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Stoneley-guided waves

Rocks in the upper crust are usually porous and/or fractured. The pores and fractures are either fully- or partially-saturated with different fluids, such as for example water or oil. In porous and fractured rocks a variety of deformation processes can occur which is still not understood in detail. For example, during deformation of partially-saturated rocks the viscous pore fluids move due to fluid pressure differences and cause dissipation (figure 1). This dissipation can attenuate seismic waves traveling through partially-saturated porous rocks and modify their reflection coefficient. Also, at the interface between rock and fluid so-called Stoneley waves (Figure 2) can propagate. In fractured rocks, Stoneley guided waves can propagate along fractures and are reflected and scattered at the fracture tips. With the help of numerical simulations we try to better understand the complex deformation and wave propagation processes in porous and fractured rocks. A better understanding of these processes is important, for example, for monitoring reservoirs for gas or CO2 storage.
 

Figure 1: Finite element mesh resolving the curved shape of of fracture tips. The fracture (black mesh) in an elastic medium (grey mesh) is filled with a viscous fluid.

 

Figure 2: Finite element simulation showing the interference of Stornley guided waves in two intersecting fractures. Shades of grey indicate the absolute particle displacement (in arbitrary units).

 

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Marcel Frehner
Beatriz Quintal
Holger Steeb

 

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