The Velocity of Elastic P Waves in Unconsolidated Material Under Low Pressure

Abstract

The growing interest in geophysical, and specifically seismic, methods to solve problems in engineering and environmental activities, calls for specific investigation into the elastic properties of unconsolidated materials lying in the uppermost part of the section. The study includes experimental measurement of the compressional velocity of soil, sand and gravel samples of different grain sizes, both dry and saturated. The wide range of velocity values obtained for saturated gravel samples required additional measurement runs, which were carried out every hour. The samples were jacketed into a hard rubber tube, which proved to be more convenient as its compressional velocity is less than that of the samples. The absolute error of measurement does not exceed 3.3%. The interpretation of the results led to the following conclusions: (1) the compressional velocity has a linear relationship with the logarithm of the pressure (semi‐logarithmic); (2) the effect of trapped air bubbles, 1–2% air saturation, significantly lowers the velocity of saturated sand samples below that of water; (3) grain packing has a more significant effect on p‐wave velocity for saturated samples than it does for dry samples; and (4) the propagation velocity may be used to estimate the porosity of the unconsolidated material, assuming the saturation and lithology are known.