A number of different mechanisms can cause attenuation of propagating seismic waves in a fractured fluid-saturated porous medium, notably geometrical spreading, displacement of pore fluid relative to the solid frame, and transmission losses and scattering. In this study, we examine these attenuation mechanisms using numerical forward simulations and a field example. The numerical methods include a quasi-static upscaling approach and wave propagation simulations. They are based on Biot's equations of poroelasticity and, hence, fractures are modeled as soft, highly porous and permeable features. The field examples include full-waveform sonic data from the Grimsel Test Site underground laboratory situated in a granodioritic rock mass, which exhibits both brittle and ductile deformation structures at various scales.