abstract

This study focuses on conducting the downhole seismic refraction test on different stratigraphic sequences within the geology of the Maltese Islands, thus allowing for analysis and comparison of the ground’s geomechanical properties. Typically, two boreholes were drilled at each location to conduct the downhole seismic test, usually down to a vertical depth of 30 metres. Two perpendicular seismic wave energy sources were used to determine the compression (P-wave) and shear (S-wave) velocities, Vp and Vs respectively, of the different ground strata. On doing so, the density of the ground was determined from the P- wave velocities and the fundamental elastic moduli were determined from S-wave velocities. Through this, the different velocity profiles, and elastic moduli of the ground materials from different areas within Malta were derived, analysed, and compared. Velocity profiles were obtained for Quaternary deposits, weak Upper Coralline Limestone, intact Upper Coralline Limestone, Blue Clay, weak Lower Globigerina Limestone and intact Lower Globigerina Limestone, allowing these to be compared. These different stratigraphic layers, of widely varying rigidity, had their corresponding average shear modulus varying accordingly, having the lowest Gmax for quaternary deposits at 0.20GPa and the highest Gmax for Lower Globigerina Limestone at 3.30GPa. Determining the shear modulus, Gmax, at small strain is of fundamental importance to the prediction of the deformation behaviour of ground materials. The stiffness parameters of the ground are fundamental in estimating the effect of foundation loading and excavation. This study attempts to provide preliminary data related to the different stiffness moduli of the various categories of ground materials across the Maltese Islands, thus providing guidance to the engineer undertaking such analyses.