Laboratory measurements show that the seismic velocities as a function of effective stress yield effective stress coefficient less than 1 and usually varying between 0.5 and 1. At the same time, theoretical analysis shows that for an idealised mono-mineral rock the effective stress coefficient for seismic velocities (or, more precisely, for elastic moduli) must always be equal to 1. The presence of heterogeneity as a possible cause of this variation is investigated in the double shell spherical model. The results show that only a small amount (less than 1%) of soft component is sufficient to cause this effect. Such soft material may be present in grain contact areas of many rocks and may explain the variation observed experimentally.


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