Effects of water and melt on seismic velocities and their application to characterization of seismic reflectors

Abstract

The effects of a silicate melt and water on seismic velocities are compared at relatively small fluid fraction (less than 20 vol.%) on the basis of a theoretical model of composite materials, to show that the two fluids are clearly characterized by the velocity ratio Vp/Vs. For a silicate melt, Vp/Vs increases with increasing fluid fraction. When a significant reflection is expected, Vp/Vs becomes much larger than 2. For water, Vp/Vs decreases as the fluid fraction increases to 10 vol.% then it increases. But it remains similar to a solid state value (about 1.8), unless the amount of water exceeds 15 vol.%. This will be a good measure to distinguish between two candidates for seismic reflectors: partially molten rocks and rocks containing free water. If a reflector can be treated as a thin low velocity layer, the velocity ratio in it can be estimated from the frequency dependence of reflection coefficient.

Crustal and upper mantle structure beneath the northeastern Tibetan Plateau from joint analysis of receiver functions and Rayleigh wave dispersions

“…Pan & Niu 2011). As the V p /V s increases with a rising fluid fraction, partial melting also has great influences on the V p /V s (Watanabe 1993). We find the LVZ in the crust of both the NW Qilian orogen and the Songpan-Ganzi terranes.…”

Lithospheric low-velocity zones associated with a magmatic segment of the Tanzanian Rift, East Africa

“…Vp/Vs ratio usually exhibits low values for felsic composition rocks (1.71 for granite, 1.78 for granodiorite) and increases for mafic rocks (1.84 for gabbro) (Holbrook et al 1992;Zhao et al 2010). The presence of fluids and partial melt are also known to control Vp/Vs ratios, the latter increasing with higher contents (Watanabe 1993).…”

Receiver function constraints on crustal seismic velocities and partial melting beneath the Red Sea rift and adjacent regions, Afar Depression

“…Fluid fractions within a medium have been shown to be clearly identifiable through the application of the velocity ratio , specifically with regard to a silicate melt [Watanabe, 1993]. The value is a continuous function of the melt fraction, with a normal increase in the fluid percentage resulting in an exponential increase in as a consequence of the rapid reduction of V s relative to V p (Figure 2 Figure 2).…”